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UPDATED TO 2015 SYLLABUS

AQA A LEVEL
BIOLOGY
SUMMARIZED NOTES ON THE THEORY SYLLABUS
AQA A LEVEL BIOLOGY

1. Biological molecules
1.1. Carbohydrates
Most carbohydrates are polymers made of long chains of
monosaccharide monomers
Monosaccharide: a molecule consisting of a single sugar
unit. It is the simplest form of carbohydrate and cannot Cellulose is found in the cell wall of plant cells and is
be hydrolysed further. It has a general formula of made from β - glucose units that form β -1,4 glycosidic
(CH2 O)n . bonds. Alternate β - glucose molecules are rotated 180
degrees in order to form these bonds. Hydrogen bonds
​ ​

Disaccharide: a sugar formed when two monosaccharides

are also formed between parallel cellulose molecules.
joined together by glycosidic bonds.
Maltose: glucose + glucose
Sucrose: glucose + fructose
Lactose: glucose + galactose
Polysaccharide: a carbohydrate which contains many
monosaccharides bonded together by glycosidic bonds.
Glycosidic bonds: covalent bonds that form between
monosaccharides in a condensation reaction
These bonds can be broken by hydrolysis: e.g. Acid
hydrolysis of non-reducing sugars to retrieve constituent
monomers (sucrose)
There are two different kinds of glucose monomers
known as α- glucose and β – glucose and their difference
lies between the position of an –OH group in their ring
structures.

Use Benedict’s test to test for reducing sugars (all

monosaccharides and some disaccharides)
Add Benedict’s reagent and heat in a water bath
Positive: forms a coloured precipitate (bigger colour
α – glucose molecules are used in macromolecules that change indicates higher concentration of reducing
store energy, e.g. glycogen and starch sugar)
β – glucose molecules are used for structural purposes, Negative: no colour change
e.g. cell walls.

Polysaccharides and their properties

Starch is a macromolecule that is found in plant cells and
is made up of two components known as amylose and
amylopectin. These components are polysaccharides that
are made from a glucose molecule and contain 1,4
glycosidic bonds. Amylopectin is branched in structure If test is negative, there still could be non-reducing sugars
and therefore also contains a- 1,6 glycosidic bonds as present
bonds form between adjacent a- glucose molecules. Hydrolyse any glycosidic bonds by heating with dilute
Amylose is helical in shape while amylopectin in hydrochloric acid
branched. Starch is highly compact and stores energy. Run Benedict’s test again
Glycogen is a macromolecule that is used for the storage If the test is positive: non-reducing sugars present
of energy is animal cells and is also made from α - glucose Iodine/potassium iodide test for starch:
molecules. The structure of glycogen is very similar to Add iodine dissolved in potassium iodide to sample
that of amylopectin; however, it is more branched and Positive: iodine changes from red-brown to blue-black
therefore contains more 1,6 glycosidic bonds. Negative: no change

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Monomers and polymers

Monomer: The simplest repeating unit of a polymer, e.g.
glucose.
Polymer: This is made from monomers joined together by
glycosidic bonds, e.g. starch,
Macromolecule: These are large and complex molecules
that are formed due to polymerisation of smaller
subunits, e.g. starch
Monosaccharides, nucleotides, and amino acids are
examples of monomers
A condensation reaction joins two molecules together to
form a chemical bond - involves the elimination of a
molecule of water.
A hydrolysis reaction breaks a chemical bond between
two molecules - involves the use of a water molecule.

Lipids
Triglycerides and phospholipids are two types of lipid
Triglycerides are made up of one molecule of glycerol Triglycerides are energy-storage molecules because
with three fatty acids attached to it The long hydrocarbon tails contain a lot of chemical
Fatty acid chains are long hydrocarbon chains with a energy
carboxylic group (-COOH) at one end. The glycerol is an They are insoluble so they don’t affect the water
alcohol containing 3 carbon atoms wherein each carbon potential of the cell
atom is attached to a hydroxyl (-OH) group The fatty acids are hydrophobic (water-repelling) so
A condensation reaction between glycerol and a fatty acid they clump together in droplets with the fatty acid
forms an ester bond (happens three times in a tails on the inside
triglyceride) Phospholipids make up the cell membrane bilayer
because
The phosphate group ‘heads’ are hydrophilic, and the
fatty acids are hydrophobic so they form a double
layer with the phosphates on the outside and the fatty
acids on the inside

A fatty acid can be saturated (all bonds in hydrocarbon

tail are single bonds) or unsaturated (some double bonds
in hydrocarbon tail)
Saturated fatty acids are more likely to be solid at room
temperature while unsaturated fatty acids are more likely
to be liquid
Phospholipids are like triglycerides but one of the fatty
acids is replaced by a hydrophilic (water- attracting)
phosphate group The centre of the bilayer is hydrophobic, so it acts as a
barrier to water-soluble substances
Emulsion test for lipids:
Shake the test substance with ethanol until it
dissolves then add to water
Positive: a milky emulsion forms (more emulsion
indicates more lipid present)
Negative: solution remains clear

Proteins

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Add a few drops of NaOH to make the solution

1.4. Protein structure
alkaline
Proteins are polymers made of long chains of amino acid Then add copper (II) sulfate solution
Positive: solution turns purple
monomers
Negative: solution remains blue
There are 20 types of amino acid but this is the general
formula: NH2 - CHR - COOH
Enzymes
Many proteins are enzymes
An enzyme is a biological catalyst that lowers the
activation energy of a metabolic reaction.
Activation energy is the energy required in any chemical
reaction to break the bonds in reactant molecules so that
new bonds are formed to make the product. An enzyme
The 20 amino acids only differ in their side chain (R-
lowers the activation energy required for the reaction.
group)
However, overall energy released during reaction is
Amino acids are joined together by peptide bonds that
maintained.
form in condensation reactions. Two amino acids joined
The effect that enzymes have on the rate of reactions can
together forms a dipeptide and many joined together
be measured in two ways:
forms a polypeptide
measuring rates of formation of products
measuring rates of decrease of substrate
By measuring the amount of product accumulated in a
period of time, the rate of the reaction can be
determined. Rate of reaction = volume of product
produced/ time. This method of measuring the effect of
an enzyme on the rate of reaction can be used with the
enzyme catalase.
By measuring the rate at which the reactants disappear
from the reaction mixture, the effect of the enzyme on
the rate of reaction can be determined. Eg: measuring the
rate at which starch disappears when the enzyme
A functional protein (e.g. an enzyme) may contain one or amylase is added.
more polypeptides Enzymes have specific active sites that are
Proteins have 4 ‘levels’ of structure: primary, secondary, complementary to the shape of the substrate. The
tertiary, and quaternary substrate is held in place at the active site by weak
Primary structure: the sequence of amino acids in the
hydrogen and ionic bonds. The combined structure is
polypeptide chain called the enzyme-substrate complex.
Secondary structure: hydrogen bonds (attractions Enzymes has two proposed modes of action known as
between the -NH and -CO groups on different side chains. lock-and-key theory and induced-fit theory.
The secondary structure is classified into 2 types: α- helix In the lock-and-key theory, the shape of the active site
(a coiled structure) and β- pleated sheets. is very precise and substrates that are not
Tertiary structure: the secondary structure is coiled and complementary to the shape of the active site cannot
folded further to form a 3D structure. Hydrogen bonds, bind. The enzyme-substrate complexes forms enable
ionic bonds (attractions between negative and positive the reaction to take place more easily.
charges on different amino acid side-chains), and
disulfide bridges form. Disulfide bridges are formed
between two cysteine amino acids when the sulfur in one
bonds to the sulfur in the other.
Quaternary structure (only in some proteins e.g.
haemoglobin, insulin, collagen): Several different
polypeptide chains are held together by bonds
Proteins have many functions in living organisms e.g.
enzymes, antibodies, transport proteins, and structural
proteins
In induced fit theory, the enzyme’s active site is not
initially an exact fit to the substrate molecule.
Testing for protiens However, the enzyme molecules are more flexible and
can change shape slightly as the substrate enters the
Biuret test for proteins:

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enzyme. This means that the enzyme molecule will group and the ribose sugar. Many of these forms a sugar-
undergo conformational changes as the substrate phosphate backbone
combines with enzyme’s active site, forming the
enzyme-substrate complex.

A DNA molecule is formed of two polynucleotide strands

held together by hydrogen bonds between specific
complementary base pairs
Adenine (A) bonds with Thymine (T) with two
hydrogen bonds
The induced fit model is a better theory than the lock and
Cytosine (C) bonds with Guanine (G) with three
key model
hydrogen bonds
Enzyme properties relate to their tertiary structure. They
An RNA molecule is generally shorter than a DNA
are very specific (i.e. will only catalyse one reaction)
molecule and it is made of one polynucleotide strand
because of the specific 3D shape of the active site
Due to the simplicity of DNA, many scientists doubted it
If the tertiary structure is altered in any way, the shape of
carried the genetic code
the active site will change so the substrate will no longer
fit causing the enzyme to become dysfunctional
The tertiary structure may be altered by pH or 1.5. DNA replication
temperature changes
The primary structure is determined by a gene so if there DNA replication is semi-conservative (one of the strands
is a mutation then the primary structure will change in each new molecule is from the original molecule)
which will also alter the tertiary structure 1. The DNA double helix unwinds
Enzymes catalyse many different intracellular and 2. The DNA helicase enzyme breaks the hydrogen
extracellular reactions and these determine structures bonds on the two polynucleotide strands (unzips
and functions from a cellular to a whole-organism level the strands)
3. Each original strand that is unwound acts as a
template strand for free DNA nucleotides to bind
Nucleic Acids to by complementary base pairing (A to T, C to G)
4. DNA polymerase joins the sugar-phosphate
DNA and RNA are both used to carry information. backbone of the new strand through
DNA stores genetic information in the nucleus of a cell condensation reactions
RNA transfers genetic information from the cell to 5. Each DNA molecule now contains one strand from
ribosomes in the cytoplasm for protein synthesis the original molecule and one strand from the
Ribosomes are formed from RNA and proteins new molecule
DNA and RNA are polymers made up of nucleotide One end of a DNA strand is called the 3’ (3-prime) end
monomers. Each nucleotide is formed from a pentose and the other is called the 5’ (5-prime) end. Two DNA
sugar, a nitrogen-containing organic base, and a strands are antiparallel: one runs from 3’ to 5’ and the
phosphate group other from the 5’ to 3’ end. Since the active site of DNA
polymerase is only complementary to the 3’ end, it will
move in opposite directions along the two new strands.

In a DNA nucleotide, the pentose sugar is deoxyribose,

and the bases are one of thymine (T), adenine (A),
guanine (G), or cytosine (C)
In an RNA nucleotide, the pentose sugar is ribose, and the
bases are one of uracil (U) , adenine (A) , guanine (G), or
cytosine (C)
Two nucleotides join via a condensation reaction that
forms a phosphodiester bond between the phosphate

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pH = -\log \[H^+\]
ATP Sodium ions (N a+ ) are used in co-transporters to
transport glucose and amino acids across cell
ATP stands for adenosine triphosphate membranes
It is a nucleotide derivative (it has a similar structure to 3−
Phosphate ions (P O4 ) are used in ATP and DNA
​

nucleotides) A phosphate ion is called a phosphate group when it

is attached to another molecule

Water
Water is vital for life and is a major component of cells

1.8. Properties of water

It is a polar molecule so it acts as a universal solvent
It is a universal solvent so metabolic reactions will occur
faster as they occur faster in solution
It is reactive as it is important for hydrolysis and
condensation reactions
It has a high specific heat capacity for buffering
temperature changes
It has a large latent heat of vaporization so it has a large
It is formed from a ribose sugar, an adenine base, and
cooling effect with little loss of water
three phosphate groups
The cohesion between water molecules helps water flow
A cell can't get its energy directly from glucose so glucose
and gives water a strong surface tension
is broken down in respiration and the energy released is
used to make ATP
This means that ATP is the immediate source of energy in
a cell
2. Cell Structure
ATP is made, diffuses to the area of the cell needing
Organisms can be prokaryotes or eukaryotes
energy, and then is quickly used
Eukaryotes are multicellular organisms that include all
The energy is stored in the the bonds between the
animals, plants, algae, and fungi
phosphate groups. To release energy, one phosphate is
Prokaryotes are smaller and simpler like bacteria
removed in a hydrolysis reaction to make ADP (adenosine
Both types of cells contain organelles which are parts
diphosphate) - catalysed by ATP hydrolase enzyme
of cells that have a specific function
This reaction can be coupled with other energy-requiring
Eukaryotic cells are more complex than prokaryotic cells
reactions in the cell so less energy is lost as heat
The phosphate removed can be added to other
compounds to increase their reactivity 2.1. Structure of Eukaryotic cells
ATP is re-made from ADP and a phosphate group
catalysed by the ATP synthase enzyme. This reaction
Cell wall: a rigid structure that surrounds cells in plants,
takes place in hydrolysis and respiration
algae, and fungi
In plants and algae, it is made of cellulose
Inorganic Ions In fungi, it is made of chitin
A cell wall supports cells and stops them from
changing shape
Inorganic ions are found in the cytoplasm and bodily
Cell-surface membrane: the membrane found on the
fluids (blood, saliva etc)
surface of animal cells and under the cell wall in plants,
They have differing concentrations and roles
algae, and fungi
They are electrically charged
It regulates the movement of substances in and out of
Iron ions (F e2+ ) are used in Haemoglobin
a cell
Haemoglobin is a protein that carries oxygen in the
It also has receptors which allow it to respond to
blood
chemicals eg hormones
Made of 4 polypeptide chains, each with an Iron ion
Nucleus: a large organelle surrounded by a double
The Iron ion is what binds to the oxygen molecule
membrane called a nuclear envelope which has openings
Hydrogen ions (H + ) determine pH (pH calculated based
called nuclear pores
on the concentration of H + ions)

