AS Level

Chapter 1
Cell Structure
 Chapter Outline
Part 1: Microscopy
• Magnification vs Resolution
• Light Microscope vs Electron Microscope

Part 2: Micrometry: How do we measure cells?

• Calibrating EPG using the Stage Micrometer (Practical 1)
• Magnification

Part 3: Cell Structure and Function

• Organelles
• Animal vs Plant cells
• Eukaryotes vs Prokaryotes
• Viruses
Updated on 12/7/21 by Beh SJ @behlogy
 LIGHT

MICROSCOPY MICROSCOPES
AND ELECTRON
MICROSCOPES

Updated on 12/7/21 by Beh SJ @behlogy

 Types of Microscopes

• Electromagnetic radiation
(light / electrons)
→ Image of specimen

1. Light microscope
2. Electron microscope
a) Transmission Electron Microscopes (TEM)
b) Scanning Electron Microscopes (SEM)

Updated on 12/7/21 by Beh SJ @behlogy

 Magnification and Resolution
1) Magnification
• Number of times an image is enlarged,
compared with the actual size of the object
• “x” sign placed in front of number

2) Resolution
• Ability to distinguish between two points clearly as
separate
• Units in nm

Increase in magnification ≠ increase in resolution

Updated on 12/7/21 by Beh SJ @behlogy

Magnification

Updated on 12/7/21 by Beh SJ @behlogy

 Resolution

High resolving power Low resolving power

Updated on 12/7/21 by Beh SJ @behlogy

 What determines resolution?
• Range of electromagnetic radiations
of different wavelengths
→ Electromagnetic spectrum

• Shorter wavelength used

→ Higher resolving power
→ Wavelength of visible light is longer than of an electron
→ Ability to distinguish between two points is lower

• Max resolution is the…

→ Shortest distance between 2 separate points
→ The max resolution = ½ wavelength used
→ Shorter wavelength = higher max resolution = lower resolution
Updated on 12/7/21 by Beh SJ @behlogy
 Resolution
For example:
• Max resolution is the…
→ Shortest distance between 2 separate points
• The max resolution = ½ wavelength used

• Wavelength of visible light = 400-700nm

• Max resolution of a light microscope = 200nm

• If closer/smaller than 200nm, points cannot be distinguished

as separate
• Can a ribosome (smallest organelle) of 25nm be seen?

Updated on 12/7/21 by Beh SJ @behlogy

 Light Microscope
• Source of electromagnetic radiation: Visible light
- Wavelength = approx. 400-700nm
- Lower energy and longer wavelength than electrons
- Focused using mirrors and glass lenses

• Highest magnification: x1,500

• Max resolution : 200nm (low)

• Advantage: Live specimens can be

viewed, image can be coloured

Used for viewing structures that can be

measured in µm
Updated on 12/7/21 by Beh SJ @behlogy
 Electron Microscope
• Source of electromagnetic radiation: Free electrons
- Wavelength = approx. 1 nm
- High energy, very short wavelength
- Must be in a vacuum environment so electron can
travel in straight lines
- Use electromagnetic lenses

• Highest magnification: x250,000

• Max resolution: 0.5 nm (high)

• Disadvantage: Only dead material can

be examined in vacuum, images are black
and white

Used for viewing structures that can be

measured in nm Updated on 12/7/21 by Beh SJ @behlogy
Updated on 12/7/21 by Beh SJ @behlogy
 Types of Electron Microscopes
a) Transmission Electron Microscopes (TEM)
b) Scanning Electron Microscopes (SEM)

Updated on 12/7/21 by Beh SJ @behlogy

 Transmission Electron Microscopes
• Beam of electrons
• Pass through (transmit) specimen
before being viewed

• 2D appearance

• Details inside cells

→ Internal structures
→ Membranes within internal
structures

Updated on 12/7/21 by Beh SJ @behlogy

 Scanning Electron Microscopes
• Lower resolution compared to TEM

• Scan surfaces of specimens

• 3D appearance

Updated on 12/7/21 by Beh SJ @behlogy

Q: List the differences between a light microscope and an electron
microscope. [4]

This is how you should ALWAYS present your answer to compare/contrast

between 2 things.

Features Light Microscope Electron Microscope

E.g. Highest Lower Higher
magnification of
microscope

• 4 marks = 4 points (1 mark is for table plus 1 just in case)

• DO NOT tick or cross in the box.
• Use comparative language.
• Make your “Feature” as specific as possible.
(e.g. instead of magnification, write highest magnification of microscope)
Updated on 12/7/21 by Beh SJ @behlogy
Q: List the differences between a light microscope and an electron
microscope. [4]

A:
Features Light Microscope Electron Microscope
Type of radiation Light Electrons
Radiation travels through Air Vacuum
Longer Shorter
Wavelength of radiation
400-700 nm 1 nm
Highest magnification of Lower Higher
microscope x1500 x250,000
Max. resolution of Lower Higher
microscope 200 nm 0.5 nm
State of specimen Live Dead

Updated on 12/7/21 by Beh SJ @behlogy

MICROMETRY HOW DO WE
MEASURE CELLS?

Updated on 12/7/21 by Beh SJ @behlogy

Units
• SI unit for length → metre (m)

Suitable units for…

• Light microscope = micrometre (μm)
• Electron microscope = nanometre (nm)

Note:
• Always measure image length in mm !!!
• Present the actual diameter of structure in μm
Updated on 12/7/21 by Beh SJ @behlogy
How do we measure cells?
Eyepiece graticule (EPG) is
on the eyepiece lens

Stage micrometer is
placed on the stage

Updated on 12/7/21 by Beh SJ @behlogy

How do we measure cells?
Two components needed:
• The stage micrometer
Shows the true value of length
(Usually 10mm in length with 100 small divisions)
Appears bigger when magnification increases

• The eyepiece graticule (EPG)

Shows 100 graticule units (100 EPG) which are in arbitrary units
Appears constant no matter the magnification

Updated on 12/7/21 by Beh SJ @behlogy

 (appears constant)

(appears larger)
Updated on 12/7/21 by Beh SJ @behlogy
How do we measure cells?
Two Steps:
1. Calibrate the EPG with the stage micrometer
• 1 EPG = __?___div = __?__ µm

2. Use the EPG to measure cell/structure

• 1 cell = ___?__ EPG =__?__ µm

Updated on 12/7/21 by Beh SJ @behlogy

1. Calibrate the EPG with the stage micrometer
1 EPG = __?___div = __?__ µm

Eyepiece Graticule

mm
Stage Micrometer
15 EPG = 10 div = 0.1 mm
1 EPG = 0.1 mm/15
= 100 μm/15
= 6.67 μm
Updated on 12/7/21 by Beh SJ @behlogy
2. Use the EPG to measure cell/structure
1 cell = ___?__ EPG =__?__ µm

Diameter of cell = 11 EPG

1 EPG = 6.67μm
Diameter of cell = 11 EPG * 6.67μm
= 73.37 μm
Updated on 12/7/21 by Beh SJ @behlogy
Now you try! 1. Calibrate the EPG with the stage micrometer
1 EPG = __?___div = __?__ µm
Q: Given that 100 divisions of the stage micrometer measures 1mm,
calibrate the EPG.

Hint 1: the stage micrometer could be magnified under the microscope

Hint 2: 10 EPG is marked 1 here!

Updated on 12/7/21 by Beh SJ @behlogy

Q: Given that 100 divisions of the stage micrometer measures 1mm,
calibrate the EPG.

A:
100div = 1mm
1div = 0.01mm
= 10µm

50 EPG = 10div
50 EPG = 10 * 10µm
= 100µm
1 EPG = 100µm / 50
= 2µm
Updated on 12/7/21 by Beh SJ @behlogy
 Magnification

• Number of times an image is enlarged,

compared with the actual size of the object
• Formula:
𝐈
𝐌=
𝐀

Remember to convert all measurements to the same units!

Pro tip:
1. Convert
2. Substitute
3. Calculate
Updated on 12/7/21 by Beh SJ @behlogy
 Exercise
Q: On an electromicrograph, a mitochondrion measures 36mm in
length.

If the magnification of the micrograph is x30,000,

what are the actual length of this organelle in µm?

M = x30,000
I = 36mm = 36*1000 μm = 36000 μm
A=I/M 1. Convert
= 36000 μm / 30 000
2. Substitute
= 1.20 µm
3. Calculate
Updated on 12/7/21 by Beh SJ @behlogy
 Exercise
Q: Given that the magnification of this electron micrograph is
x20,000, calculate the actual length of this mitochondria.

60mm

Updated on 12/7/21 by Beh SJ @behlogy

 Exercise
Q: Given that the magnification of this electron micrograph is
x200,000, calculate the actual length of this mitochondria.

M = x20,000

I = 60mm = 60 * 1000 μm
= 60,000 μm

A=I/M
= 60,000 μm / 20000
= 3 μm
1. Convert
2. Substitute
3. Calculate
Updated on 12/7/21 by Beh SJ @behlogy
 Exercise
Q: This is a micrograph taken by an electron microscope of the
egg of an Aedes mosquito.

What is the magnification of this image?

You need to physically measure the scale bar!

A = 200nm
I (measured) = 13mm = (13*1000*1000)nm
= 13000000nm
M=I/A
= 13000000 / 200
= x65,000
Magnification should always have a ‘x’
in front and expressed in whole numbers
Updated on 12/7/21 by Beh SJ @behlogy
CELL STRUCTURE
AND FUNCTION
Updated on 12/7/21 by Beh SJ @behlogy
 Cell Structure and Function
Organelles
• Functionally and structurally distinct part of a cell
• Surrounded by membranes
• For compartmentalization
– So that reactions do not interfere with each other
– Each has separate, specific function

Ultrastructure of cells
• Not necessarily surrounded by membranes
• Detailed structures of a cell
• Only can be seen under an electron microscope
Updated on 12/7/21 by Beh SJ @behlogy
Two types of cells: Prokaryotes vs Eukaryotes
on the Tree of Life

Let’s talk about this

type of cells first!

Updated on 12/7/21 by Beh SJ @behlogy

 List of Eukaryotic Cell Structures
Plant + Animal Cells included!
You need to:
1) Name it 2) Recognise it 3) Know its function

1. Cell surface membrane 9. Chloroplast

2. Nucleus, nuclear envelope and 10. Cell wall
nucleolus 11. Plasmodesmata
3. Ribosomes 12. Vacuole and tonoplast
4. RER 13. Centrioles
5. SER 14. Microtubules
6. Golgi body 15. Cilia
7. Lysosomes 16. Microvilli
8. Mitochondria

Updated on 12/7/21 by Beh SJ @behlogy

 Electromicrograph of a cell
You should learn to identify the main organelles visible here!
Updated on 12/7/21 by Beh SJ @behlogy
 1. Cell Surface Membrane
• Plasma membrane
• ~7 nm thick
• Seen as three layers at x100,000
→ Trilaminar appearance
• Partially permeable
• Made of phospholipid bilayer

Function:
Controls movement of substances into
and out of the cell

Updated on 12/7/21 by Beh SJ @behlogy

2. Nucleus
• Largest organelle!
• Has double membranes

General Function:
• Contains genetic information
for the synthesis of proteins
• Site of transcription of genes and
production of mRNA
• DNA is protected from
degradation by enzymes

Updated on 12/7/21 by Beh SJ @behlogy

Components:
1. Nuclear envelope
2. Nucleus
• Attached to ER
• 2 membranes
• Have nuclear pores
• Function: Controls movement of
substances between nucleus and
cytoplasm

2. Nucleolus
• Densest region
• Function: Site of ribosomal RNA (rRNA) synthesis
→ Site of ribosome assembly

3. Chromatin = DNA and its associated proteins

Updated on 12/7/21 by Beh SJ @behlogy
3. Ribosomes
• Smallest organelle!
• Not bound by a membrane
• Made of rRNA, that is synthesized in
nucleolus + some protein
• Has 2 subunits

Function:
• Site of protein synthesis

Updated on 12/7/21 by Beh SJ @behlogy

3. Ribosomes
Two types:
1. 80S ribosomes are
• 25nm (rmb this!)
• Larger
• Found in cytoplasm and RER
of all eukaryotes

2. 70S ribosomes are

• 18nm
• Smaller
• Found in mitochondria and
chloroplasts of eukaryotes
• Found in all prokaryotes
Updated on 12/7/21 by Beh SJ @behlogy
 4. Rough Endoplasmic Reticulum
• Extensive, connected system of membranes
• Made of cisternae (flattened membrane sacs)
• Continuous with the nuclear envelope
• Running through the cytoplasm
• 80S ribosomes are attached

Functions:
• Site of protein synthesis,
• protein modification
e.g. protein folding
e.g. glycosylation = addition of carbohydrate chains to protein
• protein transport to Golgi

Updated on 12/7/21 by Beh SJ @behlogy

 5. Smooth Endoplasmic Reticulum
• ER without ribosomes

Function:
• Site of lipid and steroid synthesis
e.g. cholesterol, steroid hormones

Updated on 12/7/21 by Beh SJ @behlogy

 6. Golgi body
• Golgi apparatus / complex
• Made of cisternae
• Have layered appearance
• No connection between members
• Not continuous with nuclear envelope

• Swellings at end of sacs for vesicle formation

• Constantly being formed and broken down
→ Being formed by: Transport vesicles from RER on cis face
→ Broken down to form: Secretory vesicles and lysosomes on trans face

Updated on 12/7/21 by Beh SJ @behlogy

6. Golgi body
Functions:
• Modification of proteins and lipid
E.g. glycosylation
phosphorylation = addition of phosphate gp to proteins
cutting / folding proteins

• Packaging molecules into vesicles for transport

• Formation of secretory vesicles for release of protein out of
the cell
• Formation of lysosomes

Updated on 12/7/21 by Beh SJ @behlogy

Production and Secretion of Proteins

Updated on 12/7/21 by Beh SJ @behlogy

Production and Secretion of Proteins

Updated on 12/7/21 by Beh SJ @behlogy

Production and Secretion of Proteins
List the cell structures involved in sequence:
OUT

Steps:
1. Synthesis of protein at ribosome / RER
2. Transport vesicle buds off RER and fuses with Golgi body
3. Modification of protein at Golgi body
4. Separation of a secretory vesicle from the Golgi body
5. Fusion of the vesicle with the cell surface membrane
6. Contents released / secretion of protein by exocytosis

Process also works to embed a protein at the cell membrane.

Updated on 12/7/21 by Beh SJ @behlogy
 Exercise
[CIE, June 2013, P13, Q5]
When mucus is secreted from a goblet cell in the trachea, these
events take place.

1. addition of carbohydrate to protein

2. fusion of the vesicle with the plasma membrane
3. secretion of a glycoprotein
4. separation of a vesicle from the Golgi apparatus

What is the sequence in which these events take place?

A: 1 → 4 → 2 → 3
Updated on 12/7/21 by Beh SJ @behlogy
 7. Lysosomes
• Very, very small
• Spherical, small sacs

Function:
Contains hydrolytic enzymes / lysozymes
• Breakdown unwanted structures via hydrolysis in an acidic enviro
→ Worn out organelles or dead cells
• In WBC, lysozymes digest bacteria
Updated on 12/7/21 by Beh SJ @behlogy
 8. Mitochondria
• Relatively large organelle
• Has double membranes
• Cristae = folded inner membrane
• Matrix = interior solution
• Contain 70S ribosomes and
small circular DNA
• Divide by binary fission
→ Have prokaryotic origin

Updated on 12/7/21 by Beh SJ @behlogy

8. Mitochondria
Functions:
• Site of aerobic respiration
→ synthesize ATP/produce energy
in the form of ATP
→ release energy

Please DO NOT write

“Mitochondria produces energy”
“Mitochondria is the powerhouse of the cell”
-.-

Updated on 12/7/21 by Beh SJ @behlogy

8. Mitochondria
Label the following electron micrograph of an animal cell.

Updated on 12/7/21 by Beh SJ @behlogy

8. Mitochondria
Q: Why are these mitochondria
shaped so differently?

A:

One is a longitudinal cross section and

the other is a transverse cross section!
Also shape may sometimes vary.

Updated on 12/7/21 by Beh SJ @behlogy

9. Chloroplasts
• Relatively large organelle
• Oval shaped
• Two membranes
• Contain chlorophyll
• Thylakoid = flattened
membrane sacs
• Grana = thylakoid stacks
• Stroma = interior solution

• Contains 70S ribosomes, small circular DNA and starch grains

• Divide by binary fission
→ Have prokaryotic origin

Updated on 12/7/21 by Beh SJ @behlogy

9. Chloroplasts

Updated on 12/7/21 by Beh SJ @behlogy

9. Chloroplasts

Updated on 12/7/21 by Beh SJ @behlogy

9. Chloroplast
Function:
• Site of photosynthesis

Two main processes in photosynthesis:

1. Light-dependent reaction (aka “light reaction”)
• Light energy absorbed and water is used to synthesise ATP

2. Light-independent reaction (aka “dark reaction”)

• ATP used to convert CO2 into glucose

Updated on 12/7/21 by Beh SJ @behlogy

 10. Cell Wall
• Thick, rigid layer
• Made of cellulose
• Permeable
• Bcs there are spaces / gaps
between fibres

Functions:
• Provide structural support
• Prevent bursting
• Limit cell size

Updated on 12/7/21 by Beh SJ @behlogy

 11. Plasmodesmata
• Strands of cytoplasm passing
through channels

Functions:
• Allows substances to pass
• From cell to cell
• Without passing through cell walls
E.g. water, sucrose, amino acids, minerals ions, ATP

• Allows more rapid transport of substances

Updated on 12/7/21 by Beh SJ @behlogy

 12. Vacuoles and
Tonoplast
• Commonly found in plant cells
• Large, permanent, central
• Surrounded by a partially permeable
membrane called tonoplast

Functions:
• Store of cell sap (contains water, ions,
minerals, salts, pigments, sugars)
• Stores waste products
• Pushes chloroplasts to edge of cell
• Gives turgidity to the cell
Updated on 12/7/21 by Beh SJ @behlogy
12. Vacuoles and
Tonoplast

Updated on 12/7/21 by Beh SJ @behlogy

13. Centrioles and Centrosomes
• Centrioles are cylindrical
• Made of 9 groups of 3 microtubules
• Not found in plant cells

Functions of Centrioles:
• Involved in cell division
→ Replicates before each cell division and moves to opposite poles
→ Centrioles are found in pairs at right angles (90o) from each other
→ Forms centrosome

• Modified centrioles are also found elsewhere e.g. in flagella / cilia

→ Acts as a Microtubule Organising Centre (MTOC)
→ Organises / assembles microtubules
Updated on 12/7/21 by Beh SJ @behlogy
 13. Centrioles and Centrosomes
Function of Centrosomes:
• It is a MTOC
• Organises / assembles microtubules
• For the formation of spindle fibres
• At opposite poles
• During cell division /mitosis

• Aid contraction of spindle

fibres to separate sister
chromatids

Updated on 12/7/21 by Beh SJ @behlogy

 14. Microtubules
• Very small (~25nm)

• Made from tubulin

→ Form dimers
→ Dimers polymerise to form long
‘protofilaments’
→ 13 protofilaments = 1 microtubule
• Long, rigid, hollow tubes

• Formed and broken down at

Microtubule Organising Centres
(MTOCs)
• E.g. centrosomes, centrioles near
flagella/cilia Updated on 12/7/21 by Beh SJ @behlogy
14. Microtubules

Updated on 12/7/21 by Beh SJ @behlogy

 14. Microtubules
Functions:
• Make up the cytoskeleton
(together with actin filaments)
→ Provides mechanical support
→ Acts as an intracellular transport system
for movement of vesicles or other components
https://www.youtube.com/watch?v=y-uuk4Pr2i8

→ Beating of flagella

• Makes up spindle fibres and centrioles

used in cell division

Updated on 12/7/21 by Beh SJ @behlogy

 15. Cilia
• Only found in eukaryotes
• Smaller in diameter than microvilli
• Also not to be confused with flagella (mostly
found in prokaryotes)
• Motile / moves rhythmically
• Complicated structure made of
microtubules

Function:
• For movement / locomotion
→ E.g. ciliated epithelial cells in lungs,
Paramecium (eukaryotic microbe)

Updated on 12/7/21 by Beh SJ @behlogy

16. Microvilli
• Only found in animal cells
• Found on epithelial cells in the
intestines and kidneys
• Finger-like extensions of the cell
surface membrane

Functions:
Increase surface area
of the cell membrane for:
• Absorption
• Secretion of enzymes
• Digestion at the cell surface
• Excretion of waste substances
Updated on 12/7/21 by Beh SJ @behlogy
 Centrifugation
Q: What happens if we rupture
cells and spin them at high
speed?

A: The larger structures will

sediment first.

Updated on 12/7/21 by Beh SJ @behlogy

 Compare and contrast the structure of
an animal cell and a plant cell. [4]

Feature Animal cell Plant cell

Similarities:

Differences:

Updated on 12/7/21 by Beh SJ @behlogy

 Similarities between
Animal and Plant Cells
• Plasma membrane
• Nucleus
• Nucleolus
• Cytoplasm
• Other organelles
(eg: mitochondria, Golgi apparatus, ribosomes, lysosomes)

Updated on 12/7/21 by Beh SJ @behlogy

 Differences between
Animal and Plant Cells
Feature Animal Cells Plant Cells
Shape of cell No fixed shape Fixed shape
Presence of Cell
Absent Present
Walls
Presence of
Absent Present
plasmodesmata
Absent, if present, Present, large and
Presence of vacuoles
small, temporary permanent
Presence of
Absent Present
chloroplasts
Presence of
Present Absent
centrosomes
Updated on 12/7/21 by Beh SJ @behlogy
 Two types of cells: Prokaryotes vs Eukaryotes
on the Tree of Life
Now let’s talk about
this type of cells!

Updated on 12/7/21 by Beh SJ @behlogy

 Prokaryotes vs Eukaryotes
Prokaryotes
• pro = before
• karyon = nucleus
• Includes all bacteria
and archaea

Eukaryotes
• eu = true
• karyon = nucleus
• Includes plants,
animals, fungi and
other microbes
Updated on 12/7/21 by Beh SJ @behlogy
 Prokaryotic Cells: A typical bacterium
• Unicellular
• Relatively smaller (1-5µm)
• Simpler in structure
• Divides by binary fission

What all bacteria do not have:

• No membrane-bound organelles
• No nucleus
DNA lies free in cytoplasm
in the nucleoid region

Updated on 12/7/21 by Beh SJ @behlogy

 Prokaryotic Cells: A typical bacterium
What all bacteria have:
• Plasma membrane
• Cytoplasm
• Peptidoglycan cell wall
→ made of chains crossed
linked by amino acids
• 70S ribosomes
• Circular DNA
• DNA is naked
→ not associated with
proteins

Updated on 12/7/21 by Beh SJ @behlogy

 Prokaryotic Cells: A typical bacterium
What is only present in some bacteria:
1) Plasmids
• Small, circular DNA
• Codes for non-essential proteins
• Several may be present

2) Pili
• Sexual reproduction
• For attachment to other
cells/surfaces

Updated on 12/7/21 by Beh SJ @behlogy

Prokaryotic Cells: A typical bacterium

3) Flagellum
• Locomotion

4) Capsule
• Outer coat, additional protection
• Attach to surfaces

5) Infoldings of plasma membrane

(mesosomes)
• For photosynthesis / nitrogen fixation
Updated on 12/7/21 by Beh SJ @behlogy
 Prokaryotes,
Mitochondria and Chloroplast
They have a lot in common!

Both have:
• Similar size
• Small, circular DNA
• 70S ribosomes
• Division by binary fission

But why???

Updated on 12/7/21 by Beh SJ @behlogy

 Prokaryotes,
Mitochondria and Chloroplast
A: The Endosymbiotic Theory
https://www.youtube.com/watch?v=FGnS-Xk0ZqU

P/S: This is not in syllabus!

Updated on 12/7/21 by Beh SJ @behlogy

 Compare and contrast the structure of a
eukaryotic cell and prokaryotic cell. [4]

Feature Prokaryotic cell Eukaryotic cell

Similarities:

Differences:

Updated on 12/7/21 by Beh SJ @behlogy

 Eukaryotic Cells
• Larger (~10-100µm in diameter)
• Has membrane-bound organelles

• Has nucleus
• DNA is linear
• DNA associated with proteins

• Larger 80S ribosomes

• Cellulose cell walls (plants)

• Chitin cell walls (fungi)
Updated on 12/7/21 by Beh SJ @behlogy
 Viruses
• Non-cellular structure
• ~50 times smaller than bacteria (20-300nm)
• Much simpler
• No plasma membrane, cytoplasm, ribosomes
• Only:
1. Nucleic acid core = DNA or RNA
2. Capsid = protein coat
- Protective coat
- May have one or two coats
3. Some viruses also have an
outer envelope made of phospholipids
4. Some proteins may be present
- e.g. haemagglutinin, neuraminidase
Updated on 12/7/21 by Beh SJ @behlogy
 Viruses
• All parasitic
• Can only reproduce by infecting living cells
• Uses protein synthesising machinery of
host cell to replicate
• Are they considered living?

Updated on 12/7/21 by Beh SJ @behlogy

 List of Eukaryotic Cell Structures
Plant + Animal Cells included!
You need to:
1) Name it 2) Recognise it 3) Know its function

1. Cell surface membrane 9. Chloroplast

2. Nucleus, nuclear envelope and 10. Cell wall
nucleolus 11. Plasmodesmata
3. Ribosomes 12. Vacuole and tonoplast
4. RER 13. Centrioles
5. SER 14. Microtubules
6. Golgi body 15. Cilia
7. Lysosomes 16. Microvilli
8. Mitochondria

Updated on 12/7/21 by Beh SJ @behlogy

 Important Terms to Remember
Pay attention to those bold, highlighted red text and what’s
emphasised in class!

In addition to the list of 16 eukaryotic cell structures to remember:

Bacteria (Prokaryote) Virus

(No membrane-bound organelles) 1. DNA / RNA
1. Nucleoid region 2. Capsid (protein coat)
3. Envelope
2. Peptidoglycan cell wall
3. Circular DNA
4. 70S ribosome
5. Plasmids

Updated on 12/7/21 by Beh SJ @behlogy

 Chapter Outline
Part 1: Microscopy
• Magnification vs Resolution
• Light Microscope vs Electron Microscope

Part 2: Micrometry: How do we measure cells?

• Calibrating EPG using the Stage Micrometer (Practical 1)
• Magnification

Part 3: Cell Structure and Function

• Organelles
• Animal vs Plant cells
• Eukaryotes vs Prokaryotes
• Viruses
Updated on 12/7/21 by Beh SJ @behlogy
 OK warning! All videos are for reference and
Videos entertainment only and ALWAYS contain too
much/ too little/irrelevant info.

Seeing the Invisible: van Leeuwenhoek's first glimpses of the microbial world:
https://www.youtube.com/watch?v=ePnbkNVdPio

Principles of electron microscopes

https://www.youtube.com/watch?v=ljTEG-B-kGc

Organelles involved in protein synthesis

https://www.youtube.com/watch?v=26y1PCkWiIc

Inner Life of the Cell (3:00 onwards – it’s hard to understand what he is saying, but the 3D animations
gives you a clue on how the cytoskeleton works and how protein production relies on it too!)
https://www.youtube.com/watch?v=FzcTgrxMzZk

Inside the living cell

https://www.youtube.com/watch?v=d4TJ4NY1IA0

Updated on 12/7/21 by Beh SJ @behlogy

 AS Level
Chapter 2
Biological Molecules
 Chapter Outline
Monomer to Polymer
Hydrolysis and Condensation

Carbohydrates
• Monosaccharide, disaccharide and polysaccharide
• Glycosidic bond
• Starch (amylose and amylopectin)
• Glycogen
Protein
• Cellulose
• Amino acids
• Benedict’s Test / Iodine Test
• Peptide bond
• Primary to quaternary structure
Lipids
• Globular vs Fibrous proteins
• Glycerol + 3 Fatty acids
• Haemoglobin
• Ester bond
• Collagen
• Triglycerides
• Biuret Test
• Phospholipids
• Emulsion Test
Water
• Hydrogen bond
Updated on 25/8/21 by Beh SJ @behlogy
Carbohydrates Proteins

Lipids

Updated on 25/8/21 by Beh SJ @behlogy

The Building Blocks of Life
4 most common elements in living
organisms:

Carbon
Hydrogen
Oxygen
Nitrogen

Also common in biomolecules:

Phosphorus
Sulpur

Updated on 25/8/21 by Beh SJ @behlogy

 Monomers
• Elements make up monomers
• Simplest repeating unit of a polymer

E.g. of monomers:
• Monosaccharides
(carbohydrate monomers)
• Amino acids
• Nucleotides
• Fatty acids, Glycerol

Updated on 25/8/21 by Beh SJ @behlogy

 Polymers
• Polymers are made of
1) Repeating monomers
2) Joined end to end
→ In a process called
polymerisation

E.g. of polymers:
• Polysaccharides – carbohydrates
• Polypeptides – protein
• Polynucleotides – nucleic acids

• Polymers can become so large in size → macromolecule

• Giant molecule
Updated on 25/8/21 by Beh SJ @behlogy
 Condensation and Hydrolysis

Condensation:
• Two molecules combine
• Removal of water

Hydrolysis:
• Molecule breaks down
• Addition of water

Updated on 25/8/21 by Beh SJ @behlogy

CARBOHYDRATES

Updated on 25/8/21 by Beh SJ @behlogy

 Carbohydrates
• Made of C, H, O
• General formula: Cx(H20)y

3 groups:

1) Monosaccharides

2) Disaccharides

3) Polysaccharides

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Roles of Carbohydrates in Living Organisms
1) Source of energy in respiration
Eg: Starch, glycogen
High C-H bonds→ energy → ATP

2) Building blocks for larger molecules

Eg: RNA, DNA, ATP, glycoproteins and
glycolipids in plasma membranes

3) Structural support
Eg: Cellulose

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Monosaccharides
• Single sugar molecule
• Soluble, sweet
• Molecular formula: CnH2nOn

Structural formula
Classified according to the
number of carbon atoms:
• 3C – triose (C3H6O3)
• 5C – pentose (C5H10O5)
• 6C – hexose (C6H12O6)

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Linear Structures vs Ring Structures
• In pentose and hexose sugars (5C and 6C)
→ Chain of carbon atoms are long enough to
close up on itself
→ Ring structure
→ More stable, more common

ribose

Updated on 25/8/21 by Beh SJ @behlogy

 Carbohydrates
Ring Structures of Glucose
• Close up at C1 and C5 You need to remember how to
draw the ring structure of
• Has 2 isomers: α- glucose and β-glucose α- glucose and β-glucose!
• Same molecular formula (C6H12O6),
same chemical substance, different form

DUDD

DUDU
Updated on 25/8/21 by Beh SJ @behlogy
Carbohydrates
Disaccharides

GALACTOSE LACTOSE

FRUCTOSE MALTOSE

GLUCOSE SUCROSE

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Disaccharides

• Soluble, sweet
• Formed from 2 monosaccharides
• Through a process called condensation
• 1 hydroxyl group (-OH) + hydrogen atom (H)
→ Produce 1 water molecule (H2O)

• Glycosidic bond formed – this is a covalent bond

• Break down of disaccharides to monosaccharides

→ Hydrolysis
→ Require addition of water
• Both reactions controlled by enzymes
Updated on 25/8/21 by Beh SJ @behlogy
 Formation of Disaccharide - Maltose
2 rings
1 ring “oxygen bridge”

You need to know how to draw products

of condensation / hydrolysis reactions!

Updated on 25/8/21 by Beh SJ @behlogy

Formation of Disaccharide - Sucrose

Updated on 25/8/21 by Beh SJ @behlogy

 Formation of Disaccharide - Lactose

• Draw the structural formula of lactose.

Updated on 25/8/21 by Beh SJ @behlogy

Formation of Disaccharide - Lactose

Updated on 25/8/21 by Beh SJ @behlogy

 Carbohydrates
Polysaccharides
• Polymers / macromolecules
• Made via condensation
• Glycosidic bonds

• Not sugars → not sweet, insoluble

• E.g. Starch, glycogen, cellulose

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Starch
• Storage molecule in plants
• Food reserve
• Amylose + amylopectin

1) Amylose
• Made from α-glucose molecules
• Linked by 1-4 glycosidic bonds
• Long, helical
• Unbranched, linear chain

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Starch

1-4 linkages

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Starch
2) Amylopectin
• Branched molecule
• Made of α-glucose molecules
• 1-4 and 1-6 glycosidic bonds
• Branches are at 1-6 linkages
• Shorter chains

1,6-glycosidic bond

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Starch

Amylopectin

Updated on 25/8/21 by Beh SJ @behlogy

Testing the Presence of Starch
• Iodine solution
• Iodine in potassium iodide solution

• Reacts with amylose in starch

• Form a starch-iodine complex

• Orange / brown → Dark blue

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Glycogen
• Storage of carbohydrates in animals
• Structure similar to amylopectin

• α-glucose
• 1-4 and 1-6 glycosidic bonds
• More branched than amylopectin
• Clumped together → forms granules
• Abundant in liver and muscle cells

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Glycogen

More branched than amylopectin

Updated on 25/8/21 by Beh SJ @behlogy

 Q: Why does excess glucose need to be
stored as starch and glycogen?
Glucose is: Starch and glycogen is:

• Soluble – increase • Inert – non reactive

concentration / decrease
water potential of cell • Insoluble – no osmotic effect on cell,
→ water would enter does not easily diffuse out of cell
→ cell volume increase,
animal cells may burst
• Compact – large quantity of energy
• Reactive – interferes with released when hydrolysed
other reactions in cell
• Glucose can be stored / mobilised
quickly – many ends for
attachment/removal of glucose

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Cellulose

• Structural role in plant cell walls

→ High tensile strength
→ Prevent cell bursting
→ Helps cell withstand turgor pressure
→ Fully permeable

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates β-glucose

Polysaccharides - Cellulose

Structure:
• β-glucose
→ Molecules of β-glucose are
rotated at 180o to each
another
→ 1-4 glycosidic bonds

• Unbranched, straight chain,

linear

→ Form fibres

Updated on 25/8/21 by Beh SJ @behlogy

Condensation of β-glucose into cellulose

β, 1-4 glycosidic bonds Updated on 25/8/21 by Beh SJ @behlogy

Condensation of β-glucose into cellulose

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Polysaccharides - Cellulose
From molecules → fibres with high tensile strength:

1) Cellulose are straight chains, can lie parallel to each other

2) Hydrogens bonds formed between cellulose molecules

→ many –OH groups in cellulose

3) Forms microfibrils and fibres

4) Fibres are arranged in a criss-cross manner

→ many gaps between fibres
→ cell wall is permeable to water, ions etc.
Updated on 25/8/21 by Beh SJ @behlogy
H bonds btwn –OH
groups of diff chains

Pro Tip: Dotted lines are often

used to represent H bonds
Updated on 25/8/21 by Beh SJ @behlogy
Carbohydrates
Polysaccharides

Updated on 25/8/21 by Beh SJ @behlogy

Carbohydrates
Testing the Presence of…

Updated on 25/8/21 by Beh SJ @behlogy

Testing the Presence of Reducing Sugars
• Reducing sugar: All monosaccharides, disaccharides
• Except sucrose

• Add 2cm3 of Benedict’s solution to 2cm3 of reducing sugar

• Copper (II) sulphate in alkaline solution
• Blue colour
• Reaction requires heating at 90oC

• If reducing sugar is present:

Cu2+ → Cu+ (in Benedict’s sol)
Blue, soluble → Red, insoluble

• Forms a brick-red precipitate

(if high conc. of reducing sugar present)
Updated on 25/8/21 by Beh SJ @behlogy
Testing the Presence of
Non-reducing Sugars (e.g. Sucrose)
1. Add 2cm3 of sample to 2cm3 of acid to hydrolyse
glycosidic bonds → monosaccharides
2. Heat at 90oC Always perform the reducing sugar test as well!

