IGCSE Biology Compliled by Mr. A.M.

Masoko For Skylimit Academy

Topic 1
Characteristics Of Living
Organisms
Characteristics of living organisms
Every living organism have certain traits that it needs to be
demonstrating in order for it to classify as “living”. There are 7
characteristics that we need to go through. If even a single one
of these characteristics are not present in an organism, then it
not alive.

1. Movement – An action by an organism or part of an

organism causing a change of position or place
2. Respiration – The chemical reactions in cells that break
down nutrient molecules and release energy for
metabolism
3. Sensitivity – The ability to detect or sense stimuli in the
internal or external environment and to make
appropriate responses
4. Growth – Permanent increase in size and dry mass by an
increase in cell number or cell size or both
5. Reproduction – The processes that make more of the
same kind of organism
6. Excretion – Removal from organisms of the waste
products of metabolism (chemical reactions in cells
including respiration), toxic materials, and substances in
excess of requirements
7. Nutrition -Taking in of materials for energy, growth and
development; plants require light, carbon dioxide, water
and ions; animals need organic compounds and ions and
usually need water

Concept and use of classification system

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Organisms can be classified into groups by features that they

share. Classification systems aim to therefore classify groups of
organisms in a systemic way, in order to reflect their
evolutionary relationships.

Before the advance of technology and science, classification

was traditionally based on morphology and anatomy. Basically
this means that if certain organisms “looked” similar and shared
similar features, then they would be classified under the same
umbrella. But it is important to understand that now, we can
more accurately classify organisms by analyzing their DNA
(rather than look at appearance alone). Organisms which share
similar DNA base sequences and protein amino acid sequences
are more likely to be closely related.

Every organism has a scientific name. The binomial system is

an internationally agreed system in which the scientific name
of an organisms is made up of two parts (Genus & Species).
The Genus is a generic term used in the classification of
living organisms or binomial nomenclature and species is a
group of organisms that can reproduce to produce fertile
offspring.

For example: Homo sapiens

It is important to understand here that Homo represents the

genus and is always starts in capital letters. Sapiens is the
species and is all lower case and in italics.

Features of organisms
All organisms are made of cells. Although the cellular structure
may be different depending on the type of organism, there are
certain things that are universally shared across every single
organism:
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 Cytoplasm
 Cellmembrane
 DNA
 Ribosomes
 Enzymes

Now, we can actually organize every single organism into 5

main categories called “kingdoms”. Here are the 5 kingdoms
that you need to know:

 Animal (i.e. Lion)

 Plant (i.e. Tree)
 Fungus (i.e. Yeast)
 Prokaryote (i.e. Bacteria)
 Protoctist (i.e. Marimo)

You need to know the features of these 5 kingdoms so that for a

given example of an organism, you can determine the kingdom
it belongs in. Here is a simple table to help you.

Now that we understand the features of the 5 kingdoms, we can

actually branch further. The syllabus wants you to understand
how to classify different “types” of animals and plants. In the
animal kingdom, animals they can further be classed as
vertebrates (with backbone) or invertebrates (no backbone).
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In the plant kingdom, plants can be classed into either flowering

plants or ferns. The table below (Oxford Revision Guide 2018)
demonstrates the further classification of both the animal and
the plant kingdom.
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Dichotomous keys
A dichotomous key is a tool that allows the user to determine
the identity of items in the natural world, such as trees,
wildflowers, mammals, reptiles, rocks, and fish. Keys consist of
a series of choices that lead the user to the correct name of a
given item.

For example, use the key to answer the question

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1. Wings present ……………………………………… Go to 2

Wings absent ……………………………………….. Go to 3
2. One pair of wings visible ………………………………. A
Two pairs of wings visible …………………………….. B
3. Three pairs of legs …………………………………………. C
Two pairs of legs …………………………………………… D

Since this insect has wins, and has two pairs visible, the answer
is B!
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Topic 2
Organisation Of An Organism
Cell structure and organisation
First of all, all organisms are mode of cells. They are like the
lego blocks of life. The syllabus wants you to know how to draw
a basic animal and plant cell, label its structures, and also
explain the functions of each of the structures too.

At a very basic level, please refer to the diagram was below.

The plant cell has everything that an animal cell has, plus some
added structures which are are in green text. The functions of
each of these structures will be discussed further down the
page.

So all cells have a cell membrane which is what allows or

disallows certain things entering and exiting the cell. The
nucleus contains genetic information (DNA) and the cytoplasm
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is a jelly-like substance in which everything else in the cell is

suspended in. The mitochondrion is the “power house’ of the
cell and the reason for this name is due to the fact that
respiration occurs here. Plants have some extra structures such
as cell walls (to support the cell) and chloroplasts for
photosynthesis. You will learn more about these in future topics.
Plants also have a permanent vacuole, whereas animal cells
have small temporary ones.