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The nucleus contains chromosomes which are made Also can be used to isolate unwanted chemicals in the
from linear DNA bound to proteins cell
It also contains one or more nucleoli
The nucleus controls the cell’s activities by controlling Cell specialisation and organisation
the transcription of DNA
The nuclear pores allow RNA to move out of the In multicellular organisms, cells become specialised to
nucleus and into the cytoplasm carry out specific functions
The nucleolus is the site of ribosome production Different types of cells have different structures to help
Mitochondrion: an oval shaped organelle with a double them carry out their functions
membrane Cells are organised into tissues, tissues into organs, and
inner membrane is folded to form cristae organs into organ systems
space inside is called the matrix Specialised cells of the same function group together
Mitochondria are the site of aerobic respiration into tissues
Chloroplast: only found in plant and algal cells Tissues of the same function form organs
Surrounded by a double membrane and contains Organs group with other organs to form organ
structures called thylakoid membranes systems eg the circulatory system
A stack of thylakoid membranes is called a granum,
and grana can be joined by thin membranes called
lamellae
Structure of prokaryotic cells
The fluid around the grana and lamellae is called the
stroma Cell wall: rigid structure that provides support to the cell
Chloroplasts are the site of photosynthesis made of murein (polymer of polysaccharides and
Golgi apparatus: an organelle made of a stack of flattened polypeptides)
sacs (called cisternae) Capsule: some bacteria have a capsule made of slime
processes and packages lipids and proteins by adding Helps protect bacteria from attack and allows groups
carbohydrate groups to them of bacteria to stick together
produces secretory enzymes Cell-surface membrane: phospholipid bilayer similar to a
forms lysosomes eukaryotic cell
The Golgi apparatus is surrounded by golgi vesicles Circular DNA: prokaryotes don’t have nuclei or proteins
that transport and store lipids associated with DNA - just a loop of circular DNA free in
Lysosome: a Golgi vesicle that contains digestive enzymes the cytoplasm
(lysozymes) eg proteases and lipases Plasmids: small extra loops of DNA that carry genes that
Can be used to break down material ingested by the can help with survival eg antibiotic resistance
cell or break down worn-out cell components not always present in prokaryotes and some
Also can release enzymes to the outside of the cell prokaryotes can have several
(exocytosis) plasmids can be transferred between prokaryotes
Rough Endoplasmic Reticulum: a system of sheet-like Flagellum: a long hair-like structure found in some
membranes that enclose a fluid filled space and covered species (some prokaryotes can have more than one)
with ribosomes used to make the cell move
The RER is the site of protein synthesis by ribosomes The cytoplasm has no membrane-bound organelles
It also provides a pathway for protein transport contains free 70S ribosomes (smaller than ribosomes
Smooth Endoplasmic Reticulum: similar to the RER but in eukaryotic cells)
doesn’t have ribosomes
Site of lipid and carbohydrate synthesis Virus structure
Ribosome: A very small organelle that either are free in
the cytoplasm or bound to the RER Viruses are non-living and acellular (not cells)
Made of two parts: larger and smaller subunits Made up of nucleic acids (DNA or RNA) surrounded by a
Come in two sizes: 80S (25nm in diameter - found in protein coat called a capsid
eukaryotic cells) and 70S (slightly smaller - found in Attachment proteins are found on the surface of the virus
prokaryotic cells) to allow it to attach to and enter a host cell
Ribosomes synthesise proteins from mRNA and
amino acids
Vacuole: A large fluid-filled sac surrounded by a
membrane called a tonoplast
Cell Division
Found in the cytoplasm of plant cells
Contains cell sap (a solution made of sugar and salts) 2.2. Mitosis
Helps to maintain pressure inside the cell and stop
plants wilting Eukaryotes replicate via mitosis

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There are two types of cell division in eukaryotes: mitosis 1. The circular DNA replicates once and plasmids
and meiosis replicate multiple times
Mitosis produces two identical ‘daughter’ cells for 2. The cell gets bigger and the loops of DNA move to
growth or repair of tissue opposite poles (ends) of the cells
Meiosis produces four genetically different daughter 3. The cytoplasm divides and a new cell membrane and
cells for reproduction cell wall begins to form
In multicellular organisms, not all cells retain their 4. Two ‘daughter’ cells are formed, each with one loop of
ability to divide but those that do follow the cell cycle circular DNA but a variable number of plasmids

The cell cycle Viruses are non-living so they cannot replicate by

themselves
The cell cycle consists of interphase and mitosis They use a host cell to replicate
Interphase is a period of growth and DNA replication that The attachment proteins bind to complementary
is divided into three stages: G1, S, and G2 receptors on the cell-surface membrane of the host
During G1 (gap phase 1), the cell grows and makes cell
organelles and proteins that will be needed They then inject their DNA or RNA into the host cell
During S (synthesis), the DNA replicates and it begins producing new viral components which
During G2 (gap phase 2), the cell grows again and are assembled into new viruses
proteins needed for cell division are made
Mitosis is a period of cell division that has four stages:
prophase, metaphase, anaphase, and telophase and Methods of studying cells
cytokinesis
Prophase: Magnification: difference in size between the image and
chromosomes (made of two identical sister the real objecT
chromatids) condense and become visible magnification: size of image/size of real object
(become shorter and fatter) Resolution: a measure of how detailed the image is
centrioles move to opposite ends of cell and start Calculated as the minimum distance apart that two
forming a network of fibres called the spindle points have to be to be able to be distinguished by the
nuclear envelope breaks down and chromosomes microscope
are free in the cytoplasm Increasing the magnification will not increase the
Metaphase: resolution
The chromosomes line up along the equator
(middle) of the cell and attach themselves to the 2.3. Types of microscopes
spindle via their centromere
Anaphase: There are two main types: light and electron
The centromeres divide and the sister chromatids
separate Light (optical) microscopes Electron microscopes
Each sister chromatid is pulled to the opposite end Use electrons to form an
Use light to form an image
of the cell image
Telophase and cytokinesis (division of the cytoplasm) Maximum resolution of 0.2 Maximum resolution of
The chromosomes reach their respective poles micrometers 0.0002 micrometers
and become longer and thinner
Maximum magnification of Maximum magnification of
The nuclear envelope reforms
x1500 x1,500,000
The cell become longer and thinner in the middle
and eventually splits into two
There are two main types of electron microscope:
Mitosis is a highly controlled process and cancer is
transmission and scanning
uncontrolled mitosis
many cancer treatments target the cell cycle to kill the
Transmission electron Scanning electron
tumor cells
microscopes (TEM) microscopes (SEM)
however this also targets normal body cells that are
rapidly dividing such as hair cells Uses electromagnets to focus
electrons into a beam that Uses electromagnets to
passes through a thin section scan the beam of electrons
Binary fission of the specimen and onto a over the surface of the
screen behind to form a specimen
Prokaryotes replicate via binary fission photomicrograph

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Transmission electron Scanning electron All cell membranes, in eukaryotic and prokaryotic
microscopes (TEM) microscopes (SEM) organisms have the same basic structure
Electrons are scattered and All cells (and many organelles in eukaryotic
organisms) are surrounded by cell membranes
Denser areas of the specimen with computer analysis, a
The cell membrane surrounding the cell is called the
absorb more electrons and so 3D image can be produced
cell-surface membrane
appear darker that show the surface of the
specimen Cell surface membranes are made up of a phospholipid
bilayer with other molecules embedded in it such as
Pros: Can use thicker proteins, glycolipids, glycoproteins, and cholesterol
Pros: Extremely high-resolution
specimens than TEM, The membrane has a ‘fluid mosaic’ structure
images
resulting image is 3D Phospholipid molecules form a continuous bilayer
Cons: Can’t use living Cons:Can’t use living which is called ‘fluid’ as the phospholipids are
specimens, specimen must be specimens, specimen must constantly moving
extremely thin, a complex be extremely thin, a Proteins can either be embedded in one phospholipid
staining process required, complex staining process layer or span the whole bilayer
high-energy electron beam required, high-energy Proteins just in one layer are normally for mechanical
could damage specimen, electron beam could support or act as receptors for eg hormones
image is not in colour, image is damage specimen, image is Proteins spanning both layers can be channel or
2D, artefacts can form on not in colour, lower carrier proteins to transport substances in and out of
image resolution than TEM cells
Glycolipids are formed from a carbohydrate bound to a
Cell fractionation and lipid molecule
They extend out of the phospholipid bilayer into the
ultracentrifugation environment outside of the cell
They act as cell-surface receptors and also help to
Cell fractionation is a method of breaking cells up and maintain the stability of the membrane
separating the components They can also allow cells to bind together to form
Step 1: Homogenisation (breaking up the cells) tissues
can be done by grinding the cells in a blender or Glycoproteins are proteins embedded in the membrane
vibrating them with a carbohydrate group attached to them
The solution should be ice-cold (to reduce activity of They act as receptors/recognition sites and help cells
enzymes that may break down organelles) and bind together to form tissues
isotonic (same pH as cell - use a buffer) They also allow immune cells to recognise self cells so
Step 2: Filtration (removing complete cells and debris) they don’t get attacked
Step 3: Ultracentrifugation (separating the organelles) Cholesterol molecules are found inside the phospholipid
Pour cell fragments into a test tube and put the test bilayer between the phospholipid molecules and they
tube into a centrifuge increase the strength of the membrane
Spin the centrifuge at low speed They pull the hydrophobic tails of the phospholipids
The heaviest organelles (nuclei) are forced to the together which limits their movement, which makes
bottom to form a thick sediment called the pellet the membrane more rigid especially at high
The liquid above is called the supernatant and is temperatures
transferred to another test tube to be spun again This helps to maintain the shape of animal cells
Spin the centrifuge faster to isolate the the (which don’t have cell walls for stability)
mitochondria in the pellet then drain the supernatant The cell-surface membrane is partially permeable which
to spin again means it lets some things through but not others
Continue the process to isolate the next heaviest etc small, lipid-soluble (non-polar) molecules can pass
Organelles are isolated in this order: nuclei, through the lipid bilayer
mitochondria, lysosomes, endoplasmic reticulum, and other molecules can only enter the cell if there is a
then ribosomes specific channel or carrier protein for them