3. Neutralize using 2cm3 of NaOH

4. Add 2cm3 of Benedict’s solution
to 2cm3 of the mixture
5. Heat at 90oC

Results:
• Brick red precipitate = Non-reducing sugar
• Even after hydrolysis, remains blue colour
= Not a SUGAR
Updated on 25/8/21 by Beh SJ @behlogy
LIPIDS

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Made of: C, H , O (sometimes, P)
Important for:
1. Energy storage
(lipids have many C-H bonds, can generate more E than carbs)
2. Structural component of membranes
3. Other specific biological functions (e.g. hormones)

Monomers:
• Glycerol
• Fatty Acids

Polymers:
• Triglycerides (Fats and oils)
• Phospholipids
Updated on 25/8/21 by Beh SJ @behlogy
Lipids
Monomers – Glycerol and Fatty Acids
Glycerol
• Has 3 carbons
• 3 –OH groups
→ functional gp

Fatty Acid
• Has an acid “head” (-COOH group)
+ a long hydrocarbon chain
→ many C-H bonds
→ hydrocarbon chain is
hydrophobic and non-polar

Updated on 25/8/21 by Beh SJ @behlogy

 Fatty Acids in Simplified Diagrams

skeletal structure

simplified, general structure

Updated on 25/8/21 by Beh SJ @behlogy

 Types of Fatty Acids
1) Saturated fatty acids
• No double bonds

2) Unsaturated fatty acids

• Has double bonds
Monounsaturated FA = 1 double bond
Polyunsaturated FA > 1 double bond

• Results in kink in hydrocarbon tail

• Lesser C-H bonds
• Lower melting point – liquid at room temp.

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Triglycerides
• 3 fatty acids + 1 glycerol
• Linked by ester bonds
• Formed via condensation reactions
• FAs can be unsat. or sat.

Ester bonds
Updated on 25/8/21 by Beh SJ @behlogy
Lipids
Polymers – Triglycerides

Properties:
• Insoluble in water
→ Bcs of the long hydrocarbon tails of
the fatty acids
→ Non-polar, hydrophobic

• Soluble in organic solvents

E.g. ether, chloroform, ethanol

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Triglycerides
Roles of Triglycerides:
1) Source of energy
→ Many C-H bonds (more than carbohydrates)
→ Higher proportion of hydrogen
→ Insoluble, compact
→ More energy can be produced per unit mass

2) Metabolic source of water

→ High ratio of H to O atoms
→ Release water during fat oxidation
→ especially for desert animals

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Triglycerides
3) Insulator
→ below dermis

4) Protection of organs

5) Buoyancy
→ blubber

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Phospholipids
• 1 fatty acid chain in
triglyceride is replaced by a
phosphate group

Composed of:
• 1 glycerol
• 2 fatty acids
• 1 phosphate gp
(PO4-)

• May also have other gps attached to

phosphate gp (represented by R)

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Phospholipids

a) Hydrophilic head
• Phosphate group
• Charged, polar
• Forms H bonds with water

b) Hydrophobic tails
• Fatty acid residues
• Hydrocarbon chains are
insoluble and non-polar
• Repels water

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Phospholipids
Roles of Phospholipids:
1) Forms a phospholipid bilayer
• With hydrophobic core
• Barrier to water-soluble substances
at membrane

2) Allow regulation of membrane fluidity

• Double bonds in unsat. fatty acid tails increases fluidity
• Sat. FA decreases fluidity

Updated on 25/8/21 by Beh SJ @behlogy

 Lipids
Polymers – Phospholipids

3) Help to hold membrane

proteins in place
• Hydrophobic interaction with
‘floating’ membrane proteins

4) Can combine with

carbohydrates to form glycolipids
• Important in cell recognition
(more in Chap 4)

Updated on 25/8/21 by Beh SJ @behlogy

 Emulsion Test for Lipids
1) Shake sample with ethanol
2) Pour mixture into a tube with water

Results:
• Transparent – no lipids
• White and cloudy – lipids present

→Lipid molecules clump together

→ Forming little groups dispersed throughout the liquid
→ Emulsion

Updated on 25/8/21 by Beh SJ @behlogy

PROTEINS

Updated on 25/8/21 by Beh SJ @behlogy

 Proteins
Made of: C, H, O, N (sometimes, S)
Examples of structures made of protein:
• Haemoglobin
• Collagen
• Components of cell membranes
• Enzymes
• Antibodies
• Keratin

Monomer: Amino acids

Dimer: Dipeptide
Polymer: Polypeptide
Updated on 25/8/21 by Beh SJ @behlogy
Proteins
Monomer – Amino acids
You need to know how to draw the
general structure of amino acids!
The general structure of amino acids:

There are 20 types of R groups

→ 20 different amino acids
Updated on 25/8/21 by Beh SJ @behlogy
Proteins
Monomer – Amino acids

skeletal structure a.a. are in an ionized

state in water
Updated on 25/8/21 by Beh SJ @behlogy
Proteins
Monomer – Amino acids You need to know
how to draw glycine!

E.g. types of amino acids

The simplest amino acid is glycine

R group = H

One more name to rmb is cysteine

R group = contains sulphur

Updated on 25/8/21 by Beh SJ @behlogy

 You DO NOT need
to memorise this!

Updated on 25/8/21 by Beh SJ @behlogy

Proteins You need to know how to draw products
of condensation / hydrolysis reactions!
Dipeptides

Peptide bond
forms between
C of –COOH and
the N of –NH2 Peptide bond

Updated on 25/8/21 by Beh SJ @behlogy

Proteins • Synthesised at the ribosome
• Condensation reaction
Polypeptides

Peptide bonds
Updated on 25/8/21 by Beh SJ @behlogy
Proteins = central carbon atom
= R group
Polypeptides

Updated on 25/8/21 by Beh SJ @behlogy

 Protein
4 types of bonds
Peptide bonds
(present in all polypeptides)

1. Hydrogen bonds
2. Disulfide bonds
3. Ionic bonds
4. Hydrophobic interactions

Updated on 25/8/21 by Beh SJ @behlogy

Peptide bonds
• Very Strong
• Covalent bonds
• Between C of –COOH and
the N of –NH2
• Result of condensation
reaction

• Present in all
polypeptides

Updated on 25/8/21 by Beh SJ @behlogy

1. Hydrogen Bonds
• Individually weak
• But many H bonds
→ cumulatively strong

• Between H of –NH / – OH group

and O of –CO group
• OR between R groups

Easily broken by:

• High temperatures
• pH changes

Updated on 25/8/21 by Beh SJ @behlogy

 2. Disulfide Bonds
• Very strong covalent bonds
• Between sulphur atoms of cysteine amino acids

cysteine

cysteine

Updated on 25/8/21 by Beh SJ @behlogy

 3. Ionic Bonds
• Between ionized amine and carboxylic acid groups
→ – NH3+ and – COO- groups
→ that are not involved in peptide bonding
• OR between charged R groups
• Weaker than disulphide bonds, but stronger than H bonds
• Easily broken down by pH changes and high temp

Updated on 25/8/21 by Beh SJ @behlogy

 4. Hydrophobic Interactions
• Between non-polar / hydrophobic R groups
• Repel and move away from water
• Weakest

Updated on 25/8/21 by Beh SJ @behlogy

Protein
4 types of bonds
Weakest → Strongest

1. Hydrophobic interactions
2. H bonds
3. Ionic bonds
4. Covalent bonds
i.e. disulphide bonds,
peptide bonds

Updated on 25/8/21 by Beh SJ @behlogy

 Proteins
4 levels of protein structure

• Primary structure
• Secondary structure
• Tertiary structure
• Quaternary structure

Updated on 25/8/21 by Beh SJ @behlogy

4 Levels of Protein Structure

Updated on 25/8/21 by Beh SJ @behlogy

 Primary Structure
• Linear sequence of amino acids
• Held together by peptide bonds
• Specific seq of amino acids
→ each with diff properties of R groups
→ Dictates folding of the polypeptide chain

Updated on 25/8/21 by Beh SJ @behlogy

 Secondary Structure
• H bonds between amino acids
• Not located directly next to each other
• Of the same polypeptide chain

Two conformations:
1) α - helix
2) β - pleated

Updated on 25/8/21 by Beh SJ @behlogy

Secondary Structure
1) α - helix
• Hydrogen bonding
• between H atom of the -NH group
and O atom of the -CO group
• 4 places ahead
• Forms spring-like structures

Updated on 25/8/21 by Beh SJ @behlogy

Secondary Structure
2) β - pleated sheets
• Hydrogen bonding
• between H atom of the -NH group
and O atom of the -CO group
• Straighter, looser form
• Parallel, flat sheets

Updated on 25/8/21 by Beh SJ @behlogy

 Tertiary Structure
• Coiling and folding of secondary structures
→ Into a precise 3D structure
• Due to interactions between R groups
• One polypeptide chain only
• Could be held by all 4 bonds – H bonds, disulfide, bonds,
ionic bonds, hydrophobic interactions

Updated on 25/8/21 by Beh SJ @behlogy

 Tertiary Structure
E.g. myoglobin
• Oxygen-carrying molecule
• Present in muscle cells
→ gives it a red colour

• Made of 1 polypeptide chain

• Has a haem group = non-amino acid group
• That binds 1 molecule of oxygen

Updated on 25/8/21 by Beh SJ @behlogy

 Quaternary Structure
• Combination of two or more polypeptide chains
• Held together by all 4 bonds

E.g. Haemoglobin, collagen

Updated on 25/8/21 by Beh SJ @behlogy

Q: Which levels of protein structure are demonstrated by a
haemoglobin molecule?

A:
primary
secondary
tertiary
quaternary

Updated on 25/8/21 by Beh SJ @behlogy

Proteins
Globular vs Fibrous Proteins
Globular Proteins Fibrous Proteins
Spherical / ball shape Long, parallel strands
Mostly tertiary, sometimes Mostly secondary structure and
quaternary structure forms fibers
Soluble Insoluble
More functional roles More structural roles
E.g. All enzymes, antibodies, some
Eg: Collagen, keratin
hormones, myoglobin, haemoglobin

Keratin
Enzyme (secondary)
(tertiary/quaternary)
Updated on 25/8/21 by Beh SJ @behlogy
 Proteins
Globular Proteins
Q: What makes globular proteins soluble?

A:
• Amino acids with non-polar / hydrophobic R groups are inside
• Amino acids with polar / hydrophilic R groups faces outside

Primary structure → Tertiary structure

Updated on 25/8/21 by Beh SJ @behlogy
 Proteins
Globular Protein – Haemoglobin
• Oxygen carrying pigment in RBC

• Has quaternary structure

• Made of 4 polypeptide chains
• 2 α-globin chains and 2 β-globin
chains

• Globular
• Amino acids with non-polar /
hydrophobic R groups are inside
• Amino acids with polar /
hydrophilic R groups faces outside
Updated on 25/8/21 by Beh SJ @behlogy
 Proteins
Globular Protein – Haemoglobin
• Each polypeptide has a haem group
→ Prosthetic group = non-amino acid group
→ Permanent part of haemoglobin

• Each haem has 1 iron ion (Fe 2+)

→ Each Fe2+ can bind 1 oxygen molecule
→One haemoglobin can bind 4 molecules of O2

Updated on 25/8/21 by Beh SJ @behlogy

 Proteins
Fibrous Protein – Collagen
• Structural protein
• High tensile strength

• Every 3rd amino acid of each

polypeptide is glycine
• Usually proline-alanine-glycine repeat
• Glycine has the smallest R group!
→ so it can be tightly wound

• 3 polypeptide chains form a triple helix

collagen molecule
→ Held by hydrogen bonds
→ Quaternary structure Updated on 25/8/21 by Beh SJ @behlogy
 Proteins
Fibrous Protein – Collagen
• Collagen molecules lie parallel
• And form covalent cross-links
• Between R groups of lysine a.a.

• Staggered ends
→ so no weak spot

• Forms fibrils and fibres

Updated on 25/8/21 by Beh SJ @behlogy

 Proteins
Fibrous Protein – Collagen

Held together by:

H Bonds

Covalent
cross-linkages

Updated on 25/8/21 by Beh SJ @behlogy

 Testing the Presence of Proteins
• Biuret reagent
• Copper (II) sulphate and dilute potassium
hydroxide
• Add 2cm3 of Biuret solution to 2cm3 of
sample

• Purple – protein present

• Blue – no protein

• NH2 groups in amine react with copper ions

→ purple

Updated on 25/8/21 by Beh SJ @behlogy

WATER

Updated on 25/8/21 by Beh SJ @behlogy

 Water
• Dipole in nature
→ O atom has slight negative charge
→ H atom has slight positive charge

• Hydrogen bonding
→ between O and H atoms of
diff water molecules
→ individually weak, cumulatively strong
→ result in many properties of water

Updated on 25/8/21 by Beh SJ @behlogy

Water
Properties of Water
1) High specific heat capacity
2) High latent heat of vaporisation
3) High latent heat of fusion
4) Water as a solvent
5) Cohesion, Adhesion and Surface Tension

Updated on 25/8/21 by Beh SJ @behlogy

1) High specific heat capacity
• Specific heat capacity = amount of heat required to
raise the temperature of 1 kg of water by 1oC

• Large amount of energy needed to raise

the temperature of water
→ Due to H bonding in water
→ Large energy needed to break H bonds

• Provide stable temperature / environment

→ Acts as buffer against sudden temperature change
→ Temperature of water does not change quickly
Updated on 25/8/21 by Beh SJ @behlogy
2) High latent heat of vapourisation
• Latent heat of vapourisation = amount of heat required to
evaporate 1 g of water

• Large amount of energy needed for

water to evaporate
→ Due to H bonding in water

• Able to remove a large amount of heat

energy from surroundings
→ Important as a cooling mechanism

Updated on 25/8/21 by Beh SJ @behlogy

 3) High latent heat of fusion
• Water also need to lose a large
amount of heat to freeze
→ Due to H bonds
→ Provide stable habitats for aquatic
organisms, less likely to freeze

• Ice is less dense than water

• Water is most dense at 4oC
• Floats → acts as insulator on
surface of frozen lakes

Updated on 25/8/21 by Beh SJ @behlogy

 4) Water as Solvent
• Water is dipolar
• Dissolves ions, polar molecules,
gases (oxygen, CO2) and waste
products (ammonia NH3 and urea)

Important for:
• Transport, removal of wastes,
secretions, medium for enzymatic
reactions

Not solvent for:

• Non-polar molecules (lipids)

Updated on 25/8/21 by Beh SJ @behlogy

5) Cohesion, Adhesion and Surface Tension
• Tend to stick to each other – cohesion
• Tend to stick to surfaces – adhesion
• Because of H bonds

Useful for:
1) Transport of water in xylem tissue of plants
→ long, unbroken column of water

2) High surface tension

→ Surface dwellers’ habitat
E.g. pond skater

Updated on 25/8/21 by Beh SJ @behlogy

Q: Describe the importance of water as an environment for fish at the
North Pole. [4]

A:
1) Water is a solvent → provides (dissolved) oxygen and remove carbon
dioxide / ammonia.
2) Water provides support / buoyancy
3) Water is liquid, so fish can move
4) Transparent, so fish can see
5) High specific heat capacity → so can provide stable temperature /
environment
6) Ice less dense than water → ice floats and act as insulator, so can
survive when water freezes
7) high latent heat of fusion, water does not freeze too easily
8) greatest density is at 4 °C → As a frozen lake warms after a cold winter,
dissolved mineral nutrients are brought to the surface.

Updated on 25/8/21 by Beh SJ @behlogy

 Chapter Outline
Monomer to Polymer
Hydrolysis and Condensation

Carbohydrates
• Monosaccharide, disaccharide and polysaccharide
• Glycosidic bond
• Starch (amylose and amylopectin)
• Glycogen
Protein
• Cellulose
• Amino acids
• Benedict’s Test / Iodine Test
• Peptide bond
• Primary to quaternary structure
Lipids
• Globular vs Fibrous proteins
• Glycerol + 3 Fatty acids
• Haemoglobin
• Ester bond
• Collagen
• Triglycerides
• Biuret Test
• Phospholipids
• Emulsion Test
Water
• Hydrogen bond
Updated on 25/8/21 by Beh SJ @behlogy
Important Things to Remember
All tests
All bonds involved
Monomer and polymer names
Role of biomolecules
Examples of polymers – structure and function

P/S: Besides the general structure of amino acids,

glycine, alpha and beta glucose, you don’t need to know
how to draw any chemical structures (from scratch
anyway), but you need to be able to recognise them

Updated on 25/8/21 by Beh SJ @behlogy

 AS Level
Chapter 3
Enzymes
 Chapter Outline
Part 1: Enzyme Structure and Mode of Action
• Enzyme Structure
• Lock and Key Hypothesis vs Induced Fit hypothesis
• Mode of Action

Part 2: Mechanism of Enzymatic Reaction

• Measuring the rate of reactions
• Time course graph

Part 3: Factors Affecting Enzymatic Reaction

• Substrate concentration
– Maximum rate of reaction (Vmax)
– Michaelis-Menten constant (Km) – a measure of affinity
• Enzyme concentration
• Temperature
• pH
• Competitive vs Non-competitive Inhibition

Part 4: Immobilised enzymes Updated on 27/7/21 by Beh SJ @behlogy

Enzyme
Structure
 What are enzymes?
• Biological catalysts
→ Can break down or
synthesize products
→ Used to control
metabolic reactions

Features:
• Speeds up / increase rate of chemical reaction
• Specific: 1 enzyme to only one / few substrates
• Unchanged at the end of the reaction
• Effective in small amounts
• High turnover number = can catalyse many reactions per unit time
Updated on 27/7/21 by Beh SJ @behlogy
 How Do Enzymes Catalyze Reactions?
• Enzymes lower the activation energy of chemical reactions
• Activation energy (Ea) = Energy needed for a chemical reaction to
successfully form products
• This provides an alternative pathway for reactions to occur

Updated on 27/7/21 by Beh SJ @behlogy

 All enzymes are globular proteins
• Spherical / ball shape
• Mostly tertiary structures, some quaternary
→ Have precise 3D structure due to interactions between R groups
• Soluble because…
→ Hydrophobic R groups are inside
→ Hydrophilic R groups faces outside

Primary structure → Tertiary structure

Updated on 27/7/21 by Beh SJ @behlogy
 Active Site of Enzymes
All enzymes have an active site
• A site where substrates bind to
• Bind = complementary in shape
to substrate

• Active site gives specificity

• Only few catalytic amino acids
at the active site!
→ Precisely positioned
→ Not necessarily beside each
other in primary seq

Updated on 27/7/21 by Beh SJ @behlogy

How do substrates bind to enzymes?
More recent!
Lock and Key Mechanism Induced Fit Mechanism
Active site does not Active site is flexible and
change shape moulds around substrate
Shape of active site is Shape of active site is
fully complementary partially complementary
to shape of substrate to shape of substrate
Result: Substrate expected Result: Better fit!
to fit exactly!

Updated on 27/7/21 by Beh SJ @behlogy

 Where do enzymes operate?
Some intracellular (inside cells)
• Cells synthesise enzymes and
retains them for internal use

Some extracellular (outside cells)

• Cells synthesise enzymes and
secrete them for external use

Updated on 27/7/21 by Beh SJ @behlogy

Mode of Action

Updated on 27/7/21 by Beh SJ @behlogy

 Mode of Action

1. Enzymes and substrates move and collide randomly

• Only collisions in the right orientation
with enough energy result in a successful reaction
• E has active site with specific shape
that is complementary to substrate

Updated on 27/7/21 by Beh SJ @behlogy

 Mode of Action

2. Formation of the enzyme-substrate complex (ESC)

• Substrates interacts with R groups of
catalytic amino acids at the active site
→ Forms temporary bonds
→ Active site changes shape to mould
around substrates (induced fit model)
→ Substrates binds strongly to active site
→ ESC is formed

Updated on 27/7/21 by Beh SJ @behlogy

Updated on 27/7/21 by Beh SJ @behlogy
 Mode of Action

3. Product formation
• Interactions of substrates with the active site…
→ Brings substrates close together
in the right position
→ Put strain on the reactants
→ So bonds to break or form more easily
→ Allow transfer of charges/groups
→ Lowers activation energy

• Products form and leave active site

• Enzymes remain unchanged
Updated on 27/7/21 by Beh SJ @behlogy
Mechanism of Enzymatic Reaction

Updated on 27/7/21 by Beh SJ @behlogy

 Rate of Reaction (ROR)
• Speed of conversion of substrate into product
• Rate is always per unit time
→ E.g. s-1 , min-1

• How to measure ROR in an experiment? By measuring the...

– Rate of product formation or
– Rate of substrate disappearance

• Steps:
1. Measure the product formation or substrate disappearance
2. Plot a time course graph
3. Find the gradient of the curve = ROR

Updated on 27/7/21 by Beh SJ @behlogy

 Catalase
• Hydrogen peroxide is a toxic metabolic product in tissues
• Catalase enzymes are found in tissues of most living organisms

• We usually obtain it from potato

/ liver / yeast extract

• ROR = Rate of oxygen released

• Collect gas or count bubbles
Updated on 27/7/21 by Beh SJ @behlogy
 Curve of Oxygen Produced Against Time
[P]

Volume of oxygen produced:

[P] increases then levels off
Speed of oxygen released:
Fast → slows down → stops
Rate of oxygen released:
High → decrease → 0

Rate = gradient
Gradient of a horizontal line = 0

Updated on 27/7/21 by Beh SJ @behlogy

Curve of Oxygen Produced Against Time

Initial rate of reaction

= gradient of the curve at 0s

Updated on 27/7/21 by Beh SJ @behlogy

 Amylase

• Problem: Both substrate and products are colourless and are

both liquid!
• Answer: Use iodine test!

• ROR = Rate that starch disappears

Updated on 27/7/21 by Beh SJ @behlogy

 Amylase
• Test samples with iodine solution at known time intervals

• Colour will change from dark blue → yellow/brown

Dark blue: Starch still present
Yellow/brown: Starch completely hydrolyzed

• Use spotting tile… or use a colorimeter to measure

the colour intensity

Updated on 27/7/21 by Beh SJ @behlogy

 Amylase
• Use colorimeter measures light absorbance in arbitary units (a.u.)
• Measures relative absorbance to a control (distilled water)

Updated on 27/7/21 by Beh SJ @behlogy

Curve of Starch Remaining Against Time
[S] Concentration of starch, [S]:
Increases then levels off
Speed of starch being used:
Fast → slows down → stops
Rate of reaction:
High → decrease → 0

Rate = gradient
Gradient of a horizontal line = 0

Updated on 27/7/21 by Beh SJ @behlogy

Curve of Starch Remaining Against Time

Initial rate of reaction

= gradient of the curve at 0s
(or in this case, as early in
the reaction as possible)

Updated on 27/7/21 by Beh SJ @behlogy

 What happens during
the course of the reaction?
To illustrate my next slide…

Pretty much the

same thing, right?

Updated on 27/7/21 by Beh SJ @behlogy

 The Course of Reaction
Why was the rate high at first, then decreased and became 0?
High [S] → Low [S] → [S] = 0
S is in excess Lesser S available No S available for binding

Observe that:
• P is not shown here but [P] increases over time, then levels off
• [E] = constant
• [S] reduces over time, then levels off
• [S] the limiting factor in this reaction
Updated on 27/7/21 by Beh SJ @behlogy
 Time Course Graphs
Rate is high bcs
more [S] is present

Note that [ES]

immediately
increases from
0 when the Rate is 0 bcs all S
reaction starts! has been used up

Updated on 27/7/21 by Beh SJ @behlogy

 How to understand graphs?
• Always read the question carefully!
– Describe = state your observations
– Explain = explain why the observations are as such
– Compare / contrast = tell me the similarities or differences
between 2 sets of data
– CONTEXT matters!

• Read the axis labels!

– x-axis = independent variable
– y-axis = dependent variable

Updated on 27/7/21 by Beh SJ @behlogy

 How to talk about graphs?
1. General trend
• First, describe how the independent variable and dependent
variable changes overall
E.g. when x increases, y increases then plateaus
• If needed, split the graph into several parts to describe and/or
explain separately

2. Comparative data quote

• Compare 2 points to support your statement
• Provide the x and y coordinates with the correct units
• You can also quote the max and min values where appropriate
• You can include some manipulative figures
E .g. the number of cases reduced by half in 2010 compared to 2008

Updated on 27/7/21 by Beh SJ @behlogy

Factors Affecting Enzymatic Reaction

Updated on 27/7/21 by Beh SJ @behlogy

 Rate of Reaction
Rate of reaction be affected by:

1. Substrate concentration, [S]

2. Enzyme concentration, [E]
3. Temperature
4. pH
5. Inhibitors

Updated on 27/7/21 by Beh SJ @behlogy

 1) Substrate Concentration
What would happen if we use a higher [S]?

Higher [S]
Volume of oxygen
produced (cm3)

Lower [S]

How do we calculate the initial ROR?

We will find that the reaction is faster / ROR is higher for the
reaction with higher [S].
Updated on 27/7/21 by Beh SJ @behlogy
1) Substrate Concentration
Low [S] High [S]

[E] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 1) Substrate Concentration

At low [S]
→ Some active sites available for binding / not all occupied
→ Few collisions between enzyme and substrate
→ Less S binds with active site
→ Few ESCs formed
→ [S] is limiting
• More active sites occupied as [S] increases
• Rate of reaction proportional to substrate concentration
Updated on 27/7/21 by Beh SJ @behlogy
 1) Substrate Concentration
At high [S]
• Rate increases to a plateau / Vmax reached
• Maximum rate of enzymatic reaction = Vmax

• All active sites are saturated

• Max no. of ESCs formed
• [E] is the limiting factor
• Further increase of [S] does not increase rate
Updated on 27/7/21 by Beh SJ @behlogy
 1) Substrate Concentration
Low [S]
• Some active sites available
• [S] is limiting

Vmax
High [S]
• At Vmax, all active
sites are saturated
• [E] is limiting

Updated on 27/7/21 by Beh SJ @behlogy

 1) Substrate Concentration
Initial Rate of
Reaction (cm3 s-1)

Vmax

Steps to find Km:

½ Vmax 1. Find half Vmax
2. Read [S] from x-axis

Km Substrate Concentration (mM)

(Michaelis-Menten
Constant)
Updated on 27/7/21 by Beh SJ @behlogy
 1) Substrate Concentration
Michaelis-Menten Constant (Km)
• Km = [S] at ½ Vmax
• Km = the [S] at which the enzyme works at half its maximum rate
• At Km, ½ the active sites are occupied by S

• Km is a measure of enzyme affinity

• Affinity = the degree of attraction
between molecules

• The higher the Km, the lower the affinity

Updated on 27/7/21 by Beh SJ @behlogy

 1) Substrate Concentration
Michaelis-Menten Constant (Km)
For an enzyme with high Km and low affinity:
• Higher [S] is needed to reach ½ Vmax
• Enzyme needs a higher [S] to reach Vmax
• Enzyme forms fewer ESC in the same unit of time
• Enzyme active site is a less good fit for substrate

Updated on 27/7/21 by Beh SJ @behlogy

1) Substrate Concentration
Michaelis-Menten Constant (Km)

Updated on 27/7/21 by Beh SJ @behlogy

 1) Substrate Concentration
Vmax and Km
• Problem: too many experiments needed for the graph to
plateau in order to find Vmax and Km
• Solution: Plot a double-reciprocal graph!
→ Plot 1/v against 1/[S]
instead of v against [S]

Updated on 27/7/21 by Beh SJ @behlogy

1) Substrate Concentration
Double Reciprocal Plot

Updated on 27/7/21 by Beh SJ @behlogy

1) Substrate Concentration
Double-reciprocal plot

Updated on 27/7/21 by Beh SJ @behlogy

2) Enzyme Concentration
Low [E] High [E]

[S] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 2) Enzyme Concentration
At low [E], as [E] increases:
• More enzymes present
• More active sites available for S to bind
• Increase in frequency of collision
• More ESCs form
→ Rate increases as [E] increases

At high [E]:
• Rate of reaction levels off
• [S] is the limiting factor
• Max number of ESCs formed

Updated on 27/7/21 by Beh SJ @behlogy

 2) Enzyme Concentration
Low [E]
• Less active sites available to bind to S
• [E] is limiting

High [E]
• Max number
of ESCs formed
• [S] is limiting

[S] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 3) Temperature
Optimum temp

Low temp High temp

[S] = constant
[E] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 3) Temperature
At low temp, as temp increases from low to
optimum:

• Increase in kinetic energy

• Enzyme and substrate molecules move faster
• Increase in collision rate between substrate
molecules and enzyme’s active site
• Substrates bind to enzyme’s active site more often
• More ESCs form
→ Rate increases as temp increases

Updated on 27/7/21 by Beh SJ @behlogy

 3) Temperature
At optimum temperature:
• Maximum rate of enzyme reaction
• Most human enzymes ~40⁰C

• Different enzymes, different optimum temperature

E.g. enzymes in Thermophilic archaea have very high optimum temp

Updated on 27/7/21 by Beh SJ @behlogy

 3) Temperature
At very high temp:
• Reaction slows down and then stops

• H bonds and ionic bonds

holding enzyme start to break
• Active site changes shape
• Enzyme is denatured
• Substrate cannot bind to the active site
• Fewer ESCs form

• Irreversible

Updated on 27/7/21 by Beh SJ @behlogy

 3) Temperature
Low temp
• As temp increases,
kinetic energy increases
• More collisions Optimum temp
• More ESCs
High temp
• Enzyme denatures
• Ionic bonds and H
bonds break, active site
changes shape

[S] = constant
[E] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 4) pH
Optimum pH

Low pH High pH

[S] = constant
[E] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 4) pH
• pH = measure of the concentration of hydrogen ions in a solution

• Low pH – acidic (more [H+], less [OH-])

• High pH – alkaline (less [H+], more [OH-])

At low/high pH:
• H+ or OH- can interact with the R groups of
amino acids in enzymes
• Disrupt ionic bonds and H bonds
• Changes the active site shape
• S cannot bind to active site
• Less ESCs form

Updated on 27/7/21 by Beh SJ @behlogy

 4) pH
At optimum pH:
• Max rate of enzyme reaction
• Beyond optimum pH - Enzyme denatures
• Different enzymes, different optimal pH

Updated on 27/7/21 by Beh SJ @behlogy

 4) pH
Optimum pH

At Low/High pH:
• Ionic bonds and H bonds break
• Active site change shape
• Less ESCs form

[S] = constant
[E] = constant
Updated on 27/7/21 by Beh SJ @behlogy
 5) Inhibitors
• Molecules which can reduce rate of an enzyme
catalysed reaction

Places where inhibitors can bind:-

1) Bind to enzyme’s active site

• Similar to substrate’s shape
• Competitive inhibitor

2) Bind to allosteric site of enzyme

• Another site on enzyme other than the active site
• Non-competitive inhibitor

Updated on 27/7/21 by Beh SJ @behlogy

 5) Inhibitors
• Two types:

Updated on 27/7/21 by Beh SJ @behlogy

 Competitive Inhibitors
• Fit into enzyme’s active site
• Similar shape to real substrate
• Competitive inhibitor competes with substrate for active site
of enzyme
• Reversible

Updated on 27/7/21 by Beh SJ @behlogy

 Competitive Inhibitors
At low [S]:
• [inhibitor] > [substrate]
• Less frequency of collisions with S
• Less ESCs forms
• Reduces rate of reaction

→ Enzyme’s function inhibited

Updated on 27/7/21 by Beh SJ @behlogy

 Competitive Inhibition
• Can be reversed/overcome by…. having more substrates

At high [S]
• [S] > [inhibitors]
• Inhibitor has less effect
• Substrate outcompetes inhibitor at high [S]
• More chances of substrate molecule
colliding and binding to active site
• More S binds to active site
• More ESCs form
→ Enzyme’s function unaffected

• Competitive, reversible inhibition

Updated on 27/7/21 by Beh SJ @behlogy

 Competitive Inhibition

Vmax stays the

same bcs it is
determined by [E]

Km increases
when competitor is present
More [S] is needed to reach 1/2 Vmax
Updated on 27/7/21 by Beh SJ @behlogy
 Non-competitive inhibition
Two types of non-competitive inhibition:
• Irreversible – e.g. in poisons (cyanide binds to enzyme in mitochondria
and inhibits ATP synthesis)
• Reversible – end product inhibition

Updated on 27/7/21 by Beh SJ @behlogy

Irreversible Non-Competitive Inhibition
• Inhibitor binds to allosteric site on enzyme
• Allosteric site = another site on enzyme
other than the active site

• Changes active site’s shape

• Disrupts H bonds and hydrophobic
interactions
→ 3D shape of enzyme affected
→ Distortion ripples

• Substrate unable to bind to active site

→ Enzyme’s function - inhibited

• Increasing substrate concentration has no
effects on enzyme activity
Updated on 27/7/21 by Beh SJ @behlogy
Irreversible Non-Competitive Inhibition

Vmax decreases
Inhibition is not reversible by increasing [S]
Lower [E] available for use

Km remains constant
Same [S] needed to reach Vmax

Updated on 27/7/21 by Beh SJ @behlogy

 Graphs
No inhibitor
Vmax
Competitive inhibitor
(Same Vmax , Higher Km )

Vmax
½ Vmax Non-competitive inhibitor
(Lower Vmax , Same Km)

½ Vmax

Km Km

Updated on 27/7/21 by Beh SJ @behlogy

 End Product Inhibition
• A form of control for
metabolic reactions
• Maintain homeostasis

In a chain of reactions
• Catalyzed by various
enzymes

• End product becomes a

non-competitive enzyme
inhibitor for upstream
reactions

Updated on 27/7/21 by Beh SJ @behlogy

 End Product Inhibition
• High amount of end product
→ binds to allosteric site on another enzyme catalyzing a
reaction in the same metabolic chain
→ Inhibits reaction

• When amount of end product (inhibitor)

decreases, rate of reaction increases again
• Inhibitors can lose its attachment on
enzyme’s allosteric site and are used elsewhere

→ Temporary/brief
→ Non-competitive, reversible inhibition

Updated on 27/7/21 by Beh SJ @behlogy

Immobilised
Enzymes
 Immobilised Enzymes
• Enzymes attached to an inert, insoluble material

How to encapsulate enzymes in alginate beads?