Whilst the above diagram but be sufficient for the core syllabus,
the extended course wants you to know two extra structures:

 Rough endoplasmic reticulum (RER)

 Ribosomes

The RER is basically a set of tubular membranes near the

nucleus which have ribosomes studded onto it, and the
ribosomes are then used for protein synthesis.

These ribosomes can ether be found on the RER (as mentioned

before) but it can be found free in the cytoplasm as well. If you
are comfortable with the first diagram, take a look at this one!
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Structure & function summary

 Cell membrane – Selective control of what goes in and out

of the cell
 Nucleus – Carries genetic material (DNA)
 Cytoplasm – Jelly like substance in which chemical
reactions take place
 Vacuole – The vacuole has many functions
 Stores/isolates harmful material
 Stores small nutrients
 Maintains water balance
 Structural support for plant cells via turgor
pressure
 Rough endoplasmic reticulum – Studded with ribosomes
 Ribosomes – Site of protein synthesis
 Mitochondria – Site of aerobic respiration (cells with high
metabolism rates will need lots of these to offer sufficient
energy)
 Cell wall – Structural support for plant cells
 Chloroplast – Site of photosynthesis in plant cells

Levels of organisation
There are levels of organisation that you need to be aware of.
As we discussed before, the smallest unit of a living thing is a
cell. So that’s a good place to start. A group of cells are called
tissues, a group of tissues are called organs, and a group of
organs are then called organ systems. Take a look here:

 Cell– The smallest structural and functional unit of an

organism
 Tissue – Group of cells with similar structures working
together to perform a shared function
 Organ – Structure made up of a group of tissues, working
together to perform specific functions
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 Organ system – Group of organs with related functions,

working together to perform body functions

Now some cells have structures that help them with their
particular function. There are a couple of these examples that
CIE wants you to know:

 Ciliatedcells
 Root hair cells
 Xylem vessels
 Palisade cells
 Nerve cells
 Red blood cells
 Sperm and egg cells

Each of the things above will naturally be covered in more detail

in other topics in the syllabus and therefore will not be covered
here.

Size of specimens
In the lab, a lot of biology is done under a microscope. For
example, we can’t exam the cells of a human tissue with our
naked eyes right? Therefore the purpose of the microscope is to
magnify ourspecimen so that it appears bigger for us to be able
to actually see.

Naturally, the CIE syllabus wants you to be able to perform

basic equations regarding magnification, the image size (of the
specimen) and the actual size (of specimen). Please memorize
the following equation:
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In an exam, they will always give you 2 out of the 3 factors in

the equation and tell you to find the missing one. Just apply the
formula above and it will be a walk in the park!
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Topic 3
Movement In And Out Of Cells
Diffusion
Diffusion is the net movement of particles from a region of their
higher concentration to a region of their lower concentration
down a concentration gradient, as a result of their random
movement.

The constant random movement of particles (and their kinetic

energy) allows diffusion to occur. Ultimately this means that
particles will spread out.

It is also important to understand that diffusion is quite often

how molecules move in and out of our cells through the cell
membrane.

For example, the diagram below demonstrate a cell surrounded

by nutrients (red dots). We can see that on the left, there are a
lot more nutrients outside the cell than inside the cell.
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By diffusion, the nutrients will diffuse into the cell (from higher
to lower concentration) until the number of nutrients inside and
outside the cell are balanced.

There are certain factors that affect the rate of diffusion:

 Surface area
 The larger the surface area, the higher the rate of
diffusion. This is because more molecules at a
given time will be diffusing.
 Temperature
 The higher the temperature, the higher the rate
of diffusion. This is because molecules are faster
and have more kinetic energy with higher
temperatures.
 Concentration gradients
 The higher the concentration gradient, the higher
the rate of diffusion.
 Distance
 The shorter the distance, the higher the rate of
diffusion. This is quite self-explanatory. The
shorter the distance the particles have to move,
the quicker the process is going to be.
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Osmosis
Concept of osmosis

Osmosis is the net movement of water molecules from a region

of high water potential (dilute solution) to a region of low water
potential (concentrated solution) through a partially permeable
membrane.

Think of osmosis as the diffusion of water across a partially

permeable membrane. When we are talking about water, we
cannot use the term ‘concentration’ anymore because a
concentration denotes the amount of substance dissolved in
water.

Because water cannot be dissolved in water, we need to use

another term instead: Water potential.