Transport across cell Movement across cell-surface

membranes
membranes
Substances can move across the cell membrane by
2.4. Structure of the cell-surface diffusion, osmosis, or active transport

membrane Diffusion

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Diffusion is the net movement of particles from areas of protein

high concentration to areas of low concentration until the The molecule/ion is deposited in the cell and the
concentration in all areas is equal carrier protein reverts to its original shape
It is a passive process (does not require energy from Co-transporters are a type of carrier protein that bind
ATP) two molecules/ions at the same time
It occurs due to the random movement of particles One of the molecules moves down its concentration
The concentration gradient is the path from the area gradient and this moves the other against its
of high concentration to the area of low concentration concentration gradient
- particles move down a concentration gradient An example of this is the transport of glucose using
Simple diffusion occurs when the molecule can move sodium in the ileum
freely through the cell-surface membrane Sodium moves into the cells down its concentration
This occurs for small, non-polar molecules such as gradient and this moves glucose in as well, against its
oxygen and carbon dioxide concentration gradient.
To increase the rate of simple diffusion, increase the
surface area of the exchange surface
Facilitated diffusion involves the use of channel and Cell recognition and the
carrier proteins
Large molecules and charged ions aren’t able to move immune system
via simple diffusion as they can’t pass through the
lipid membrane All cells have antigens on their surfaces that allows them
Instead, they move through channel or carrier to be recognised by other cells
proteins down a concentration gradient The immune system can distinguish antigens on body
It is a passive process (doesn’t require energy from cells (self) and antigens on foreign objects (non-self)
ATP) The immune system can identify pathogens (disease-
Channel proteins form pores that allow a specific causing organisms), abnormal body cells (cancerous/
molecule to pass through infected cells), cells from other individuals of the same
Carrier proteins change shape when a specific species (eg organ transplants) and toxins.
molecule binds to them and deposit the molecule
inside the cell 2.5. Immune response
To increase the rate of facilitated diffusion, increase
the number of channel/carrier proteins on the The immune response has three main stages: phagocytosis,
exchange surface cell-mediated response, and humoral response

Osmosis Phagocytosis
Osmosis is the diffusion of water molecules Phagocytes are a type of white blood cell that engulf and
It is the movement of water from an area of high destroy pathogens
water potential to an area of low water potential
It is a passive process (doesn’t require energy from 1. The pathogen releases chemical products that attract
ATP) the phagocyte towards it
Water potential is the ‘concentration’ of water 2. The phagocyte attaches to the receptors on the
A solution with a high concentration of solute will have a surface of the pathogen and recognises the foreign
low water potential and vice versa antigens
To increase the rate of osmosis, increase the water 3. The cytoplasm of the pathogen engulfs the pathogen
potential gradient, make the exchange surface thinner, inside a membrane called a phagosome
and increase the surface area of exchange surface 4. Lysosomes fuse with the phagosome and break down
the pathogen
Active Transport 5. The phagocyte displays the pathogen’s antigens on its
cell-surface membrane and is referred to as an APC
Active transport is the movement of substances from an (antigen-presenting cell)
area of low concentration to high concentration against
the concentration gradient B-cells (see below) can also engulf pathogens and
It is an active process (requires energy from ATP) become APC’s
Carrier proteins are used in active transport (as well as
facilitated diffusion) Cell-mediated response
The molecule or ion binds to a specific carrier protein
ATP breaks down into ADP and a Pi group which T cells are a type of immune cell
releases energy to change the shape of the carrier

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There are two types of T cells: helper T cells and Passive immunity is when the body receives antibodies
cytotoxic T cells from a different organism
Receptors on specific helper T cells are complementary to Natural passive immunity: after a baby receives
the antigens the APC is presenting and this causes the antibodies from its mother in breast milk or through
helper T cell to undergo clonal expansion (clones itself) the placenta
and release cytokines (chemical signals) Artificial passive immunity: after you get a vaccine
The helper T cells activate cytotoxic T cells which search containing antibodies against a pathogen
for infected body cells
When they find an infected cell, they inject it with Active immunity Passive immunity
perforin which causes the cell membrane to Involves exposure to the Does not involve exposure to
disintegrate and the cell to die pathogen’s antigens antigens
No memory cells are
Humoral response Memory cells are produced
produced

The cytokines from the helper T cells activate B cells that Protection is not immediate Protection is immediate
are specific to the antigens on the APC Immunity is short term as
Immunity is long term as
The B-cell undergoes clonal expansion and after a while, the antibodies
memory cells are formed
differentiation (specialisation) into plasma cells and are broken down
memory B cells
Plasma cells produce antibodies that fit the antigens on
the APC HIV and AIDS
Antibodies are proteins made by B-cells that have a
binding site complementary to antigens on a specific HIV (Human Immunodeficiency Virus) is made up of two
pathogen single strands of RNA enclosed in a capsid and
Antibodies are made of four polypeptide chains, two surrounded by a lipid envelope in which are embedded
called heavy chains and two called light chains. They attachment proteins
each have two binding sites HIV infects and kills helper T cells and eventually this
Antibodies have a variable region (the binding sites) leads to weakened immunity which is AIDS (Acquired
and a constant region that is the same for all Immune Deficiency Syndrome )
antibodies This makes the individual very susceptible to other
Antibodies bind to antigens to create a antigen- secondary diseases that eventually kill the person
antibody complex Antibiotics don’t work against viruses because antibiotics
Antibodies can attack different aspects of bacteria that viruses don’t have
cause agglutination (pathogens stick together so Antibiotics like penicillin work by inhibiting enzymes
they can be engulfed easier) that form peptide cross-linkages in the murein cell
act as markers to stimulate the phagocytes to walls
engulf the pathogen This causes the wall the burst and the bacterium to
neutralise the pathogen so it can’t enter any body die
cells Viruses don’t have cell walls so antibiotics won’t work
Memory B-cells circulate in the blood to activate the against them
secondary immune response in case the pathogen
attacks again
If it encounters the pathogen again, it will undergo Antibodies and Medicine
clonal expansion, differentiate into plasma cells, and
release antibodies
However some pathogens can evade the immune
2.6. Monoclonal antibodies
system a second time due to antigenic variation
Monoclonal antibodies are antibodies that can be
(changing their surface antigens) and this means that
isolated and cloned
the secondary immune response won’t be triggered
They can be used in medicine to target medication to a
specific cell type by attaching a drug to the antibody
Active and passive immunity They can also be used in medical diagnosis eg the ELISA
test
Active immunity is when the immune system produces its
own antibodies and memory cells: The ELISA test
Natural active immunity: after you catch a disease,
you become immune The ELISA test is a medical test that can be used to test
Artificial active immunity: after you get a vaccine for anything that the body makes antibodies against
containing dead or weakened pathogens

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1. Antigens from the pathogen we are testing for is ratio. In warmer climates, animals have adaptations
bound to the bottom of a surface eg large ears to maximise surface area to volume
2. A sample of the patient’s blood plasma is added. If ratio
they have antibodies against the antigen, these will Smaller animals tend to have a higher metabolic rate
bind now so they end up losing more heat than larger animals
3. The surface is rinsed to remove any unbound antigen
4. A secondary antibody with an enzyme attached is
added to the surface. This antibody is specific for the
first antibody and if the first antibodies were present
in step 2, the secondary antibodies will bind to them
now
5. The well is rinsed again
6. A substrate for the enzyme is added. If the enzyme is
present, the substrate will change colour and this
indicates that the patient has the antibodies in their
blood and is infected with the pathogen.

Ethical use of monoclonal antibodies

and vaccines
3.2. Gas Exchange
Ethical issues with monoclonal antibodies

The production of monoclonal antibodies involves Gas exchange in single-celled

animals organisms
Testing monoclonal antibodies on humans often comes
with the risk of severe side-effects Single celled organisms are very small compared to
multicellular organisms and have a large surface area to
Ethical issues with vaccines volume ratio
Therefore, gas exchange occurs across the cell surface
All vaccines are tested on animals before use on humans membranes directly into the cytoplasm of the cell
Testing vaccines on humans can be risky
To achieve herd immunity, should governments make
vaccination compulsory? Gas exchange in insects
Insects have openings in their body called spiracles that
3. Organisms exchange lead to air-filled tubes called trachea
The trachea that branches off into smaller tracheoles
substances with their that extend into the body systems of the insect
There is a short distance between a tracheoles and an
environment insect body cell so oxygen and carbon dioxide can

3.1. Surface area to volume ratio

Organisms need to exchange substances with their
environment
The rate of transfer depends on the surface area to
volume ratio
Smaller animals have a higher surface area: volume ratio
than larger animals
In single-celled organisms, nutrients can directly diffuse
through the cell wall into the organism
However, multicellular organisms have to use
transport systems to move nutrients around the body
because diffusion alone would be too slow
Surface area to volume ratio affect heat exchange
Smaller animals in colder climates tend to have a
compact shape to reduce their surface area to volume

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directly diffuse counter-current mechanism

Gas exchange across the leaves of

dicotyledonous plants
The gas exchange system on a plan is similar to that of
insects
Short, fast diffusion pathway
No living cell is far from the external air supply
Gases move in and out of pores called stomata to the gas
exchange surface which is the surface of mesophyll cells
Mesophyll cells have a very high surface area so they
are adapted for gas exchange
The stomata can open and close to control the
Gases move in and out in three ways:
exchange of gases
Down a diffusion gradient: As cells respire, they use
Plants have adapted to control water loss
up oxygen and release carbon dioxide which causes a
Gas exchange leads to water loss so there is a trade-
concentration gradient to form and this causes the
off between the two
respiratory gases to be exchanged
When plants have enough water, the guard cells
Mass transport: Insects use rhythmic abdominal
around the stomata are turgid and this keeps the
movements to push air in and out of the spiracles
stomata open for gas exchange
The ends of the tracheoles are filled with water:
When plants do not have another water, the guard
during normal activity, this makes respiration harder
cells become flaccid and this closes the stomata to
but during exercise, the water leaves the tracheoles to
conserve water
make respiration easier which provides the insect
Some plants are adapted to survive in particularly dry
with a ‘boost’
conditions
They are called xerophytes
Gas exchange in fish They have a thick cuticle so less water can escape
The leaves may roll up so the stomata on the lower
In fish, gas exchange occurs in the gills epidermis on the leaf are not exposed to the outside,
Gills are made up of gill filaments that are thin plates reducing the water potential gradient and water loss
covered in small structures called lamellae The leaves may have hairs to trap a layer of moist air
This increases the surface area of the gills to speed up near the surface of the leaves to reduce the water
diffusion potential gradient
The lamellae have many small capillaries and a very The stomata may be in pits and grooves
thin cell membrane so that there is a small distance The leaves have a small surface area to volume ratio
for diffusion Both insects and plants have to compromise between
Water follows across the gills in the opposite direction efficient gas exchange and the limitation of water loss
to blood so that the diffusion gradient is maintained Efficient gas exchange would mean that a lot of water
and as much gas as possible diffuses - known as a is lost
Minimising the amount of water lost would result in
very inefficient gas exchange

Gas exchange in humans

In humans, gas exchange takes place in the lungs
The lungs are made up of lots of small air chambers
that increase surface area

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The trachea (windpipe) branches off into two bronchi, Tidal volume: the volume of air in each breath (0.4 -
which split into many small bronchioles, that end in 0.5 dm3 for healthy adults)
tiny air sacs called alveoli Ventilation rate: number of breaths/ minute (usually
15)
ventilation rate = tidal volume x breathing rate
Forced expiratory volume (FEV): the volume of air that
can be expelled in 1 second
Forced vital capacity (FVC): maximum volume of air
that can be breathed out after a deep breath in
Tuberculosis (TB) results in the formation of small lumps
in the lungs called ‘tubercles’
This damages the gas exchange surface so tidal
volume is reduced
The alveolar epithelium is the site of gas exchange in This leads to coughing up blood, chest pain, shortness
humans of breath, and fatigue
There are millions of alveoli so there is a huge surface Fibrosis is when scar tissue forms in the lungs
area for gas exchange this can be due to an infection or exposure to
The alveolar wall is only one cell thick so there is a substances like asbestos
very short diffusion distance Scar tissue cannot expand as much as normal tissue
The alveoli are covered in capillaries so tidal volume and FCV decrease
Ventilation (breathing) is made up of two stages: this results in shortness of breath and chest pain
inspiration (breathing in) and expiration (breathing out) Asthma attacks cause the airways to become narrow so
Ventilation is controlled by the diaphragm and the breathing becomes difficult
intercostal muscles (muscles around the ribs) This reduces FEV and leads to wheezing and
shortness of breath
Emphysema occurs as a result of smoking or exposure to
air pollution
Foreign particles become trapped in the alveoli
causing inflammation
Phagocytes are attracted to the area and produce an
enzyme that breaks down the elastic walls of the
lungs
This results in the alveoli becoming damaged and
unable to recoil to expel air
Leads to shortness of breath and wheezing as well as
an increased ventilation rate
Research in the 1950s -60s showed a link between
smoking and several types of cancer
This resulted in health warnings being printed on
During inspiration:
cigarette packets
The external intercostal muscles contract, while the
Many studies have documented the link between air
internal intercostal muscles relax which pulls the ribs
pollution and various diseases
upwards and outwards
This resulted in upper limits being placed on the
The diaphragm contracts, causing it to flatten
amount of pollution that can be emitted and taxes on
The volume of the thorax increases and the pressure
cars that pollute
in the lungs decreases so air is sucked into the lungs
Inspiration is active - it requires energy
During expiration: 3.3. Digestion and Absorption
The external intercostal muscles and the diaphragm
relax Digestion
The volume of the thorax decreases, increasing the
pressure and this causes air to be forced out Digestion involves large biological molecules are
Expiration is passive - it doesn’t normally require hydrolysed into smaller molecules that can be absorbed
energy (except during exercise) across cell membranes
In mammals, carbohydrates are digested by
Lung disease and ventilation amylases (breaks down starch into maltose) in the
mouth and small intestines
Lung disease can be diagnosed by measuring lung
function