1. Mix enzymes with sodium alginate

2. Add drop by drop into calcium chloride
3. Pack them into a column
Updated on 27/7/21 by Beh SJ @behlogy
 Immobilised Enzymes
4. Run substrates through the column
5. Collect products from bottom

You can run the products through the column again and again to
increase % yield.

Updated on 27/7/21 by Beh SJ @behlogy

 Why Use Immobilised Enzymes?
• Can easily reuse enzymes
• Can easily recover enzymes
• Longer shelf-life of enzyme

• Less purification / downstream processing needed

→ Bcs it keeps product enzyme-free so no effect on product quality
• Allow continuous production
→ Can obtain more product per unit time
• Reduces end product inhibition

• Enzymes more tolerant of pH changes

• Thermostable – less likely to denature at high temp
Updated on 27/7/21 by Beh SJ @behlogy
 Tolerance to pH and temperature
• Alginate protects enzyme
→ As enzyme is less exposed to solution
→ Less H+ or OH- penetrate the alginate beads
→ Shape of active site of immobilised enzyme is less disrupted

• 3D structure of enzyme is stabilized

→ H bonds vibrate less at high temp
→ Fewer bonds within immobilised
enzyme break
→ Active site less likely to change shape
→ Less denaturation

Updated on 27/7/21 by Beh SJ @behlogy

 Chapter Outline
Part 1: Enzyme Structure and Mode of Action
• Enzyme Structure
• Lock and Key Hypothesis vs Induced Fit hypothesis
• Mode of Action

Part 2: Mechanism of Enzymatic Reaction

• Measuring the rate of reactions
• Time course graph

Part 3: Factors Affecting Enzymatic Reaction

• Substrate concentration
– Maximum rate of reaction (Vmax)
– Michaelis-Menten constant (Km) – a measure of affinity
• Enzyme concentration
• Temperature
• pH
• Competitive vs Non-competitive Inhibition

Part 4: Immobilised enzymes Updated on 27/7/21 by Beh SJ @behlogy

 Useful Videos
• Explaining enzyme inhibitors and the graphs
https://www.youtube.com/watch?v=p08Z_fyErWg

Links below cover things you SHOULD NOT write in exams

but maybe helpful for your understanding / for your entertainment!

• How to speed up chemical reactions (and get a date)

https://www.youtube.com/watch?v=OttRV5ykP7A

• How do you explain Michaelis Menten to a kid?

(this is more detailed than needed)
https://www.youtube.com/watch?v=6KCRfY5mwkU

Updated on 27/7/21 by Beh SJ @behlogy

 AS Level
Chapter 4

Cell Membranes and Transport

 Chapter Outline
Part 1: Cell Membrane Part 2: Cell signalling
Structure and Function
• Fluid mosaic model Part 3: Transport Across Membrane
• Roles of: 1. Diffusion
– Phospholipids 2. Facilitated diffusion
– Cholesterol 3. Osmosis
– Proteins 4. Active transport
– Glycolipids 5. Endocytosis and exocytosis
– Glycoprotein

Updated on 13/7/21 by Beh SJ @behlogy

 CELL MEMBRANE
STRUCTURE AND FUNCTION
 Cell Surface Membrane
• Aka plasma membrane
• ~7nm thick

Roles:
1. Controls movement of substances
into and out of the cell
E.g. Nutrients, waste products

2. Semi-permeable
Barrier to water-soluble substances
Allow passage of lipid-soluble substances

Updated on 13/7/21 by Beh SJ @behlogy

 Plasma Membrane
And many more roles due to the many
components found at the membrane:

3. Cell signaling
4. Cell recognition
5. Cell-to-cell adhesion
6. Site for enzymes to catalyse reactions
7. Anchoring for the cytoskeleton
8. Form H bonds with water for stability
Updated on 13/7/21 by Beh SJ @behlogy
 Fluid Mosaic Model
• Fluid – phospholipids and protein molecules are able to
move about and diffuse sideways within its monolayer

• Mosaic – proteins interspersed / scattered within membrane

Updated on 13/7/21 by Beh SJ @behlogy

Components of the Plasma Membrane
1. Phospholipids
2. Cholesterol
3. Proteins
4. Glycolipids
5. Glycoproteins

Updated on 13/7/21 by Beh SJ @behlogy

 1. Phospholipids Chap 2 Recap

• 1 fatty acid chain in

triglyceride is replaced by a
phosphate group

Composed of:
• 1 glycerol
• 2 fatty acids
• 1 phosphate gp
(PO4-)

• May also have other gps attached to

phosphate gp (represented by R)

Updated on 13/7/21 by Beh SJ @behlogy

 1. Phospholipids Chap 2 Recap

a) Hydrophilic head
• Phosphate group
• Charged, polar
• Forms H bonds with water
→ Role: Stabilise membrane

b) Hydrophobic tails
• Fatty acid residues
• Hydrocarbon chains are
insoluble and non-polar
• Repels water
Updated on 13/7/21 by Beh SJ @behlogy
 1. Phospholipids

b) Hydrophobic tails
• Point inwards facing each other
→ Form hydrophobic core
Roles:
Barrier to water-soluble substances
Allow passage to lipid-soluble substances

Only lipid-soluble, small, uncharged molecules can diffuse

through the phospholipid bilayer

• Also! Fatty acids help maintain fluidity of membrane

Updated on 13/7/21 by Beh SJ @behlogy
 What affects membrane fluidity?
1. Temperature
• Higher temperature, higher kinetic energy, more fluid

2. Ratio of unsaturated to saturated fatty acids

• More unsaturated FA, higher unsat:sat ratio, more fluid
→ Unsaturated FA has C=C which cause kinks
→ Phospholipids more loosely arranged
• More saturated FA, lower unsat:sat ratio, less fluid
→ No kinks, more area for phospholipids to interact

Updated on 13/7/21 by Beh SJ @behlogy

 What affects membrane fluidity?
3. Length of phospholipid tails
• The longer the tails, less fluid
→ More surface area for interaction between tails

4. Cholesterol!

The cell can maintain fluidity during higher / lower

temperatures by changing the ratio of unsat:sat, length of tails,
and adding cholesterol!

Updated on 13/7/21 by Beh SJ @behlogy

2. Cholesterol

• Small molecule
• Has hydrophilic head and hydrophobic tail
• Fit between the phospholipid molecules
• Not found in prokaryotes’ cell membrane

Roles:
1) Regulates fluidity of membrane
2) Stabilises the membrane
esp the hydrophobic layer
3) Block passage of very small ions
through membrane
Updated on 13/7/21 by Beh SJ @behlogy
 How Cholesterol Regulates
Membrane’s Fluidity
a) At lower temperatures, less fluid
• Cholesterol increases fluidity / decrease rigidity
→ Prevent close packing of the phospholipid tails

b) At higher temperatures, more fluid

• Cholesterol decreases fluidity
→ Reduce mobility of phospholipids

Overall, it stabilizes the membrane

Updated on 13/7/21 by Beh SJ @behlogy

 3. Membrane Proteins
Types based on their position in the membrane:

Extrinsic / peripheral proteins

• Inner or outer surface of the membrane

Intrinsic / integral proteins

• Extend into hydrophobic core
• Maybe mobile or fixed
(attached to structures)
• Some are transmembrane proteins
→ Span across the membrane
→ E.g. transport proteins
Updated on 13/7/21 by Beh SJ @behlogy
 Structure of Intrinsic Proteins
• Have both hydrophobic
and hydrophilic regions

• Hydrophobic regions
→ interacts with hydrophobic core / fatty acids tails of phospholipids

• Hydrophilic regions
→ Extend into aqueous external environment inside/ outside the cell
Updated on 13/7/21 by Beh SJ @behlogy
 3. Membrane Proteins
Roles:
1) Transport proteins
Passage for ions / charged / polar / larger molecules through
membrane

Two types:
• Channel proteins
→ For facilitated diffusion

• Carrier proteins
→ For facilitated diffusion /
active transport

Updated on 13/7/21 by Beh SJ @behlogy

 3. Membrane Proteins
Channel proteins:
• Highly specific
• Channel /pore is water-filled
→ Hydrophilic R-groups on amino acids face inwards towards channel
→ E.g. aquaporins = channel protein for water

• Can be gated / can open and close

→ E.g. voltage-gated or ligand-gated

Updated on 13/7/21 by Beh SJ @behlogy

 3. Membrane Proteins
Carrier proteins:
• Highly specific
• Conformational change occurs when it
interacts with the ion / molecule

• Binding sites that alternately open to one

side of the membrane then the other
• Constantly flip between two shapes

→ E.g. sodium-potassium pump

Pumps 2 K+ in, 3 Na+ out

Updated on 13/7/21 by Beh SJ @behlogy

 3. Membrane Proteins
Roles:
1) Transport proteins
– channel and carrier proteins

2) Enzymes

3) Receptor for cell signalling

molecules

4) Anchoring cytoskeleton
– maintain cell shape

5) Cell-to-cell adhesion
Updated on 13/7/21 by Beh SJ @behlogy
 4. Glycolipids
• Glyco = carbohydrate chain
• Glycolipid = carbohydrate chains
attached to phospholipids
• Glycoprotein = carbohydrate chains
attached to protein

• Carbohydrate chains face outside of cell

• Form a sugary coat on the cell = glycocalyx

Updated on 13/7/21 by Beh SJ @behlogy

 4. Glycolipid
Roles:
1) Interacts with water to stabilize membrane structure
• Able to form H bonds with water molecules

2) Cell-to-cell adhesion

3) Cell recognition
• Acts as cell surface antigens / markers
• Macromolecules on cell surface membrane
• Antigen: foreign substance that triggers
immune response
• To distinguish self from non-self

Updated on 13/7/21 by Beh SJ @behlogy

 5. Glycoprotein
Roles:
1) Interacts with water to stabilize membrane structure
• Able to form H bonds with water molecules

2) Cell-to-cell adhesion

3) Cell recognition
• Acts as cell surface antigens / markers
• Macromolecules on cell surface membrane
• Antigen: foreign substance that triggers
immune response
• To distinguish self from non-self

Updated on 13/7/21 by Beh SJ @behlogy

 5. Glycoprotein
Roles:

4) Receptor for cell signalling molecules

(for glycoprotein only, not glycolipid)

Updated on 13/7/21 by Beh SJ @behlogy

Components of the Plasma Membrane

Updated on 13/7/21 by Beh SJ @behlogy

 CELL
SIGNALLING
 Cell Signalling
• Cell signalling = How cells detect and respond to stimuli
• Also, how cells communicate
→ Involves ligands = specific chemicals as signalling molecules
→ Leads to specific responses

Updated on 13/7/21 by Beh SJ @behlogy

 Cell Signalling
The Process:
1. Ligands secreted from cells
(E.g. adrenaline from adrenal gland)

2. Ligands transported via bloodstream

to target cells

3. Ligands bind to cell surface receptors

on target cells (E.g. liver / muscle cell)
• Receptor is specific and complementary
in shape to ligand
• Shape of receptor changes when ligand binds
→ Signal passed into the cell (transduction)

Updated on 13/7/21 by Beh SJ @behlogy

 Cell Signalling
4. Receptor activates G protein

5. G protein triggers production of many

secondary messengers by enzyme
• 2o messengers are small and soluble

6. 2o messengers triggers enzyme cascade

• Catalysed by enzyme kinases and
phosphatases
• Cause signal amplification

7. Enzymes carry out specific response

Updated on 13/7/21 by Beh SJ @behlogy
 Examples of Specific Responses

E.g. Adrenaline

• Activated enzymes breakdown

glycogen into glucose
• More glucose available for
respiration
• More energy in the form of
ATP produced More on this in A2!

Updated on 13/7/21 by Beh SJ @behlogy

 Examples of Specific Responses
Specific response depends on the ligand!
Responses can include:
• Increase transcription of a gene
• Movement
• Change in cell shape
• Secretion
• Activation of an enzyme
• Altered metabolism
• Opening an ion channel
• Using the altered receptor directly as
a membrane-bound enzyme

Updated on 13/7/21 by Beh SJ @behlogy

 Types of Ligands
1. Water-soluble ligands
• Cannot pass through membrane
• Recognised by receptor at plasma membrane
• E.g. adrenaline, glucagon

2. Lipid-soluble ligands
• Can pass through membrane
• Can diffuse directly across the
cell surface membrane
• Bind to intracellular receptors
in the cytoplasm or nucleus
• E.g. steroids, oestrogen

Updated on 13/7/21 by Beh SJ @behlogy

TRANSPORT ACROSS MEMBRANE
 Transport Across the Plasma Membrane

5 mechanisms of transport between the cell and its environment:

1. Simple diffusion
2. Facilitated diffusion Passive process
3. Osmosis (Does not require energy)

4. Active transport Active process

5. Endocytosis and Exocytosis (Requires energy)

Updated on 13/7/21 by Beh SJ @behlogy

 1. Simple Diffusion
Definition:
• Net movement of molecules
• From a region of high concentration to low concentration
• Down the concentration gradient
• Until equilibrium

• In cells, this occurs across a

phospholipid bilayer

• Passive transport
→ No ATP used
→ Result of random particle
movements
Updated on 13/7/21 by Beh SJ @behlogy
 1. Simple Diffusion
Substances that can pass through the phospholipid bilayer
using simple diffusion are:

1) Non-polar/ lipid soluble

2)Uncharged
E.g. oxygen and carbon dioxide

3) Small
E.g. water molecules (osmosis)

Polar, water soluble, charged molecules are unable to pass

through the hydrophobic core of the phospholipid bilayer
Updated on 13/7/21 by Beh SJ @behlogy
Factors Affecting Rate of Simple Diffusion
1) Steepness of the concentration gradient
• Greater the difference in concentration,
the steeper the conc. gradient, the higher the rate

2) Temperature
• The higher the temperature, the higher the kinetic energy of
molecules/ions, the higher rate of diffusion

3) Nature of molecules / ions

• Smaller, non-polar molecules diffuse faster +

4) Surface area to volume ratio (SA:V)

• As the object size decreases, the SA:V increases, the shorter
diffusion distance, the higher the rate of diffusion
Updated on 13/7/21 by Beh SJ @behlogy
Surface Area to Volume Ratio (SA:V)
• As the object size decreases, the SA:V increases, the shorter
diffusion distance, the higher the rate of diffusion

Agar block with

pH indicator
Petri dish
with acid

Updated on 13/7/21 by Beh SJ @behlogy

 2. Facilitated Diffusion
Definition:
• Diffusion through membrane transport proteins
• From a region of high concentration to low conc.
• Down a concentration gradient

• Involves channel proteins and carrier proteins

Allow passage of ions and polar molecules

• Passive transport
→ No ATP required

Updated on 13/7/21 by Beh SJ @behlogy

 2. Facilitated Diffusion
Polar, water soluble, charged molecules are unable to pass
through the hydrophobic core of the phospholipid bilayer

Substances that can pass through using facilitated diffusion

via transport proteins are:

1) Large or water-soluble molecules

E.g. glucose, amino acids

2) Ions or polar molecules

E.g. Na+ and Cl-

Updated on 13/7/21 by Beh SJ @behlogy

Factors Affecting Rate of Facilitated Diffusion
1) Steepness of the concentration gradient

2) Temperature

3) Number of transport proteins available

• Channel proteins: Open or close

4) Surface area of the membrane

• Large surface area able to fit more transport proteins

Updated on 13/7/21 by Beh SJ @behlogy

 Example Question
In an investigation, animal cells were exposed to different concentrations of
glucose. The rate of uptake of glucose into the cells across the plasma membrane
was determined for each concentration. Figure below shows the results.

Explain how the results of the investigation support the idea that glucose enters
cells by facilitated diffusion. [2]
Updated on 13/7/21 by Beh SJ @behlogy
 Example Question
It’s facilitated diffusion:
• Because the rate of uptake increases with increasing
glucose concentration, up to a plateau
• At high concentration, all transport proteins in use

If it is passive diffusion:
• Rate would continue to rise

If it is active transport:
• Rate would be independent of concentration
(except at low concentration)

Updated on 13/7/21 by Beh SJ @behlogy

 3. Osmosis
*** There is no such thing as water concentration!***
We use the term water potential (Ψ)
Ψ = the tendency of water molecules to move from one area to
another

Definition:
• Diffusion of water
• From a region of high water
potential to low water potential
• Down the water potential gradient
• Across a partially permeable membrane
• Until equilibrium

• Passive transport
→ No use of ATP Updated on 13/7/21 by Beh SJ @behlogy
 Visking Tube Experiment
• In control, starch cannot diffuse out
→ Water diffuse in by osmosis
→ Raised water level
→ Solution in beaker remains blue
when heated with Benedict’s solution

• In experiment, maltose diffuses out

→ Less water diffuse in by osmosis
→ Solution in beaker results in brick-red precipitate
when heated with Benedict’s solution

Updated on 13/7/21 by Beh SJ @behlogy

 Water Potential, Ψ depends on…
1. How much water there is in relation to solutes
• Concentration of the solution
• More solutes present, water more likely to move in
• Solute potential, Ψs -ve value
• Ψs of pure water = highest = 0
• As concentration increases, Ψs more negative = Ψs decreases

2. How much pressure is applied to it

• More pressure, more likely to move out
• Pressure potential, Ψp +ve value

• Esp in plant cells, bcs they have cell wall

Ψ = Ψs + Ψ p Overall -ve value

Updated on 13/7/21 by Beh SJ @behlogy
 Example Question
The diagram shows the water potential of three cells.

In which directions will there be net movement of water by

osmosis to or from cell X?

Updated on 13/7/21 by Beh SJ @behlogy

 Osmosis in Animal Cells
No cell wall, no pressure potential!
Ψ = Ψs

1) When the external solution is hypertonic to the

animal cell
• Higher concentration of solutes outside
• Water potential outside is lower (more negative)
• Water diffuse out of cell by osmosis
• Cell shrinks

2) When the external solution is isotonic to the

animal cell
• Water potential outside animal cell similar to
cell’s content
• No net gain/loss of water
• Cell maintains its shape
Updated on 13/7/21 by Beh SJ @behlogy
 Osmosis in Animal Cells
3) When the external solution is hypotonic to
the animal cell
• Lower solute concentration outside
• Water potential outside animal cell higher
(less negative)
• Water diffuse into cell by osmosis
• Cell volume increases and bursts (lyse)

P/S: words like crenation and haemolysis only apply to RBC,

not other animal cells
Updated on 13/7/21 by Beh SJ @behlogy
 Osmosis in Plant Cells
1) When the external solution is hypertonic to the plant cell

• Higher conc of solutes outside cells

→ Lower / more negative water potential outside
→ Water leaves the plant cells by osmosis
→ Water potential in cells decreases

Result:
• Protoplast shrinks, pull away from the cell wall
→ Plasmolysis occurs
• No pressure on cell wall, so Ψp = 0
External solution has
• Therefore Ψ = Ψs only passed through cell wall

Updated on 13/7/21 by Beh SJ @behlogy

 Osmosis in Plant Cells
2) When the external solution is isotonic to the plant cell

• Plant cell and solution are in the

state of equilibrium
• No net movement of water

• Protoplasm just began to shrink

away from cell wall
→ Incipient plasmolysis
• No pressure on cell wall, so Ψp = 0
• Therefore Ψ = Ψs only

Updated on 13/7/21 by Beh SJ @behlogy

 Osmosis in Plant Cells
3) When the external solution is hypotonic to the plant cell

• Low conc of solutes outside cells

→ Higher water potential outside
→ Water diffuse into cells by osmosis
→ Water potential in cells increases

Result:
• Protoplast pushes against cell wall
→ The cell becomes turgid
• Ψp or turgor pressure in cells builds up
• Increases water potential of the cell further

• Water potential in cell, Ψ = Ψs + Ψp

Updated on 13/7/21 by Beh SJ @behlogy
 Osmosis in Plant Cells

Ψp = 0 Ψp = 0 Ψ = Ψ s + Ψp
Ψ = Ψs Ψ = Ψs
Onion cells Onion cells

Updated on 13/7/21 by Beh SJ @behlogy

 Example Question
The stalk of a dandelion flower is a hollow tube. Pieces of the stalk are cut as
shown and placed in sucrose solutions of different water potentials.

Which diagram shows the piece that is placed in the sucrose solution with the
highest water potential?

Updated on 13/7/21 by Beh SJ @behlogy

 4. Active Transport
Definition:
• Movement of molecules or ions through
carrier proteins
• From a region of low concentration to
high concentration
• Against the concentration gradient

• Using energy in the from of ATP

→ Needed for conformational change of
carrier protein

• Result in cells having a higher

concentration of ions compared to the
external environment
Updated on 13/7/21 by Beh SJ @behlogy
 4. Active Transport
E.g. sodium-potassium pump

• Carrier protein
• Pumps 2 K+ in, 3 Na+ out
• Result: inside of cell becomes less
positively charged than outside

• Uses 1 ATP

• Carrier protein also acts as enzyme ATPase

• Has binding sites for Na+ and K+, and an active site for ATP
• ATP → ADP + Pi + energy
• Needed for conformational change of carrier protein

Updated on 13/7/21 by Beh SJ @behlogy

 4. Active Transport
Roles:
• Sodium-potassium pumps in cells
→ Important in nerve impulses

• Transport of ions from soil via root hairs

→ Contributes to root pressure

• Hydrogen pumps in cells

→ Translocation of sucrose into phloem

• Absorption in the intestines

• Reabsorption in the kidneys

Updated on 13/7/21 by Beh SJ @behlogy

Updated on 13/7/21 by Beh SJ @behlogy
 5. Endocytosis and Exocytosis
• Mechanism to transport large quantities of substances
• Requires energy in the form of ATP

• Endocytosis = into cell

– Phagocytosis = solids
– Pinocytosis = liquids
• Exocytosis = out of cell

Updated on 13/7/21 by Beh SJ @behlogy

 Mechanism of Phagocytosis
Membrane fuses to
form endocytic vesicle
/ phagosome.

Phagocyte is attracted The vesicle fuses with

to bacteria. Bacterial lysosomes containing
antigens binds to hydrolytic enzymes, that
receptors on the cell catalyses hydrolysis.
membrane.
Enzymes break down
protein / DNA / lipid
Membrane infolds. peptidoglycan / carb.
Pseudopodia forms.
The bacterium is
Bacteria is engulfed. killed and digested
within the vesicle.
https://www.youtube.com/watch?v=JnlULOjUhSQ
https://www.youtube.com/watch?v=iZYLeIJwe4w
Updated on 13/7/21 by Beh SJ @behlogy
 Exocytosis
• Substances packaged into
secretory vesicles
→ Fuse with cell surface membrane
→ Release contents

• E.g. Secretion of digestive

enzymes and hormones from cells

https://www.youtube.com/watch?v=U9pvm_4-bHg

Updated on 13/7/21 by Beh SJ @behlogy

 Chapter Outline
Part 1: Cell Membrane Part 2: Cell signalling
Structure and Function
• Fluid mosaic model Part 3: Transport Across Membrane
• Roles of: 1. Diffusion
– Phospholipids 2. Facilitated diffusion
– Cholesterol 3. Osmosis
– Proteins 4. Active transport
– Glycolipids 5. Endocytosis and exocytosis
– Glycoprotein

Updated on 13/7/21 by Beh SJ @behlogy

 Related Videos (for fun)
Basic Components Of Cell Membrane Explained Under 9 Minutes!!!
https://www.youtube.com/watch?v=Aly7AFh46lg

Receptors: Signal Transduction and Phosphorylation Cascade (Prof Dave Explains)

https://www.youtube.com/watch?v=VatdTJka3_M

Cell Membranes and Cell Transport: Molecules like to Move it, Move it
https://www.youtube.com/watch?v=Ptmlvtei8hw

Osmosis and Water Potential (Amoeba Sisters)

https://www.youtube.com/watch?v=L-osEc07vMs

An educational game lol

https://www.biomanbio.com/HTML5GamesandLabs/Cellgames/celldefensehtml5page
.html
Updated on 13/7/21 by Beh SJ @behlogy
 AS Level
Chapter 5

The Mitotic
Cell Cycle
 Chapter Outline
Part 1: Cell Division
• Structure of chromosomes
• Importance of mitosis
• Interphase
• Mitosis
• Cytokinesis

Part 2: Regulation of the Cell Cycle

• Importance of telomeres
• Stem cells in cell replacement and tissue repair
• Cancer and tumour formation

Updated on 13/7/21 by Beh SJ @behlogy

 Part 1:
Cell Division
Terminology
 DNA Packaging
• Double-helix DNA is
associated with histone
proteins

• DNA wraps and coils around

histone proteins
→ Forms chromatin
→ Found in this form usually

During cell division,

• DNA undergoes further
coiling
→Form tightly-coiled DNA =
chromosomes

https://www.dnalc.org/resources/3d/07-how-dna-is-packaged-basic.html Updated on 13/7/21 by Beh SJ @behlogy

 Structure of Chromosomes
• 1 chromosome has several thousand genes
• Genes = DNA that codes for protein
• Locus (plural: loci) = Position of gene in a chromosome

• If same species, genes are on same

chromosome and loci
→ But different alleles
• Alleles = Different forms of one gene
• Can be dominant or recessive
• E.g. Gene - Eye colour
Alleles - Allele for blue eyes, allele for brown eyes etc.
Updated on 13/7/21 by Beh SJ @behlogy
Homologous Chromosomes
• Pairs of chromosomes found
in diploid cells
• 1 maternal and 1 paternal

• Similar centromere’s position

• Similar chromosome’s size and shape
• Same genes, different alleles

Updated on 13/7/21 by Beh SJ @behlogy

Sister Chromatids – Duplicated DNA
• During interphase, DNA
replication occurs in the nucleus
• One chromatid becomes two
• Two identical chromatids
= Sister chromatids

• Sister chromatids have identical

copies of genes

• Centromere holds two

chromatids together

Updated on 13/7/21 by Beh SJ @behlogy

Structure of Chromosomes

Updated on 13/7/21 by Beh SJ @behlogy

 Karyogram
• Total set of chromosomes in an organism
• Arranged in order of size
• No. of chromosomes are specific to each species
• Human diploid cells have 23 sets of chromosomes (2n = 46)

2 types of
chromosomes:
Sex chromosomes
and autosomes

Updated on 13/7/21 by Beh SJ @behlogy

Mitosis
 Introduction to Cell Division
• All cells in our body are genetically identical (except gametes)
• Cells reproduce / divide
• To pass copies of genes to daughter cells

Updated on 13/7/21 by Beh SJ @behlogy

 Diploid vs Haploid cells
Diploid cells have two complete sets of chromosomes (2n)
• 1 maternal, 1 paternal
• Somatic cells = all cells except for gametes / germ cells
• In humans: 2n = 46
→ Result of mitosis

Haploid cells have only one set of chromosomes (n)

• Gametes / germ cells
• In humans: n = 23
→ Result of meiosis (aka reduction division)

Updated on 13/7/21 by Beh SJ @behlogy

 Importance of Mitosis
• Results in two genetically identical daughter cells
→Same number of chromosomes as parent cell

Biological importance:
1) Maintaining number of chromosomes
→Ensuring genetic stability /
→New cells can retain function

2) Growth of multicellular organisms

3) Cell replacement / Tissue repair

4) Asexual reproduction
E.g. vegetative reproduction in plants / cloning
Updated on 13/7/21 by Beh SJ @behlogy
 The Mitotic Cell Cycle
1) Interphase
• G1
• S
• G2

2) M phase (mitosis)
• Prophase
• Metaphase
• Anaphase
• Telophase

3) Cytokinesis

Cell Cycle and Mitosis [3D Animation]

https://www.youtube.com/watch?v=xsrH050wnIA
Updated on 13/7/21 by Beh SJ @behlogy
 Interphase
1) G1 phase
• Growth of cell
• Synthesis of proteins and other substances

2) S phase
• DNA Replication
→ Result in sister chromatids

3) G2 phase
• Growth
• Duplication of centrioles
→ Form 2 pairs of centrosomes
• Repair of DNA replication errors
Updated on 13/7/21 by Beh SJ @behlogy
Mitosis

• Prophase
• Metaphase
• Anaphase
• Telophase

Updated on 13/7/21 by Beh SJ @behlogy

Prophase
• Condensation of chromatin
→ Become shorter and thicker
→ Appearance of chromosomes
→Visible as two, sister chromatids

• Spindle fibres form

→ Start attaching to centromere
• Centrosomes move to opposite poles
• Nuclear envelope breaks down
• Nucleolus breaks down

Updated on 13/7/21 by Beh SJ @behlogy

Metaphase
• Centrosomes reach opposite poles
• Spindle fibres are fully formed

• Chromosomes line up at the

metaphase plate / equator

• Chromosomes attached to spindle

fibres at centromere / kinetochore

(protein structure at
centromere where
spindle attach to)

Updated on 13/7/21 by Beh SJ @behlogy

 Anaphase
• Centromere of each chromosome divides
• Sister chromatids split at the centromere

• Spindle microtubules shorten

• Chromatids pulled to opposite poles

→ with centromeres leading towards poles

Updated on 13/7/21 by Beh SJ @behlogy

Telophase
• Chromatids reach the poles

• Chromosomes decondense
→ Become long and thin

• Nucleolus reforms
• Nuclear envelope reassembles

• Spindle fibres breaks down

• Cytokinesis starts
Updated on 13/7/21 by Beh SJ @behlogy
 Onion root tip cells

Broad bean root tip cells

Updated on 13/7/21 by Beh SJ @behlogy

Cytokinesis
• Division of the cell’s cytoplasm

In animal cells:
• Cell membrane drawn together
→ By contractile ring of
microfilaments
→ Forms a cleavage furrow
→ Creating a drawstring effect

• Cell membrane fuses

• To divide cell into two
• Organelles are shared out
Updated on 13/7/21 by Beh SJ @behlogy
Cytokinesis
In plants:
• Vesicles transported to equator
→ To form cell plate at equator
• Cell wall laid down

• So cytoplasm divided into two

• Organelles are shared out

Updated on 13/7/21 by Beh SJ @behlogy

Which row is correct for a human cell just before the cell enters prophase of
mitosis?

number of
number of nuclear envelope
molecules of DNA spindle present
chromatids present
in nucleus
A 46 46 yes no

B 46 92 yes yes

C 92 46 no yes

D 92 92 no yes

Updated on 13/7/21 by Beh SJ @behlogy

Name process K and L.

Updated on 13/7/21 by Beh SJ @behlogy

 Phase G1 S G2 P M A T+C G1

Number of
46 46 46 46 46 92 92 46
chromosomes

Number of
chromatids /
46 92 92 92 92 92 92 46
Number of DNA
molecules

Amount of DNA
x1 x2 x2 x2 x2 x2 x2 x1
/ Mass of DNA

Updated on 13/7/21 by Beh SJ @behlogy

 Part 2:
Regulation of
the Cell Cycle
 Checkpoints
• The duration of the cell cycle differs for diff types of cells
• The cell cycle is tightly controlled and coordinated
• Checkpoints prevent the cycle from continuing when
mistakes are made or DNA is damaged

Updated on 13/7/21 by Beh SJ @behlogy

Telomeres
Telomeres
• A region of repetitive nucleotide sequences
• At the end of a chromatid

Role:
• DNA replicating enzymes stops a little before
the end of DNA molecules
• Telomeres prevent loss of genes at the tips of
each chromatid

BUT….
• Telomeres get shorter with each mitotic division
• So after many rounds of mitosis, aged cells
eventually lose vital genes and die
• When cells age….we age too!
Updated on 13/7/21 by Beh SJ @behlogy
Telomeres

Updated on 13/7/21 by Beh SJ @behlogy

If only we can repair telomeres….
• The enzyme telomerase repairs telomeres
• It stops the telomeres from getting shorter each time a
chromosome is replicated

• But telomerase is NOT normally active in human body cells!

• It is however active in stem cells.