 For a very dilute solution, because it has a lot of water, it

has a high water potential.
 For a very concentrated solution, because it has less water,
it has a low water potential.
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So lets apply this concept to osmosis. Refer to this diagram:

The left hand side of the beaker has less solutes dissolved so
therefore the solution is more dilute (or less concentrated)
compared to the right hand side.

Under normal circumstances, the sugar themselves will diffuse

across from RHS to the LHS via diffusion (as we talked about
earlier). However, the sugar molecules are too large to pass
through the partially permeable membrane, and therefore
cannot diffuse.
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Water molecules however can pass by freely through the

membrane. The molecules will travel from the region of high
water potential to low water potential so therefore in this case,
water will move from the RHS to the LHS via osmosis.

Osmosis in plant & animal cells

Cell membranes are partially permeable so cells absorb or

remove water via osmosis.

Consider these scenarios:

1. Adding cell into pure water / dilute solution

 There will be higher water potential outside the
cell than inside the cell and therefore water will
move in to the cell
 As water enters the cells they become ‘turgid’
 An animal cell can burst if too much water
enters
 A plant cell has support from its cell wall
and therefore will most likely maintain its
turgidity without bursting
2. Adding cell into concentrated solution
 There will be higher water potential inside the cell
than the outside and therefore water will move
out of the cell
 As the water moves out, cells become ‘flaccid’
 An animal cell can become crenated if too
much water is lost
 A plant cell can become plasmolysed if too
much water is lost. This is when the
cytoplasm shrinks due to the loss of water
but the cell wall fails to shrink due to its
tough structure. The cytoplasm eventually
tears away from the cell wall.
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Active transport
Active transport is the movement of particles through a cell
membrane from a region of lower concentration to a region of
higher concentration using energy from respiration.

Active transport is used in cases where diffusion or osmosis

cannot be relied upon. For example, what if a cell wanted to
absorb extra nutrients from outside the cell despite having a
higher concentration of those nutrients inside the cell? Diffusion
wouldn’t work because the concentration gradient is going the
opposite way. These situations are encountered frequently in:

 Plant root hairs

 Villi epithelial cells

Active transport uses energy to oppose the concentration

gradient and forcefully transport molecules against it. Here is a
simple diagram to help you visualize how it all works:
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In the cell membranes of all cells, there are certain embedded

protein molecules that carry out this process. The protein
basically ‘captures’ the molecules from one side of the cell, and
it changes shape in a way to transport the captured molecules
to the other side of the cell. Energy (from respiration) is
required to alter the protein shape (referred as ATP in the
diagram).

Topic 4
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Biological molecules
Carbohydrates

Carbohydrates are made of carbon,hydrogen, and oxygen. They

are used as a source of energy for the body. There are three
types of carbohydrates: Monosaccharides, disaccharides,
and polysaccharides.

 Monosaccharides (i.e. glucose) are the most simplest form

of sugars. They are a single unit and they cannot be
broken down any further to make a more simpler sugar.
 Disacchardies are literally two monosaccharides joined
together.
 Polysaccharides are large chains of monosaccharides
joined together
 Starch is a polysaccharide made of large chains of
glucose
 Glycogen is another polysaccharide made of large
chains of glucose
 Cellulose is a polysaccharide made of large chains
of glucose
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Fats/oils

Fats are made of carbon, hydrogen, and oxygen. The oxygen

content is lower than in carbohydrates. Fats have various
purposes in the body:

 Source of energy. In fact, they have two times higher

energy content than carbohydrates!
 Heat insulation
 Myelin sheath formation
 Cell membrane formation

Fats are made up of three fatty acid units attached to a single

unit of glycerol:
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Proteins

Proteins are made of carbon, hydrogen, oxygen, nitrogen, and

sometimes sulfur or phosphorus. They are extremely important
to the body and serve many different functions. here are a few:

 Growth
 Tissue repair
 Cell membrane formation
 Source of energy

Proteins are made of long chains of amino acids chemically

bonded to each other. There are about 20 different amino acids
that are found in the human body. Different combinations of
these amino acids will give rise to different proteins.

For example, each of the different colored circles represent a

different amino acid. They are joined in a specific sequence as
shown below:
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It is really important to understand here that the final 3D

structure of a protein is derived from the specific interactions
between amino acids that are joined in the protein chain.

The sequence of amino acids in the chain therefore creates the

final shape of the protein, and the shape is what gives the
protein its function.

This diagram below represents how a single chain of amino

acids eventually turn into a complex 3D protein structure with a
specific function.
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For example: Consider amino acids A B C D and E. The hyphens

represent a chemical bond between the amino acids.