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membrane-bound disaccharides attached to the cell This pattern is shown on a disassociation curve:
membrane of epithelial cells in the small intestine
(breaks disaccharides into monosaccharides)
Lipids are digested by lipase and bile salts in the small
intestine
Bile salts break up lipids into small droplets called
micelles in a process of emulsification
Lipase breaks down the micelles into fatty acids and
glycerol
Proteins are broken down by peptidases
Endopeptidases break peptide bonds in the middle of
the molecule
Exopeptidases break peptide bonds on the ends of
the molecule
Dipeptidases break peptide bonds in dipeptidases
The partial pressure of carbon dioxide (pCO2) has an
effect on the haemoglobin disassociation curve
Absorption This is called the Bohr effect
Cell respiration produces CO2, which increases pCO2
In mammals, absorption occurs along the cell membrane This causes the affinity of haemoglobin for oxygen to
of epithelial cells in the small intestine decrease, so it disassociates with oxygen
Glucose and amino acids are absorbed by sodium ion co- The disassociation curve shifts to the right so more
transporter oxygen is released
Sodium ions are actively transported out of the Different organisms have different types of haemoglobin
epithelial cell into the lumen of the small intestine Organisms that live in areas with a low pO2 (high
They then diffuse back in and take glucose with them altitudes) have haemoglobin with higher affinity for
through the co-transporter oxygen (disassociation curve is shifted to the left)
Organisms that live in areas with a high pO2 have
3.4. Mass Transport haemoglobin with a lower affinity for oxygen
(disassociation curve is shifted to the right)

Mass Transport in Animals The circulatory system

Haemoglobin Mammals have a double circulatory system (blood passes
through the heart twice in a complete circuit through the
In animals, haemoglobin carries oxygen around the body body)
haemoglobin is a protein with a quaternary structure -
Blood travels to the lungs to be oxygenated then
it is made up of four chains, each with a haem group travels to the heart, then through the arteries to body
Each haem group contains an iron ion so each cells where oxygen moves to the body cells
molecule of haemoglobin has four oxygen binding
Deoxygenated blood then returns to the heart via the
sites veins
Haemoglobin binds with oxygen in the lungs to form
\
oxyhaemoglobin Arteries carry blood away from the heart and veins return
It then disassociates from oxygen when it reaches blood to the heart
body cells The pulmonary artery supplies deoxygenated blood to
Depending on the partial pressure of oxygen, the
the lungs where it picks up oxygen and the pulmonary
saturation of haemoglobin will change vein returns it to the heart
partial pressure of oxygen (pO2) = concentration of
The vena cava is the largest vein in the body that
oxygen returns deoxygenated blood to the heart and the
When there is a high pO2 (at the lungs), haemoglobin
aorta is the largest artery that takes blood from the
has a high affinity for oxygen so combines with it to heart
form oxyhaemoglobin
The hepatic artery supplies the liver with blood and
When there is a low pO2 (at body cells), haemoglobin the hepatic vein removes deoxygenated blood from
has a low affinity for oxygen so it disassociates from it the liver
The hepatic portal vein takes nutrient-rich blood
from the small intestine directly to the liver
The renal artery supplies the kidneys with blood and
the renal vein removes deoxygenated blood

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3. Diastole: atria and ventricles relax, atrioventricular

valves open

Structure of blood vessels

Arteries carry blood away from the heart to the rest of

the body
Walls are thick and muscular with elastic tissue to
stretch and recoil to maintain pressure as the heart
beats
Structure of the human heart and the cardiac cycle Inner endothelium (lining) is folded to allow the artery
to stretch
The heart is divided into four chambers: the left and right
The biggest artery, the Aorta, divides into smaller
atria and the left and right ventricles
arteries which divide into even smaller arterioles
The left and right sides of the heart are divided by the
Veins carry blood to the heart from the rest of the body
septum
Walls are thinner and lumen is larger to decrease
The ventricle has a thicker wall than the atrium and
resistance as blood is at a low pressure
the left ventricle has a thicker wall than the right
Contains valves to keep the blood flowing in the right
ventricle
direction
There are valves between the atria and the ventricles
Smaller vessels called venules join together to form
(atrioventricular valves) and between the blood
veins which combine to form the biggest vein, the
vessels and the heart chambers (semilunar valves)
Vena Cava
Capillaries are the site of exchange of substances
between the blood and body cells
Their walls are only one cell thick to minimise the
diffusion distance
They are the width of one red blood cell to bring the
red blood cells close to cells to minimise diffusion
distance
They form capillary beds to maximise surface area for
exchange

During the cardiac cycle, the pressure in different heart

chambers changes and valves open & close

1. Atrial systole: atria contract, ventricles relax,

atrioventricular valves are open
2. Ventricular systole: atria relax, ventricles contract,
semilunar valves are open

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Formation of tissue fluid Xylem and water transport

Tissue fluid is the fluid surrounding cells in tissues Water moves up the xylem due to cohesion (water
It is formed from blood plasma molecules sticking together) and tension (suction due to
At the arteriole end of the capillary bed, the loss of water molecules)
hydrostatic pressure in the blood vessels is higher It is a passive process (does not require energy)
than the tissue fluid
Water and small molecules are pushed out of gaps in 1. Water is constantly evaporating from the leaves
capillary walls and leave the blood plasma 2. This creates a water potential gradient from the roots
At the venule end of the capillary bed, the hydrostatic to the leaves
pressure in the tissue fluid is higher than in the blood 3. The water column moves up the plant down this
vessels water potential gradient
Water and small molecules re-enter the blood plasma
This causes a constant renewal of tissue fluid which Phloem and transport of sugars
leads to efficient exchange
Sugars move through the phloem according to the mass
Excess tissue fluid is drained by the lymphatic system
flow theory of translocation
which returns it to the circulatory system
It is an active process (requires energy)
Cardiovascular disease 1. Sugars and solutes are actively transported from the
source, through companion cells, and loaded into the
Cardiovascular disease begins with the formation of an
phloem
atheroma
2. This causes the water potential of the phloem to
If the endothelium of an artery is broken then white
decrease so water moves into the phloem from the
blood cells and lipids gather under it and form a
xylem
plaque called an atheroma
3. This creates a high pressure in phloem at the source
If many atheromas form in the coronary arteries then
end
this is called coronary heart disease (CHD)
4. The pressure at the phloem at the sink end is lower
An aneurysm forms when the inner layers of the artery
than at the source end
are forced through the outer layers
5. Solutes move through the phloem from the source to
This can occur due to a atheroma
the sink down a pressure gradient
If the aneurysm bursts, it can cause a haemorrhage
6. Solutes are used up in respiration or stored at the
Thrombosis is a blood clot that can dislodge and block a
sink end
blood vessel elsewhere in the body
This can form as a result of an atheroma rupturing
the inner lining or an artery
If atheromas in the coronary arteries block blood flow to
the heart then this can lead to a myocardial infarction
(heart attack)
this can lead to chest pain, shortness of breath and
death of the heart muscle
Risk factors for CHD include:
High blood cholesterol due to a diet high in saturated
fats
Cigarette smoking
High blood pressure due to lack of exercise, being
overweight, or chronic stress
Some people also have a genetic disposition to CHD
so they should reduce their risk by minimising other
risk factors

Mass Transport in Plants

In plants, mass transport takes place in the xylem and the
phloem
Radioactive tracers can be used to investigate mass flow
The xylem transports water and small mineral ions
One leaf of a plant is exposed to radioactive CO2
from the roots up the plant
(containing 14-C instead of 12-C)
The phloem transports sugars up and down the plant

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The 14-C will then be combined into the sugars non-overlapping: base triplets don’t overlap or
produced by the plant share their bases
Photographic film can then be used to track how the A gene’s position on a chromosome is called a locus
radioactive carbon has moved through the plant Most of the DNA in eukaryotes is non-coding
Ringing experiments can be used to investigate mass flow There are non-coding repeating sequences between
A ring of bark that includes the phloem but not the genes
xylem is removed from a woody stem of a plant Within genes, there are non-coding sequences called
The plant is left for some time until a bulge forms introns between the coding sequences (exons)
above the ring
If the fluid from the bulge is tested, it will contain a 4.2. DNA and protein synthesis
higher concentration of sugars than the fluid below
the ring The genome of a cell is the complete set of genes that it
This shows that there is a downwards flow of sugars contains
The proteome of a cell is the full range of proteins
4. Genetic information, that the cell is able to produce
The proteome will be different for different types of
variation and relationships cell
Protein synthesis has two main parts: transcription and

between organisms translation

Transcription
4.1. DNA, genes, and chromosomes
Transcription is the production of mRNA from DNA
DNA is stored differently in prokaryotes and eukaryotes mRNA is a single polynucleotide strand that contains
groups of three bases called codons

1. DNA helicase unwinds the DNA double helix

2. Free RNA nucleotides bind to the exposed bases on
one of the DNA strands (the ‘template’ strand)
3. RNA polymerase joins the sugar-phosphate backbone
of these RNA nucleotides
4. In prokaryotes, the RNA strand formed is now mRNA.
In eukaryotes, it is called pre-mRNA and it needs to be
spliced (edited) to remove the introns and leave just
the exons before it can be called mRNA

Eukaryotes Prokaryotes
Translation
Linear DNA Circular DNA
Associated with proteins Not associated with proteins Translation is the production of polypeptides from the
Contained in nuclear Not contained in a sequence of codons carried by mRNA
membrane membrane It involves tRNA (transfer RNA) which is a single strand
of RNA folded into a ‘clover-leaf’ shape
On one end is a specific sequence of three bases
Mitochondria and chloroplasts in eukaryotic cells have
circular DNA like prokaryotes called an ‘anticodon’, and on the other end is an
DNA contains genes amino acid binding site
Genes are sequences of DNA that can code for the
1. the mRNA attaches itself to a ribosome and the tRNA
amino acid sequence of a polypeptide brings amino acids
Genes that don’t code for a polypeptide code for
2. A tRNA molecule that has a complementary anticodon
functional RNA eg tRNA (transfer RNA) or rRNA for the first codon on the amino acid binds to it
(ribosomal RNA)
3. Another tRNA that is complementary for the the
Every three bases in the DNA sequence codes for a second codon binds
different amino acid
4. The amino acids they carry are joined together by the
The genetic code is
ribosome
universal: the same base sequence codes for the 5. This continues until the ribosome meets a codon
same amino acid in different organisms
called a ‘stop’ codon and this signals for translation to
degenerate: some amino acids are coded for by stop and the polypeptide to detach
more than one base sequence (eg UAU and UAC
both code for tyrosine)

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All the active processes in transcription and translation Every time the DNA divides, independent segregation
occur with the energy released from the breakdown of occurs (chromosomes line up randomly)
ATP into ADP + Pi This results in genetically different daughter cells

4.3. Genetic Diversity

Genetic mutations involve a change in the base sequence
of a chromosome (eg ATG becomes ACG)
Can occur during DNA replication
Involve base deletion (eg ATGA becomes AGA) and
base substitution (eg CTA becomes CAA)
Not all base substitutions can cause a change in an amino
acid
The genetic code is degenerate which means that one
amino acid can be coded for by more than one triplet
sequence
Eg both GAA and GAG code for glutamic acid so if a
substitution mutation occurs that changes GAA to
GAG, there won’t be a change to the amino acid
sequence
Mutagenic agents can increase the risk of gene mutation
Eg UV light and X-rays

Meiosis and genetic variation 4.4. Genetic diversity and adaptation

Meiosis is the process that forms four haploid daughter Genetic diversity is the number of different alleles in a
cells from one diploid parent cell population
The daughter cells produced are genetically different Genetic diversity can be increased by new mutations
from each other in the DNA or through migration of a new population
This is used in the body during meiosis to form (called gene flow)
gametes Genetic diversity is one of the factors that enable
natural selection and evolution to occur
1. Before meiosis, the DNA replicates and condenses to A ‘genetic bottleneck’ reduces genetic diversity
form chromosomes made up of two sister chromatids Eg everyone in the population is descended from just
2. Meiosis 1: the chromosomes arrange themselves into a few people (the founder effect)
23 homologous pairs which are then separated Or an event occurs that causes a large number of
3. Meiosis 2: the sister chromatids are separated
organisms to die so their offspring have less diversity
4. This results in four haploid daughter cells