• As well as in cancer cells :O

Updated on 13/7/21 by Beh SJ @behlogy

Stem Cells
Stem Cells
Features:
• Stem cells are undifferentiated

• Able to continually divide via mitosis

→ Bcs stem cells have telomerase
→ Telomeres do not shorten each cell
cycle

• When it divides, it can produce:

→ A stem cell that divides
→ A cell that differentiates / specializes

• Many genes not switched off Updated on 13/7/21 by Beh SJ @behlogy

Stem Cells
Roles:
• Form cells that can differentiate
• Divides to give continuous supply of stem cells
• For:
→ Cell replacement
→ Tissue repair
→ Growth

“Levels” of stem cells potency:

• Totipotent
• Pluripotent
• Multipotent P/S: No need to memorise
names of stem cells!
Updated on 13/7/21 by Beh SJ @behlogy
 P/S: No need to memorise
names of stem cells!
Updated on 13/7/21 by Beh SJ @behlogy
 Stem Cells
• Stem cell therapy
- Introduce new adult stem cells
- Promote healing of injury/disease

Updated on 13/7/21 by Beh SJ @behlogy

Cancer Cells
Cancer
• Most common disease in
developed countries
• 1 in 4 deaths

Most common cancers:

• Prostate cancer – men
• Breast cancer – women

• Shows us the importance of controlling cell division precisely

Updated on 13/7/21 by Beh SJ @behlogy

Cancer
• Result of uncontrolled mitosis
→Cells divide repeatedly
→Cell cycle checkpoints not controlled

• Short interphase
→ DNA replication is error-prone
→ Very little growth

Updated on 13/7/21 by Beh SJ @behlogy

Factors Which Increases Risk of Cancer
• Mutagen = substance that causes mutation
• Carcinogen = Cancer-causing substance
• Not all / only some mutagens are carcinogenic
• Cancer is usually not due to a single mutation
in a cell

• Mutagens:
1) Ionising radiation (e.g. X-rays, gamma rays)
2) UV light
3) Free radicals
4) Chemicals (e.g. tar, ethidium bromide, mustard gas)
5) Virus infection (e.g. HPV, HIV)
Updated on 13/7/21 by Beh SJ @behlogy
Factors Which Increases Risk of Cancer
Other factors that increase chances of cancer:
• Hereditary predisposition
• Tobacco smoking
• Obesity

Updated on 13/7/21 by Beh SJ @behlogy

 What Causes Cancers?
• Mutation in genes that controls cell division
→ Oncogenes (mutated gene that causes cancer) is switched on
→ Proto-oncogene (normal gene) converted to an oncogene
→ Tumour suppressor genes switched off

• Cancer cells pass on mutations and oncogenes to daughter cells

• Cancer cells can escape DNA repair during interphase as well
→ result in accumulated mutations

Updated on 13/7/21 by Beh SJ @behlogy

 What Causes Cancers?
Accumulated mutations in cancer cells can cause:

• Immune system does not recognize the cells as foreign and

destroy it → avoid detection of immune system
• No programmed cell death

• Has telomerase → Can divide indefinitely

• Do not respond to extra/intracellular signals to stop dividing
• Mitosis is no longer inhibited by cell to cell contact
→ no contact inhibition

• Loss of function / cell specialization

• Form mass of undifferentiated cells = tumour
Updated on 13/7/21 by Beh SJ @behlogy
 Tumour Development
1) Carcinogens cause mutations in gene which controls cell division
→ oncogene switched on

2) Cancerous cell escape cell death and immune system

3) Cell undergoes rapid and uncontrolled mitosis

→ grows into mass of unspecialised cells = tumour

4) Tumour grows, then displaces and compresses surrounding tissues

5) Growth of blood capillaries into tumour (angiogenesis)

Updated on 13/7/21 by Beh SJ @behlogy

Tumours
• Mass of undifferentiated cells
• No specific functions
• Result of uncontrolled cell division

• Has abnormal changes in cell shape

• Can grow, then displace and
compress surrounding tissues

• Has high demand of nutrients

• As tumour grows, there can be
growth of blood capillaries into
tumour
• To deliver nutrients
Updated on 13/7/21 by Beh SJ @behlogy
 Types of Tumours

a) Benign
• Do not spread from their site of origin
• E.g. warts, ovarian cysts, brain tumours

b) Malignant
• Can spread throughout the body (metastasise)
• Can undergo metastasis
→Through blood and lymphatic system
→Invade and destroys other tissues in other areas
→Can result in secondary growth

Updated on 13/7/21 by Beh SJ @behlogy

 Chapter Outline
Part 1: Cell Division
• Structure of chromosomes
• Importance of mitosis
• Interphase
• Mitosis
• Cytokinesis

Part 2: Regulation of the Cell Cycle

• Importance of telomeres
• Stem cells in cell replacement and tissue repair
• Cancer and tumour formation

Updated on 13/7/21 by Beh SJ @behlogy

Videos for fun…as usual
Mitosis: The Amazing Cell Process that Uses Division to Multiply!
https://www.youtube.com/watch?v=f-ldPgEfAHI

TED-Ed: How do cancer cells behave differently from healthy ones? - George Zaidan
https://www.youtube.com/watch?v=BmFEoCFDi-w

The Science of Aging

https://www.youtube.com/watch?v=BkcXbx5rSzw

WHAT CAN STEM CELLS DO?

https://www.youtube.com/watch?v=K7D6iA7bZG0

Updated on 13/7/21 by Beh SJ @behlogy

 AS Level
Chapter 6

Nucleic Acids and

Protein Synthesis
 Chapter Outline
Part 1 : Nucleic Acids
• Structure of Nucleotides
• RNA and DNA
• Semi-Conservative DNA Replication

Part 2 : Protein Synthesis

• Triplet Code → Codon → Anticodon
• Transcription
• RNA processing
• Translation
• Gene mutation
→ Sickle cell anaemia
Updated on 13/7/21 by Beh SJ @behlogy
Part 1:
Nucleic Acids
 Introduction to Genetic Materials
Characteristics:
1) Ability to carry instructions/information
• Blueprint for the construction and behaviour of cells

2) Ability to be copied
• Pass on exact copy of information to
daughter cells

DNA structure discovered by

James Watson and Francis Crick, 1953
(but is quite contraversial...) Rosalind
Franklin

Updated on 13/7/21 by Beh SJ @behlogy

 Nucleic Acids
• Monomer = Nucleotides

• Polymer = Polynucleotides
• 2 types:
1. DNA
2. RNA

• Bond between adjacent monomers after condensation

= phosphodiester bond
• Bond between complementary base pairs
= hydrogen bond

Updated on 13/7/21 by Beh SJ @behlogy

 Nucleotides
A nucleotide consists of:
1) Nitrogenous base : Purine or pyrimidine base
2) Pentose sugar : 5 carbon sugar, either deoxyribose or ribose
3) Phosphate group: Negatively charged, making DNA a negatively
charged molecule

Updated on 13/7/21 by Beh SJ @behlogy

 Nitrogenous Bases
2 major types of nitrogenous bases:

Purine bases – has 2 rings:

1) Adenine
2) Guanine
(Pure As Gold)

Pyrimidine bases – has 1 ring only:

1) Cytosine
2) Uracil (RNA only)
3) Thymine (DNA only)

Updated on 13/7/21 by Beh SJ @behlogy

 Complementary Base Pairs
• Pairing of bases is precise
• Purine always binds with pyrimidine
• So DNA molecule has same width throughout

Updated on 13/7/21 by Beh SJ @behlogy

 RNA
• RNA = Ribonucleic acid
• Single-stranded
• Forms a single helix

3 components in ribonucleotides:
1) Nitrogenous base
2) Ribose sugar
• Contains 1 oxygen atom more
than deoxyribose
3) Phosphate group

• Bases: A, U, C, G

Updated on 13/7/21 by Beh SJ @behlogy

P/S: Adenosine Triphosphate (ATP) is a RNA nucleotide

Updated on 13/7/21 by Beh SJ @behlogy

 DNA
• DNA = Deoxyribonucleic acid

• Double-stranded
• Forms a double helix
• Have complementary base pairs
• Longer than RNA

3 components in deoxyribonucleotides:
1) Nitrogenous base
2) Deoxyribose sugar
• Contains 1 oxygen atom less than ribose
3) Phosphate group

• Bases: A, T, C, G
Updated on 13/7/21 by Beh SJ @behlogy
What are the
structural
differences btwn
RNA and DNA?

Updated on 13/7/21 by Beh SJ @behlogy

 Structure of DNA
• Linking of nucleotides occurs in nucleus during S phase of cell cycle
• DNA replication occurs
• Bond between adjacent monomers after condensation =
phosphodiester bond

Structure of DNA:
1) DNA has a sugar-phosphate
backbone
• Formed by alternating sugar
and phosphate groups
→ Linked by phosphodiester bonds
• Strong covalent bonds

Updated on 13/7/21 by Beh SJ @behlogy

Phosphodiester
bond

Updated on 13/7/21 by Beh SJ @behlogy

 Structure of DNA
2) Complementary base pairing occurs
between opposite strands

• 2 H bonds between A = T
• 3 H bonds between G ≡ C
→ H bonds hold the 2 strands together
→ Easily broken for transcription to
RNA or replication

• Pairing of bases is precise

• Purine always binds with pyrimidine
→ 2 rings + 1 ring = 3 rings
→ Distance between strands always the same
Updated on 13/7/21 by Beh SJ @behlogy
 Hydrogen Bonding
• Between complementary bases of 2 DNA strands

• Important for 3D structure of molecule

• Many H bonds give stability
• But individual H bonds are weak

Important for DNA replication / transcription

• So strands can be separated
• H bonds only form between specific bases
→ Fewer mistakes
• H bonds also can easily reform without chemical reaction

Updated on 13/7/21 by Beh SJ @behlogy

 Structural Characteristics of DNA
3) Two strands of polynucleotides are
antiparallel
• Run in opposite directions
• One strand runs from the 5’ to 3’
direction
• The other runs from 3’ to 5’

Deoxyribose sugar Updated on 13/7/21 by Beh SJ @behlogy

 Two strands of polynucleotides are antiparallel

It’s called 5’ bcs it is 5’

2’ 3’
nearest to carbon 5 1’
4’ 4’
1’ 5’
3’ 2’

5’ 2’ 3’
1’ 4’
4’
3’ 1’
2’ 5’
It’s called 3’ bcs it is
nearest to carbon 3

Updated on 13/7/21 by Beh SJ @behlogy

 5’ 3’

You only need to be given what one end of

the DNA molecule is called to label the rest!

5’ 3’
Updated on 13/7/21 by Beh SJ @behlogy
Example question:
5’ – T A A A G C C C T A – 3’
Given the sequence of DNA above, calculate:
• the total number of bases in the length of DNA
• the number of purines and pyrimidines
• the number of H bonds in the DNA molecule

A:
DNA is double-stranded so…. A = T, whereas G ≡ C so…..
Total no. of bases = 10 * 2 = 20
5’ – T A A A G C C C T A – 3’
3’ – A T T T C G G G A T– 5’
Complementary base pairing occurs so….
5’ – T A A A G C C C T A – 3’ No. of H bonds = (2*6) + (3*4) = 24
3’ – A T T T C G G G A T– 5’

No. of purines = 10
No. of pyrimidines = 10
Updated on 13/7/21 by Beh SJ @behlogy
Semi-Conservative
DNA Replication
 Semi-Conservative DNA Replication
• Occurs in the nucleus during
S phase of interphase
• Requires ATP

• Enzymes needed:
1. Helicase
• To break H bonds to separate 2 DNA strands
2. DNA polymerase
• To synthesise a new strand of DNA (in the 5’ to 3’ direction)
• To catalyse the formation of phosphodiester bond
• Proofreads DNA
3. DNA ligase
• To join DNA fragments together
• To catalyse the formation of phosphodiester bonds
Updated on 13/7/21 by Beh SJ @behlogy
 Semi-Conservative DNA Replication
1. DNA double helix unwinds
• The whole DNA molecule is unwound eventually

2. Helicase break H bonds

• 2 DNA strands separated
• Both strands are used as templates

Updated on 13/7/21 by Beh SJ @behlogy

 Semi-Conservative DNA Replication
3. Free, activated DNA nucleotides are
activated with 2 additional phosphates
• Have 3 phosphate groups in total
• Free in the nucleus for synthesis of new
strands

• The bases of activated nucleotides form

H bonds with bases on each exposed
parent DNA strands
• According to the rules of
complementary base pairing

Updated on 13/7/21 by Beh SJ @behlogy

 Semi-Conservative DNA Replication
4. DNA polymerase attach to each of the two separated
parental strands
• Catalysing the formation of a new strand of DNA

• DNA pol links activated nucleotides together

→ By removing 2 phosphate groups (which are then recycled!)
→ Catalysing phosphodiester bond formation
• DNA pol also proofreads DNA!

• This occurs step-by-step

• Process continues along whole

DNA molecule

Updated on 13/7/21 by Beh SJ @behlogy

 Semi-Conservative DNA Replication
Slight complication:
• DNA polymerase attach to each of the
two separated parental strands
→ But the two enzymes move in opposite
directions
→ And new DNA strand always formed in DNA pol moving from 3’ to 5’
the 5’ to 3’ direction 3’

5’
3’
New DNA formed from 5’ to 3’
3’
Process able to continue
as DNA unwinds

5’
New DNA formed from 5’ to 3’
5’ 3’
Another DNA pol needed
to start process again
5’
DNA pol moving from 3’ to 5’
Updated on 13/7/21 by Beh SJ @behlogy
 Semi-Conservative DNA Replication
• One DNA strand is synthesised continuously
→ Called the leading strand
• The other is synthesised in sections known as Okazaki fragments
→ Called the lagging strand

• The fragments are joined by an enzyme, DNA ligase

→ catalyses the formation of phosphodiester bonds

Updated on 13/7/21 by Beh SJ @behlogy

Semi-Conservative DNA Replication
Result:
• 2 DNA molecules
• Each containing 1 original strand
and 1 newly synthesised strand
→ This is why it’s called
semi-conservative replication

Updated on 13/7/21 by Beh SJ @behlogy

Other models of DNA Replication

DNA molecule has DNA molecule has 1 Each DNA strand

either 2 old strands parental strand and 1 has mixture of old
or 2 new strands new strand and new strands

Updated on 13/7/21 by Beh SJ @behlogy

 How did scientists know it was semi-conservative ?
• Meselson & Stahl’s experiment (1958)

• Using bacteria E.coli & different nitrogen sources

1) Grow E.coli with nitrogen-15

→ For many generations
→ So all DNA contains only 15N in their nucleotides
→ Extract & centrifuge samples to separate DNA
→ Result: All DNA is heavy

2) Transferred to a medium with nitrogen-14

→ Allowed E.coli to divide again (1 gen, approx. 20min)
→ Centrifuge & observe
→ Result: All DNA is a hybrid. Has 1 heavy strand + 1 light strand.
→ Repeated for 2 more generations & DNA is observed

Updated on 13/7/21 by Beh SJ @behlogy

 15N

1 heavy DNA 14N

2 hybrid DNA

2 light DNA
2 hybrid DNA

6 light DNA
2 hybrid DNA

Updated on 13/7/21 by Beh SJ @behlogy

Updated on 13/7/21 by Beh SJ @behlogy
 Other models of DNA Replication
The results prove that DNA replication followed the
semi-conservative model, NOT conservative or disruptive.

Updated on 13/7/21 by Beh SJ @behlogy

Part2:
Protein Synthesis
How does the nucleus control all
activities of the cell?
• Cell’s activities refer to chemical reactions in the cells
• All chemical reactions are controlled by enzymes
• All enzymes are made of proteins

Therefore,
• DNA contains information for the
synthesis of proteins
• Genome = Total set of genes in a cell
• Gene = Region of DNA which
codes for a polypeptide
→ Determines the exact sequence
of amino acids
→ Determine primary structure of proteins
→ As well as secondary, tertiary and quaternary structures
Updated on 13/7/21 by Beh SJ @behlogy
 Intro to Protein Synthesis
Process:
1) Transcription
• DNA is copied to mRNA
• Takes place in nucleus

2) RNA processing
• mRNA modification in nucleus

3) Translation
• mRNA is translated into
polypeptide chain
• Takes place at ribosomes at
RER/cytoplasm
Updated on 13/7/21 by Beh SJ @behlogy
How do genes code for proteins?
• Triplet code = sequence of
3 nucleotide bases in DNA
→ Codes for 1 amino acid

But there’s….
4*4*4 = 64 possible different
triplet codes and only 20
amino acids

• More than 1 triplet code can

code for the same amino acid
→The triplet code is degenerate
→ E.g. CAA, CAG, CAT and CAC
code for amino acid, valine

Updated on 13/7/21 by Beh SJ @behlogy

Things needed for Protein Synthesis
Types of RNA involved in protein synthesis:

• Messenger RNA (mRNA)

• Transfer RNA (tRNA)

– 20 types

• Ribosomal RNA (rRNA)

– made in nucleolus
– makes up the ribosome

Updated on 13/7/21 by Beh SJ @behlogy

 mRNA
• Single-stranded
• AUCG
• Copy of the gene that codes for a polypeptide
• Made in nucleus and moves to ribosome (small
subunit)
• Codes for sequence of amino acids

• Codon = set of 3 bases on mRNA

• mRNA sequences is a series of codons
• mRNA sequence of codons dictate which amino
acids will be added to polypeptide chain
Updated on 13/7/21 by Beh SJ @behlogy
• Set of 3 bases on mRNA Codon
• Read by tRNA
• Start codon: AUG
→ Starts translation
→ Every 1st amino acid in a polypeptide chain - methionine

• Stop codon: UAA, UAG, UGA (textbook got this wrong!)

→ Stops translation and production of polypeptide chain

Updated on 13/7/21 by Beh SJ @behlogy

 tRNA
• Made in nucleus
• Found in cytoplasm and ribosome
• Single stranded – 3 loops
→ Clover-leaf shaped

• 20 diff types of tRNA for 20 diff amino

acids

• Carries a specific amino acid to

ribosomes

• Anticodon = specific exposed 3 bases on

one loop
→ Anticodon forms complementary base
pairs with codon mRNA at ribosome
→ Different tRNA type, different anticodon
Updated on 13/7/21 by Beh SJ @behlogy
 tRNA
• tRNA holds amino acids in place, side by side
→ for peptide bond formation
→ at the ribosome
• tRNA molecules can be reused after leaving
ribosome

Updated on 13/7/21 by Beh SJ @behlogy

 rRNA
• Single-stranded
• Made in nucleolus
• Make up ribosomes

P/S: no need to
memorise the
numbers

Updated on 13/7/21 by Beh SJ @behlogy

 Ribosomes
• rRNA + some proteins = ribosomes
• Site of protein synthesis
aka translation

Two ribosomal subunits:

• Small subunit – binding site for mRNA

• Large subunit
– 3 sites: E, P, A
– P and A are 2 binding sites for tRNA carrying amino acids
to bind to mRNA
– E site = tRNA exit site
– Also contains peptidyl transferase to catalyse the
formation of peptide bond to form polypeptide
Updated on 13/7/21 by Beh SJ @behlogy
 Things needed for Protein Synthesis
• Occurs in the nucleus during
G1 or G2 phase of interphase
• Requires ATP

• Enzymes needed:
1. Helicase
• To break H bonds to separate 2 DNA strands
2. RNA polymerase
• To synthesise a new strand of RNA (in the 5’ to 3’ direction)
• To catalyse the formation of phosphodiester bond
3. Peptidyl transferase
• To catalyse the formation of peptide bond
Updated on 13/7/21 by Beh SJ @behlogy
Transcription
 Stage One: Transcription
1. DNA double helix unwinds
• Only part of the DNA (gene) unwinds

2. Helicase break H bonds

• 2 DNA strands separated
• Only 1 strand is used as template

non-transcribed strand

transcribed / template strand

Updated on 13/7/21 by Beh SJ @behlogy
 Stage One: Transcription
3. Free, activated RNA nucleotides
• Form H bonds with bases on
DNA template strand
• According to the rules of
complementary base pairing

Updated on 13/7/21 by Beh SJ @behlogy

 Stage One: Transcription
4. RNA polymerase attach to template
• To catalyse the formation of mRNA

• RNA pol joins activated RNA nucleotides together

→ Catalysing phosphodiester bond formation

• mRNA forms in the 5’ to 3’ direction

→ This is the primary transcript
or pre-mRNA

Updated on 13/7/21 by Beh SJ @behlogy

 Stage Two: RNA processing
• The pre-RNA has exons and introns
• Introns = non-coding sequences
→ Removed via RNA splicing
• Exons = coding sequences
→ Joined together to form mature mRNA
→ Mature mRNA leaves the nucleus via the nuclear pore

Updated on 13/7/21 by Beh SJ @behlogy

Translation
 Stage Three: Translation
1. mRNA binds to ribosome
• Translation starts at START codon of mRNA
(Codon: AUG, codes for methionine)

2. tRNA carries specific amino acid to ribosome

• Binds to large subunit of ribosome
• Has specific tRNA / anticodon for
the amino acid
• Anticodon of tRNA complementary base pair
to codon on mRNA by forming H bonds

Updated on 13/7/21 by Beh SJ @behlogy

 Stage Three: Translation
3. A second tRNA molecule with amino acids
binds with the next codon on mRNA
• Two tRNAs hold amino acids in place, side
by side for peptide bond formation
• 2 tRNAs are bound at the ribosome at a time

Updated on 13/7/21 by Beh SJ @behlogy

 Stage Three: Translation
4. Peptidyl transferase in the ribosome
→ Catalyses the formation of peptide bond
between the two amino acids

Updated on 13/7/21 by Beh SJ @behlogy

 Stage Three: Translation
5. Ribosome moves along one codon more
on the mRNA
• In the 5’ to 3’ direction
• Next tRNA arrives and amino acids are
added one at a time
• Previous tRNA detaches, moves away
and is recycled

• Polypeptide is released when STOP

codon (UAA, UAG, UGA) reached

Updated on 13/7/21 by Beh SJ @behlogy

Updated on 13/7/21 by Beh SJ @behlogy
 Stage Three: Translation
• Polyribosomes are often used
→ One mRNA may have many ribosomes binding to it
→ Many polypeptides of the same type can be made
from 1 mRNA
→ Fast response to the cell’s changing requirements

Updated on 13/7/21 by Beh SJ @behlogy

 Stage Three: Translation
• mRNA is short-lived / labile.
• Production of protein is only for a short period of time
• But why?

• Gene expression can be controlled

→ Gene expression = process of DNA → protein
→ Prevents too much product forming
→ Efficient for energy use

Updated on 13/7/21 by Beh SJ @behlogy

Protein Synthesis Summary

Updated on 13/7/21 by Beh SJ @behlogy

Q: How do we know which codon codes for what amino acid?
A: Use a codon table!

Updated on 13/7/21 by Beh SJ @behlogy

 Example question:
Given the sequence of the DNA template strand below, what is the
sequence of amino acids in the resulting polypeptide chain?
DNA TAC TGA ATA GCC CCG AAA ATT
mRNA
Polypeptide

Updated on 13/7/21 by Beh SJ @behlogy

Mutation
 Mutations
• Random change in the
nucleotide sequence of a gene
→ Affects phenotype
(appearance/trait) of organism
https://www.youtube.com/watch?v=oTMpl8_9684

Two types of mutations:

a) Chromosome mutation
• Change in structure/number
of chromosomes

b) Gene mutation

Updated on 13/7/21 by Beh SJ @behlogy

 Gene Mutations
• Mutations = change in the
sequence of bases in a gene
→ Causes altered codons in mRNA
seq
→ May alter amino acid seq of
polypeptide chain

• Can result in new alleles

• Alleles = diff forms of one gene

Updated on 13/7/21 by Beh SJ @behlogy

 Gene Mutations
Types of gene mutations:
• Substitution
• Insertion/Addition
• Deletion

• The simplest form of mutation

involves only a change of one
nucleotide
→ Point mutations / base mutations

Updated on 13/7/21 by Beh SJ @behlogy

 Base substitution
• Only one nucleotide is replaced by another

Can result in one of 3 types of mutations:

1. Silent mutation
= triplet code / codon still codes for the same amino acid
2. Nonsense mutation
= stop codon is introduced
3. Missense mutation
= triplet code / codon codes for a different amino acid

Updated on 13/7/21 by Beh SJ @behlogy

 Base substitution

Result in STOP codon

Triplet code Premature chain Only one amino acid
changed but termination of the polypeptide
amino acid seq is Subsequent amino seq is affected
unaffected acids are not
If amino acid has side chain
formed with different property,
tertiary structure is more
affected.
Normal, Incomplete, Faulty protein, may
functional non-functional be still functional
protein polypeptide
Updated on 13/7/21 by Beh SJ @behlogy
 Base Insertion + Base Deletion
Results:
1. Frameshift mutations
• Deleting/inserting one nucleotide of DNA will change which
bases are read together
• All subsequent codons are affected, all subsequent amino
acids affected
• Faulty, non-functional protein

2. Nonsense mutation
• Result in STOP codon
• Premature chain termination
• Subsequent amino acids are not formed
• Incomplete, non-functional polypeptide
Updated on 13/7/21 by Beh SJ @behlogy
 Base Insertion + Base Deletion

Q: But what if 3 nucleotides are deleted at once?

A:
Can result in the deletion of 1 codon
Deletion of 1 amino acid
Protein may still be functional!

Updated on 13/7/21 by Beh SJ @behlogy

 Example: Sickle Cell Anaemia
• Inherited blood disorder
• Affects the structure of the haemoglobin
• Cause: Base substitution in gene coding for β-globin

Updated on 13/7/21 by Beh SJ @behlogy

 Example: Sickle Cell Anaemia
P/S: the textbook is
wrong and yes, you
need to rmb that T is
replaced by A.

• Base substitution in gene coding for β-globin results in

→different mRNA codon
→different tRNA brings a different amino acid to ribosome

→ leads to a change of 6th amino acid in polypeptide chain

→Altered primary structure
→Glutamic acid is polar whereas valine is non-polar
→Changed secondary, tertiary and quaternary structure
Updated on 13/7/21 by Beh SJ @behlogy
 Example: Sickle Cell Anaemia
Mutated β-globin causes:
• Hb molecule to become less soluble
• Stick tgr to form fibres
• Less able to bind oxygen

• Cells to become crescent /

sickle-shaped
• Sickled red blood cells are prone to
rupture (haemolysis)
• May block blood vessels

Updated on 13/7/21 by Beh SJ @behlogy

 Example: Sickle Cell Anaemia
• A person that inherited TWO
mutated β-globin alleles (HbS) will
have sickle cell anaemia
→ One from mother, one from father

• A person with ONE HbS and ONE HbA

(normal β-globin allele) is only a
carrier
→ Still have one working copy
→ No disease

Updated on 13/7/21 by Beh SJ @behlogy

 Chapter Outline
Part 1 : Nucleic Acids
• Structure of Nucleotides
• RNA and DNA
• Semi-Conservative DNA Replication

Part 2 : Protein Synthesis

• Triplet Code → Codon → Anticodon
• Transcription
• RNA processing
• Translation
• Gene mutation
→ Sickle cell anaemia
Updated on 13/7/21 by Beh SJ @behlogy
 Videos!
On the controversy of Watson & Crick’s experiment:
•Rosalind Franklin vs. Watson & Crick - Science History Rap Battle
https://www.youtube.com/watch?v=35FwmiPE9tI

•DNA Replication
(in a bit more detail than you need to know)
https://www.youtube.com/watch?v=5qSrmeiWsuc
https://www.youtube.com/watch?v=TNKWgcFPHqw

•Transcription and Translation: From DNA to Protein

https://www.youtube.com/watch?v=bKIpDtJdK8Q

•Banana DNA Experiment

https://www.youtube.com/watch?v=pdDP9OcqcbA

A little unrelated, but pretty inspiring:

•How simple ideas lead to big discoveries
https://www.youtube.com/watch?v=F8UFGu2M2gM
Updated on 13/7/21 by Beh SJ @behlogy
 AS Level
Chapter 7
Transport In Plants
 Chapter Outline
Part 1: Plant Anatomy
• Structure of Xylem Vessel Elements, Phloem Sieve Tubes and
Companion Cells

Part 2: Transport of Water and Ions

Part 3: Translocation of Sucrose

Updated on 12/8/21 by Beh SJ @behlogy

Transport of Water and Ions
*always explain in terms of water potential

1) Symplastic pathway
2) Apoplastic pathway
• Casparian strip

Mechanisms for movement of water in xylem vessels

1) Transpirational pull
2) Cohesion and adhesion of water
3) Root pressure

Symplastic/apoplastic pathway to mesophyll cell surface

Transpiration
• Factors that affect transpiration rate
• Measuring rate of transpiration using a potometer
• Xerophytes: adaptations to reduce transpiration
Updated on 12/8/21 by Beh SJ @behlogy
Translocation of Sucrose

Loading of sucrose

Mass flow

Unloading of sucrose

Updated on 12/8/21 by Beh SJ @behlogy

 AS Level
Chapter 7
Transport In Plants
Plant Anatomy
Q: Why do all multicellular plants and animals
need transport systems anyway?
A:
• Small SA:V ratio compared to unicellular organisms
• Longer distance for gas/ nutrients/ hormones to reach tissues
• Higher demand for nutrients/gas
• Diffusion alone too slow to satisfy needs
• Efficient supply of gas/ nutrients/ hormones needed!

→ Transport systems reduce diffusion

distance, can transports large amounts
of required materials and is efficient!

Updated on 12/8/21 by Beh SJ @behlogy

Note: We study dicotyledonous plant anatomy only.
Monocotyledonous plants have similar transport system
but their xylem and phloem are placed differently. Updated on 12/8/21 by Beh SJ @behlogy
 Transverse
section of ROOT

Longitudinal
section of ROOT Updated on 12/8/21 by Beh SJ @behlogy
Transverse section of ROOT

Updated on 12/8/21 by Beh SJ @behlogy

Transverse section of ROOT

Updated on 12/8/21 by Beh SJ @behlogy

Longitudinal section of ROOT

epidermis

Root cap

Updated on 12/8/21 by Beh SJ @behlogy

 Transverse
section of STEM

Longitudinal
section of STEM
Updated on 12/8/21 by Beh SJ @behlogy
Transverse section of STEM

Updated on 12/8/21 by Beh SJ @behlogy

Updated on 12/8/21 by Beh SJ @behlogy
Longitudinal section of STEM

epidermis

cortex

Vascular
bundle

pith

Updated on 12/8/21 by Beh SJ @behlogy

Longitudinal section of STEM

pith

xylem

Updated on 12/8/21 by Beh SJ @behlogy

Updated on 12/8/21 by Beh SJ @behlogy
Upper Transverse section of LEAF
epidermis
Palisade
mesophyll Collenchyma
Spongy
mesophyll

Vascular
bundle

Lower
epidermis
Xylem Cambium
Phloem
Updated on 12/8/21 by Beh SJ @behlogy
Updated on 12/8/21 by Beh SJ @behlogy
Updated on 12/8/21 by Beh SJ @behlogy
Ground Tissue
Parenchyma vs Collenchyma vs Sclerenchyma cells
Ground tissue includes all tissues except vascular bundle and epidermis.

Parenchyma Collenchyma Sclerenchyma

Thickness of cell wall Thin Thicker Thickest (has lignin)
Cortical cells of
Cortical cells, pith
Example Outer cortical cells harder stems / tree
cells, mesophyll cells
branches
Photosynthesis,
Non-living, only for
Functions storage of nutrients, Structural support
support
cell division
Updated on 12/8/21 by Beh SJ @behlogy
Vascular Tissue
Xylem and Phloem
• Tissues are made of many cells

Different types of cells in xylem tissue:

a) Vessel elements / Xylem vessels
b) Tracheids
c) Fibres
d) Parenchyma cells

Different types of cells in phloem tissue:

a) Sieve tube elements
→ joined end to end to form sieve tubes
b) Companion cells
Updated on 12/8/21 by Beh SJ @behlogy
Xylem
Functions:
a) Structural support
b) Transport of water from root to
leaves to atmosphere

Appearance under microscope:

• Cell wall have lignin bands,
in addition to cellulose
• Lignin = strong, hard, waterproof
substance

→ Bands can have diff patterns

(e.g. rings, spiral, reticulated)
→ Thicker cell wall observed
→ Safranine dye stains xylem in red Updated on 12/8/21 by Beh SJ @behlogy
 Xylem Vessels
• Also known as vessel elements
• Made of elongated cells joined end to end

Structure and how its related to its function:

• Xylem vessels are non-living

→ No cytoplasm, no organelles, hollow lumen

→ So more space for greater volume of water

to flow
→ Less resistance to flow of water

Updated on 12/8/21 by Beh SJ @behlogy

Xylem Vessels
• Thick cell walls made of cellulose
→ Structural support
→ Allows adhesion of water

• Cell walls contain lignin

→ Prevents inward collapse as xylem vessel is
under tension
→ Also waterproof to prevent loss of water

• No end walls
→ Less resistance to flow of water
→ Forms a continuous tube joined end to end
Updated on 12/8/21 by Beh SJ @behlogy
Xylem Vessels
Large lumen
→ Large volume of water can be transported

• Pits
→ Formed from plasmodesmata
→ No lignin
→ Allow lateral movement of water
→ To connect to all parts of plant
→ If there is a air bubble blocking vessel, pits
allow water to move out into another xylem
vessel and bypass airlock

Updated on 12/8/21 by Beh SJ @behlogy

 Phloem
Function:
• Transport of assimilates (e.g. sucrose, amino acids)
• From source = site of synthesis of photosynthetic products
• To sink = site where assimilates are stored/used
• Via translocation

Different types of cells in phloem tissue:

a) Sieve tube elements
→ joined end to end to form
sieve tubes
b) Companion cells

Updated on 12/8/21 by Beh SJ @behlogy

Sieve Tube Elements
• Elongated sieve elements are joined end to
end Form continuous column = sieve tube

• Sieve elements are living cells

Structure and how its related to its function:

• Have many plasmodesmata

→ Allow loading of sucrose from companion cells
→ For rapid entry of water near source

• Strong cellulose walls

→ Prevent excessive cell bulging under pressure

Updated on 12/8/21 by Beh SJ @behlogy

 Sieve Tube Elements
• Has few organelles
• Has cellulose cell wall, plasma
membrane, few mitochondria and
ER only
• Has no nucleus, ribosomes,
vacuoles

• Has peripheral cytoplasm that

lines cell walls

→ Less resistance to flow

→ Maximum volume of phloem sap
containing assimilates transported

Updated on 12/8/21 by Beh SJ @behlogy

 Sieve Tube Elements
• Have sieve plate = perforated cell wall
• Have many sieve pores
• Cytoplasm of cells are connected

→ Reduce barrier to flow

→ Prevent cell bulging under pressure for phloem
→ Sieve pores become plugged with callose to
prevent loss of phloem sap after damage
(a clotting mechanism)

Updated on 12/8/21 by Beh SJ @behlogy

Sieve Tube Elements

Callose is stained
red at sieve plate

Updated on 12/8/21 by Beh SJ @behlogy

 Companion Cells
• Next to & closely associated with sieve
element

Structure and how its related to its function:

• Many mitochondria
→ For ATP production via aerobic respiration
→ For active transport in translocation

• Many ribosomes / RER

→ For polypeptide production

• Numerous plasmodesmata across cell walls

→ Transport assimilates into sieve tube
Updated on 12/8/21 by Beh SJ @behlogy
Companion Cells

Updated on 12/8/21 by Beh SJ @behlogy

Updated on 12/8/21 by Beh SJ @behlogy
 AS Level
Chapter 7
Transport In Plants
Transport of Water and Ions
 Introduction to Transport in Plants
Main substances transported:
a) Gases (e.g. carbon dioxide, oxygen)
b) Products of photosynthesis
(e.g. sucrose)
c) Mineral ions
d) Water

In general, plants have slower transport

than animals. Why?
→ Lower requirement for oxygen & glucose
→ Lower energy requirements
→ Lower rate of respiration

Updated on 12/8/21 by Beh SJ @behlogy

 Transport of Gases
• Using simple diffusion
• Leaves are thin and flat
• Have branching shape and a network
of air spaces
→ High SA:V ratio
→ Effective for diffusion
→ No specialized transport system for gases

Updated on 12/8/21 by Beh SJ @behlogy

Transport of Water and Ions
*always explain in terms of water potential

1) Symplastic pathway
2) Apoplastic pathway
• Casparian strip

Mechanisms for movement of water in xylem vessels

1) Transpirational pull
2) Cohesion and adhesion of water
3) Root pressure

Symplastic/apoplastic pathway to mesophyll cell surface

Transpiration
• Factors that affect transpiration rate
• Measuring rate of transpiration using a potometer
• Xerophytes: adaptations to reduce transpiration
Updated on 12/8/21 by Beh SJ @behlogy
 Transport of Water and Soil

Mineral Ions Root Hair

• Similar pathway for both
• From roots upwards
Cortex

Endodermis

Xylem
(in root to stem to leaves)

Mesophyll Cells
(in leaves)

Stomata

Atmosphere
Updated on 12/8/21 by Beh SJ @behlogy
 Movement of Water From
Root cap: Soil → Root Hairs
• Impermeable to water
• Tough – protection Root
hairs

Root hairs :
• Permeable to water
• Mineral ions taken up by facilitated
diffusion / active transport
• Long, thin extensions of epidermal cells
• Able to reach into spaces between soil
particles
• Delicate – replaced after a few days Root cap
• Large surface area
→increased area of absorption
Updated on 12/8/21 by Beh SJ @behlogy
 Movement of Water From
Soil → Root Hairs
• Soil has a higher water potential, Ψ than root hair’s cytoplasm
• Cytoplasm of root hair cell has high concentration of ions and
organic substances (proteins, sugars)

• Water diffuses down water potential gradient via osmosis

• Through the partially permeable cell surface membrane

• Into vacuole & cytoplasm

of root hair cells

Updated on 12/8/21 by Beh SJ @behlogy

 Movement of Water From
Root Hairs → Cortex
• Root hairs has higher water potential than cortex
• Water moves down the water potential gradient via osmosis
• From root hairs → cortex

3 possible routes for

root hairs → cortex:

1) Apoplastic pathway
• Through cell wall or intercellular spaces
• Between cellulose / lignin fibres
→ there is adhesion of water to cellulose
• Water does not cross membranes and enter cells
Updated on 12/8/21 by Beh SJ @behlogy
 Movement of Water From
Root Hairs → Cortex
2) Symplastic pathway
• Through cytoplasm
• Travel cell to cell via plasmodesmata

3) Vacuolar pathway

Updated on 12/8/21 by Beh SJ @behlogy

 Movement of Water From
Root Hairs → Cortex → Endodermis
The endodermis has a Casparian strip
= suberized cell wall
→ Has waxy band of suberin in cell walls
→ Impermeable to water

In endodermal cells:
→ Apoplast pathway is blocked

• Only way to cross endodermis

is through cytoplasm
→ Symplast pathway

Updated on 12/8/21 by Beh SJ @behlogy

 Movement of Water From
Root Hairs → Cortex → Endodermis
• Water and ions must pass through endodermal cells
→ So transport of mineral ions can be controlled
→ At carrier proteins of plasma membrane of endodermal cells
→ Absorbed by active transport and contributes to root pressure

Updated on 12/8/21 by Beh SJ @behlogy

 Movement of Water From
Endodermis → Xylem
• Water continues to move down the water potential gradient
• Across pericycle (layer of cells just below endodermis)
→ Pits in cell walls of xylem vessels

Updated on 12/8/21 by Beh SJ @behlogy

 Movement of Water in
Xylem Vessels from Roots to Leaves
Soil

• Roots have a higher water potential than leaves

→ Water moves down water potential gradient Root Hair

→ Water moves up xylem from roots to leaves

Cortex

Endodermis

Xylem
(in root to stem to leaves)

Mesophyll Cells
(in leaves)

Stomata

Atmosphere
Updated on 12/8/21 by Beh SJ @behlogy
 Movement of Water from
Xylem in Leaves → Atmosphere
• Xylem has a higher water potential than atmosphere
• Water moves down water potential gradient

• After evaporation, intercellular airspaces are also saturated

with water vapour
• Air space also has higher water potential than atmosphere
Updated on 12/8/21 by Beh SJ @behlogy
 Movement of Water from
Xylem in Leaves → Atmosphere
P/S: Do not confuse process names!

1. Apoplast / symplast pathway

2. Evaporation

1. Apoplast / symplast pathway

3. Diffusion of
water vapour

Updated on 12/8/21 by Beh SJ @behlogy

 Transpiration
• Loss of water vapour from leaves
• Side effect: cooling of plant
2 ways:
1. Via stomata
• Diffusion of water vapour from airspace to atmosphere
• Only occurs when stomata is open
→ For gas exchange
→ Entry of carbon dioxide for photosynthesis and exit of oxygen

2. Via cuticle
• Loss of water vapour through cuticle
on leaf surface
• Very small amount of water loss
Updated on 12/8/21 by Beh SJ @behlogy
 Transpiration
• Loss of water vapour from leaves
• Side effect: cooling of plant
2 ways:
1. Via stomata
• Diffusion of water vapour from airspace to atmosphere
• Only occurs when stomata is open
→ For gas exchange
→ Entry of carbon dioxide for photosynthesis and exit of oxygen

2. Via cuticle
• Loss of water vapour through cuticle
on leaf surface
• Very small amount of water loss
Updated on 12/8/21 by Beh SJ @behlogy
 Q: What helps water to move upwards
against gravity without a pump?
A:
1) Transpiration pull +
cohesion and adhesion of water
2) Root pressure

Updated on 12/8/21 by Beh SJ @behlogy

 1) Transpiration Pull, Cohesion and Adhesion
• During transpiration, water vapour diffuse out via
stomata
• Water evaporates from mesophyll cell wall surface
and lowers water potential at leaves

• Roots have a higher water potential than leaves

→ Water moves down water potential gradient
→ Water moves up xylem from roots to leaves

• Transpiration from leaves creates transpiration pull

Updated on 12/8/21 by Beh SJ @behlogy

1) Transpiration Pull, Cohesion and Adhesion
• Tension is set up in xylem vessels due to:
→ H bonds between water molecules
→ Cohesion between water molecules in xylem and in root cells
→ Adhesion of water molecules to cellulose
cell wall of xylem vessels

Updated on 12/8/21 by Beh SJ @behlogy

 1) Transpiration Pull, Cohesion and Adhesion
Result of transpiration pull, cohesion and adhesion:

• This creates a continuous column of water

→ Extending from root hairs to stomata in leaves

• Cohesion and adhesion result

in cohesion-tension in transpiration pull
→ Ccause trees to reduce its girth
when rate of transpiration is high
• Also capillarity / capillary action
= ability of fluid to move upwards
against gravity in narrow spaces
Updated on 12/8/21 by Beh SJ @behlogy
 2) Root pressure
* Not as significant compared to transpiration pull in causing water to move up

• Casparian strip at the endodermis block apoplast pathway

→ So water and ions must pass through endodermal cell
→ Solutes are actively pumped across membranes into xylem
vessels in root
→ This active transport requires ATP

• Xylem vessel in root increases

in solute concentration
→ Lowers water potential
→ Results in more water uptake
from soil
→ Increases hydrostatic
pressure at roots
Updated on 12/8/21 by Beh SJ @behlogy
Updated on 12/8/21 by Beh SJ @behlogy
 Rate of Transpiration
Factors affecting rate of transpiration:
• Humidity
→ At low humidity, steeper water potential gradient
→ High rate of diffusion of water vapour out of stomata

• Wind speed / air movement

→ In moving air, water vapour around leaf is blown away
→ Steeper water potential gradient
→ High rate of diffusion of water vapour

• Water availability
→ More water available, steeper water potential gradient
→ Reduced water availability causes stomata to close
Updated on 12/8/21 by Beh SJ @behlogy
 Rate of Transpiration
• Temperature
→ Rise in temperature, higher kinetic energy
→ Higher rate of evaporation from surface of spongy mesophyll
→ BUT at very high temp, stomata closes so transpiration slows

• Light intensity
→ At high light intensity, increased rate of transpiration bcs stomata
opens more widely due to increased photosynthesis
→ BUT at very high light intensity, stomata closes to prevent loss of
water so transpiration (and photosynthesis) slows

• Stomatal aperture
→ Increased width of stomatal aperture allows more water vapour
to diffuse out
Updated on 12/8/21 by Beh SJ @behlogy
 How to Measure the
Rate of Transpiration?
• Use a potometer!

• Assume:
• Rate of transpiration Used to reset position
of air bubble
= rate of water uptake by plant
• Only an approximation, as
not all water taken up is
used for transpiration

Ensure that it’s

water-tight!

Updated on 12/8/21 by Beh SJ @behlogy

 Potometer
Procedure:
1. Leafy twig cut in a slanted
manner
→ Cut underwater to prevent
formation of airlock in xylem
that will slow down transpiration
2. Twig is put into a potometer
3. Expose to different sets of
conditions for a set time
4. Measure rate of air bubble
movement / rate of water loss

Updated on 12/8/21 by Beh SJ @behlogy

 Xerophytes
• Plants living in places
• With short supply of water
• Which have special
structural adaptations
→ To reduce water loss

Examples of xerophytes:
• Marram grass
• Cactus
• Sitka spruce
• Phlomis italica
• Cardon

Updated on 12/8/21 by Beh SJ @behlogy

 Special Adaptations
1. Rolled leaves
• Exposing tough, waterproof cuticle to the outside air
• Stomata enclosed in humid space within the rolled leaves
→Reduce water potential gradient Marram
between air space and atmosphere grass

2. Hairs / Trichomes
• In folded inner surface
• Trap water vapour / layer of moist air
near leaf surface Phlomis italica
→ Reduce water potential gradient between air space
and atmosphere

Marram
grass Phlomis italica
Updated on 12/8/21 by Beh SJ @behlogy
Special Adaptations Marram
grass
3. Sunken stomata
• Minimise effect of wind
• Traps layer of moist air
→ Reduce water potential gradient
between air space and atmosphere

4. Stomata only present on lower/inner

surface
→ Shaded, so reduces evaporation
• Surface directly exposed to air currents
has no stomata

5. Reduced no. of stomata

Sitka spruce

Updated on 12/8/21 by Beh SJ @behlogy

 Special Adaptations Cardon
Cactus

6. Leaves reduced to spines /

needles / small leaves
→ Reduces surface area for
transpiration
→ Also added protection against
animals

Sitka spruce

Updated on 12/8/21 by Beh SJ @behlogy

 Special Adaptations

7. Thick waterproof, waxy cuticle

• Increased distance decreases
rate of diffusion
• Reduced water loss through
cuticle

• Waxy, reflects some light

• Reduces heat load and
evaporation

8. Multilayered epidermis
• Increase distance for diffusion
of water vapour to cuticle

Updated on 12/8/21 by Beh SJ @behlogy

Other Adaptations to Gain More Water
• Stems that store water
→ Swollen, succulent cardon stems

• Flattened, photosynthetic Cardon

cactus stems Cactus

• Deep and extensive roots

Updated on 12/8/21 by Beh SJ @behlogy

 AS Level
Chapter 7
Transport In Plants
Translocation of Sucrose
 Translocation
• Transport of assimilates within plants
→ Soluble, organic substances
→ Made by plant via photosynthesis
→ E.g. sucrose, amino acids

• Transported as phloem sap in phloem tissue

Updated on 12/8/21 by Beh SJ @behlogy

Source:
• Site of synthesis of photosynthetic
Source → Sink
products
• Loading of sucrose into sieve tube here
• E.g. Mesophyll cells of leaves

Sink:
• Site where assimilates are stored/used
for growth
• Unloading of sucrose from sieve tube here
• E.g. Roots, fruits, tubers
• Can be below / above source

• Phloem sap able to flow upwards or

downwards in a sieve tube
• But one way direction in 1 sieve tube at
any 1 time Updated on 12/8/21 by Beh SJ @behlogy
 Mesophyll cells →
Mesophyll cells near sieve tubes
• Photosynthesis occurs in
mesophyll cells
→ Produce glucose
→ Converted into sucrose

• Transport of sucrose from

mesophyll cells to mesophyll
cells nearer to phloem via:-
1) Symplast pathway
2) Apoplast pathway

Updated on 12/8/21 by Beh SJ @behlogy

Loading of Sucrose into Sieve Tubes
• Active transport involved
→ Requires ATP

• H+ ions in companion cells are

pumped out
→ Into mesophyll cell wall /
intercellular space
→ By a proton pump

• H+ ions gradient builds up

Updated on 12/8/21 by Beh SJ @behlogy

Loading of Sucrose into Sieve Tubes
• H+ ions re-enter companion cells down conc. gradient
→ using the sucrose/H+ co-transporter protein
→ Sucrose is cotransported together into companion cell
against conc. gradient

• H+ ions is transported via

facilitated diffusion
• Sucrose is transported via
secondary active transport

Updated on 12/8/21 by Beh SJ @behlogy

Loading of Sucrose into Sieve Tubes
• Sucrose diffuse from companion cell into sieve tube
→ Down the concentration gradient
→ Via plasmodesmata

Updated on 12/8/21 by Beh SJ @behlogy

 Translocation in Sieve Tubes
Near source:
→ Presence of sucrose lowers water
potential in sieve tube element

→ Water enters sieve tubes via

osmosis
→ Down water potential gradient

→ Increases hydrostatic pressure in

sieve tube near source
(think of this as a “push”)

Updated on 12/8/21 by Beh SJ @behlogy

 Translocation in Sieve Tubes
Near sink:
• Lower hydrostatic pressure Higher h.p.
in sink
→ Due to removal of sucrose

• Phloem sap with sucrose

move from region of high to
low hydrostatic pressure
→ Down hydrostatic pressure
gradient
→ Towards sink

• Result in mass flow =

movement of fluids down a
hydrostatic pressure gradient
Lower h.p.
Updated on 12/8/21 by Beh SJ @behlogy
 Translocation in Sieve Tubes
• Water in sieve tubes near sink has
higher hydrostatic pressure than
xylem vessels
→ Water move back to xylem vessels
→ Down the hydrostatic pressure
gradient

Lower h.p.

Higher h.p.
Updated on 12/8/21 by Beh SJ @behlogy
 Unloading of Sucrose from Sieve Tubes

• Sink has lower concentration of

sucrose than sieve tube
→ Sucrose move down its
concentration gradient
→ Transported to sink via diffusion

Updated on 12/8/21 by Beh SJ @behlogy

Unloading of Sucrose from Sieve Tubes
At sink:
• Sucrose converted into
glucose, fructose, starch
→ Catalysed by Enzymes
→ Use for respiration, growth or
storage

Updated on 12/8/21 by Beh SJ @behlogy

 Chapter Outline
Part 1: Plant Anatomy
• Structure of Xylem Vessel Elements, Phloem Sieve Tubes and
Companion Cells

Part 2: Transport of Water and Ions

Part 3: Translocation of Sucrose

Updated on 12/8/21 by Beh SJ @behlogy

Transport of Water and Ions
*always explain in terms of water potential

1) Symplastic pathway
2) Apoplastic pathway
• Casparian strip

Mechanisms for movement of water in xylem vessels

1) Transpirational pull
2) Cohesion and adhesion of water
3) Root pressure

Symplastic/apoplastic pathway to mesophyll cell surface

Transpiration
• Factors that affect transpiration rate
• Measuring rate of transpiration using a potometer
• Xerophytes: adaptations to reduce transpiration
Updated on 12/8/21 by Beh SJ @behlogy
Translocation of Sucrose

Loading of sucrose

Mass flow

Unloading of sucrose

Updated on 12/8/21 by Beh SJ @behlogy

 Videos
Vascular Plants = Winning! - Crash Course Biology #37
(May help with plant anatomy)
https://www.youtube.com/watch?v=h9oDTMXM7M8

Transportation in Plants (Animated, too simple, but gives a good overview)

https://www.youtube.com/watch?v=JFb-CWlz7kE

Measurement of Transpiration Rates using a Potometer

https://www.youtube.com/watch?v=gXocZZDDPaw

The Pressure Flow Model (aka Translocation, Animated)

https://www.youtube.com/watch?v=3OEd8WDxg1U

Updated on 12/8/21 by Beh SJ @behlogy

 AS Level
Chapter 8

Transport in
Mammals
 Chapter Outline
Part 1: The Circulatory System
• Blood Vessels: Arteries, Veins and Capillaries
• Blood Plasma vs Tissue Fluid
• WBCs and RBCs

Part 2: Transport of oxygen and carbon dioxide

• The Haemoglobin Dissociation Curve
• Bohr effect and Transport of CO2

Part 3: The Heart

• Structure of the Heart
• Cardiac Cycle
• Control of Heart Beat

Updated on 23/9/21 by Beh SJ @behlogy

 The
Circulatory
System
 The Cardiovascular System
• Aka circulatory system
• Includes blood vessels, blood, lymph
and heart

Needed for:
1. Transport of nutrients and oxygen
around the body
2. Disposal of waste materials (e.g.
carbon dioxide, urea)
3. Transport of hormones
4. Circulate WBCs and RBCs in body

Updated on 23/9/21 by Beh SJ @behlogy

 Closed, Double Circulatory System
Closed
• Blood is contained in blood vessels
• Always in heart, arteries, veins or
capillaries

Double
• Blood passes through the heart twice,
in one complete circuit

Updated on 23/9/21 by Beh SJ @behlogy

 Closed, Double Circulatory System
1 circuit = 2 circulations

1) Pulmonary circulation
• Circulation through the lungs and heart

2) Systemic circulation
• Circulation through other parts of the
body and heart except the lungs

Updated on 23/9/21 by Beh SJ @behlogy

 Other Circulatory Systems

*don’t need to remember the details

Updated on 23/9/21 by Beh SJ @behlogy

 Blood Vessels
Arteries
• Carry blood away from heart
• Carries oxygenated blood
(except for pulmonary artery)

Capillaries
• Exchange vessels, bring blood
close to tissues
• Link arteries and veins

Veins
• Carry blood towards the heart
• Carries deoxygenated blood
(except for pulmonary vein)
Updated on 23/9/21 by Beh SJ @behlogy
 Arteries
• Shorter distance from heart
→ Blood travels at high pressure in arteries

Appearance:
• Well-defined oval shape
• Thick wall
• Narrow lumen in relation to thickness of wall
• Folded endothelium

Arterial wall has 3 layers:

1. Tunica intima / endothelium
2. Tunica media
3. Tunica externa
Updated on 23/9/21 by Beh SJ @behlogy
 Arteries
1. Tunica intima / endothelium
• Squamous epithelial cells
= flattened cells
• One-cell thick
• Smooth surface facing lumen

Updated on 23/9/21 by Beh SJ @behlogy

 Arteries
2. Tunica media
• *THICCCEST*
• 3 components:
Collagen fibres, elastic fibres
and smooth muscle

3. Tunica externa
• 2 components:
Only collagen fibres and elastic fibres

Updated on 23/9/21 by Beh SJ @behlogy

 Arteries
Roles of each component:

1) Collagen fibres
• Withstand high pressure
• Prevents rupture of vessels

2) Elastic fibres
• Allows vessel to stretch to
withstand high pressure
• When blood enters at lower pressure,
it recoils to give blood a small push to
increase blood pressure
→ Smooths out pulsatile flow
→ Maintains blood pressure
Updated on 23/9/21 by Beh SJ @behlogy
Arteries
3) Smooth muscle – many layers
• Maintains blood pressure
• Contract / relax to change volume of blood delivered
• Keep blood moving forwards

• When muscles relax: • When muscles contract:

→ Arterioles become wide → Arterioles become narrow
(vasodilation) (vasoconstriction)
→ Increase blood flow → Reduce blood flow
Updated on 23/9/21 by Beh SJ @behlogy
 Arteries → Arterioles
Arteries further from heart have less elastic fibres
and more smooth muscles

1. Elastic arteries
• Closer to the heart
• Elastic fibres can stretch / recoil to withstand high
pressure and smooth out pulsatile flow

2. Muscular arteries/arterioles
• Further from the heart
• Smooth muscles can contract / relax to control volume of
blood flow
• Become narrower so blood flow slows down
→ More time for exchange of gases and nutrient with tissues
Updated on 23/9/21 by Beh SJ @behlogy
 Capillaries
Vessel diameter = ~7µm
Made of endothelial / squamous epithelial cells

Adapted for function

1) One-cell thick
→ Short diffusion distance

2) Has pores/gaps between endothelial cells

→ Allow some smaller components of blood to
pass through
→ E.g. water, ions, glucose
→ Allow formation of tissue fluid

Updated on 23/9/21 by Beh SJ @behlogy

 Capillaries
3) Small lumen diameter
→ Slows down flow of blood
→ Bring RBC close to body tissue
→ Blood pressure in capillaries are lower

4) Have high surface area

→ Network of capillaries form a capillary bed
→ Allows more exchange

Updated on 23/9/21 by Beh SJ @behlogy

 Veins
• Return blood to the heart
• Low blood pressure
• Slower blood flow than in artery
• Also has 3 layers

Appearance:
• Irregular/ flattened oval shape
• Wide lumen in relation to thickness of wall
• Thin tunica media
→ less elastic tissue and less smooth muscle
• Tunica intima / endothelium not wavy

Updated on 23/9/21 by Beh SJ @behlogy

 Veins
Features:
1. Presence of valves
→ To prevent backflow of blood
→ Ensure blood flows towards heart
→ Valves close the pathway when
blood travels opposite direction

2. Surrounded by skeletal muscles

→ When skeletal muscle contracts,
pushes blood towards heart

Updated on 23/9/21 by Beh SJ @behlogy

Updated on 23/9/21 by Beh SJ @behlogy
 Blood Vessels

Arteries Arterioles Capillaries Venules Veins

Networks of capillaries
result in high surface area. Velocity in veins in lower than
arteries due to low blood pressure.
Movement of blood depends only on
the contraction of skeletal muscles.

Blood pressure pulsates in the arteries due

to the pumping of the heart. Elastic fibres
help smooth out pulsatile flow.

Pressure in veins is very low due

to distance from heart pump.
Updated on 23/9/21 by Beh SJ @behlogy
Remember
Collagen is the
main component Summary
of Tunica External

Updated on 23/9/21 by Beh SJ @behlogy

 Blood Plasma vs Tissue Fluid
(good solvent, high specific heat capacity)

(e.g. glucose, amino acids, lipid)

(e.g. urea)
(e.g. CO2, O2)

Blood that is
centrifuged

Updated on 23/9/21 by Beh SJ @behlogy

 Tissue Fluid
• Aka interstitial fluid
• Bathes cells
• Medium for exchange
of materials between
cells and blood

• Formed from blood

plasma
• Returned to blood
eventually

Updated on 23/9/21 by Beh SJ @behlogy

 Formation of Tissue Fluid
from Blood Plasma
• Due to differences in blood pressure at
arterial and venous ends
• Blood pressure in arterioles is higher
than blood pressure in venules

• Blood plasma flow out into tissue spaces

→ Through endothelial pores of capillaries
→ Form tissue fluid

P/S: If pressure is too high, tissue fluid may

accumulate in tissues and cause oedema!
Arterioles have smooth muscles to reduce blood
pressure to avoid this Updated on 23/9/21 by Beh SJ @behlogy
Formation of Tissue Fluid
from Blood Plasma
• But gaps are small so filtration occurs
→ Larger plasma proteins cannot pass through
→ RBCs cannot pass through

Updated on 23/9/21 by Beh SJ @behlogy

 Composition of Tissue Fluid
• Similar to blood plasma’s composition

Tissue fluid contains:

• Water, gases, glucose, fatty acids, urea, ions
• Smaller proteins (e.g. antibodies)
→ Overall lower protein conc. than plasma
• Some WBCs (e.g. phagocytes can leave eventho large)
• Lower O2 conc. than plasma
• NO platelets
• NO large proteins
• NO RBC

Updated on 23/9/21 by Beh SJ @behlogy

Return of Tissue Fluid to Blood
• Blood pressure at venous end is lower than at arterial end
• Solute concentration is higher in the blood plasma of
capillary due to large, dissolved proteins
• So at venous end, some tissue fluid returns to the blood

Updated on 23/9/21 by Beh SJ @behlogy

Return of Tissue Fluid to Blood
90% is returned to blood, through endothelial gaps
10% moves into lymphatic vessels and becomes lymph
→ Lymph is returned to blood via the subclavian veins near heart

Updated on 23/9/21 by Beh SJ @behlogy

Summary

Updated on 23/9/21 by Beh SJ @behlogy

 Components of Blood
(good solvent, high specific heat capacity)

(e.g. glucose, amino acids, lipid)

(e.g. urea)
(e.g. CO2, O2)

Blood that is
centrifuged

Updated on 23/9/21 by Beh SJ @behlogy

 WBCs and RBCs
• Both made in the bone marrow
• From same type of stem cells

Updated on 23/9/21 by Beh SJ @behlogy

 White Blood Cells
• Aka leucocytes
• Made in bone marrow
• Function: fighting diseases

2 types of WBCs:
1) Phagocytes
2) Lymphocytes

Updated on 23/9/21 by Beh SJ @behlogy

1) Phagocytes
• Produced throughout life

Function:
• Patrol in blood, tissues and organs
• Remove dead cells and pathogens
→ By phagocytosis (Chap 4)

• Involved in non-specific defense

→ responds to many different non-self antigens

Appearance:
• Lobed nuclei
• Granular cytoplasm – due to many vesicles

Updated on 23/9/21 by Beh SJ @behlogy

 1) Phagocytes
E.g. Neutrophils
• Multi-lobed nucleus
• Have receptor proteins on its membrane
→ To identify pathogens as non-self

• When there is an infection, large numbers

are released from bone marrow
→ Accumulate at site of infection

• Short-lived (few hours-days)

→ Dies after digesting pathogens
→ Dead neutrophils form pus :O
Updated on 23/9/21 by Beh SJ @behlogy
1) Phagocytes
E.g. Monocyte → Macrophage
• Lobed nucleus / kidney-bean shaped
• Larger than neutrophils
• Have receptor proteins on its membrane
→ To identify pathogens as non-self

• Monocytes = circulate in blood

→ Mature into macrophages when it leaves
blood and enter organs

• Long-lived cells
• Macrophages found in organs such as
liver, lungs, spleen, kidney, lymph nodes
Updated on 23/9/21 by Beh SJ @behlogy
 2) Lymphocytes
• Produced in bone marrow before birth

Function:
• Involved in specific immune responses
→ responds to only specific non-self antigens
• Mature lymphocytes circulate in the blood
and lymph
→ Accumulate at sites of infection

Appearance:
• Smaller than phagocytes
• Large round nucleus
• Little cytoplasm

Updated on 23/9/21 by Beh SJ @behlogy

2) Lymphocytes
2 main types:
Both made in bone marrow, but mature in different places and
have different functions

1. B-lymphocytes (B cells)
• Mature in bone marrow
• Produces antibodies

2. T-lymphocytes (T cells)
• Mature in thymus
• Does NOT produce antibodies

• Both work together to defend the immune system

Updated on 23/9/21 by Beh SJ @behlogy
Blood Smear

Updated on 23/9/21 by Beh SJ @behlogy

 Differentiating WBCs from RBCs
1) Contains nucleus

2) Mostly larger than erythrocytes

(except for lymphocytes)

3) Spherical / irregular in shape

• Do not have a biconcave disc shape

4) Phagocytes have granular cytoplasm

Updated on 23/9/21 by Beh SJ @behlogy

 Red Blood Cells
• Aka erythrocytes
• RBCs are short lived (120 days)
• Function: transport oxygen to body tissues

Features:
1) Small and flexible
• Diameter about 6-8μm
• Able to squeeze through
capillaries (7μm)
• Reduce diffusion distance

Updated on 23/9/21 by Beh SJ @behlogy

 Red Blood Cells
2) Biconcave Disc
• Increases surface area
• For diffusion of oxygen to cells

3) No nucleus, no mitochondria, no ER
• More room for haemoglobin
• Maximise the number of oxygen
carried by RBC

Updated on 23/9/21 by Beh SJ @behlogy

 Haemoglobin
• Found in large quantities in RBC

• Globular protein
• Made of 2 alpha-globin chains and
2 beta-globin chains
→ Quaternary structure
• Each chain contains 1 haem group that has Fe2+
→ Binds 1 molecule of O2 each

• Hb bind with oxygen to form oxyhaemoglobin in lungs

• Hb + 4O2 → HbO8

Updated on 23/9/21 by Beh SJ @behlogy

Transport of
O2 and CO2
The Haemoglobin Dissociation Curve
As partial pressure of oxygen (pO2) increases, percentage
saturation of haemoglobin with oxygen increases

At capillaries in lungs:
O2 supply high → so pO2 is high
Hb is highly saturated with O2

At respiring tissues:
O2 demand high for aerobic respiration →so pO2 is low
Hb release O2 , Hb is less saturated with O2

Updated on 23/9/21 by Beh SJ @behlogy

Why is the curve S-shaped?
Hemoglobin exhibits cooperative binding / allosteric effects, as
oxygen binding increases the affinity of hemoglobin for more oxygen!
At high pO2 (lungs)
• All haem groups are fully occupied
• Hb fully saturated
• Curve levels off

At increasing pO2 :
• Binding of 2nd and 3rd O2 is easier than 1st
• Small increase in pO2, results in large
increase in % saturation

At low pO2 (respiring tissue)

1st O2 combine with haem group
→ Hb changes shape

Updated on 23/9/21 by Beh SJ @behlogy

Why is this important?
At lungs:
O2 supply high
• pO2 in lungs are higher Bind O2
pO2 high, %saturation high
• pO2 in respiring tissues are lower

As blood travel from capillaries in

lungs to tissues:
• Small decrease in partial pressure
leads to a large decrease in %
saturation
→ Allows more oxygen to be released
/ dissociate from Hb
→ Affinity of Hb to oxygen decreases
at low pO2 At respiring tissues:
O2 demand high for aerobic
respiration
Release O2
pO2 low, %saturation low
Updated on 23/9/21 by Beh SJ @behlogy
 Factors that affect
affinity of Hb to O2
• pO2 – High pO2, increases affinity of Hb to O2

• pCO2 – High partial pressures, decreases affinity of Hb to O2

→ Result in the Bohr effect / Bohr shift
→ Where the curve shifts to the right

Updated on 23/9/21 by Beh SJ @behlogy

 The Bohr Effect
• Where the affinity of Hb to O2 is affected by pCO2

• High pCO2 decreases affinity of Hb to O2

→ Therefore a higher pO2 is needed to meet the
same % saturation
Christian Bohr, 1904
→ The curve shifts to the right

• Bohr effect increases dissociation

of oxyhaemoglobin in actively
respiring tissues

Updated on 23/9/21 by Beh SJ @behlogy

 The Bohr Effect
Lung tissues have
• Higher pO2 , Lower pCO2

Respiring tissues have

• Lower pO2, Higher pCO2
→ oxyhaemoglobin dissociates
more readily
→ more oxygen to meet the
demand for aerobic
respiration

Updated on 23/9/21 by Beh SJ @behlogy

 How does high pCO2 decrease
the affinity of Hb to O2 ?

1. CO2 diffuses from respiring tissue → blood plasma → RBC

2. Carbonic anhydrase in cytoplasm of RBC converts CO2 to

carbonic acid
→ This maintains a steep concentration gradient for diffusion of
carbon dioxide from tissues to blood

3. Carbonic acid dissociates into hydrogencarbonate ions and

hydrogen ions
→ decrease in pH Updated on 23/9/21 by Beh SJ @behlogy
 blood plasma HHb

O2

respiring
cells

4. HCO3- diffuse from RBC → blood plasma

Cl- move into RBC to balance out negative charge (chloride shift)

5. H+ combines with Hb to form haemoglobinic acid (HHb)

→ Hb has higher affinity for H+ than oxygen
→ H+ lowers affinity of Hb for oxygen
→ HHb also prevents pH from decreasing / acts as buffer

6. Hb releases oxygen
O2 diffuses from RBC → blood plasma → respiring cells
Updated on 23/9/21 by Beh SJ @behlogy
Numbers circled correspond to the steps in previous slides!

dissociation

O2 release

chloride shift

CO2 is only transported in the form of

hydrogencarbonate ions only 85% of the time!!!
Updated on 23/9/21 by Beh SJ @behlogy
Transport of CO2 from Respiring Tissues
85% Hydrogencarbonate ions
• Carbonic anhydrase converts
CO2 to carbonic acid
• Carbonic acid dissociates
• HCO3- diffuse from RBC into plasma

5% Dissolve in blood plasma

• Remains as CO2 molecules

10% Carbaminohaemoglobin
• Diffuse into RBC
• Bind to terminal amine of Hb
• Form carbaminohaemoglobin

Updated on 23/9/21 by Beh SJ @behlogy

 Transport of CO2 to Lungs
• Low pCO2 , high O2 in alveoli
• Processes are reversed!