During meiosis 1, the sister chromatids cross over (swap

Natural selection and evolution
alleles of genes)

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Natural selection results in species becoming better

adapted to their environment over time

1. Random mutations result in new alleles of genes

2. Individuals may have an allele that gives them a
reproductive advantage (ie better at survival and
reproduction)
3. These individuals are more likely to survive and pass
on their genes whereas individuals with a different
allele that is less advantageous will be less likely to
survive and pass it on
4. Over time, the frequency of the advantageous allele in
the population increases Eg human birth weights: very small and very large
5. This results in the population being better adapted to babies are less likely to survive but medium-sized
their environment over time babies have a good chance of survival
Over time, human birth weight tends to shift to a
These adaptations can be anatomical, physiological, or
medium weight
behavioural
Anatomical: the structure of an organism’s body
changes 4.5. Species and taxonomy
Physiological: the processes inside an organism’s body
change Organisms are classified according to a phylogenic
Behavioural: organisms act in a way that increases classification system
their chance of survival This type of system groups species based on their
Two of the types of natural selection are directional evolutionary origins and relationshipS
selection and stabilising selection It ensures there are no overlaps between groups
Directional selection occurs when individuals with an The most common phylogenic system uses the
extreme characteristic are more likely to survive and hierarchies domain, kingdom, phylum, class, order,
reproduce family, genus, and species
Eg: humans are in
Domain: Eukarya
Kingdom: Animals
Phylum: Chordate
Class: Mammals
Order: Primates
Family: Hominade
Genus: Homo
Species: sapiens
Organisms are usually referred to by their genus and
species, eg Homo sapiens and this is called the
binomial naming system
Two organisms are said to be from the same species if
they are able to mate to produce fertile offspring
Courtship behaviours are used to help classify species
Eg antibiotic resistance in bacteria: the bacteria that Organisms use courtship behaviour to attract a mate
have the most resistance are more likely to survive
from the right species
Over time, the population of bacteria increases in These can be simple eg release of a chemical or
resistance
involve complex routines and displays
Stabilising selection occurs when individuals with
The more closely related species are, the more similar
moderate characteristics are more likely to survive and
their courtship behaviour
reproduce

4.6. Biodiversity within a community

Biodiversity refers to the variety of different organisms
It can refer to different habitats, from a small garden
to the Earth
Species richness is a measure of the number of different
species in a community

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An index of diversity is a numerical way of measuring Again, many differences mean that the organisms
diversity are distantly related
It is calculated by the following formula: The immunological responses that are caused by their
antibodies
An antibody from a similar organism will be
recognised by the immune system
New gene technology has caused a change in how genetic
N = the total number of organisms of each species diversity is investigated
and n = the total number of organisms of each We are now able to sequence DNA which is more
species accurate than looking at measured/observable
Biodiversity can be reduced through farming techniques characteristics
A monoculture is when farmers grow only one type of
plant Investigating variation within a species
Only a small number of organisms can be
supported so biodiversity is reduced Variation in a species is caused by genes and/or the
The use of herbicides to kill weeds reduces plant environment
diversity Variation exists within a species and between species
Animals that rely on the weeds could die To study variation within a species
Using pesticides to kill insects that eat the crops
reduces insect diversity 1. Choose a random sample of organisms
2. Collect statistical data from the sample
Birds and small mammals that feed on the insects
could die 3. Calculate a mean and standard deviation for the
Removing hedgerows to create bigger fields for sample
machinery destroys the habitat of the organisms that 4. Interpret the results
live in the hedgerows 1. The mean is used to compare variation
Woodland clearance reduces the number of trees and between samples
2. The standard deviation is used to compare
the diversity of tree species while also destroying the
habitat of the organisms that live in the woodland variation within a sample
Due to a growing human population, agriculture is more
important than ever but it is essential to balance
improving agriculture with conserving biodiversity.
5. Energy transfers in and
Strategies to do this may include:
Protecting endangered species
between organisms
Restricting development of some areas by designating
them as National Parks, SSSIs (Sites of Special 5.1. Photosynthesis
Scientific Interest) or AONBs (Areas of Outstanding
Natural Beauty) Photosynthesis is the process by which plants convert
Provide subsidies to farmers to encourage them to sunlight, water and carbon dioxide into glucose and
conserve biodiversity oxygen
It occurs in a series of steps but the overall reaction is
4.7. Investigating diversity 6CO2 + 6H2O → C6H12O6 + 6O2
The plant then stores the glucose until it is needed to
Diversity within a species or between many species can release energy in the form of ATP during respiration.
be measured genetically
This can be done by investigating The Light-Dependent Reaction
The frequency of measured/observable
The light-dependent reaction is the first step of
characteristics that are found in the organisms
photosynthesis
Organisms that look similar are probably closely
This takes place in the Thylakoid membranes of
related
chloroplasts and involves the green pigment chlorophyll
The base sequence of DNA and/or mRNA
Chlorophyll is found in structures called photosystems
The more similar the DNA and mRNA, the more
that are embedded in the thylakoid membrane
closely related the organisms are
Between the photosystems is a system of protein
If there are many differences, the organisms are
channels called electron carriers that make up the
not closely related
electron transport chain
The amino acid sequence of the proteins formed from
In the light-dependent reaction, ATP, reduced NADP, and
the mRNA
O2 are made by phosphorylation
Related organisms will have similar DNA so similar
amino acid sequences

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Non-cyclic phosphorylation forms ATP, reduced NADP, and 1. An enzyme called rubisco (ribulose bisphosphate
O2 carboxylase) is used to combine CO2 with a molecule
called ribulose bisphosphate (RuBP) to form 2
1. Light energy strikes photosystem 2 (PSII) and is
molecules of glycerate phosphate (GP)
absorbed
2. 2xGP is converted to three molecules of TP using the
2. The light energy excites electrons in the chlorophyll in
energy from the breakdown of 2ATP to ADP and
PSII
2NADPH to NADP
3. These electrons, now at a higher energy level, move
3. Some TP is converted to glucose. The rest is used to
down the electron transport chain to photosystem 1
regenerate RuBP using the energy from the
(PSI)
breakdown of 1 ATP to ADP
4. As they move down the electron transport chain, they
lose energy that is used to actively transport H+ ions The ATP and NADPH used in the Calvin Cycle come from the
into the thylakoid against their concentration gradient light-dependent reaction
5. The H+ ions flow down their concentration gradient
through the ATP synthase enzyme channel which Optimum conditions for photosynthesis
causes ADP and a phosphate group (Pi) to be
combined to form ATP High light intensity of a certain wavelength
6. The H+ ions bond to NADP to form reduced NADP The higher the intensity (brightness) of the light, the
(NADPH) more energy it can carry
Only certain wavelengths of light are absorbed by the
Cyclic phosphorylation only forms ATP chlorophyll found in plants (chlorophyll a and b and
carotene)
1. Light strikes PSI
The red and blue wavelengths are absorbed
2. The electrons are moved in a loop from PSI to the
The green wavelengths are reflected which is why
electron carriers and back again
plants look green
3. This process only forms small amounts of ATP
A temperature of about 25C
This is the optimum temperature for the enzymes
The Light-Independent Reaction (the Calvin Cycle)
involved in photosynthesis
The light-independent reaction, also known as the Calvin At high temperatures, the stomata of the plant will
Cycle, is the second stage of photosynthesis close to avoid water loss, limiting the rate of
It takes place in the stroma of the chloroplasts transpiration and photosynthesis
It forms a molecule called triose phosphate (TP) that can A CO2 concentration of about 0.4%
be converted into glucose and other substances The concentration of CO2 in the air is 0.04% but
increasing this to 0.4% increases the rate of
photosynthesis

5.2. Respiration
Respiration produces ATP

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There are two main types: aerobic and anaerobic The link reaction converts pyruvate to acetyl coenzyme A
aerobic respiration takes place in the presence of air
anaerobic respiration takes place without air but is
less efficient than aerobic respiration

Glycolysis

Glycolysis is the first stage in aerobic respiration and the

only stage in anaerobic respiration.
Involves converting one molecule of glucose into two
molecules of pyruvate, forming 2ATP and 2 molecules of
reduced NAD (NADH)
It occurs in the cytoplasm of cells.
1. Pyruvate is decarboxylated by the removal of one
carbon as CO2 and forms acetate with the conversion
of one NAD to NADH
2. It is then combined with coenzyme A to form acetyl
coenzyme A
3. The acetyl coenzyme A is then transported to the next
stage of respiration

The Krebs Cycle

Forms NADH and CO2 from a series of oxidation-

reduction reactions
It takes place in the matrix of mitochondria

1. Glucose is phosphorylated to glucose phosphate

using the phosphate group from the breakdown of
ATP to ADP and Pi
2. Another Pi group is added from the breakdown of
another ATP, forming hexose bisphosphate
3. Hexose bisphosphate is then split to form two
molecules of triose phosphate
4. Triose phosphate is oxidised (loses hydrogen),
1. Acetyl coenzyme A combines with a 4-carbon
forming two molecules of pyruvate as well as two
molecule to form a 6-carbon molecule.
NADH and 4ATP
2. The 4-carbon molecule is converted to a 5-carbon
4ATP are made, but 2ATP are used up in the process, so molecule by releasing CO2 and forming one NADH
overall, 2ATP are produced from NAD.
In aerobic respiration, the pyruvate moves onto the next 3. The 5-carbon molecule is converted back into the
stage (the link reaction) original 4-carbon molecule, forming one CO2, two
In anaerobic respiration in animals, the pyruvate is NADH from NAD, one ATP from ADP + Pi, and one
converted to lactate, regenerating NAD and allowing FADH from FAD.
glycolysis to continue 1. FAD is a coenzyme like NAD but with a slightly
In plants and yeast, the pyruvate is converted to different structure though it has a similar
ethanal and then ethanol (alcohol) function
4. The ATP, NADH and FADH move on to the next stage
The Link Reaction of respiration while the 4-carbon molecule combines
with another acetyl coenzyme A and carries out the
cycle again.

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Oxidative Phosphorylation (the Electron Transport Use the heat to heat a sample of water and measure
Chain) the temperature change
Use this value to work out the energy content of the
The final stage of respiration, forming ATP and H2O biomass
The other steps of respiration create some ATP but
not as much as this stage Measures of chemical energy stores for plants
It takes place across the inner mitochondrial membrane
The plant's chemical energy formed and stored can be
measured using GPP and NPP.
GPP (Gross Primary Production) is the total amount of
light energy that a given area of plants converts to
chemical energy through photosynthesis.
It is the chemical energy stored in plant biomass in a
given area or volume.
Some of the Energy will be lost as heat during
respiration, and this is called R (respiratory losses)
NPP (Net primary production) is the store of energy that
is available for growth and reproduction
NPP = GPP - R
1. NADH and FADH from the Krebs Cycle break down Gross and net primary production is often expressed as a
into NAD and FAD, releasing hydrogen rate (e.g. kJ per m2 per year)
2. These hydrogen atoms are broken down into H+ ions
and electrons Calculating net production for consumers
3. The electrons move down the chain of electron
carriers in the mitochondrial membrane (the electron Consumers store chemical energy that they have gotten
transport chain) and release energy as they do so from eating plants and animals.
4. This energy is used to pump the H+ ions across the This chemical energy is stored in the biomass of the
membrane against their concentration gradient animal\
5. The H+ ions diffuse back across the membrane Net production is calculated as N = I - (F +R)
through the ATP synthase enzyme channel N = net production
6. This movement provides energy for the enzyme to I = energy in ingested food
combine ADP and Pi to form ATP F = energy lost in faeces and urine
7. The H+ ions and the electrons from the electron R = energy lost through respiration
transport chain combine with O2 to form H2O Net production is also referred to as secondary
production
Glucose is not the only respiratory substrate - lipids and To calculate the % efficiency of energy transfer, work out
amino acids break down to form molecules that enter the the net production as a percentage of the ingested
Krebs Cycle and are used for respiration. energy

5.3. Energy and Ecosystems Production and Farming practices

Plants synthesise organic compounds from atmospheric Farming practices can make the transfer of energy within
or aquatic carbon dioxide. food webs more efficient.
Some of these compounds are used during respiration, Food webs show how energy is transferred between
but the rest forms plant biomass. organisms
Energy transfer can become less efficient in food
Measuring biomass webs due to pests or energy losses in respiration
Farming practices like using herbicides and pesticides
Measuring the dry mass can eliminate losses because of pests
Heat a sample of biomass until it is dry, then weigh it Keeping animals indoors in a warm environment
Scale up the mass to work out the mass of the entire reduces energy losses through respiration
organism
Measuring the mass of carbon
5.4. Nutrient Cycles
The mass of carbon is usually taken as 50% of the dry
mass Within natural ecosystems, minerals and nutrients are
Measuring the chemical energy content recycled
Weigh a sample of biomass and then burn it Two examples of this are the nitrogen and phosphorus
completely cycles

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The Nitrogen Cycle 4. When plants and animals die, saprobionts release
phosphate ions back into the soil, where plants can
take them up
5. Fertilisers also release phosphate ions into the
ground so plants can take them up.