• HHb releases its H+ ion binds O2

→ forms oxyhaemoglobin

1. Hydrogencarbonate ions diffuses back into RBC

→ Binds with H+ and converted back into carbonic acid
→ Carbonic acid converted back to CO2 + H2O
→ Diffuse into alveoli

2. Dissolved CO2 in blood plasma

→ Diffuse into alveoli

3. CO2 from carbaminohaemoglobin

→ Diffuse into alveoli
Updated on 23/9/21 by Beh SJ @behlogy
The Heart
 Chapter Outline
• Structure of the heart

• The Cardiac Cycle

Atrial Systole
Ventricular Systole
Diastole
Blood Pressure

• Control of heart beat

Sinoatrial node (SAN)
Atrioventricular node (AVN)
Purkyne/Purkinje tissue

Updated on 23/9/21 by Beh SJ @behlogy

 Structure of the Heart
• Made of cardiac muscle

(Atrioventricular valve)

(Atrioventricular valve)

Updated on 23/9/21 by Beh SJ @behlogy

 Structure of the Heart

Blood passes through

the heart twice, in one
complete circuit
→ Closed, double
circulatory system

Updated on 23/9/21 by Beh SJ @behlogy

 Structure of the Heart
The walls of the ventricles are thicker than atria walls. WHY?
• Because ventricles need high force to pump blood to the
whole body

The left ventricle has a thicker wall than the right. WHY?
• Because the right ventricle only pumps blood to the lungs, left
ventricles pumps blood to the entire body
• If the right ventricle pumps with too high pressure, tissue fluid
may accumulate in lungs

Updated on 23/9/21 by Beh SJ @behlogy

 Structure of the Heart
• Coronary arteries = the blood vessels that supply cardiac
muscle with oxygenated blood

Updated on 23/9/21 by Beh SJ @behlogy

 The Cardiac Cycle
• Normal heart rate is ~75 beats per minute (bpm)
• Average length of one cardiac cycle is 0.8s

• 3 stages in the cardiac cycle:

1) Atrial systole
2) Ventricular systole
3) Diastole

Systole = contraction/pumping
Diastole = relaxation/filling
Updated on 23/9/21 by Beh SJ @behlogy
 Atrial Systole Ventricular Systole Diastole
Length (s) 0.1 0.3 0.4
Atria contract Ventricles contract -
What
happens? Atria and
Ventricles relax Atria relax
ventricles relax
Valves in vena cava and
Valves that are
Atrioventricular valves Semilunar valves pulmonary veins,
open
atrioventricular valves
Blood flows Vena cava and pulmonary
Atria Ventricles
from… veins
Blood flows Atria and trickles into
Ventricles Aorta and pulmonary artery
to… ventricles
Valves in vena cava and
Valves that are Atrioventricular valves Semilunar valves
pulmonary veins, semilunar
closed (produces “lub” sound) (produces “dub” sound)
valves

Updated on 23/9/21 by Beh SJ @behlogy

Differences in pressure in atria, ventricles and arteries directly cause valves to open/close!

Atrial Systole Ventricular Systole Diastole

Atria contract Ventricles contract -
What
Atria and
happens? Ventricles relax Atria relax
ventricles relax
Atrial Atrial pressure > Much lower than ventricular Low, but increasing as blood
Pressure Ventricular pressure pressure fills heart
Lower than atrial pressure Ventricular pressure >
Ventricular Low, but increasing as blood
Pressure but increasing as blood fills Atrial pressure
fills heart
heart High, rapidly increasing.
Pressure in arteries >
High, also rapidly increasing
Pressure in High, but slowly decreasing Ventricular pressure
as blood is pumped through
Arteries as blood flows to entire body High, but slowly decreasing
aorta
as blood flows to entire body
Updated on 23/9/21 by Beh SJ @behlogy
Changes in Blood Pressure in Atria,
Ventricles and Arteries
• Changes with stage of cardiac cycle
• Usually measure the pressure on the left side
of heart → Left atrium, left ventricle, aorta
• Due to higher pressure and larger diff in pressure
compared to right side

Generally:
• Atrial pressure is relatively low because it has thinner walls
and exert less force
• Atrial pressure increases during atrial systole

• Ventricular pressure increases during ventricular systole

• Aortic pressure increases during ventricular systole
Updated on 23/9/21 by Beh SJ @behlogy
Atrial systole Ventricular Diastole
systole

Updated on 23/9/21 by Beh SJ @behlogy

 1 Atrial contraction begins
2 Blood enters ventricles
3 Ventricular systole begins;
6 AV valves close
* 4 Ventricles contract
5 Semilunar valves open;
Blood flows into arteries
5 6 Semilunar valve closes
7 7 Ventricles empty and relax
8 AV valves open;
4 Blood starts to fill atria
9 Blood trickles into ventricle
** Pressure slightly increases
when valves close
1 * 8
2
3 9

Updated on 23/9/21 by Beh SJ @behlogy

Changes in Ventricular Volume
Atrial Ventricular
Diastole
systole systole

Ventricular volume drops

during ventricular systole

Updated on 23/9/21 by Beh SJ @behlogy

 Changes in Pressure in Blood Vessels
• Pulsates due to ventricular contraction
• Systolic pressure = the maximum blood pressure in the arteries
• Diastolic pressure = the minimum blood pressure in the arteries

• Human blood pressure is usually between 80-120mmHg

Updated on 23/9/21 by Beh SJ @behlogy

 Control of Heart Beat
• Cardiac muscles are myogenic
• Myogenic = contraction initiated by muscles itself, not by
nervous impulses from outside

• How is the cardiac cycle initiated and coordinated?

• Wave of excitation/electrical impulse is passed through the:
1) Sinoatrial node (SAN)
2) Atrioventricular node (AVN)
3) Purkyne tissue

Updated on 23/9/21 by Beh SJ @behlogy

 Sinoatrial node (SAN)
• Determines rhythm of heart
• Also called pacemaker
• Found at the wall of the right atrium

1. SAN / pacemaker sends out waves of excitation

/ electrical impulses

2. Impulses spreads across atria

→ Both atria contract simultaneously
→ Result in atrial systole

• But non-conducting tissue prevents impulses from reaching the

ventricles
→ So atria and ventricles do not contract at the same time

• Wave of excitation passed to AVN

Updated on 23/9/21 by Beh SJ @behlogy
 Atrioventricular node (AVN)
• AVN is found between the atria
• Prevents atria and ventricles from
contracting at the same time
• Acts as a relay station

3. There is a time delay of ~0.1-0.2 seconds

→ Allows atria to empty
→ And ventricles to fill

4. AVN sends wave of excitation to

ventricles
• Wave of excitation passed to Purkyne
tissue

Updated on 23/9/21 by Beh SJ @behlogy

 Purkyne tissue
• Tiny bundles of connecting fibres
• Found at base of ventricles

5. Purkyne tissue conducts excitation to base of septum / ventricles

6. Electrical impulses spreads upwards in ventricle walls

→ Ventricle muscles contract from base upwards
→ Ventricles force blood up
from base
→ Result in ventricular
systole

Updated on 23/9/21 by Beh SJ @behlogy

 Refractory Period
• Period of insensitivity to any stimulation
• ~0.4seconds
• Atrial and ventricular muscles relax
• Diastole

SAN AVN Purkyne tissue Refractory Period

Updated on 23/9/21 by Beh SJ @behlogy
 Electrocardiography (ECG)
• Used to measure heart rate and regularity
• Records wave of electrical activity
• P = atrial systole
• QRS = ventricular systole
• T = diastole

Updated on 23/9/21 by Beh SJ @behlogy

 Calculating heart rate in bpm from
an electrocardiogram
bpm = beats per minute

1. Find how long 1 cardiac cycle is

1 cardiac cycle = 0.8 s

1 cardiac cycle = 0.8s
2. Find how many cardiac cycles
occur per minute

1 min ÷ 0.8 s
= 60 s ÷ 0.8 s
= 75bpm Updated on 23/9/21 by Beh SJ @behlogy
 Summary
Atrial Ventricular
systole systole Diastole

Updated on 23/9/21 by Beh SJ @behlogy

 Chapter Outline
Part 1: The Circulatory System
• Blood Vessels: Arteries, Veins and Capillaries
• Blood Plasma vs Tissue Fluid
• WBCs and RBCs

Part 2: Transport of oxygen and carbon dioxide

• The Haemoglobin Dissociation Curve
• Bohr effect and Transport of CO2

Part 3: The Heart

• Structure of the Heart
• Cardiac Cycle
• Control of Heart Beat

Updated on 23/9/21 by Beh SJ @behlogy

 Videos
• Haemoglobin Dissociation Curve + Bohr Effect
https://www.youtube.com/watch?v=vj8c2jiYI2g
• Respiratory System, part 2: Crash Course A&P #32
(How Blood Cells Exchange Oxygen and CO2 2:23
Partial Pressure Gradients 2:41
How Hemoglobin Binds to Gases in the Blood 4:40)
https://www.youtube.com/watch?v=Cqt4LjHnMEA

Updated on 23/9/21 by Beh SJ @behlogy

 AS Level
Chapter 9

Gas Exchange
 Chapter Outline
Structure and function of…. …and its components:
• Lungs • Ciliated epithelium
• Trachea • Goblet cells
• Bronchi • Mucous glands
• Bronchioles • Cartilage
• Alveoli • Smooth muscles
• Elastic fibres
• Squamous epithelium

* You must be able to identify it in micrographs and draw plan diagrams

Updated on 12/8/21 by Beh SJ @behlogy

Updated on 12/8/21 by Beh SJ @behlogy
 1. Trachea

P/S: This is an overly-simplified diagram Updated on 12/8/21 by Beh SJ @behlogy

1. Trachea

Updated on 12/8/21 by Beh SJ @behlogy

 1. Trachea

The C-shape cartilage ring is a distinctive feature of the trachea.

Updated on 12/8/21 by Beh SJ @behlogy

1. Trachea
1 lumen
2 ciliated epithelium
3 submucosal layer
4 hyaline cartilage
6 mucous glands
5+7 smooth muscles

Updated on 12/8/21 by Beh SJ @behlogy

 Ciliated epithelium layer
1. Trachea

Goblet cell

Mucous glands in submucosal layer

Updated on 12/8/21 by Beh SJ @behlogy

1. Trachea
hyaline cartilage

smooth muscle

Updated on 12/8/21 by Beh SJ @behlogy

 Structures and Functions
Structure Function
• Produce mucus (made of glycoprotein called mucin)
Goblet cells
• Mucus traps pathogens, dust etc.
Mucous • Produce mucus (made of glycoprotein called mucin)
gland • Mucus traps pathogens, dust etc.
• Cilia move mucus to back of throat / away from lungs
Cilia
→ to be swallowed

Mucous glands
Updated on 12/8/21 by Beh SJ @behlogy
 Structures and Functions
Structure Function
• Keeps airway open → Low air resistance
Cartilage • Prevents airway from collapsing during breathing
due to changes in air pressure

Updated on 12/8/21 by Beh SJ @behlogy

 Structures and Functions
Structure Function
Elastic • Can stretch during inspiration, recoil during expiration
• When elastic fibres stretch: increase surface area
fibres • When elastic fibres recoil: expel air
• When it relaxes: the airway widens
Smooth
(e.g. during exercise)
muscle
• When it contracts: the airway constricts

Updated on 12/8/21 by Beh SJ @behlogy

 2. Bronchus
Smooth muscles

P/S: This is an overly-simplified diagram Updated on 12/8/21 by Beh SJ @behlogy

 2. Bronchus

The cartilage plates are distinctive features of the bronchus.

Updated on 12/8/21 by Beh SJ @behlogy

 3. Bronchiole
Ciliated cells (very few)

Elastic fibres

Ciliated
epithelium

Lumen

P/S: This is an overly-simplified diagram

Terminal bronchioles (nearer to bronchus) have some smooth muscle
Respiratory bronchioles (further from bronchus) do not have smooth
muscle and have less cilia Updated on 12/8/21 by Beh SJ @behlogy
 3. Bronchiole

Bronchioles do not have cartilage, are close to alveoli and may have
a folded epithelium when its smooth muscle (if present) contracts.

Updated on 12/8/21 by Beh SJ @behlogy

4. Alveolus

Updated on 12/8/21 by Beh SJ @behlogy

 4. Alveolus

blood vessel

Alveoli have a simple squamous epithelium

and is the site of gaseous exchange.

Updated on 12/8/21 by Beh SJ @behlogy

 4. Alveoli
Features to enable rapid diffusion of gases:

1) One-cell thick
• Squamous epithelial cells
→ Short diffusion distance between air & blood

2) Collectively large surface area for diffusion

Updated on 12/8/21 by Beh SJ @behlogy

3) Walls have elastic fibres (elastin)
• Allow alveoli to increase in volume
• When elastic fibres stretch: increase surface area
• When elastic fibres recoil: expel air
• Prevent bursting Updated on 12/8/21 by Beh SJ @behlogy
4) Surrounded by network of capillaries
• Maintain a steep diffusion gradient
• Blood is also slowed in the capillaries
• Good ventilation of the lungs and good circulation of blood
maintains the necessary concentration gradients for carbon
dioxide and oxygen

High pO2
Low pCO2

High pO2
Low pCO2 Low pO2
High pCO2

Updated on 12/8/21 by Beh SJ @behlogy

What is the number of cell surface membranes through which
oxygen must pass?

O2

Answer: 5
Updated on 12/8/21 by Beh SJ @behlogy
 What is the number of cell surface membranes through which
CO2 must pass?

CO2 in RBC produced from HCO3-

CO2 in dissolved (85%) or released from Hb (10%)
blood plasma (5%)
Answer: 4 or 5
Updated on 12/8/21 by Beh SJ @behlogy
Acute Respiratory Distress Syndrome (ARDS)
Normal
COVID-19

Updated on 12/8/21 by Beh SJ @behlogy

 Summary
Ciliated Hyaline Smooth Site of gas
epithelium cartilage muscle exchange
Trachea ✓ ✓ ✓ 
Bronchus ✓ ✓ ✓ 
Bronchiole ✓   
Alveolus    ✓

Structure Function
Goblet cells + • Produce mucus (made of glycoprotein called mucin)
Mucous gland • Mucus traps pathogens, dust etc.
Cilia • Cilia move mucus to back of throat
Cartilage • Prevents airway from collapsing
Elastic fibres • Can stretch during inspiration, recoil during expiration
Smooth • When it relaxes: the airway widens
muscle When it contracts: the airway constricts
Updated on 12/8/21 by Beh SJ @behlogy
 AS Level
Chapter 10
Infectious Diseases
 Chapter Outline
• What is an infectious disease? What are pathogens?

• Causes, Transmission, Symptoms, Treatment, Prevention and Global

Distribution of:
1. Cholera
2. Malaria
3. HIV/AIDS
4. Tuberculosis
5. Smallpox (just the causative agent)

• Antibiotics
e.g. Penicillin
Why it doesn’t it work on viruses?
• Antibiotic Resistance (Cause + Consequence + Prevention)

Updated on 12/10/21 by Beh SJ @behlogy

 What is a disease?
• Ill-health / sickness
• Cause reduced effectiveness of functions
• Illness with a set of symptoms
• Poor physical, mental or social well being

Updated on 12/10/21 by Beh SJ @behlogy

 What is an infectious disease?
• Disease caused by a pathogen
• Cause harm to health of host
• Can be passed from one organism to another
i.e. communicable / transmissable

Updated on 12/10/21 by Beh SJ @behlogy

 What are pathogens?
• Pathogens = Parasitic disease-causing microorganisms
• Can be prokaryote/eukaryotes
• E.g. bacteria, virus, protoctists, fungi

Pathogens….
1) Gain entry to host
2) Colonise host tissue
3) Damage host’s tissues
4) Resist host defences

• We need to break the disease transmission cycle to prevent

disease from spreading
Updated on 12/10/21 by Beh SJ @behlogy
 Types of Pathogens
Virus Bacteria Protoctist (or protist) Fungi
Non-living Prokaryote Mostly unicellular eukaryotes Eukaryotes

Examples of protists that you may know

(they do not cause disease and are not
pathogens though!)

Updated on 12/10/21 by Beh SJ @behlogy

 Viruses
• Non-cellular structure
• No plasma membrane, cytoplasm, ribosomes

• Only:
1. DNA or RNA
2. Protein coat = capsid
- Protective coat
- May have one or two coats
3. Many viruses also have a
lipid envelope
4. Some proteins may be present
- e.g. haemagglutinin, neuraminidase

Updated on 12/10/21 by Beh SJ @behlogy

 Viruses
• All parasitic
• Can only reproduce by infecting living cells
• Uses protein synthesising machinery of
host cell to replicate

Updated on 12/10/21 by Beh SJ @behlogy

 Bacteria
What all bacteria do not What all bacteria have:
have: • Plasma membrane
• No membrane-bound • Cytoplasm
organelles • Peptidoglycan cell wall
• No nucleus → made of chains crossed
DNA lies free in cytoplasm linked by amino acids
in the nucleoid region
• 70S ribosomes
• Circular DNA
• DNA is naked
→ not associated with
proteins

Updated on 12/10/21 by Beh SJ @behlogy

Guess the type of causative organisms!
Is it a Bacteria/Virus/Protoctist?
bacteria protoctist virus

bacteria virus virus

Updated on 12/10/21 by Beh SJ @behlogy
 Types of Pathogens
Disease Type of causative agent Name of causative agent (pathogen)

Cholera Bacteria Vibrio cholerae

Plasmodium falciparum /
Malaria Protoctist
P. malariae / P. vivax / P . ovale

HIV/AIDS (Retro)virus Human Immunodeficiency Virus

Tuberculosis Bacteria Mycobacterium tuberculosis / M. bovis

Smallpox Virus Variola virus

Updated on 12/10/21 by Beh SJ @behlogy

 Terminology to describe Distribution of Disease
• Endemic: a disease that exists permanently in a particular region or
population.
Malaria is a constant worry in parts of Africa.
Malaria is endemic in Africa.

• Epidemic: An outbreak of disease that attacks many peoples at about the

same time and may spread through one or several communities.
Remember the SARS epidemic in Malaysia?

• Pandemic: When an epidemic spreads throughout the world.

HIV/AIDS is a worldwide pandemic.

Updated on 12/10/21 by Beh SJ @behlogy

Cholera
Cholera
Causative organism: Vibrio cholerae – a bacterium
• comma-shaped, has flagella, motile

Updated on 12/10/21 by Beh SJ @behlogy

 Cholera
Transmission:
• Large numbers of Vibrio cholerae found in faeces of infected people
• Infected person's faeces / sewage contaminates food / water.
• Houseflies land on faeces and contaminate food / water.
• Uninfected person eats contaminated food / water.
Food-borne
Faecal-oral route
Water-borne

Updated on 12/10/21 by Beh SJ @behlogy

Cholera

Symptoms and Effects:

1. If bacteria is not killed by stomach acid,
bacteria reaches the small intestine
2. Bacteria secretes choleragen toxin
3. Toxin binds to complementary receptor on intestinal
epithelial cell and enters via endocytosis
4. Disrupts function of intestine epithelium lining
5. Loss of chloride ions and sodium ions from epithelial cells.
6. Water potential decreases, water moves out from blood
down water potential gradient by osmosis through partially
permeable membrane.
7. Causes severe diarrhoea + dehydration
Updated on 12/10/21 by Beh SJ @behlogy
Cholera
More Symptoms:
• Diarrhoea
• Severe dehydration
• Loss of water and salts
• Weakness and Fatigue
• Low Blood Pressure
• Weight Loss
• Vomitting

Diagnosis:
• Microscopical analysis of faeces

Updated on 12/10/21 by Beh SJ @behlogy

 Cholera
Treatment:
• Oral rehydration therapy
• Use oral rehydration solution (ORS) that has glucose/salts

• Ensure that fluid intake = fluid losses in urine & faeces

• Maintain osmotic balance of blood and tissue fluids
• Almost all treated patients survive

Updated on 12/10/21 by Beh SJ @behlogy

 Cholera
Prevention:
• Proper sewage treatment to break transmission cycle
• Chlorinate water to kill bacteria before drinking
• Drink bottled water
• Vaccination only offers short-term protection
(no longer recommended)

Updated on 12/10/21 by Beh SJ @behlogy

 Cholera
Global Distribution:
• Endemic in developing countries
• E.g. West and East Africa, Afghanistan, Latin America, parts of Asia

• Outbreak also follows natural disasters (or war)

• E.g. chlolera epidemic during the 2010 earthquake in Haiti, 2014-
2016 outbreak in Yemen

• Due to absence of proper sanitation – no treatment of faecal

waste → water supply contaminated
• Poor hygiene and poor living conditions
• Lack of education about transmission
Updated on 12/10/21 by Beh SJ @behlogy
 Cholera
Other Problems:

• Many different strains of Vibrio cholerae

• E.g. classical/O1, El Tor, O139
• Each strain that caused a pandemic is more
virulent than the last

• Adults can be reinfected and have cholera

again
• This is also why it is hard to vaccinate
against cholera

Updated on 12/10/21 by Beh SJ @behlogy

 Malaria
Causative organism : Plasmodium – a protoctist (eukaryote), parasite
• In humans, malaria is caused by
Plasmodium falciparum (75%) , P. malariae, P. ovale, P. vivax (20%).
• Appearance changes depends on life cycle stages
• Plasmodium is most motile during initial infective stages

Vector/Transmission:
• Vector = Organism that carries a disease from
a person to another/from an animal to a human
• Insect vector = Female Anopheles mosquitoes
• Only female take blood meals to supply eggs with nutrients
• Also through blood transfusions, use of unsterile
needles, and can pass across placenta from mother
Updated on 12/10/21 by Beh SJ @behlogy
 Malaria
Mos takes a blood meal from infected person and then takes a blood
meal from uninfected person.

Life cycle of Plasmodium:

1. Plasmodium’s gametes fuse, multiplies in and form infective
stages in mosquitoes
2. When mos takes a blood meal, parasite enters host with mos's
anticoagulant and saliva.
3. Infective stages of parasite enter bloodstream and then liver cells
4. Parasite matures in liver cells, then leaves liver to enter RBCs
5. Parasites multiply in RBCs, causing RBCs to lyse
6. Parasites are released and infect other RBCs
7. Parasites picked up by another mos in a blood meal
Updated on 12/10/21 by Beh SJ @behlogy
 2 3

1
4

1 7

Updated on 12/10/21 by Beh SJ @behlogy

 (liver cells)

(sporozoites = infective stages)

(RBC)

You do not need to know the

names of the diff stages/forms
– just know that there are many.
Updated on 12/10/21 by Beh SJ @behlogy
 Malaria
Symptoms:
• Fever
• Anaemia
• Nausea
• Headaches
• Muscle pain
• Shivering
• Sweating
False-colored electron
• Enlarged spleen micrograph of a Plasmodium

Diagnosis:
• Microscopical analysis of blood
• Dip stick test for malaria antigens in blood
Updated on 12/10/21 by Beh SJ @behlogy
 Malaria

Treatment:

• Anti-malarial drugs
• E.g. quinine, chloroquine, artemisin
• Chloroquine inhibits protein synthesis and prevents parasite from
spreading within the body
• Proguanil inhibits sexual reproduction of Plasmodium in the mos

• Combination therapy = where multiple drugs are used at the same

time → used to prevent drug-resistance

Updated on 12/10/21 by Beh SJ @behlogy

 Malaria
Prevention:
• No vaccine for malaria (more in Chap 11)

1. Use prophylactic / preventive drugs (e.g. chloroquine)

2. Reduce no. of mosquitoes

• E.g. spray insecticides, spread oil over water surface to prevent mos
breeding, breed fish than feeds on larva, spray Bacillus thuringiensis
bacteria to kill mos larvae etc.

3. Prevention of bites (best method)

• E.g. use mos nets, soak mos nets in insecticide, mos repellent, don’t
expose skin when mos are active at dusk etc.

Updated on 12/10/21 by Beh SJ @behlogy

 Malaria
Global Distribution:
What can you tell about the distribution of malaria from the map below?

Updated on 12/10/21 by Beh SJ @behlogy

 Malaria
Global Distribution:
Malaria is endemic in….
• Tropical areas
• Sub-tropical areas

WHY?
• The vector, the Anopheles mosquito, survives and breeds in
hot and humid areas
• Needs still/stagnant water to reproduce
• Plasmodium reproduces within the mosquito at >20oC
• Eradicated outside tropics (e.g. USA, Italy)

Updated on 12/10/21 by Beh SJ @behlogy

 Malaria
Other Problems:
• Drug-resistant Plasmodium (e.g. chloroquine)

• Insecticide-resistant mosquitoes (e.g. DDT)

– DDT is most common insecticide
– Insecticides used also killed other organisms
– Reduction in mos also caused lost in immunity to malaria in
local community, making them more vulnerable when the
diseased returned.

• Global warming has resulted in spread of mos

– More warm areas for mos to breed and survive
Updated on 12/10/21 by Beh SJ @behlogy
Acquired Immune Deficiency
Syndrome (AIDS)
 HIV/AIDS
Causative organism: human immunodefiency virus (HIV) – a virus
• It is a RNA virus / retrovirus
• Contains single-stranded RNA as genetic material

• Has protein coat/capsid made of capsomeres

• Has outer viral envelope made of a lipid
bilayer and proteins (mostly derived from host)
• Has viral glycoproteins on the outer envelope

• Has 2 enzymes:
1. Reverse transcriptase uses RNA
as template to produce DNA
in host cell
2. Protease: cleave/process new viral proteins
Updated on 12/10/21 by Beh SJ @behlogy
Life cycle of HIV:
HIV/AIDS
1. The viral RNA and reverse transcriptase (RT) enters T helper
lymphocytes
2. RT converts RNA into DNA
3. The viral DNA is incorporated into the host DNA
4. The cells’ machinery is used to express viral proteins
(through transcription and translation)
5. Viral proteins are assembled into many new viruses

Updated on 12/10/21 by Beh SJ @behlogy

 HIV/AIDS
Transmission:
• Through direct exchange of body fluids
• Virus is unable to survive out the human body

• Semen, vaginal fluids during unprotected sexual intercourse

• Blood transmission via infected blood transfusion / contaminated
syringes
• Mother to baby transmission across placenta / breast milk

• Having multiple sex partners allows virus to spread more widely

• Anal intercourse increases risk of transmission
• Bcs less natural lubrication in the rectum, so rectal lining is easily
damaged and virus can pass from semen into blood

Updated on 12/10/21 by Beh SJ @behlogy

Updated on 12/10/21 by Beh SJ @behlogy
 HIV/AIDS
Symptoms and Effects:
• Slow infection - virus can stay dormant for years
• Changes surface proteins to hide from immune system

• HIV infects cells of the immune system, called helper T cells

• This destroys helper T cells / cause their no. to decrease
• Body is unable to defend itself against infection

Symptoms of HIV infection:

• Flu-like symptoms and then symptomless
(since virus can stay dormant for years)

• Opportunistic infections can occur as result of a compromised

immune system
• Collection of opportunistic diseases associated with
immunodeficiency caused by HIV = AIDS
Updated on 12/10/21 by Beh SJ @behlogy
 AIDS
Symptoms and Effects:

Opportunistic infections:
• oral thrush
• pneumonia
• cancers like Kaposi's sarcoma
• neurogenerative diseases like dementia
• tuberculosis (TB)
• malaria
• malnutrition
• weight loss
• diarrhoea
• fever
• sweating

Updated on 12/10/21 by Beh SJ @behlogy

 HIV/AIDS
Treatment:
• No cure for AIDS

• Drug therapy can only slow down the onset of AIDS.

– Zidovudine (similar to the nucleotide base that contains
Thymine) binds to viral enzyme reverse transcriptase and
inhibits it
– Drugs can target viral protease and inhibit it as well
– However, drugs are expensive and have a variety of side effects

• Combination therapy
– Must follow a strict pattern and timing of medication
– If not followed properly, patients may develop strains of HIV
resistant to drugs.
Updated on 12/10/21 by Beh SJ @behlogy
Prevention:
HIV/AIDS
• No vaccine
• Use of condoms, femidoms and dental dams

• HIV testing promoted in high-risk groups

(e.g. male homosexuals, prostitutes, injecting
drug users, sex partners)
• Contact tracing = where the person with HIV traces people he/she
has put at risk of infection and that person to provided a HIV test

• Discourage needle sharing

• Donated blood is screened for HIV and heat-treated to kill viruses
• Take PrEP, a prophylactic drug (not in syllabus)

Updated on 12/10/21 by Beh SJ @behlogy

 HIV/AIDS
• Control mother-to-child transmission using drugs
• HIV+ women of high income countries should avoid
breastfeeding to reduce transmission
– BUT HIV+ women of low/middle income countries advised
to breastfeed bcs milk provides protection against other
diseases and lack of clean water outweigh the risk of
transmitting HIV

Other problems:
• In Africa, difficult to reach people for widespread testing in
rural areas
• Many symptomless carriers
• Drugs are expensive and cause many side-effects
Updated on 12/10/21 by Beh SJ @behlogy
 HIV/AIDS
Global Distribution:
• Pandemic in whole world
• But especially high numbers/high prevalence in Africa

Updated on 12/10/21 by Beh SJ @behlogy

Tuberculosis
 Tuberculosis
Causative organism: Mycobacterium tuberculosis, M. bovis – bacteria
• M. tuberculosis causes TB in humans
• M. bovis causes TB in cows, humans and other mammals

Updated on 12/10/21 by Beh SJ @behlogy

 Tuberculosis
Transmission:
Mycobacterium tuberculosis
• By aerosol infection
• Pathogen is in airborne droplets
• Infected person coughs / sneezes
• Uninfected person breathes in droplets

Mycobacterium bovis
• From infected cows / cattle
• Eat undercooked contaminated meat
• Drink unpasteurised milk containing bacteria

Updated on 12/10/21 by Beh SJ @behlogy

 Tuberculosis
Symptoms and Effects:
• Site of primary infection = lungs
• Secondary infections in lymph nodes,
bones and gut

• Slow infection - many infections are controlled by immune system

and people don't suffer symptoms, cannot pass on the disease
• Bacteria may be activated after many years when immune system
weakened by other infections (e.g. HIV)
• The incubation period is few weeks to few years

Updated on 12/10/21 by Beh SJ @behlogy

 Tuberculosis
Symptoms and Effects:
• Racking cough
• Coughing blood
• Chest pain
• Shortness of breath
• Fever
• Sweating
• Loss of appetite
• Weight loss

Diagnosis:
• X-ray
• Microscopical examination of sputum
(mucus and pus) for bacteria
Updated on 12/10/21 by Beh SJ @behlogy
Tuberculosis
Treatment:
• Long treatment time bcs bacteria is
slow growing and not very responsive to drugs.

• Combination therapy
– Use multiple antibiotics
– E.g. streptomycin, isoniazid,
rifampicin
– Prevent drug-resistance

• DOTS (direct observation treatment, short

course) makes sure patients take medicine
regularly and reduce spread of drug resistance
Updated on 12/10/21 by Beh SJ @behlogy
 Tuberculosis
Prevention:
• Bacillus Calmette–Guérin (BCG) vaccine
• Derived from M. bovis

• Isolate patients which are in infectious stages

• Contact tracing and TB screening for early
detection to prevent spread
• Cattle tested for TB
• Pasteurise milk

Updated on 12/10/21 by Beh SJ @behlogy

 Tuberculosis
Other Problems:
• Multiple drug-resistant TB (MDR-TB) and extremely drug-
resistant TB (XDR-TB)
• Drugs like isoniazid is no longer effective

• Not completing the course of treatment / partial treatment of

TB in patients increase risk of drug resistance

Updated on 12/10/21 by Beh SJ @behlogy

 Tuberculosis
Global Distribution:
• Endemic all over the world
• Especially high rates in cities due to high migration rate
• And in overcrowded areas where there is poverty
• Also where HIV/AIDS is prevalent

Updated on 12/10/21 by Beh SJ @behlogy

Correlation between global distributions of AIDS and TB

Updated on 12/10/21 by Beh SJ @behlogy

Correlation between global distributions of AIDS and TB
• Countries where many people have AIDS, also have a high
death rate from TB
• Many people who have AIDS die of TB

WHY?
• TB is an opportunistic infection
• People who are HIV+ are more susceptible to TB
• Dormant TB more likely to become active if person is HIV+

Updated on 12/10/21 by Beh SJ @behlogy

 Smallpox
*only need to know the causative organism
Causative Organism: Variola virus

Transmission:
• By aerosol infection
• Air droplets from sneezing and coughing
• Contact with contaminated clothing and bedding

Symptoms:
• Fever, headache, severe fatigue,
severe back pain, vomiting
• Rash
• Develop into abscesses with
fluid and pus
• The abscesses would break
open and scab over
• Scab would fall off and leave scars
Updated on 12/10/21 by Beh SJ @behlogy
 Smallpox
Treatment:
• NO cure, although some antiviral drugs may help to
prevent it from getting worse

Prevention:
• Smallpox vaccine

Fortunately, smallpox has been ERADICATED

…..through vaccines!

Updated on 12/10/21 by Beh SJ @behlogy

Antibiotics
How Penicillin was discovered by Alexander Fleming

Updated on 12/10/21 by Beh SJ @behlogy

 Antibiotics
• Antibiotics are drugs used
• Usually derived from microorganisms
• To kill / inhibit growth of bacteria
• Without harming the infected organism

• Bacteriocidal antibiotics – kill bacteira

• Bacteriostatic antibiotics – inhibit bacterial growth
• Prevent spread of bacteria within body

• Harmless to human cells

• Have NO affect on viruses

Updated on 12/10/21 by Beh SJ @behlogy

 Antibiotics
How do antibiotics work?
• Inhibit bacterial cell wall synthesis
• Inhibit activity of specific membrane protein / glycoprotein
→ Block binding to cells
• Block specific enzyme action
• Inhibit protein synthesis and
nucleic acid synthesis
→ Target enzymes that
bacteria have, but humans
don’t

Note: many diff antibiotics

each have diff mechanisms
Updated on 12/10/21 by Beh SJ @behlogy
E.g. Penicillin
In the absence of penicillin:
• Bacterial cell wall is made of peptidoglycans
• When bacteria cells grow, it secretes autolysins
• Autolysins make tiny holes to allow the cell wall to stretch
• New peptidoglycans formed
• Peptidase enzyme form cross-links
between peptidoglycan chains
• To form cell wall

Updated on 12/10/21 by Beh SJ @behlogy

 E.g. Penicillin
In the presence of penicillin:
• Penicillin inhibits peptidase enzyme
• Stops formation of cross-links between peptidoglycan
polymers in the cell wall

• Autolysins make tiny holes to allow the cell wall to stretch

• New peptidoglycans formed but cannot link up
• Cell wall is weaker
• Cells walls unable to withstand
turgor pressure
• When water moves in by osmosis,
bacteria lyses and dies
• Penicillin is only effective when
bacteria is growing
Updated on 12/10/21 by Beh SJ @behlogy
 Antibiotics
Diff antibiotics are effective against diff bacteria.

Antibiotic discs can be placed in a Petri dish with growing bacteria.

The diameter of the zone of inhibition shows how effective the
antibiotic is. Which antibiotic is the most effective?

Updated on 12/10/21 by Beh SJ @behlogy

 Why antibiotics do not affect viruses
• Viruses do not have peptidoglycan cell wall, have protein coat
• Viruses do not have their own metabolism, rely on host cells
• Viruses have no cell structure / very few organelles
→ Very few sites for antibiotic to act on
• Viruses live inside host cells, out of reach of antibiotics

• Antivirals exist
→ Usually target viral
glycoproteins on
viral envelope
→ Prevent binding of
virus to host cells
→ Inhibit specific
viral enzymes Updated on 12/10/21 by Beh SJ @behlogy
 Antibiotic Resistance
• When antibiotics are no longer effective against bacteria
• Antibiotic resistance can be spread from bacteria to bacteria

E.g.
• Many bacteria have penicillinase enzymes can break down penicillin
• Become resistant to penicillin

Caused by:
• Spontaneous/random mutation in bacteria
• Mutation cause change in protein/production of new protein that
cannot be targeted by antibiotics

Updated on 12/10/21 by Beh SJ @behlogy

 Antibiotic Resistance
• Natural selection enables resistance genes to spread

• Antibiotic is the selection pressure

• Antibiotics only kill bacteria that are non-resistant

• Resistant bacteria survive and reproduce

• Antibiotic resistance gene is spread to next gen and other bacteria

Updated on 12/10/21 by Beh SJ @behlogy

 Antibiotic Resistance
Antibiotic resistant genes are usually found in plasmids!
Bacteria can spread antibiotic resistance genes using:
• Vertical transmission
→ pass plasmids down to daughter cells by binary fission
• Horizontal transmission
→ pass plasmids to other bacteria by conjugation

Updated on 12/10/21 by Beh SJ @behlogy

Updated on 12/10/21 by Beh SJ @behlogy
 Antibiotic Resistance
Cause:
• Due to patients not
completing the course of
antibiotics given
→ Treatment may not be
completed so some
susceptible bacteria survives
→ Bacteria replicates and have
increased chance of mutation
/ becoming resistant

Updated on 12/10/21 by Beh SJ @behlogy

 Antibiotic Resistance
Consequences:
• Bacteria can carry several antibiotic resistance genes
• Develop multiple resistance
• Become a “super bug”
• E.g. methicillin-resistant Staphylococcus areus (MRSA)
→ Wounds do not heal and are continually infected with bacteria

• Cannot be killed/inhibited by common antibiotics

• Need to be controlled by stronger antibiotics
→ But we are running out of antibiotics!

Updated on 12/10/21 by Beh SJ @behlogy

 Antibiotic Resistance
Solution:
• We need to discover more new antibiotics
• We can slightly alter/modify chemical structure of known
antibiotics to produce new antibiotic
• BUT discovery takes time!

• The best solution is….

PREVENTION!
• Only use antibiotics when
prescribe
• Always finish the course of
antibiotics

Updated on 12/10/21 by Beh SJ @behlogy

 Chapter Outline
• What is an infectious disease? What are pathogens?

• Causes, Transmission, Symptoms, Treatment, Prevention and Global

Distribution of:
1. Cholera
2. Malaria
3. HIV/AIDS
4. Tuberculosis
5. Smallpox (just the causative agent)

• Antibiotics
e.g. Penicillin
Why it doesn’t it work on viruses?
• Antibiotic Resistance (Cause + Consequence + Prevention)

Updated on 12/10/21 by Beh SJ @behlogy

 Links
• If you are obsessed with colourful maps showing global distribution…
http://www.who.int/gho/map_gallery/en/

• Antibiotic resistance
http://time.com/4767683/bacteria-antibiotic-resistance-superbugs/

• Malaria Lifecycle
https://www.youtube.com/watch?v=1v55yg0RfoY

Updated on 12/10/21 by Beh SJ @behlogy

 AS Level
Chapter 11
Immunity
 Chapter Outline
Part 1: The Immune Response
• Definitions: Immune response, antigen, self and non-self
• Phagocytes
– Antigen presentation
• B-lymphocytes
– Antibodies: structure and function
– plasma cells and memory cells
• T-lymphocytes
– T helper cells and T killer cells

Part 2: Vaccination and Monoclonal Antibodies

• Active vs Passive // Natural vs Artificial Immunity
• Vaccination
– How smallpox was eradicated
– Why vaccines aren’t effective for other diseases
• Monoclonal antibodies

Updated on 12/8/21 by Beh SJ @behlogy

Chapter 11
(Part 1)

The Immune
Response

Updated on 12/8/21 by Beh SJ @behlogy

 Definitions
• Immunity = Protection against diseases
• Immune system = The body’s defense system

Updated on 12/8/21 by Beh SJ @behlogy

 Lines of Defences Against Diseases
• To prevent infectious diseases from entering and spreading

1) First line of defence

• External, non-specific

2) Second line of defence

• Internal, non-specific immune
response
• Involves phagocytes

3) Third line of defence

• Internal, specific immune response
• Involves lymphocytes

• Both non-specific and specific defences work together to protect the

body against diseases
Updated on 12/8/21 by Beh SJ @behlogy
 Definitions
• Antigens
• 2 types: Self and Non-self
• In general, antigens are macromolecules on cell surfaces
• E.g. protein, glycoprotein, glycolipid, polysaccharides etc.

1) Non-self antigens = macromolecules that activates an

immune response
• Macromolecules are found on…
– foreign materials’ surface (e.g. pathogen, allergen)
– surface membrane of infected host cells
→ Stimulates production of antibodies
Representative diagram showing
diff antigens on surface of diff pathogen

Updated on 12/8/21 by Beh SJ @behlogy

 Definitions
2) Self antigens / cell marker
• Macromolecules on cell surface membranes of host cells
• Cell surface antigens do NOT trigger body’s immune system
• No antibodies are produced
Self-antigen markers function to prevent the immune system from attacking
cells of the body it is protecting.

Note: when we say antigen in general, we are usually

referring to NON-self antigen though
Updated on 12/8/21 by Beh SJ @behlogy
 Definitions
Immune response = the body’s immune reaction towards
non-self antigens

Involves WBCs that made in bone marrow (Chap 8)

1) Phagocytes
(mostly non-specific defence)
• Neutrophils
• Monocytes
→ which mature into macrophages
2) Lymphocytes
(mostly specific defence)
• B-lymphocytes
• T-lymphocytes
Updated on 12/8/21 by Beh SJ @behlogy
 Chap 8 Recap *yellow highlights = points important for this chap!

1) Phagocytes
• Produced throughout life

Function:
• Patrol in blood, tissues and organs
• Remove dead cells and pathogens
→ By phagocytosis (Chap 4)

• Involved in non-specific defense

→ responds to many different non-self antigens

Appearance:
• Lobed nuclei
Updated on 12/8/21 by Beh SJ
• Granular cytoplasm – due to @behlogy
many vesicles
 Chap 8 Recap

1) Phagocytes
E.g. Neutrophils
• Multi-lobed nucleus
• Have receptor proteins on its membrane
→ To identify pathogens as non-self

• When there is an infection, large numbers

are released from bone marrow
→ Accumulate at site of infection

• Short-lived (few hours-days)

→ Dies after digesting pathogens
→ Dead neutrophils formUpdated on 12/8/21 by Beh SJ
pus :O@behlogy
 Chap 8 Recap

1) Phagocytes
E.g. Monocyte → Macrophage
• Lobed nucleus / kidney-bean shaped
• Larger than neutrophils
• Have receptor proteins on its membrane
→ To identify pathogens as non-self

• Monocytes = circulate in blood

→ Mature into macrophages when it leaves
blood and enter organs

• Long-lived cells
• Macrophages found in organs such as
Updated on 12/8/21 by Beh SJ
liver, lungs, spleen, kidney, lymph@behlogynodes
 Role of Macrophages
• Initiates / starts the immune response

Mechanism:
1) Has various receptor proteins of cell surface
• Can detect non-self antigens
• Non-specific

2) Engulf pathogen / foreign material via

phagocytosis (Chap 4)
• Fusion of phagocytic vacuole with lysosome

Updated on 12/8/21 by Beh SJ

@behlogy
 Role of Macrophages
• Initiates / starts the immune response

Mechanism:
3+4) Cuts up pathogen using lysozymes

5) Antigens presented on its cell surface

→ Macrophages act as
antigen-presenting cells (APC)

6) Some cell fragments released by exocytosis

• APCs can activate / stimulate lymphocytes

P/S: other APCs can include B cells and other types of

Updated on 12/8/21 by Beh SJ
phagocytes too! @behlogy
 Chap 8 Recap

2) Lymphocytes
• Produced in bone marrow before birth

Function:
• Involved in specific immune responses
→ responds to only specific non-self antigens
• Mature lymphocytes circulate in the blood
and lymph
→ Accumulate at sites of infection

Appearance:
• Smaller than phagocytes
• Large round nucleus
Updated on 12/8/21 by Beh SJ
• Little cytoplasm @behlogy
 Chap 8 Recap

2) Lymphocytes
2 main types:
Both made in bone marrow, but mature in different places and
have different functions

1. B-lymphocytes (B cells)
• Mature in bone marrow
• Produces antibodies

2. T-lymphocytes (T cells)
• Mature in thymus
• Does NOT produce antibodies

Updated on 12/8/21 by Beh SJ

• Both work together to defend the immune system
@behlogy
Lymphocytes
• Millions of different types of B and T-lymphocytes with
receptors of different shapes
• SPECIFIC = each type of lymphocyte responds to 1 type of
antigen only
E.g. Each type of B cell produce 1 type of antibody receptor
→ which responds to 1 type of antigen only

• So the body can respond to almost any type of pathogen

Updated on 12/8/21 by Beh SJ @behlogy

 Lymphocytes
• Only mature lymphocytes can circulate in the blood & lymph and
carry out immune responses
• Have telomerase to divide continuously
activation and
maturation differentiation Only plasma cells
produce antibodies!

1
Plasma cells
Mature B
Naïve B cells
cells
B memory
cells
Stem cell
T helper
T helper cells
memory cells
Mature T
Naïve T cells
cells
T killer cells T killer
2 / cytotoxic T cells memory cells
Updated on 12/8/21 by Beh SJ
@behlogy
1

Maturation of B-Lymphocytes
1) All B cells are formed in the bone marrow before birth
→ Genes in B cells that code for antibodies code for diff types of
antibodies for diff types of B cells

2) Forms a specific antibody that acts as glycoprotein receptor on

surface membrane of B cells
→Binds to specific antigen that is complementary in shape

3) B lymphocytes divides and mature in bone marrow

→ Mature B lymphocytes circulate in blood and concentrate in liver,
spleen & lymph nodes

Updated on 12/8/21 by Beh SJ @behlogy

1

Antibodies
• Aka immunoglobulins
• Globular glycoproteins
(carbo part not shown in diagram)

Made of 4 polypeptide chains:

• 2 heavy chains
• 2 light chains
→ Quaternary structure

• Held together by disulfide bonds

→ gives stability

Updated on 12/8/21 by Beh SJ @behlogy

 1

Three regions: Antibodies

1) Variable region (Fab)

• Every chain has a variable region = 4 in total

• Provide 2 identical antigen-binding sites

• Specific for binding antigen

→ Complementary shape to antigen
→ Shape determined by primary structure
= specific seq of amino acids
• R groups at antigen-binding site forms H
bonds and ionic bonds with specific antigen

Sequence of amino acids at the variable region is

different for each type of antibody
→ Each type of antibody binds different antigens
Updated on 12/8/21 by Beh SJ @behlogy
 1
Antibodies
2) Constant region (Fc)

• Formed by light and heavy chains

• When circulating in blood: binds to receptors on phagocytes
• When antibody acts as B cell receptor:
attach to cell surface membrane of B cell

• Gives antibody class

Aiyo no need to remember lah

Updated on 12/8/21 by Beh SJ @behlogy

 1

Antibodies
3) Hinge region

• Held by disulfide bridges

• Gives flexibility when binding to antigen

Updated on 12/8/21 by Beh SJ @behlogy

1

Action of AntibodiesAgglutination
Prevent entry into cell Attach to flagella

Lysis of pathogen Opsonisation

Neutralise toxins

Updated on 12/8/21 by Beh SJ @behlogy

 1

Action of B-Lymphocytes
1. Pathogens invade

2. Antigen presentation cell formation

3. Only specific B lymphocytes has receptors with the

complementary shape to antigen will be activated
→ Clonal selection

4. B cell divides by mitosis

→ Clonal expansion

5. Activated B cells develop into plasma cells and memory cells

Updated on 12/8/21 by Beh SJ @behlogy
 1

Plasma Cells
• Short lived (few weeks)
• Do not divide and do not have telomerase

• Produce and secrete antibodies rapidly

→ by exocytosis
→ into blood plasma, lymph, lungs and stomach lining

• Antibodies are glycoproteins

→ So plasma cells have extensive network of RER
and Golgi

Updated on 12/8/21 by Beh SJ @behlogy

 1

Memory Cells
• Long-lived, remain in circulation
• Has telomerase

• Provides long term immunity

• Last for many years/lifetime

• Enable faster response during

2nd invasion of same antigen,
as many memory cells are circulating

• During 2nd invasion, it divides rapidly (clonal expansion)

→ Form more plasma cells → more antibodies
→ Infection is destroyed before symptoms develop
→ Body immune to pathogen

Updated on 12/8/21 by Beh SJ @behlogy

 Secondary response
1
• Faster response
Memory cells • Many memory cells circulating
• More cells specific for pathogen, higher
chance of encountering pathogens quickly
• More plasma cells formed
• More antibodies produced
• No symptoms developed

Primary response
• Slower response
• Only a few B cells specific
to the antigen is present
• Individual becomes ill

Updated on 12/8/21 by Beh SJ @behlogy

1
Summary of Action by B-Lymphocytes

Updated on 12/8/21 by Beh SJ @behlogy

2
Maturation of T-Lymphocytes
1) All T cells produced in bone marrow before birth

2) Maturation in thymus gland

→ thymus shrinks after puberty

• Produce specific T cell receptors on cell

surface membrane
→ Binds to specific antigen that is complementary in shape
→ T cell receptor’s structure similar to antibodies

3) Mature T cells circulate in blood and lymph

Updated on 12/8/21 by Beh SJ @behlogy

 2
Action of T-Lymphocytes
1. Pathogens invade APC

2. Antigen presentation cell formation

3. Only specific T lymphocytes has

receptors with the complementary shape
to antigen will be activated
→ Clonal selection

4. T cell divides by mitosis

→ Clonal expansion

5. Activated T cells develop into

T helper cells and T killer cells

Updated on 12/8/21 by Beh SJ @behlogy

 2
T Helper Cells
Functions:

1) Secrete cytokines / interleukins which….

a) Stimulate specific B cells

• To divide and develop into plasma cells & memory B cells
• Increased antibody levels

b) Stimulate macrophages
• To carry out phagocytosis more
vigorously

c) Stimulate killer T cells

• To divide and produce more toxins
Updated on 12/8/21 by Beh SJ @behlogy
 2
T Helper Cells
Functions:

2) Form T helper memory cells

• Secondary response
• Long term immunity

Updated on 12/8/21 by Beh SJ @behlogy

 2

Cytotoxic T Killer Cells

Functions:

1) Seeks out infected host cells (including APC, cancer cells)

and pathogens and destroys them

a) Attach to surface of cells

b) ‘Punch’ holes into cells

c) Secrete toxins into cells

• E.g. hydrogen peroxide,
perforin https://www.youtube.com/watch?v=ntk8XsxVDi0

Updated on 12/8/21 by Beh SJ @behlogy

2

Cytotoxic T Killer Cells

Functions:
2) Forms killer T memory cells
• Secondary responses
• Long term immunity as it is long-lived

Updated on 12/8/21 by Beh SJ @behlogy

A Summary of the
Phagocyte +
Immune Response
Antigen

APC

Humoral (antibody-mediated) Cell-mediated

immune response immune response

B T

Plasma T
Memory T killer helper
cell cell
B cell cell

H2O2
T killer
memory
perforin
cell

cytokines
T helper
memory
cell
antibodies
toxins
Updated on 12/8/21 by Beh SJ @behlogy
Chapter 11
(Part 2)

Vaccination and
Monoclonal
Antibodies

Updated on 12/8/21 by Beh SJ @behlogy

 Types of Immunity
• Active: Own immune response is activated
– Own lymphocytes are activated by antigens
– Own antibodies are made
– Takes time, not immediate
– Memory cells formed → results in long-term immunity

• Passive: Immune response is NOT activated

– Own lymphocytes cells not activated
– NO plasma cells to produce antibodies
– Protection is immediate
– NO memory cells formed → only short-term immunity

Updated on 12/8/21 by Beh SJ @behlogy

 Active Natural Active Artificial Passive Natural Passive Artificial
Immunity Immunity Immunity Immunity
E.g. Catching a cold E.g. Vaccination E.g. Maternal E.g. Antibodies or
antibodies antitoxins
Natural: Antigens Artificial: Antigens
from the Natural: Artificial: Antigens
are introduced via 1) Antibodies pass are introduced via
environment injection into vein or from mother to injection
muscle / consumed infant through
placenta Antibodies are
→ activate the collected from
immune response → remain for blood of donor /
artificially months animals who are
vaccinated or suffer
Antigens can be 2) Breast milk that from the same
attenuated / made is colostrum-rich disease
harmless (e.g. heat- has antibody (IgA)
that prevents → Contains the
treated, cut up, specific antibodies
inactivated toxins) growth of against the specific
bacteria/viruses in antigen
Antigen used could the stomach of
be dead or alive infant
Updated on 12/8/21 by Beh SJ @behlogy
Active Natural
Immunity Antibody Levels after Infection

Updated on 12/8/21 by Beh SJ @behlogy

Active Artificial
Immunity Antibody Levels after Vaccination
Vaccination activates the immune system
→ producing memory cells that result in
long term immunity

Updated on 12/8/21 by Beh SJ @behlogy

Passive Natural Antibody Levels in the Blood of
Immunity
Fetus or Infant

Maternal antibody passed

from placenta to fetus drops in
concentration after birth
→ protection is temporary

Updated on 12/8/21 by Beh SJ @behlogy

Passive Artificial Antibody Levels after
Immunity
Antibody Injection
Concentration of antibody increases
immediately but decreases over time
→ only provides short-term protection

Updated on 12/8/21 by Beh SJ @behlogy

Vaccination
Effective Vaccines Ineffective Vaccines
Provide sufficient antigens
→ To mimic or copy natural infections Do not mimic natural infections
→ To form sufficient plasma and memory → No plasma and memory cells formed
cells for long-term protection
Do not give lifetime protection
→ Require booster injections
→ To stimulate secondary response in
order to give protection
Give lifetime protection against pathogen
→ Pathogen unable to developed in
Do not provide sufficient protection
immunised person
against pathogen
→ Maybe due to pathogen’s high
mutation rate or ability to hide from
immune system
E.g. vaccines using live pathogens, E.g. vaccines using dead pathogens,
smallpox vaccine cholera vaccine
Updated on 12/8/21 by Beh SJ @behlogy
 Vaccination
Two modes:

1. Mass vaccination
• Vaccinate a large number of people at the same time

2. Ring vaccination
• Perform contact tracing with infected person
• Vaccinate the area of community the person
is in / people who was in contact with the person

• Aim: Vaccinate a high proportion of the population

→ To achieve herd immunity
Updated on 12/8/21 by Beh SJ @behlogy
 Herd Immunity
• Mass vaccination results in
in herd immunity

• Less chance of
transmission of disease
→ Reduce pool of infected
people in the community
→ Fewer people can catch the
disease and be source of
infection

• Protection of those
unvaccinated /
immunocompromised as
disease does not spread
Updated on 12/8/21 by Beh SJ @behlogy
 Common Barriers to Vaccination
1) Poor response to vaccines
• People that are immunocompromised

• People who lack protein (malnutrition)

• Less antibodies made

2) Pathogens can mutate rapidly (antigenic variation)

• Form diff strain with diff antigens
• Memory cells are unable to recognise pathogen that has major
changes in antigen structure

3) Pathogens can escape from immune system (antigenic concealment)

• By living inside cells / covering bodies with host proteins /
suppressing immune system

Updated on 12/8/21 by Beh SJ @behlogy

How was smallpox eradicted?
An effective vaccine was developed!
https://www.youtube.com/watch?v=yqUFy-t4MlQ

• Same vaccine used everywhere

→ Variola virus – stable, low mutation rate

• Use live virus so strong immune response

→ similar Vaccinia virus
• One dose enough to give life-long immunity,
no boosters needed

• Vaccine is heat stable

• Easy to administer
→ Use bifurcated needle
→ Needle can be sterilized and reused
Updated on 12/8/21 by Beh SJ @behlogy
How was smallpox eradicated?
Mass vaccination was very successful!
This is due to:

• High percentage of population immunized

→ Low cost needed to for mass production
of vaccine
→ Many volunteers became vaccinators
→ Result in herd immunity

• Infected people were easy to identify

→ Few symptomless carriers
→ Can perform contact tracing and
ring vaccination
→ Can isolate cases to prevent spread
Updated on 12/8/21 by Beh SJ @behlogy
 Why isn’t TB eradicated already?
• BCG vaccine is available!
• It is not eradicated even though there is a high coverage of
vaccination!

Why?
• Vaccination does not work in adults >35yo

• High percentage cover (above 90%) needed to achieve herd

immunity
→ Not yet done in every country

• Difficult for surveillance / to ensure vaccination

→ Due to high birth rates and high migration rates
→ Latent TB is symptomless and found in 1 in 4 people

Updated on 12/8/21 by Beh SJ @behlogy

Why can’t we vaccinate against malaria?
• No effective vaccines against malaria

Why?
• Protoctists are eukaryotes
→ Many more genes than bacteria & viruses

• Display diff antigens on its cell surface for:

→ Diff species / strains
→ Different stages of its life cycle

• Parasite changes antigens during infection

→ Diff genes coding for antigens switch on during infection

• Plasmodium parasite hides in liver and RBCs

Updated on 12/8/21 by Beh SJ @behlogy
 Why can’t we vaccinate against cholera?
• No effective vaccines against cholera
• Oral vaccination only gave limited protection as it was excreted

Why?

• Many different strains of cholera

→ Bacterium mutates

• Vibrio cholerae lives in the host’s intestines

→ Beyond reach of antibodies

Updated on 12/8/21 by Beh SJ @behlogy

 Monoclonal Antibodies (Mabs)
• Monoclonal = only 1 type of antibody, specific for 1 antigen

Problem:
• B cells that divide by mitosis DO NOT produce antibodies
• Plasma cells that secrete antibodies DO NOT divide

Solution:
Fuse plasma cells + cancer/myeloma cells
→ hybridoma cells that CAN divide and
CAN produce antibodies
Updated on 12/8/21 by Beh SJ @behlogy
 Monoclonal Antibodies (Mabs)
How to produce?

1) Inject foreign antigen (e.g.

pathogen) into mice

2) Allow time for immune response to

occur

3) Collect plasma cells from spleen

3) Fuse plasma cells with cancer cells to

produce hybridoma cells
→ Use fusogen for fusion
Updated on 12/8/21 by Beh SJ @behlogy
 Monoclonal Antibodies (Mabs)

4) Clone hybridoma cells

→ Use HAT medium for hybridoma growth

5) Screen for cell secreting desired antibody

→ By separating cells and culture in individual wells
→ Select only one type

6) Grow hybridoma cells in large scale culture

Updated on 12/8/21 by Beh SJ @behlogy
 Usage of Mabs
1) Diagnosis
• Monoclonal antibodies have same specificity and detects only
one antigen
→ Can distinguish between diff pathogens / strains
→ Fast diagnosis than having to culture pathogen
→ Less labour intensive
→ Quicker diagnosis = quicker treatment

• Can be tagged with a fluorescent label/dye

→ Can detect location of tissues expressing antigen
→ E.g. cancer cells, blood clots

• Cheap, safe, fast results, easy to use, accurate

(Also used in blood typing, pregnancy tests etc.) Updated on 12/8/21 by Beh SJ @behlogy
 Usage of Mabs
2) Treatment
• Used to target specific diseased cell by binding to receptors on
its cell surface
→ Can kill cell by stimulating the immune system
→ Can attach radioactive substance / drug to Mabs to kill cell

• Can bind to antigens on pathogens

E.g. Herceptin, Mab used
→ Result in artificial passive immunity to treat breast cancer

Updated on 12/8/21 by Beh SJ @behlogy

 Problems of using Mabs in Treatment
Problems:
• Causes some side effects
• Antibodies made in animals recognised as non-self
• Trigger immune response in humans
→ Allergic reaction

• Remains in the body for short period of time as it is destroyed

• Need to be administered more than once in small amounts

Updated on 12/8/21 by Beh SJ @behlogy

 Problems of using Mabs in Treatment
Solution: Humanise Mabs
1) Alter genes that code for heavy and light chains of antibodies
→ Code for human antibodies instead of mice and rabbit’s

2) Changing type and position of sugar groups attached to heavy chains

→ Arrangement of sugar groups same as human antibodies

Example names of Mabs:

• Infliximab
• Rituximab
• Ipilimumab

P/S: No need to memorise details of diagram or Mabs names lah

Updated on 12/8/21 by Beh SJ @behlogy
 Chapter Outline
Part 1: The Immune Response
• Definitions: Immune response, antigen, self and non-self
• Phagocytes
– Antigen presentation
• B-lymphocytes
– Antibodies: structure and function
– plasma cells and memory cells
• T-lymphocytes
– T helper cells and T killer cells

Part 2: Vaccination and Monoclonal Antibodies

• Active vs Passive // Natural vs Artificial Immunity
• Vaccination
– How smallpox was eradicated
– Why vaccines aren’t effective for other diseases
• Monoclonal antibodies

Updated on 12/8/21 by Beh SJ @behlogy

 Videos! (again, warning – some of these things are not in your syllabus)
B-lymphocytes
• https://www.youtube.com/watch?v=JnXxU5XAVWU

T-lymphocytes
• https://www.youtube.com/watch?v=WdCiaIS2LV4

TED-Ed: How does your immune system work? - Emma Bryce

• https://www.youtube.com/watch?v=PSRJfaAYkW4

TED-Ed: Why it’s so hard to cure HIV/AIDS

• https://www.youtube.com/watch?v=0TipTogQT3E

How do Pregnancy Tests Work?

• https://www.youtube.com/watch?v=aOfWTscU8YM

Updated on 12/8/21 by Beh SJ @behlogy

Paper 3 (practical) notes
2.1 Testing for biological • if the colour turns lilac/purple, proteins are present

molecules Qualitative data

Reducing sugars Descriptive data from observations and not
• add equal volumes of the food sample and Benedict's measurements such as colour changes
reagent
• heat in a water bath at 80°C Qualitative food test errors
• colour changes from blue, green, yellow, orange, brick 1) difficult to judge colours, especially if concentrations
red ppt are low resulting in light colours or small colour
• for accuracy, make sure the tubes are left in the water changes
bath for the same amount of time 2) temperature of the water bath may not remain
constant throughout heating

Improvements for qualitative tests

1) use a thermostatically controlled water bath
2) use a colourimeter
3) place a white card or tile behind the test tubes

Quantitative data
Numerical data such as measurements
Non-reducing sugars Quantitative food test errors
• first hydrolyse the food sample with hydrochloric acid 1) difficulty in comparing colours
and heat
2) when determining an unknown concentration, it may
• then neutralise the sample with an alkali (e.g., NaOH, be between 2 concentrations
Na2CO3)
3) temperature not being constant for all samples
• add an equal volume of Benedict’s reagent to the test
tube, heat in a water bath at 80°C and observe the 4) DCPIP test for vitamin C – drops fall on the sides of
colour change (refer to the above diagram) the test tubes

Iodine test for starch Improvements for quantitative tests

Add a few drops of iodine solution to the sample 1) carry out more experiments with a narrower or wider
range of concentrations
• if a blue black colour is quickly produced, starch is
2) plot a graph with results to estimate the unknown
present
• if the iodine solution remains yellow-brown, starch is 3) use a colourimeter to help compare colour changes
absent better
4) use a white card/tile to observe the colour changes
Emulsion test for lipids better
5) use a thermostatically controlled water bath
• shake sample with ethanol till it dissolves
6) in the DCPIP vitamin C test, use a wider test tube, or
• add water
a wide-mouthed one
• if lipids are present solution turns into a cloudy white 7) repeat and take the average
suspension

Biuret test for proteins Dilutions

• take 2 cm3 of food sample to be tested in a test tube
• add 2 cm3 of 5% sodium hydroxide solution and shake
• then add a few drops of 1% copper sulphate solution
and observe colour change
• or if biuret reagent is available then add equal volumes
V1
starting
of that to the food sample and observe colour change
volume
• if the colour remains blue, no proteins are present

1 www.alevel-notes.weebly.com
a) Simple dilutions 3.2 Factors that affect enzyme
E.g., reducing the concentration by a factor of 2 to make a
10 cm3 solution (this differs depending on the question)
activity
volume of solution / volume of distilled Effect of heavy metals such as copper sulphate
concentration / %
cm3 water / cm3 and lead nitrate
10 0 1.0
• can act as inhibitors
8 2 0.8 • may cause protein to clot or coagulate
6 4 0.6 • may denature proteins
4 6 0.4 • may breakdown bonds so that can alter tertiary and
quaternary structures

Immobilising enzymes
+ 8 cm3 1% Errors
solution + 6 cm3 1% + 4 cm3 1%
solution solution 1) beads are not of equal sizes
2) beads stuck to the sides of the tube and to each
3) other
4) forceps may cause damage to the beads
5) difficulty to introduce drops using syringe
0.8% solution
6) test tube is not vertical and test tubes are not of
0.6% solution equal sizes
0.4% solution 7) temperature/pH is not controlled (if they are not the
b) Serial dilutions factors under investigation)
Reducing the concentration by a factor of 10 to make a
10 cm3 solution Improvements
1) use sieve with equal size holes to produce beads
with equal diameter
2) use wider test tubes
3) stain the beads for clearer movement
4) use retort stand to make the test tube vertica
5) use test tubes with equal sizes
6) use spatula or spoon instead of forceps
7) control temperature using thermostatic waterbath
8) control PH using buffer

3.1 Mode of action of enzymes 4.2 Movement of substances into

Errors in enzyme experiments and out of cells
1) counting bubbles is inaccurate due to different Effect of changing surface area to volume ratio on
bubble sizes, bubbles being too fast to count,
diffusion using agar blocks of different sizes
bubbles too small so they’re missed
2) temperature or pH not constant (if they aren’t the Errors
factors being investigated) 1) difficulty in cutting the agar squares into equal
3) difficulty in judging endpoint e.g., with renin the dimensions
coagulation may not be clear 2) difficulty in judging the colour change
3) agar is not of equal depth
Improvements in enzyme experiments 4) pigmentation of the agar is not even
5) agar may be damaged during cutting
1) use a gas syringe to measure volume of gas instead
of counting bubbles
2) control temperature using a thermostatically Improvements
controlled water bath 1) use moulds for preparing agar squares with equal
3) control pH by using buffers dimension
4) use a colourimeter/put a white tile or card to judge 2) use a different indicator with clearer endpoint
colour change
2 www.alevel-notes.weebly.com
3) use wider or narrower range of concentrations (in Decide how the independent variable should be
case of determining unknown) changed within a suitable range to provide accurate
4) place a white card below the beaker for better results.
judgment of colors
• concentration – simple/serial dilution
Effects of immersing plant tissues in solutions of • temperature – water bath/freezer
different water potentials • pH – use buffers
• moving air – fan
How to keep fair comparison in osmosis experiments?
• humidity – plastic bag/calcium hydroxide
• when using petri dishes, use petri dishes of the sme
size
• when using sucrose or any other solution, put the Decide the number of values at which
same volume in each dish measurements are recorded
• when using onion epidermis or potato, must be of • a minimum of 5 measurements,
the same size • replicates or more measurements around a specific
• leave the potato or onion epidermis in the solution value
for the same period of time
Appropriate controls
General precautions in osmosis experiments
• replacing a solution with the same volume of water
• cover the petri dishes during the experiment to avoid
evaporation of water which can affect concentration • denaturing an enzyme by boiling
of the solution
• when using droppers or syringes, use separate Decide which variables to standardise
droppers for each solution or wash and dry after • volume (must be suitable for apparatus)
each step
• concentration
• while preparing a slide of onion epidermal cells,
• temperature
lower the coverslip gently to avoid any air bubbles
• pH
Errors • biological material (e.g., same species, age, storage
conditions, time of year, mass)
1) time tissues left in solutions is not enough to • humidity (plastic bag/Ca(OH)2)
observe complete plasmolysis
• apparatus
2) evaporation of solution may take place and therefore
change the concentration
3) difficulty in judging degree of plasmolysis Measuring area
4) difficulty in cutting the samples into correct
• count areas covering half or more of a grid square as
dimensions
one whole square
5) parallex error may occur during reading the lengths
• don’t count areas less than half of a square

Improvements
1) leave for longer time in the solutions Constructing a table
2) prepare more concentrations with narrower/wider • the first column should be the independent variable
range (give examples and method of dilution) followed by the dependent variable
3) cover the petri dish to avoid evaporation of solution • the heading should have the units
4) prepare more than one sample in each solution • units should be in “standard form” i.e., not mol/dm3
5) ensure that the tissues are completely immersed in instead mol dm-3
the solutions • the slash (/) should only be used to separate the
heading from the unit e.g., concentration / mol dm-3
4.2.2 Expectations for each mark • all the figures in the table should be the same
number of significant figures or decimal places
category (Paper 3)
independent variable / unit dependent variable / unit
Making decisions about measurements or
observations

3 www.alevel-notes.weebly.com
Line graphs

Rate of reaction
• 1/t or 1000/t
• or the gradient of a line
• each axis should be scaled using multiples of 1, 2, 5, • the unit used is s-1
10 for each 20 mm square on the grid
• never use multiples of 3
• do not extrapolate unless they’ve asked you to Types of errors
a) systematic errors – from apparatus, doesn’t affect the
Presenting anomalous results
trend in results
Put a circle on the graph away from the line and put a key b) random errors – e.g., from variability of biological
to state that the circled point(s) represents ana material, may affect trend and accuracy
anomalous result
Calculating error
How to present the curve
smallest division
• straight line – e.g., effect of enzyme concentration error =
on rate of enzyme catalysed reaction, if obvious that 2
points lie on a straight line
• smooth curve – only if intermediate values fall on the error
% error =
curve measurement
• single results – draw straight lines between points;
this indicates uncertainty about the result for the
values of independent variable • when 2 readings are taken (e.g., syringe, burette,
change in something), the error is multiplied by 2.