Eutrophication

Fertilisers are used to replace nutrients that are lost when

crops are harvested.
They can be natural (e.g. manure) or artificial.
Using too much fertiliser can cause the minerals in them
to leach out into waterways, where they can cause
serious environmental issues such as eutrophication.

1. Mineral ions are leached from the soil and wash into
rivers and lakes
2. This causes the rapid growth of algae on the surface
of the lake
Nitrogen fixation is when the nitrogen gas in the 3. Large amounts of algae block out light to the aquatic
atmosphere is turned into nitrogen compounds in the plants below
soil, e.g. nitrates. 4. These plants die as they are unable to carry out
Bacteria can do it in the root nodules of plants such as photosynthesis
peas, beans, and clover. 5. Bacteria feed on the dead plants and respire, using up
It can also take place during lightning strikes. the dissolved oxygen in the water
Ammonification is when nitrogen compounds in animal 6. Fish and other animals die as there is not enough
waste or dead organisms are turned into ammonia by oxygen left for them
decomposing bacteria called saprobionts.
Nitrification is when ammonium ions in the soil are
changed into nitrogen compounds (e.g. nitrates) by 6. Organisms respond to
nitrifying bacteria.
Denitrification is the opposite of nitrogen fixation: when changes in their internal and
nitrates in the soil are converted into nitrogen gas by
denitrifying bacteria. external environments
This process takes place in anaerobic conditions such
as waterlogged soils. 6.1. Survival and Response
The Phosphorus Cycle Organisms survive by responding to changes in the
environment (internal and external)
Internal: ensuring that body temperature and pH are
optimal for metabolism
External: avoiding harmful environments or changing
by responding to changes in the environment.
Plants respond to external stimuli like gravity and light
Plants grow towards the light to maximize
photosynthesis
They sense gravity so that shoots and roots grow the
correct way
A plant’s growth response to a stimulus is called a
tropism.
a positive tropism is a growth towards a stimulus
1. Phosphate ions in rocks are transferred to the soil by a negative tropism is growth away from a stimulus
weathering. Phototropism is the growth response of plants to lights
2. Mycorrhizal bacteria in plant roots take phosphate shoots have a positive phototropic response: they
ions into the plants grow towards the light
3. Phosphate ions are then transferred to animals when roots have a negative phototropic response: they
they eat the plants and lost from the animals in waste grow away from the light
products Gravitropism is the growth response of plants to gravity

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shoots are negatively gravitropic: they grow away Kinetic responses (kineses) are when organisms move in
from the direction of the pull of gravity response to a non-directional stimulus.
roots are positively gravitropic: they grow towards the eg woodlice have a kinetic response to humidity: in a
direction of the pull of gravity humid environment, they move and turn less often so
Plants respond to stimuli by using growth factors they will stay where they are but in a dry
Growth factors are chemicals that speed up or slow environment, they move and turn more so that it is
down plant growth more likely they will move into a humid environment
Auxins are a group of growth factors that stimulate this helps to keep them in humid conditions which
growth by causing cells to elongate stops them from drying out
In shoots, high concentrations of auxin cause cell
elongation but in roots, high concentrations of auxin Control of heart rate
inhibit cell elongation
IAA (Indoleacetic Acid) is a type of auxin that is produced Cardiac muscle is a myogenic muscle: it will contract on
in the shoots of flowering plants its own without a nerve impulse
It moves around the plants to control tropisms The heart has a pacemaker that regulates these
Phototropism: IAA accumulates on the more shaded side contractions called the SAN (sinoatrial node)
of shoots and roots
This causes shoots to bend towards the light and
roots to bend away from the light

Gravitropism: IAA accumulates on the underside of 1. The SAN sends out a wave of electrical activity over
shoots and roots the atria of the heart which causes the left and right
This causes shoots to bend upwards and roots to atria of the heart to contract at the same time
2. The ventricles of the heart are separated from the
bend downwards
atria by a band of non-conducting tissue, so the signal
passes through the AVN (atrioventricular node)
1. the AVN has a slight delay which allows the
atria to fully empty before the ventricles
contract
3. The impulse travels from the AVN down the bundle of
His (a bundle of nerves that are found in the septum
or middle of the heart) and into the Purkyne tissue
4. The Purkyne tissue causes the ventricles to contract
simultaneously from the bottom up

The rate that the SAN sends out impulses is

unconsciously controlled by the brain and the autonomic
Taxes and kineses nervous system (made up of the sympathetic and
parasympathetic nervous system)
Taxes and kineses are simple responses that keep simple The sympathetic nervous system helps the body get
organisms in a favorable environment ready for action while the parasympathetic nervous
Tactic responses (taxes) are when organisms move system calms it down
towards or away from a directional stimulus (eg light) A part of the brain called the medulla oblongata
eg woodlice have a phototaxic response (taxes controls the SAN
response to light): they move away from a light source The medulla oblongata receives impulses from
this helps them to remain in dark conditions that baroreceptors (pressure receptors) and
will keep them safe from predators chemoreceptors (chemical receptors)

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Baroreceptors are found in the aorta and carotid The retina of the eye contains lots of photoreceptor
arteries (in the neck) and are stimulated by high cells that detect light
and low blood pressure When light hits a photoreceptor, it causes an action
Chemoreceptors are found in the aorta, the carotid potential to form and sends a nerve impulse to the
arteries, and the medulla and respond to changes in brain via the optic nerve
O2 concentration and blood pH There are two types of photoreceptors: rods and cones
If blood pressure is too high, baroreceptors send Rods are very sensitive to light but they have low
impulses to the medulla oblongata which passes them to visual acuity. They can only detect monochromatic
the parasympathetic nervous system. This causes the light (black and white)
SAN to reduce heart rate this is because many rods join to one neuron so
If blood pressure is too low, then impulses are sent the signal is amplified but the light from two close
along the sympathetic nervous system which points cannot be told apart
increases heart rate. Cones are less sensitive to light but they have high
If blood CO2 is too low (which causes blood pH to be visual acuity. They can detect light in colour (red,
high), chemoreceptors send impulses to the medulla green, and blue)
oblongata which passes them to the parasympathetic this is because one cone joins to one neuron so
nervous system. This causes heart rate to decrease the signal is weaker but the light from two close
If blood O2 is too low (which causes blood pH to be points can be told apart
low), then impulses are sent along the sympathetic
nervous system which increases heart rate. 6.3. Nervous Coordination
6.2. Receptors Nerve Impulses

Receptors are specific to one kind of stimulus (will only Nerve impulses are transmitted via neurons
detect one kind of stimulus) and there are many different There are three types of neurons: sensory, motor, and
kinds of receptors relay
An example of a receptor is a Pacinian Corpuscle Sensory neurons transmit impulses from receptors
it is a mechanoreceptor (ie detects pressure and like Pacinian corpuscles to the CNS (central nervous
vibrations) and it is found in the skin system)
Motor neurons transmit impulses from the CNS to
effectors eg muscles
Relay neurons transmit signals between sensory and
motor neurons

\
A Pacinian Corpuscle is made up of the end of a sensory
neuron wrapped in layers of connective tissue called
lamellae
When a stimulus is applied, eg a nudge, the lamellae When a neuron is at rest, the inside of the cell membrane
deform and press on the sensory nerve ending is more negative than the outside
This causes sodium ion channels called stretch-mediated This is called the resting potential and it is about
sodium ion channels to open and sodium ions to enter -70mV
the cell The resting potential is maintained by sodium-
This causes a change in the charge of the cell called a potassium pumps
generator potential Sodium-potassium pumps move sodium out of
If this generator potential reaches above a certain the neuron and potassium into the neuron
threshold, it will form a nerve impulse known as an The cell membrane is permeable to potassium but
action potential not sodium, so the potassium inside the neuron
Another example of a receptor is a photoreceptor (light moves out down its concentration gradient
receptor) in the retina of the eye.

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This means that overall there are more positive A synapse is where an impulse is transferred from one
charges on the outside of the cell than on the neuron to another
inside which results in the inside being more There is a small gap between the neurons called the
negative than the outside synaptic cleft
When a neuron is stimulated, the membrane is The area of the neuron before the synapse is called
depolarised and forms an action potential the presynaptic knob and it is filled with vesicles
Na+ voltage-gated channels open so Na+ ions move containing chemicals called neurotransmitters
into the membrane, depolarizing it In a cholinergic synapse, the neurotransmitter is
If this depolarization reaches the threshold acetylcholine
potential of about -55mV, it is the beginning of an
action potential
Na+ voltage-gated channels then close and K+
voltage-gated channels open, repolarising the
membrane
Too much K+ enters the membrane, causing
hyperpolarisation
Ions diffuse back to their original places during the
refractory period and the next section of the
membrane is stimulated.
The refractory period ensures that the impulse only
travels in one direction as the membrane cannot be
stimulated

1. An action potential reaches the presynaptic knob

2. This causes Ca2+ voltage-gated channels to open and
calcium ions to enter the presynaptic knob
3. The influx of Ca2+ ions causes vesicles containing the
neurotransmitter acetylcholine to fuse with the
membrane of the presynaptic knob
4. Acetylcholine is released into the synaptic cleft and
binds with receptors on the postsynaptic membrane
5. This triggers Na+ ion channels to open which results
in Na+ entering the membrane and depolarisation
occurring
6. The acetylcholine in the synaptic cleft is broken down
Myelination of a nerve affects the speed of nerve by the acetylcholinesterase enzyme so that the post-
synaptic membrane is not triggered multiple times
transmission
A myelinated nerve is covered by a myelinated sheath There are only receptors on the post-synaptic membrane
which is an electrical insulator
so the impulse only travels in one direction
It is made up of a layer of Schwann cells wrapped
(unidirectionality)
around the neuron
The gaps between the neurons are called the Nodes
of Ranvier
6.4. Skeletal Muscles
Depolarisation only occurs at these Nodes of Ranvier
Skeletal muscles are stimulated to contract by nerves and
so the impulse jumps from one Node to the next,
act as effectors
speeding it up
This is called saltatory conduction They are set in antagonistic pairs: in a pair, one muscle
The bigger the axon diameter, the faster the speed of pulls one way and the other muscle pulls the other way.
transmission The skeleton is incompressible (rigid) so the muscles
Temperature affects the speed of transmission - it is have something to pull against.
fastest at 40C but decreases at any other temperature
Structure of a muscle
Synaptic Transmission
Skeletal muscle is made up of long muscle fibre cells

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The cell membrane of a muscle fibre is called the reticulum

sarcolemma and it folds in on itself to form a network 2. The sarcoplasmic reticulum releases Ca2+ into the
of T-tubules sarcoplasm (cytoplasm)
The sarcoplasmic reticulum is the endoplasmic 3. Ca2+ binds to a receptor on tropomyosin and this
reticulum of muscle cells. It stores and releases Ca2+ causes it to uncover the binding sites on the actin
ions that are needed for muscle contraction. filament
Muscle cells contain a lot of mitochondria which 4. The troponin head binds to the binding site
release the energy needed for muscle contraction. 5. ATP on the troponin head is converted to ADP + Pi,
Muscle fibres have long organelles called myofibrils that releasing energy and this causes the troponin head to
contract bind, forming an actin-myosin cross bridge
6. This pulls the actin filament across the myosin
filament, which shortens the myofibril
7. Another ATP attaches to the troponin head, which
causes it to detach from the actin filament and be
ready to bind onto another
8. This continues as long as the muscle cell is
depolarised.