Bar graphs
• bars should not be shaded
Suggesting improvements to a
• they need to be clearly labelled procedure/modification
• y-axis usually starts from 0 Improvements
• all bars should be of equal thickness
1) standardise relevant variables
• leave equal spacing between bars (including a space
between the first bar) 2) use a measurement method for the dependent
variable which is more accurate
3) collect more data by taking replicates to obtain
mean

Modifications
• if the bar chart is for two classes, make the bars of 1) change a different independent variable
the 2 classes attached to each other but leave even 2) standardise all other variables including a previously
spacing between intervals used independent variable
3) use an improved method to obtain accurate and
precise results for dependent variable

Taking measurements from photographs

Measure in mm not cm.
Histograms
Bars should be attached to each other, have the same
thickness and the x-axis should have intervals e.g., 12-14,
14-16 and not categories.

4 www.alevel-notes.weebly.com
Plan diagrams Artery
• show outlines only
• make proportions of tissues/cells in the diagram the
same as the observed section

High power drawings (of cells)

Plant cell walls should be shown as double lines with a
middle lamella between cells

Vein

Show any details of content of cells; draw what you see

not what you know should be present.
• use a sharp pencil
• no overlapping lines
• no shaky lines
• never use a ruler
• don’t press too hard on the pencil as this makes it
difficult to erase in case of mistakes
Blood smear
Disclaimer: The following examples of plan diagrams are
generalised drawings. In the actual question, the drawing
that should be drawn may differ, and these are examples
for your assistance when attempting such questions.
Stages of mitosis

Pseudo-stratified columnar ciliated epithelium

cells

Bronchus

Alveoli

5 www.alevel-notes.weebly.com
Trachea Longitudinal section of a root

Bronchiole

What to avoid when drawing plan diagrams

Xerophyte leaves

6 www.alevel-notes.weebly.com
Transverse sections of plants

7 www.alevel-notes.weebly.com
IG@thebiologyjotterbook

Contributed by Ruifang for The Biology Jotter Book

 Qualification
Accredited

A LEVEL

Teacher guide
BIOLOGY A
BIOLOGY B
(ADVANCING BIOLOGY)
H020, H420, H022, H422
For first teaching in 2015

Biological Drawing
Version 2

www.ocr.org.uk/biology
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

CONTENTS
Introduction to biological drawing 3

Guidance for biological drawing 4

Drawing from a microscope slide 8

Teacher resource 1 – common errors activity 15

Student Resource 1 – Drawings, graphs and tables checklists 16

Would you prefer a

Word version?
Did you know that you can save this pdf
as a Word file using Acrobat Professional?
Simply click on File > Save As Other . . .
and select Microsoft Word
(If you have opened this PDF in your browser you will need
to save it first. Simply right click anywhere on the page and
select Save as . . . to save the PDF. Then open the PDF in
Acrobat Professional.)
We value your feedback
If you do not have access to Acrobat Professional there We’d like to know your view on the resources
we produce. By clicking on the icon above
are a number of free applications available that will also you will help us to ensure that our resources
convert Word to PDF (search for pdf to word converter). work for you.

2 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

INTRODUCTION TO
BIOLOGICAL DRAWING
This biological drawing skills handbook has been developed to
support GCE Biology A H020/H420 and GCE Biology B H022/
H422.

Why bother?
The ability to draw, label and annotate biological specimens
is an important and useful biological skill. These days students
may well challenge the need for making biological drawings,
particularly given the ease of using digital photography for
record-keeping. So how can it be justified? The following points
help to provide a rationale for developing biological drawing
skills:

• Accurate observation and attention to detail is

encouraged. Having to draw a biological specimen not
only increases the amount of time spent examining the
specimen, which in itself will aid learning, but requires a
much greater level of accurate observation than a casual
examination.
• Active recording aids memory. The educational
philosophy behind this is neatly summarised in the
well-known Chinese proverb:
I hear and I forget
I see and I remember
I do and I understand
Confucius
• The drawing provides a permanent record of what
has been observed. There is a historic tradition within
biology of providing accurate records of specimens so that
the images could be used for future reference purposes.
Today’s taxonomists are often indebted to the illustrators Figure 1: Antique botanical illustration of
of the 17th and 18th centuries, particularly where the Limon vulgaris (lemon tree), including detail
‘type’ (reference) specimen may only exist as an illustration. of flowers and fruit.
Even today, when digital photography can be used to
store images, artists are still often commissioned to record
biological specimens of interest by drawing or painting.
This is particularly true for flowering plants. This is partly
because all the features of interest can be combined in
one or several scientifically accurate, but aesthetic, images
with great clarity (see Figure 1).

3 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

GUIDANCE FOR
BIOLOGICAL DRAWING
What equipment is needed?
• Sharp pencil - HB is generally preferred, but H, 2H or B (for • Make the drawing large enough. If the specimen is a
emphasis) can all be used according to preference. relatively large structure such as a plant or a section of
• Pencil sharpener - A nail file may also be useful to keep an organ, it should normally occupy more than half the
the point really sharp. available space on the page. In microscopy, individual
cells drawn at high power should be about one to several
• Eraser
centimetres in diameter.
• Ruler - For label lines.
• Correct mistakes. If you make a mistake, use a good
• Plain paper quality eraser to rub out the lines completely.
• Include a title. Include a title stating what the specimen is.
General Principles • Include a scale. Include a scale if relevant (see Labelling
below). If you are drawing from a microscope, it is useful
When assessing biological drawing, marks are awarded for to state the combined magnification of the eyepiece plus
both quality of drawing and labelling. The latter may include objective lenses used when making the drawing, e.g. x100
annotation. The general principles described below apply to (low power) or x400 (high power). Note, though, that this
all types of biological drawing: is not the same as recording the scale.

• Use a sharp pencil only. Don’t use pens or coloured

pencils. Labelling
• Use clear, continuous lines. A line which encloses a When labelling biological drawings, follow the guidance below:
shape, such as a circle, should join up neatly without
obvious overlap. Overlapping lines is a common error in • Use a sharp pencil.
hastily drawn sketches and is easily spotted and penalised • Label all relevant structures, including all tissues in the
by examiners. case of microscopy.
• Don’t use any form of shading. This includes stippling, • Use a ruler for label lines and scale bars.
cross-hatching and shading. Students find this is a hard
• Label lines should start exactly at the structure being
instruction to follow, and it is sometimes difficult to justify.
labelled; don’t use arrowheads.
Although shading may help to make the drawing look
more realistic and/or to discriminate between areas of the • Arrange label lines neatly and make sure they don’t
specimen, it does not represent a permanent structural cross over each other. It is visually attractive, though not
feature. Artistic impression is certainly not what is required. essential, if the length of the label lines is adjusted so
that the actual labels are right or left justified, i.e. line up
• Accuracy is paramount. It shows good observation.
vertically above each other on either side of the drawing.
Remember that observation is assisted by understanding,
so a good knowledge of theory goes alongside good • Labels should be written horizontally, as in a textbook, not
drawing. Pay particular attention to the outlines of written at the same angle as the label line.
structures and to the relative proportions of different parts • As previously mentioned, a title, stating what the
of the specimen. Don’t draw what you think you should specimen is, should be added at the top or bottom of the
see, for example text book style drawings. Draw what you drawing.
observe. • Add a scale bar immediately below the drawing if
• Guidelines can help. Faint sketching of the main areas of necessary (see below).
the specimen which can later be erased may help. Some
students find a simple grid helps them.
• Magnification and illumination. To help in the
drawing process it is often useful to use a hand lens or a
magnifying glass for larger specimens and, for microscopy,
both low and high power lenses when making preliminary
observations. Field biologists usually carry a hand lens
as standard equipment. Dissection, and drawing from a
dissection, is greatly aided by good illumination of the
specimen by a lamp and by a tripod lens placed over the
material where possible.

4 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Annotating Unfamiliar specimens

Annotation adds concise notes about the structures labelled As stated above, the same basic principles of drawing
on a biological drawing. It is often used to draw attention to technique apply to all drawings and specimens. Nevertheless,
features of particular biological interest, either structural (such it can be daunting for a student if they are asked to draw
as shape, size, colour, hairiness) or functional. something they have not seen before or in a new situation, for
example a plant growing in a field, a fungal colony growing on
See Figure 3, 4e, 4h and 5e for examples of annotation in this an agar plate or an unfamiliar slide. Assessment questions will
booklet. always be phrased so that it is clear exactly what is required
and any relevant information the student is not expected to
know will be provided. The important thing to remember is
Scale and magnification to follow instructions carefully and to observe and draw the
actual specimen and not try to guess what should be visible.
It is useful to give an indication of the scale/magnification For example, roots should not be drawn on a plant growing in
of a drawing, particularly for large specimens drawn without the field if they are not visible.
the aid of a microscope. The actual size of a plant or leaf, for
example, may be impossible to judge simply from a drawing. Specimens should be studied carefully before any drawing is
For drawings made using microscopes, if the actual scale or undertaken, noting particularly where the outlines of structures
magnification is not given, it may be useful simply to indicate are going to be delimited in the final drawing. Depending on
whether a low or high power lens was used, preferably the the subject, separate, more detailed drawings may be useful to
actual magnification achieved by the combined eyepiece and highlight features of particular biological interest.
objective lens, usually just below the title.
The following figures are good biological drawings. Figure 2
Calculating scale/magnification of a drawing shows a drawing made from a heart dissection and Figure 3
shows two flowers during a fieldwork exercise.
Scale, or magnification, is simply how much bigger or smaller
the drawing is compared with the actual specimen. Calculate
as follows:
1. Measure between two appropriate points of the drawing
(e.g. total length or width).
2. Measure between the same two points of the specimen.
3. Divide measurement 1 by measurement 2.

5 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 2: Drawing of the base of the aorta showing the aortic (semilunar) valve through which blood leaves the left
ventricle of a mammalian heart. (Note the fibrous swelling at the middle of the cusps may not be present in some
mammalian hearts.) This is a good biological drawing, fully labelled, and clearly showing detail from the dissection,
although care should be taken to ensure lines do not overlap or are left incomplete. Also, a scale bar is not present.

6 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 3: The difference in arrangement of the sepals in two species of buttercup, Ranunculus bulbosus and R. repens.
Again, this is a good biological drawing, showing specific details of the flowers and labelling them accordingly.
However, care should be taken to ensure lines do not overlap or are left incomplete. Also, a scale bar is not present.

7 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

DRAWING FROM A
MICROSCOPE SLIDE

Low power drawings Examples

The purpose of a low power drawing is usually to show the Figures 4a-i show photomicrographs and low and high
distribution of the main tissues within an organ, for example power drawings of a section of mouse pancreas. Two
in a transverse section of a stem or a trachea. Students are versions of each drawing are shown, one based on tracing
required only to identify the tissues and to delimit the different the photomicrograph and one an example of an acceptable
tissues with boundary lines. No individual cells should be drawing of the same structure/cells completed by an able
drawn. There should be no mysterious gaps between tissues. student. Students are not expected to produce facsimiles
The temptation is to try to make the drawing look like the of what they observe, but drawings should show an
specimen, hence the tendency to fill spaces with cells. The final understanding, realistic proportions and recognition of key
drawing is basically a map – accurate details of the cells can features.
only be revealed at high power.
Figures 5a - e show photomicrographs and low and high
Follow these guidelines: power drawings of transverse sections of leaves of beech
(Fagus) from sunny and shaded conditions. A student would
• Identify the different tissues, using high power to help if
not be expected to have seen sections of this leaf before and
necessary
would be given sufficient information to make the drawings
• Draw all tissues and completely enclose each tissue by based on knowledge of the specification content.
lines
• Don’t draw individual cells
• Accuracy is important – the specimen will not necessarily
look like a textbook drawing. For example, vascular
bundles in a stem may vary in size and shape.
• A representative portion may be drawn if the structure is
symmetrical, e.g. a wedge or half of a transverse section
of root or stem, or in the case of a leaf, half a midrib and a
small portion of the adjacent lamina.

High power drawings

The purpose of high power drawings is to show as much
accurate detail as microscopy will allow. It is important to
realise that the high power and low power drawings are
complementary – neither on its own looks like the whole
specimen being viewed, but the combination would allow
someone to reconstruct the structure being drawn. As with low
power drawings, students often fall into the trap of wanting
the drawing to ‘look like what they see down the microscope’
and draw a lot of cells, none accurately.

• Draw only a few representative adjacent cells (assessment

questions will usually give specific instructions about what
exactly is required.) If all the cells are similar, then three
cells is often sufficient to show both cell structure and the
way in which cells are arranged in relation to each other.
In such a case, detail of only one cell may be needed, with
outlines only of adjacent cells just to show their relative
positions.
• Don’t shade in nuclei – just draw the outline. Similarly with
nucleoli.

8 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 4a: Photomicrograph of part of a section of the pancreas of a mouse taken at low power.

Figure 4b: Low power plan trace of one lobule from Figure 4c: Low power plan of the same lobule as
the pancreas shown in Figure 4a showing an islet of in Figure 4b but drawn by a student. This is a good
Langerhans and one acinus. attempt at drawing the lobule shown in Fig 4a,
although some lines are thicker than others.

9 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 4d: High power photomicrograph of the pancreas shown in Figure 4a. The acinus drawn in Figures 4e and 4f is outlined.

Figure 4e: High power drawing of the acinus Figure 4f: High power drawing of the acinus outlined
outlined in Figure 4d, obtained by tracing and in Figure 4d, drawn from the slide by a student. This is
fully labelled. a good attempt at drawing the acinus from Figure 4d,
although there are some overlapping lines.

10 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 4g: High power photomicrograph of an islet of

Langerhans from the pancreas shown in Figure 4a.
The chain of four cells drawn in Figures 4h and 4i is
outlined.

Figure 4h: High power drawing of the chain of four cells outlined in Figure 4g, obtained by tracing.

Figure 4i: High power drawing of the chain of four cells outlined in Figure 4g, drawn from the slide by a student. Again, this is a
good attempt at the drawing but this student needs to be careful they do not overlap lines.

11 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 5a: Photomicrograph of a transverse section of the lamina of a shade leaf of beech (Fagus) taken at low power.

Figure 5b: This is a low power plan of the beech leaf section shown in Figure 5a drawn by a student. The student has correctly
drawn and labelled the different tissues, rather than drawn individual cells.

12 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 5c: Photomicrograph of a transverse section of the lamina of a sun leaf of beech taken at low power.

Figure 5d: This is a low power plan of the beech leaf section shown in Figure 5c drawn by a student. Again the student has
correctly drawn and labelled the different tissues, rather than drawing individual cells.

13 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Figure 5e: This is a student drawing at high power detailing cells in a transverse section of the lamina of a shade
leaf of beech (a different part of the same leaf shown in Figure 5a). The student has correctly included a title and
scale bar. The student has labelled the drawing and there is good use of annotation. The drawing itself is detailed
and clear. (Note: the cell walls of all the plant cells have been drawn; this is because they were visible with the
microscope and slide used. It is not always possible to see this much detail using a classroom light microscope).

14 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

TEACHER RESOURCE 1 –
COMMON ERRORS ACTIVITY
Figure 1 below shows a drawing of a transverse section of Helianthus stem at low power. The left
hand half of the drawing shows some common errors that are avoided in the right hand half. Students
could be asked to try to spot the errors.

Left-hand side Right-hand side

Figure 1: Transverse section of a young Helianthus stem showing some common drawing errors in
the left-hand half of the drawing. The right-hand half shows examples of good technique.

15 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

LEARNER CHECKLISTS –
GRAPHS, TABLES AND DRAWINGS

Instructions for teachers

Introduction
In line with the new DfE subject criteria for GCE Biology qualifications (available here: https://www.gov.uk/government/publications/
gce-as-and-a-level-for-science), a number of practical skills will be assessed as part of the Practical Endorsement (directly-assessable
practical skills) and within the examinations (indirectly-assessable practical skills). This includes presenting data and observations in
graphs, tables and drawings.

All the practical skills that must be covered as part of the teaching and learning within the new Biology qualifications can be found
in Module 1 of the new OCR Biology specifications:

Biology A – H020, H420,

http://www.ocr.org.uk/qualifications/as-a-level-gce-biology-a-h020-h420-from-2015/

Biology B (Advancing Biology) – H022, H422,

http://www.ocr.org.uk/qualifications/as-a-level-gce-biology-b-advancing-biology-h022-h422-from-2015/

The following pages contain three optional checklists that can be given to learners to help self-evaluate their graphs, tables and
drawings.

16 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Drawings
The following practical Learning Outcomes relate to biological drawing:
Module 1: Development of practical skills in biology (Biology A and Biology B),
1.1.2(c) presenting observations and data in an appropriate format
1.2.1(f ) present information and data in a scientific way (Practical Endorsement)
1.2.2(e) production of scientific drawings from observations with annotations (Practical Endorsement).
Drawing skills are also part of many of the Learning Outcomes throughout the biological content e.g.:
2.1.1(d), 3.1.1(g), 3.1.2(e)(ii), 3.1.3(b)(ii), 4.1.1(e)(ii), 5.1.2(b)(ii), 5.1.2(c)(ii), 5.1.2(c)(iii), 5.1.4(c)(ii) (Biology A).
2.1.1(c)(ii), 2.2.1(b)(ii), 2.2.4(c)(i), 3.1.1(b)(ii) (Biology B).

Here is a checklist you can use for your drawings,

1 Your drawing and its label lines must be done with a really sharp pencil
(not a pen).
2 Your drawing should take up at least half the page / space available.

3 Lines need to be clear and continuous – not ragged or broken – and no

shading or colouring is allowed.
4 Ensure the proportions are correct, i.e. different areas are the right size
relative to each other, and that your drawing is a true likeness of the
specimen that you are drawing.
5 Label all the different areas of tissue that you have shown, writing the
words in pencil or pen.
6 Rule the label lines (in pencil). Don’t let the label lines cross each other and
do not write on the label lines.
7 Make sure the label lines touch the part you are labelling.

8 Annotations - add concise notes about the structures/features labelled on

your drawing.
9 Include a scale - add a scale bar immediately below the drawing if
necessary.
10 Include a title stating what the specimen is.

LOW POWER TISSUE PLAN

Remember: A low power tissue plan defines the extent of areas of different tissues but does NOT
show any individual cells.

17 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Graphs
The following practical Learning Outcomes relate to graph drawing:
Module 1: Development of practical skills in biology (Biology A and Biology B),
1.1.2(c) presenting observations and data in an appropriate format
1.1.3(d) plotting and interpreting suitable graphs from experimental results, including:
(i) selection and labelling of axes with appropriate scales, quantities and units
(ii) measurement of gradients and intercepts.
1.2.1(f ) present information and data in a scientific way (Practical Endorsement).
Graphs must also be covered under the biology mathematical skills requirements,
See maths skills M1.3, M1.7, M3.1, M3.2, M3.3, M3.4, M3.5, M3.6.

Here is a checklist you can use for your graphs,

S Size of the graph: does the bit with actual plotted points in take up at least
half the paper?
P Plotting: is every data point within half a little square of where it should be?

L Line of best fit: if there’s a trend in your data, is it indicated with a smooth
curve or straight line?
A Axes right way round: the thing you changed (independent variable) along
the bottom; the thing you measured (dependent variable) up the side.
T Title: have you included a title that tells you what this graph shows?

A Axis labels: name of each variable with the right unit symbol.

18 © OCR 2019
A Level Biology and A Level Biology B (Advancing Biology) Biological drawing

Tables
The following practical Learning Outcomes relate to tables:
Module 1: Development of practical skills in biology (Biology A and Biology B),
1.1.2(c) presenting observations and data in an appropriate format
1.2.1(d) make and record observations/ measurements (Practical Endorsement)
1.2.1(f ) present information and data in a scientific way (Practical Endorsement).
Tables must also be covered under the biology mathematical skills requirements,
See maths skills M1.3, M3.1.

Here is a checklist you can use for your tables,

1 All raw data in a single table with ruled lines and border.
2 Independent variable (IV) in the first column; dependent variable (DV)
in columns to the right (for quantitative observations) OR descriptive
comments in columns to the right (for qualitative observations).
3 Processed data (e.g. means, rates, standard deviations) in columns to the far
right.
4 No calculations in the table, only calculated values.

5 Each column headed with informative description (for qualitative data) or

physical quantity and correct units (for quantitative data); units separated
from physical quantity using either brackets or a solidus (slash).
6 No units in the body of the table, only in the column headings.

7 Raw data recorded to a number of decimal places appropriate to the

resolution of the measuring equipment.
8 All raw data of the same type recorded to the same number of decimal
places.
9 Processed data recorded to up to one significant figure more than the raw
data.

19 © OCR 2019
Did you know?
OCR Resources: the small print Need to get in touch?
OCR’s resources are provided to support the delivery of OCR
qualifications, but in no way constitute an endorsed teaching
If you ever have any questions about
method that is required by OCR. Whilst every effort is made OCR qualifications or services (including
to ensure the accuracy of the content, OCR cannot be held administration, logistics and teaching)
responsible for any errors or omissions within these resources.
We update our resources on a regular basis, so please check the
please feel free to get in touch with our
OCR website to ensure you have the most up to date version. Customer Support Centre.
This resource may be freely copied and distributed, as long as
the OCR logo and this small print remain intact and OCR is
General qualifications
acknowledged as the originator of this work. Telephone 01223 553998
Facsimile 01223 552627
Our documents are updated over time. Whilst every effort is made
to check all documents, there may be contradictions between Email general.qualifications@ocr.org.uk
published support and the specification, therefore please use the
information on the latest specification at all times. Where changes
are made to specifications these will be indicated within the
www.ocr.org.uk
document, there will be a new version number indicated, and a
summary of the changes. If you do notice a discrepancy between
the specification and a resource please contact us at:
resources.feedback@ocr.org.uk.

OCR acknowledges the use of the following content: N/A

Whether you already offer OCR qualifications, are new to OCR, or

are considering switching from your current provider/awarding
organisation, you can request more information by completing the
Expression of Interest form which can be found here:
www.ocr.org.uk/expression-of-interest

Please get in touch if you want to discuss the accessibility of

resources we offer to support delivery of our qualifications:
resources.feedback@ocr.org.uk

Looking for a resource?

There is now a quick and easy search tool to help find free resources
for your qualification:
www.ocr.org.uk/i-want-to/find-resources/

OCR is part of Cambridge Assessment, a department of the University of

Cambridge. For staff training purposes and as part of our quality assurance
programme your call may be recorded or monitored.

© OCR 2019 Oxford Cambridge and RSA Examinations is a Company

Limited by Guarantee. Registered in England. Registered office
The Triangle Building, Shaftesbury Road, Cambridge, CB2 8EA.
Registered company number 3484466. OCR is an exempt charity.
 REVISION’S SHEET
ERRRORS AND IMPROVEMENTS:

A) Qualitative food test:

ERRORS:

• Difficulty in judging colours especially if the

concentrations are low resulting in light colours
• If heating, then temperature is not constant, as there is
heat loss to the surrounding
• Different molecules in the same solution,may interfere
with the colours
Improvements:

• In case of heating,Thermostatic water bath to control

the temperature and avoid the heat loss to the
surrounding
• Colour standard to help judging the colour
• Use black card/ white card to help judging colours

B) Quantitative food test:

ERRORS:

• In case of determining unknown concentration: it can

be between 2 known concentrations
• Difficulty in comparing colours
• Temperature varies due to heat loss to the
surrounding
 • In case of vitamin c :Drops fall on the walls of the test
tube
Improvements:

• Use more concentrations with narrower range(

smaller gaps between concentrations) or wider range(
wider gaps between the concentrations), you can
then plot a graph with the obtained results and find
your unknown by reading off the graph, this provides
more accurate estimate for the unknown
• Use colourimeter or colour standards to help judge
the colour better
• Use black card or white card to enable you from
observing the colour difference
• Use thermostatic water bath to avoid heat loss to the
surrounding
• In case of introducing drops , use wider test tube
• Use graduated pipette for more accurate volumes
than the syringe
• Repeat and take average
How to make Benedicts test more reliable
• Use the same volume of the substance under test.
• Use the same volume of the reagent.
• Leave the tubes in the same water bath for the same period
of time.

C) OSMOSIS

How to keep fair comparison in osmosis experiments?

1-When using petri dishes , use petri dishes of the sme
size.
2- When using sucrose or any other solution , put the
same volume in each dish.
3- When using onion epidermis or potato, must be of the
same size.
4- Leave the potato or onion epidermis in the solution for
the same period of time.

General precautions in osmosis experiments:

1- Cover the petri dishes during the experiment to void

evaporation of water which cn affect concentration of the
solution.
2- When using droppers or syringes , use separate
dropper for each solution or wash and dry it after each
step.
3- During preparing slide of onion epidermal cells ,lower
the cover-slip gently to avoid trapping of any air bubbles.

ERRORS:

• Time tissues left in solutions is not enough to

observe complete plasmolysis
• Evaporation of solution may take place and therefore
change the concentration
• Difficulty in judging degree of plasmolysis
• Difficulty in cutting the samples into correct
dimensions
• Parallex error may occur during reading the lengths

IMPROVEMENTS:
• Leave for longer time in the solutions
 • Prepare more concentrations with narrower / wider
range (give examples and method of dilution)
• Cover the petri dish to avoid evaporation of solution
• Prepare more than one sample in each solution
• Ensure that the tissues are completely immersed in
the solutions

D) ENZYME REACTIONS:

ERRORS:
• Difficulty in judging the endpoint (in amylase, the
colour with iodine is not clear, in renin the
coagulation is not clear, colour of litmus paper with
urease is not clear)
• Temperature/PH is not controlled (if they are not the
factors under investigation)
• Bubbles of different sizes and are all counted as
one,bubbles may be too fast for counting, gas
expansion interfere with the results, small bubbles
may be missed as not seen (in case of counting
bubbles with catalyse (potato)or with yeast)
• Paper may stick to the walls and bottom of the test
tube ( in case of litmus paper with urease, or paper
dipped in catalyse then placed in hydrogen peroxide
or paper dipped in iodine then placed in amylase
using splint)

IMPROVEMENTS:
 • Use colorimeter instead of judging colors or use
black or white card against the tube for better color
comparison
• Measure the volume of gas using gas syringe
instead of counting bubbles
• Use wider test tubes( in case of beads or paper)
• Control temperature using thermostatic water bath
• Control PH using buffer

Effect of heavy metals such as copper sulphate and lead

nitrate
➢ Can act as inhibitors.
➢ May cause protein to clot or coagulate.
➢ May denature proteins.
➢ May breakdown bonds so that can alter tertiary and quaternary
structures.

E) AGAR:

ERRORS
• Difficulty in cutting the agar squares into equal
dimensions
• Difficulty in judging the color change
• Agar is not of equal depth
• Pigmentation of the agar is not even
• Agar may be damaged during cutting

Improvements
• Use moulds for preparing agar squares with equal
dimension
• Use different indicator with clearer end point
 • Use wider or narrower range of concentrations (In
case of determining unknown)
• Use black card or white card below the beakers for
better judgment of colors

F) IMMOBILIZTION OF ENZYME (BEADS FORMATION)

ERRORS
• Beads are not of equal sizes
• Beads stuck to the sides of the tube and to each
others
• Forceps causes damage the the beads
• Difficulty to introduce drops using syringe
• Test tube is not vertical and also test tubes are not of
equal sizes (in case of catalyse beads and hydrogen
peroxide)
• Temperature/PH is not controlled ( if they are not the
factors under investigation)

IMPROVEMENTS
• Use sieve with equal size holes to produce beads
with equal diameter
• Use wider test tubes
• Stain the beads for clearer movement
• Use resort stand to make the test tube vertical, use
test tubes with equal sizes (in case of catalyse beads
and hydrogen peroxide)
• Use spatula or spoon instead of forceps
• Control temperature using thermostatic water bath/
control PH using buffer

G)COLOR DIFFUSION
 Diffusion of pigment from inside the tissue
(potato/beetroot) to the solution by the influence of salt or
temperature( due to the damge of cell membrane/cell wall)

ERRORS
• Difficulty in judging intensity of diffused color
• Some pigments remain on the surface of the tissue
despite washing
• Temperature is not controlled
• Time is not enough for the diffusion to take place
• There is narrow range of concentrations

IMPROVEMENTS
• Use color standards with degree of color and its name
for better colour describtion , or numbered colour
scale with images showing the color
• Use white/black card against the test tubes for
comparing colors
• Control the temperature using thermostatic water bath
• Dry the samples with paper towel before placing them
in the solutions
• Leave the samples for longer time in solutions
• Use wider range of concentration so the colors
produced show difference

H) Density of solution

(a method for determining the concentration)

In this method you pigment the unknown concentration by
adding 2 drops of methylene blue to 5 cm3 of the
unknown in a petri dish, u then take a drop of the
pigmented solution and introduce it to test tubes of
 prepared known concentration, the drop produced will
move according to its concentration:
• If the drop has higher concentration than the
solution, it will move down
• If the drop has lower concentration than the solution,
it will move up
• If the drop has same concentration as the solution , it
stays

ERRORS
• Difficulty in introducing drops by syringe
• The pigmentation is too dark/ or too light
• The drop ruptures (it releases a flow which may be
not clear)
• Too wide/narrow range of concentrations
• Test tube is not vertical

IMPROVEMENTS
• Use dropper instead of syringe
• Increase or decrease the drops of the stain ( to make
the pigment lighter or darker) or use different stain
• Prepare wider/narrower range of concentration
• Use resort stand to make the test tube vertical

I) VISKING TUBES
Take care to open it using water

ERRORS
• Difficulty in opening and tying the visking tube
• Visking tubes is too short /or too long
• Difficulty in rinsing the visking tube ( water may enter
inside it)
• Mixing is not constant
• Drops may not of equal volume
 • Difficulty in judging color

IMPROVEMENTS
• Make the visking tube longer or shorter
• Dry the visking tube with paper towel before placing in
the beaker
• Mix using electronic mixer
• Use syringe instead of dropper to introduce equal
volume of solution and indicator
• Use color standard/ or colorimeter

TYPES OF VARIABLES:

First independent variable

- The variable that affects the results by the experiment for
example
▪ In an experiment to investigate the effect of light intensity on
the rate of photosynthesis, light intensity is the independent
variable.
▪ In an experiment to investigate the effect of change in pH on
the activity of amylase, pH is the independent variable.

- It is represented on the X- axis of the graph.

Second Dependent variable

- It is the results obtained in the investigation, for example
▪ In an experiment to investigate the effect of light intensity on
the rate of production of oxygen, the volume of oxygen
produced per unit time is dependent variable.
▪ In an experiment to investigate the effect of change in pH on
the activity of amylase by measuring the time needed for
complete breakdown of a certain volume of starch, time is
the dependent variable.
- It is represented on the Y- axis of the graph.

TYPES OF CHARTS AND GRAPHS

A) Smoth curve
• You will always join all your points either by ruler or
hand
• If there is trials and means , you will always plot the
mean only, except if you were asked to plot trial 1
and trial 2 you will use a key and produce the
following figure

B) Histogram:
in the histogram all the bars will be attached to each
others, and will be of same thickness, your x axis will be
inervals (e.g 12-14, 14-16) rather than categories or
numbers
C) Bar chart
If he need a bar or histogram , he will ask you to plot a
chart, if he needs a smooth curve or line graph he willask
you to plot a graph
• In the bar chart, your x axis will be letters or
categories rather than numbers
• All the bars should be of equal thickness
• You will leave equal spacing between the bars , this
includes the space before the first bar as the below
figure (b):
 N.B: the bar chart may for 2 classes e.g animal a and
animal b , in that case you will make the bars of the 2
classes (a and b) attached to each others but leave even
spacing between the intervals as the below figure shows:

INDICATORS:
1-Effects of litmus.
➢ In acidic medium its colour is red.
➢ In an alkaline medium its colour is blue.

2-Universal indicator
It is a pH indicator.

pH Descripti colour
on
0-3 Strong Red
acid
3-6 Acid Orange/yel
 low
7 Neutral Green
8-11 Alkaline Blue
11-14 Strong Violet /
alkali purple

3-Cobalt chloride paper

➢ It is used as an indicator for water.
➢ The dry cobalt chloride paper (anhydrous )has a blue
colour.
➢ In presence of water (when hydrated) its colour changes to
pink or mauve.

4-Hydrogen carbonate indicator.

➢ Its original colour is red .
➢ Increase in concentration of carbon dioxide makes it yellow (
increase in concentration of carbon dioxide makes the
medium acidic as carbon dioxide is acidic gas) .
➢ Decrease in concentration of carbon dioxide makes it purple
.
➢ ( removal carbon dioxide from the medium makes it
alkaline).

5-Methylene Blue: Oxygen Indicator

➢ methylene blue indicator is blue when oxygen is present
➢ If oxygen is removed from the solution, the blue color
disappears.

6-Bromothymol Blue pH Indicator

➢ Bromthymol blue changes color over a pH range from 6.0
(yellow) to 7.6 (blue)
➢ It is a good indicator of dissolved carbon dioxide (CO2) and
other weakly acidic solutions

7-Potassium permanganate
➢ It is a strong oxidizing agent. It dissolves in water to give
intensely pink or purple solutions
➢ Concentrated sulfuric acid reacts with potassium
permengnate forming acidic solution
➢ Acidic solutions of permanganate are reduced to the faintly
pink manganese(II)
alevel-notes.weebly.com
alevel-notes.weebly.com
alevel-notes.weebly.com
alevel-notes.weebly.com