ATP and phosphocreatine provide the energy for muscle

contraction
ATP is produced in three ways:

1. Aerobic respiration in the cell’s mitochondria

2. Anaerobic respiration (but this leads to the build-up of
lactic acid)
3. ATP-Phosphocreatine (PCr) system:
1. ADP + Phosphocreatine → ADP + Creatine
2. Phosphocreatine is stored inside cells and can
Myofibrils are made up of bundles of myosin and actin release ATP very quickly
filaments 3. The creatine is broken down into creatinine
Myosin bundles are thick and actin bundles are thin and removed from the blood via the kidneys
A myofibril is made up of short units called 4. This system is anaerobic and alactic (doesn’t
sarcomeres produce lactic acid)
The ends of the sarcomeres are called the Z-line
The middle is the M-line Muscles are a mix of slow-twitch and fast-twitch muscle
Around the M-line is the H-zone, that only contains fibres
myosin
The sliding filament theory explains how sarcomeres Slow-twitch Fast-twitch
contract Contract slowly Contract fast
The myosin and actin fibrils slide over each other Good for short bursts of
Good for endurance
instead of contracting themselves speed
This causes the muscle cell to reduce in length and Used for posture Used for fast movement
the muscle to contract
Red because they are rich in Whitish because they don’t
oxygen-storing myoglobin have much myoglobin
Myofibril contraction

The myosin filament has many troponin ‘heads’ that can 6.5. Homeostasis
bind to actin
However, when a muscle is relaxed, a protein called Homeostasis involves using physiological systems to
tropomyosin covers the binding sites on the actin maintain a constant internal environment within fixed
filament limits
A stable internal temperature and pH is very important to
ensure that the enzymes involved in metabolic reactions
are working at their optimum temperatures
Negative feedback is used in homeostasis
If the stimulus is above the limits, then the body will
work to reduce it and vice versa
1. An action potential arrives at a muscle cell and This allows for greater control over the levels of the
spreads down the T-tubules into the sarcoplasmic stimulus

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Maintaining blood glucose concentration Water is filtered in the nephrons in the kidney

Glucose is used as a source of energy by many cells in the

body so it is important to keep it at a constant level
Blood glucose levels rise after a carbohydrate-containing
meal and fall between meals
Blood glucose is controlled by two hormones: insulin and
glucagon
Both are secreted by groups of cells in the pancreas
called islets of Langerhans
insulin is secreted by β cells and glucagon is secreted
by α cells
The nephrons are found in the cortex and medulla
If blood glucose is too high: layers of the kidney
The hormone ADH is used in the regulation of blood
1. The pancreas detects the rise in blood glucose
glucose
2. β cells start secreting insulin and α cells stop
It is secreted by the posterior pituitary gland in the
secreting glucagon brain
3. Insulin binds to receptors on liver and muscle cells It controls how permeable the distal convoluted
4. Muscle cells are stimulated to take up more glucose tubule (DCT) and collecting duct is to water which
5. Liver cells are stimulated to take up glucose and changes the amount of water that is reabsorbed
initiate glycogenosis (making glycogen from glucose)
6. Blood glucose levels drop back to normal If blood water potential is too high:

If blood glucose is too low: 1. Osmoreceptors in the hypothalamus detect the rise in
water potential
1. The pancreas detects a drop in blood glucose 2. The posterior pituitary gland releases less ADH into
2. β cells start secreting glucagon and α cells stop the blood
secreting insulin 3. Less ADH results in the DCT and collecting duct
3. Glucagon binds to receptors on liver cells becoming less permeable to water so less water is
4. This activates glycogenolysis (breaking down glycogen reabsorbed
into glucose) and gluconeogenesis (making glucose 4. This results in a larger amount of more dilute urine
from fats) being produced
5. Liver cells release glucose into the blood
6. Blood glucose levels return to normal If blood water potential is too low:

Maintaining the water potential of blood 1. Osmoreceptors in the hypothalamus detect the drop
in water potential
Water potential of blood is regulated by the kidneys 2. The posterior pituitary gland releases more ADH into
the blood
3. More ADH results in the DCT and collecting duct
becoming more permeable to water so more water is
reabsorbed
4. This results in a smaller amount of more concentrated
urine being produced

7. Genetics, Population,
Evolution, and Ecosystems

In the kidneys, water is filtered out of the blood and

7.1. Inheritance
then reabsorbed so that the water potential of blood
Genotype: the genetic constitution of an individual (what
can be controlled
genes they have)
Phenotype: the expression of this genetic constitution
and its interaction with the environment (what genes are
expressed)
Genes can have different versions, called alleles

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normally there are two alleles of a gene, but The Chi-squared test is a statistical test used to see if the
sometimes there can be more results of an experiment support the expected result.
Alleles can be dominant or recessive The test tells you if you can reject the ‘null hypothesis’
Dominant alleles will always be expressed if they are - which is that there is no significant difference
present while it takes two copies of a recessive allele between the expected data and observed data
to allow it to be expressed If you reject the null hypothesis, it means that there is
In fruit flies, the allele for normal wings (N) is a significant difference between the expected and
dominant, and the allele for vestigial wings (n) is observed data.
recessive You can use the chi-squared test to compare the
NN and Nn will all result in a fly with normal wings, goodness of fit of observed phenotypic ratios with
while only nn will result in a fly with vestigial wings expected ratios.
Co-dominant alleles will both be expressed if they are To run the chi-squared test, you need to calculate a test
present statistic and then compare it with a critical value in a
In humans, blood type is determined by genes: Io is table.
the allele for blood type O, Ia is the allele for blood
type A, and Ib is the allele for blood type B. 1. Use this formula to work out the test statistic:
Ia and Ib are co-dominant and Io is recessive
So Ia Ia and Ia Io will result in type A, Ib Ib and Ib Io
will result in type B, and only Io Io will result in type O
Ia Ib will result in type AB
You can show inheritance on an inheritance diagram like
a Punnet square
This can be used for both monohybrid and dihybrid
crosses
Monohybrid cross: when the inheritance of only one
characteristic is studied
Dihybrid cross: when the inheritance of two
characteristics is studied. 2. Find the right critical value from the chi-squared table:
1. Work out the degrees of freedom (number of
classes - 1) and the probability level (p-value)
you are using (usually 0.05)
2. Look in the corresponding row and column of
the table to find the critical value
3. Compare the critical value and the test statistic
1. If the test statistic is > or = to the critical value,
there is a significant difference between the
observed and the expected results, so the null
hypothesis can be rejected
2. If the test statistic is < the critical value, there is
no significant difference and the null
hypothesis cannot be rejected
You can predict the ratios of the offspring with a genetic
diagram 7.2. Populations
Sometimes the actual ratio will be very different from
the expected ratio A population is a group of organisms of the same species
This can be due to sex linkage, autosomal linkage, or occupying a particular space at a particular time that can
epistasis potentially interbreed.
Sex linkage is when the two genes are on the same sex Species exist as one or more populations of organisms
chromosome (X or Y) so they tend to be linked to the sex eg there are populations of the American Black Bear
of the offspring (Ursus americanus) in America and Canada
Autosomal linkage is when the two genes are on the Even though the populations are separate, they are
same autosomal chromosome (any of the other still part of the same species
chromosomes) so they tend to be inherited together The gene pool is the complete range of alleles present in
Epistasis is when one gene affects the expression of the a population
other Allele frequency is how often an allele occurs in a
population
The Chi-Squared Test It’s usually calculated as a percentage of the total
population eg 43%

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The Hardy-Weinburg Principle Speciation is when a new species forms from an existing
species
The Hardy–Weinberg principle provides a mathematical This is due to reproductive isolation which can be
model, which predicts that allele frequencies will not allopatric or sympatric
change from generation to generation. Allopatric speciation occurs due to geographical isolation
However, the principle only is used under certain Eg a population of the species gets trapped in an
conditions isolated area with a different climate
population is large Over time, the population will adapt to the new
no immigration or emigration climate leading to changes in allele frequency and
no mutations therefore changes in phenotype
no natural selection This results in individuals from the two populations
random mating being unable to breed to form fertile offspring, which
There are two Hardy-Weinburg equations: means that they are different species.
p+q =1 Sympatric speciation is similar to allopatric speciation but
p2 + 2pq + q 2 = 1 doesn’t require geographical isolation
p is the allele frequency of the dominant allele and q This can be due to:
is the allele frequency of the recessive allele polyploidy (different chromasome numbers)
seasonal isolation (different individuals have
different flowering or mating seasons)
7.3. Evolution and speciation mechanical isolation (differences in genetalia
prevent mating)
Individuals within a population of a species may show a
behavioral isolation (incorrect or differnet
wide range of variations in phenotype.
courtship rituals prevent mating)
This is because of genetic and environmental factors
Genetic drift is when a change in allele frequency occurs
Genetic variation stems from mutation, meiosis, and
by chance
random fertilisation of gametes
This can lead to evolution but often has a greater
Environmental variation is due to environmental
effect in smaller populations
factors such as climate, lifestyle, and food availability
The diversity of life on earth today results from millions of
Only genetic variation results in evolution
years of evolutionary change
Evolution is a change in allele frequency over time
Natural selection is one method by which evolution
occurs but it can also occur due to artificial selection 7.4. Populations in Ecosystems
1. There is variation amongst individuals in a population Populations of different species form a community
due to different individuals having different alleles A community and the non-living components of its
2. Selection pressures such as predation, competition, environment together form an ecosystem.
and disease, create a struggle for survival Ecosystems can range in size from the very small to
3. The individuals that are best adapted to the selection the very large.
pressures survive A habitat is a place where an organism lives
4. They then pass their alleles to their offspring Each species has its own niche in a habitat
5. This results in a greater proportion of the population This niche depends on adaptation to both abiotic
having the more desirable alleles (non-living, eg temperature) and biotic (living, eg the
6. Over time, the frequency of the desired allele presence of predators) conditions
increases, which is evolution. If two organisms occupy the same niche then there
will be competition between them and the better-
Selection can be stabilising, directional, or disruptive
adapted organism will survive
Stabilising and directional selection were discussed in
The carrying capacity of an ecosystem is the maximum
Chapter 4
population of a species that it can support
Disruptive selection is when individuals with extreme This varies due to abiotic and biotic factors
phenotypes are more likely to survive and reproduce abiotic factors include the amount of light, water, or
Eg in a bird population, birds with large beaks can eat
space available
large seeds and birds with small beaks can eat small
biotic factors include interspecific competition
seeds but birds with medium-sized beaks can eat (competition between species), intraspecific
neither
competition (competition within a species), and
Over time, most birds end up having either large or predation
small beaks
Investigating a population
Speciation

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To investigate a population, you need to take a random (this is called the climax community)
sample of the area you are investigating
For slow-moving or non-motile organisms, use a
quadrat to measure the % cover of the species at
different points in the habitat
For motile organisms, use the mark-release-recapture
method.

The mark-release-recapture method:

1. Capture a sample of a species

2. Mark them in a way that will not affect their chances
of survival
3. Release them, and wait for a sufficient amount of time
to allow them to distribute evenly into the population
(usually a week) At each stage of succession, the species inhabiting the
4. Take a second sample and count the number of ecosystem change it so that it becomes more favourable
marked organisms in this sample for new species that then outcompete the previous
5. Use this equation to work out an estimate of species
population size. Conservation often involves managing succession and
preventing the climax community from forming
Assumptions made when using the mark-release- In Scotland, there are large areas of open moorland
recapture method: that support many species of plants and animals. If
The marked sample has had enough time and succession is allowed to take place, eventually the
opportunity to mix back in with the population moorland will become woodland which would lead to
The marked individuals are just as adapted to survive the loss of the moorland and some of the animals
as the unmarked ones living there.
There are no changes in population size (due to birth, To prevent this from happening, animals are allowed
death etc) while the study was being carried out to graze the land to prevent trees from growing and
Ecosystems are dynamic systems which means that they managed fires are lit, which causes moorland plants
are constantly changing to grow back but not shrubs and trees
The way they change is called succession
Primary succession happens on newly-exposed rock Conservation
eg formed by a volcanic eruption
Secondary succession happens in an area that has no Conservation is the protection and management of
vegetation but still has soil eg land after a forest fire species and habitats in a sustainable way
There is often conflict between human needs and
Stages of succession conservation
Careful management is needed to balance economic
1. To start with, the abiotic conditions of the habitat are
and ecological goals
quite harsh as there is no soil.
There are many methods of conservation - some focus on
2. Pioneer species such as lichens begin to grow on the
protecting a particular species and others focus on the
rocks which break them down, releasing minerals
ecosystem as a whole
3. When the lichens die, they form a thin layer of soil on
Fishing quotas are limits to the numbers of fish that
which other species eg moss can grow
can be caught, which help protect fish stocks
4. As the soil deepens, larger plants can take root such
Zoos can run breeding programs for endangered
as grasses and flowering plants
species, eventually returning the animals to the wild
5. Shrubs, ferns, and small trees take root, out-
National parks and other protected areas allow for
competing the grasses. As succession progresses, the
habitats to be protected and managed
biodiversity of the habitat increases
Seedbanks preserve the seeds of endangered plants
6. Eventually, the soil is deep enough to support
so they can be re-grown
hardwood trees. This is the final stage of succession -
Conflicting evidence may have to be considered in
the ecosystem is supporting the largest and most
regards to conservation
complex community of plants and animals that it can

8. The control of gene

expression

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Totipotent cells can divide and produce any type of body

8.1. Alteration of the sequence of bases
cell
in DNA can alter the structure of When they go on to develop, they only translate a
small portion of DNA
proteins
This results in them becoming specialised
Totipotent cells are found for a short time in embryos
Gene mutations can arise during DNA replication
These include: Pluripotent stem cells are also found in embryos but they
Addition: one or more bases are added to the DNA are able to specialise into fewer types of cells than
totipotent stem cells
sequence (eg ATG becomes ACTG)
Unipotent stem cells can only differentiate into one kind
Deletion: one or more bases are deleted from the
DNA sequence (eg ATG becomes AG) of cell
eg cardiac stem cells can only differentiate into
Substitution: one or more bases are swapped for
cardiomyocytes
another (eg ATG becomes ACG)
Induced pluripotent stem cells (IPS) can be produced
Inversion: a sequence of bases is reversed (eg ATG
from adult somatic stem cells
becomes GTA)
Duplication: one or more bases are repeated (eg This is done by adding the appropriate protein
ACTG becomes ACTCTG) transcription factors
Translocation: a sequence of bases is moved from
one location in the genome to another - this can be to 8.3. Regulation of transcription and
a new spot on the same chromosome or to a
translation
completely different chromosome
Gene mutations occur spontaneously but the rate of
In eukaryotes, the transcription of target genes can be
mutation can be increased by mutagenic agents
stimulated or inhibited
mutagenic agents include UV radiation, ionising
This occurs due to specific transcription factors
radiation, some chemicals, and some viruses moving from the cytoplasm to the nucleus
Mutations can result in a different amino acid sequence
Once in the nucleus, they bind to specific DNA sites
in the encoded polypeptide.
near their target genes
Sometimes, only one triplet code is changed
This has the effect of activating (stimulating or
Since the genetic code is degenerate (more than one increasing) transcription of the gene or repressing
triplet codes for the same amino acid), a mutation
(inhibiting or decreasing) transcription.
changing one triplet code may not change the amino
The steroid hormone oestrogen is an example of a
acid sequence of a protein
transcription factor
Eg: both TAT and TAC code for the amino acid
Oestrogen binds to an oestrogen receptor to form an
Tyrosine. Therefore, a substitution mutation changing oestrogen-oestrogen receptor complex
TAT to TAC will not change the amino acid sequence This complex moves from the cytoplasm to the
of the protein
nucleus where it binds to its DNA target site
Mutations can cause a ‘frame shift’ - there is a change in
This complex acts as an activator of transcription by
all the base triplets that follow
helping RNA polymerase to bind to the DNA strand
Eg a deletion mutation will cause a frameshift
In the base sequence ATT ATG GAC CAT, if the first T is
deleted, it will read as ATA TGG ACC AT… so all the
triplets will change and so will the sequence of amino
acids

8.2. Gene expression is controlled by a

number of features
Stem cells

There are two types of cell: somatic and stem Epigenetic control of gene expression
Usually, only part of a cell’s DNA is translated
Eg a muscle cell has the same DNA as a hair cell but Epigenetics involves heritable changes in gene function,
different portions of the DNA are translated which without changes to the base sequence of DNA.
makes the cells different This determines whether a gene is ‘switched on’ or
‘switched off’ by changing how easily the gene can be
Stem cells are body cells that can differentiate into many
different kinds of cell transcribed
There are several types of stem cell Some epigenetics can be inherited

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To switch a gene off, more methyl groups (-CH3) are Determining the genome of simpler organisms, such as
added to the DNA bacteria and viruses, allows the proteome of the
This makes it harder to transcribe the gene organism to be determined.
To switch a gene on, more acetyl groups (-COCH3) are The proteome is the protein sequences that are made
added to the DNA by the genome
This makes it easier for the DNA to be transcribed This may have many applications, including the
Epigenetics can lead to the development of genetic identification of potential antigens for use in vaccine
diseases and cancer production.
Fragile-X syndrome is a condition where there is More complex organisms have non-coding DNA and
increased methylation of the FMR1 gene because of a regulatory genes, so it is more difficult to work out the
deletion mutation proteome from the genome
This causes the gene to be switched off and the Sequencing methods are continuously updated and have
protein it codes for not to be produced become automated.
Increased acylation of genes that cause mitosis can Eg pyrosequencing. It is a new method that can
lead to cancer sequence up to 400 million DNA bases in under 10
hours
RNA interference in translation
8.5. Recombinant DNA technology
In eukaryotes and some prokaryotes, translation of the
mRNA produced from target genes can be inhibited by
Recombinant DNA technology involves the transfer of
RNA interference
fragments of DNA from one organism, or species, to
Specific RNA molecules called siRNA (small interfering
another
RNA) and miRNA (microRNA) disrupt the translation
The genetic code is universal, so the transferred DNA
siRNA cuts up the mRNA so it can’t be translated
will be translated in the cells of the recipient organism
miRNA binds to the mRNA to block translation
An organism that has had DNA transferred to it is
called transgenic
Gene expression and cancer
Fragments of DNA can be produced in several ways:
Using reverse transcriptase to convert mRNA to cDNA
A mass of abnormal cells is called a tumour
Using restriction enzymes to cut a fragment
There are two main types of tumour: benign and
containing the gene from the DNA
malignant
Benign tumours are not cancerous. They grow slowly Using a ‘gene machine’ to create the desired section
of DNA
and are often harmless but some can become
malignant tumours
Amplifying DNA fragments
Malignant tumours are cancerous. They grow rapidly
and invade surrounding tissue. Pieces of the tumour
Fragments of DNA can be amplified by in vitro and in vivo
can break off and spread to other parts of the body
techniques.
through the blood or lymphatic system
In vitro refers to techniques that involve a lab
Tumours can be caused by changes to genes called
In vivo refers to techniques that use living cells
tumour suppressor genes and proto-oncogenes
The polymerase chain reaction (PCR) is an example of an
If many methyl groups (-CH3) bind to tumour
in vitro method
suppressor genes, it becomes harder to transcribe the
PCR makes millions of copies of a DNA fragment in a
gene and this means that mitosis isn’t regulated
few hours
Unregulated mitosis then leads to cancer
If many acetyl groups (-COCH3) bind to proto- 1. A reaction mixture containing the DNA sample, free
oncogenes that promote mitosis, they become nucleotides, DNA polymerase, and DNA primers
oncogenes which then stimulated cells to divide (short pieces of DNA that are complementary to the
uncontrollably ends of the DNA strand) is set up
Increased concentrations of the hormone oestrogen can 2. The mixture is heated to 90C to break the hydrogen
play a part in the development of some breast cancers bonds between the complementary strands of DNA
3. The mixture is then cooled to between 50 and 65C so
8.4. Using genome projects that primers can anneal (bind) to the strands
4. The mixture is heated again to 72C for the DNA
Sequencing projects have read the genomes of a wide polymerase to form new complementary strands of
range of organisms, including humans. DNA
The Human Genome Project mapped the entire 5. In every cycle of PCR, the volume of DNA doubles
genome of several volunteers in 2003

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Medicines and vaccines can be made cheaply and

easily using recombinant DNA technology
However, companies may limit the use of
potentially life-saving products to increase profit,
and these techniques can be used unethically eg
to produce ‘designer babies’
Recombinant DNA can be used for ‘gene therapy’ -
altering defective genes to treat genetic disorders and
cancer

8.6. DNA probes and genetic

fingerprinting
DNA probes and DNA hybridisation

Recombinant DNA technology is an example of an in vivo DNA probes are used to locate specific alleles of genes
method: This can be used to see if a person has an allele that
could lead to them having a genetic disorder
1. The required DNA fragment is isolated using DNA probes are short strands of DNA that have a
restriction endonucleases, reverse transcriptase, or a complementary base sequence to the target allele
gene machine. The DNA is cut to form ‘sticky ends’ They also have a fluorescent or radioactive label
that allow it to join to other DNA strands attached which means they can be seen under UV
2. The DNA is inserted into a loop of plasmid DNA that light or in an X-ray
acts as a vector. Ligases are used that act as ‘glue’ to If they meed the target allele, they bind (hybridise)
join the DNA to the strands by forming with it
complementary sticky ends
3. This recombinant plasmid is transferred to a 1. A sample of DNA is digested into fragments using
bacterium and the transformed bacteria are identified restriction enzymes and separated through a process
and isolated. known as electrophoresis
1. ‘Marker genes’ can be inserted into the 1. Electrophoresis is discussed in detail later in
plasmid as well so that bacteria that have this section
taken up the plasmid can be identified 2. The separated DNA fragments are then transferred to
2. These marker genes can cause the bacterium a thin nylon membrane and the fluorescently -
to fluoresce (glow) under UV light or give the labelled DNA probe is added
bacterium to be resistant to a certain antibiotic 3. If the allele is present, the DNA probe will bind to it
4. The bacteria are allowed to grow and divide and the 4. The membrane is then exposed to UV light and if the
protein produced from the recombinant plasmid is allele is present, a fluorescent band will appear
isolated.
DNA probes can also be used in DNA microarrays
1. Promotor and terminator regions are regions
A microarray is a glass slide with different DNA probes
of the DNA that start or stop transcription
attached to it
2. These may need to be added to the DNA or
When using a microarray, the DNA probes are not
may already be present in the plasmid DNA
labelled; the human DNA that is added is
Recombinant DNA technology has many benefits to The human DNA binds to any of the DNA probes that
humans but also poses some ethical issues are complementary
Crops can be transformed to give higher yields or be When the tray is rinsed and looked at under UV light,
more nutritious (eg Golden Rice) only certain spots on the microarray will light up. This
However, genetically identical crops are more indicates which alleles are present.
susceptible to disease, weeds may breed with DNA probes can be used to screen for:
genetically modified crops to create ‘superweeds’, Inherited conditions such as Huntingdon’s disease.
and seeds from genetically modified crops can This disease does not have any effects until around
contaminate organic crops age 40 so screening for it can allow a person to start
Industrial enzymes can be formed using recombinant the treatments as soon as possible
DNA technology, which reduces costs Responses to specific drugs such as cancer
However, this will make some companies more medications that will only be effective if a person has
powerful as they can take out patents on certain a specific mutation in an allele
enzymes and charge others for using them Health risks such as genetic predispositions to
developing certain cancers. This can allow people to

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make lifestyle choices that reduce their risk. 7. The electrical current is turned off and the DNA is
This screening can be used in genetic counselling transferred to a thin nylon membrane in a process
known as Southern Blotting
Genetic fingerprinting 8. Under UV light, bands of DNA can be seen which is
the individual’s genetic fingerprint
Some of the non-coding DNA of an organism is made up 9. Two genetic fingerprints can be compared and if both
of Variable Number Tandem Repeats (VNTRs) fingerprints have a band on the same location on the
They are long chains of repeating base sequences eg gel, it means they have the same number of VNTRs
ATGCATGCATGCATGCATGC… and are therefore related
The number of repeats varies between individuals
and can be compared Genetic fingerprinting can be used in:
This is known as genetic fingerprinting Paternity tests, to determine who the father of a child
is
Gel electrophoresis is used to separate DNA fragments to Forensic science, to compare suspects’ DNA with that
make a genetic fingerprint found at the crime scene
Medical diagnosis, to diagnose genetic disorders and
1. A DNA sample is obtained and the areas containing
cancer
VNTRs are amplified using PCR
2. DNA fragments corresponding to the length of the
VNTRs are obtained and a fluorescent tag is added
3. The DNA mixture is placed in a slab of gel and is
covered with a buffer solution that conducts
electricity
4. An electrical current is passed through the gel
5. DNA is negatively charged so the fragments move
towards the positive electrode
6. Smaller, more charged DNA fragments move further
than heavier, less charged ones

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Biology

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