Protein 1: A-B-C-D-E
Protein 2: A-C-B-D-E

In the example above, protein 1 has amino acids A through to E

joined in order. Protein 2 on the other hand, has a slightly
different amino acid sequence. Just from this slight difference in
amino acid order, protein 2 will be completely different from
protein 1 in terms of its function and structure. This is super
important for you to understand.

Food testing
We can test for starch, monosaccharides, proteins, and fats in a
given sample via the following tests:

 Starch test [Starch test]

 Add a few drops of iodine solution
 Blue/black coloration means starch is present

 Benedict’s test [Monosaccharide test]

 Add equal amount of benedicts solution into a
solution of food and boil gently
 A colour change (from blue) signifies presence
and quantity of monosacchrides
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 Biuret test [Protein test]

 Add an equal amount of sodium hydroxide to a
solution of food and mix
 Add a few drops of 1% copper sulphate
 A violet colour signifies the presence of protein

 Emulsion test [Fat test]

 Dissolve food in ethanol and pour the solution
into a clean tube of water
 White emulsion signifies the presence of fat

DNA structure
As you may already know, genetic information is stored inside
our DNA. Whilst you do not need to go into full depth with this,
CIE wants you to understand the generic structure of a DNA
molecule:
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So first of all, a DNA has a double helix structure whereby two

strands are coiled together. Each strands have chemicals called
bases. The double helix is held together via pairs of bases that
are attracted to each other from one strand to the other.

Bases will always pair up in the same way. Adenine (A) will
always pair with Thymine (T). Cytosine (C) will always pair with
Guanine (G). The diagram above demonstrates this pairing (i.e.
green is always bonded to purple and pink is always bonded to
blue).

Water
CIE wants you to understand the importance of water. Indeed,
water is essential to the human body for many things. One of
these things being the fact that water is an important solvent.
This means that nutrients and wastes can be dissolved in water
so that it can be transported around the body. Moreover,
majority of our chemical reactions inside our bodies are
controlled by enzymes. Enzymes cannot work unless it is in
solution (i.e. in the presence of water).
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Topic 5
Enzymes
A catalyst is a substance that increases the rate of chemical
reaction and is not changed by the reaction itself.

An enzyme is a biological catalyst that catalyzes many

important reactions inside an organism (such as respiration)
and therefore necessary to sustain life.
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How enzymes work is more easily shown through a diagram.

Take a look below:

Firstly, a substrate is a substance that an enzyme acts on. It is

important to understand that enzymes are very specific, and
the reason for their specificity lies in their active sites – a region
of an enzyme that binds to a particular substrate. The shape of
the active site of an enzyme is complementary to only one
specific substrate.

As demonstrated in the diagram, the green substrate has a

shape that pairs impeccably with the shape enzyme’s active
site. As the enzyme binds with the substrate, an enzyme-
substrate complex is formed. The reaction then occurs on the
enzyme and the enzyme-product complex is formed. The
products eventually leave the enzyme.

Enzyme activity VS Temperature & pH

There are certain factors that can impact enzyme activity. The
two of which CIE wants you to know is temperature and pH. But
before getting into the details, you need to know that all
enzymes have an optimum temperature and an optimum pH.
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These are certain temperatures or pH in which a particular

enzyme work best in, and it can vary between different
enzymes.

For any given enzyme, if the conditions stray too far from the
optimum, then denaturation can occur. This is when extreme
non-ideal conditions (i.e. high temperatures or very low/high
pH) causes chemical bonds in the enzymes to break apart. This
results in the change in shape of the enzyme’s active site.
Remember, the active site has a very special shape and it fits
only one specific type (and shape) of substrate. An alteration in
active site will therefore cause the enzyme to lose function.

Temperature and enzyme activity

Low temperatures reduce the rate of chemical reactions in

general. This is because molecules need to collide with one
another and have enough energy for a reaction to occur. In low
temperatures, molecules are traveling at lower speeds (less
energy) and therefore the rate of successful collisions are lower.
Moreover, even when collisions do occur, the molecules may
have insufficient kinetic energies to begin with, and therefore
the reaction may not occur. Enzyme activity is therefore low in
low temperatures. It is important to note however, that low
temperatures do not denature enzymes.

Higher temperatures generally increase the rate of chemical

reactions. Molecules are faster and have more kinetic energy.
This means that rate of successful molecular collisions are
higher,and most molecules will have sufficient energy required
for the reaction. However, temperatures that are far beyond the
optimum temperature of the enzymes can start to denature it,
and reduce enzyme activity as a result. Most enzymes have an
optimum temperature of approximately 37 degrees in the
human body, and start getting denatured at above 50 degrees.
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pH and enzyme activity

The optimum pH of an enzyme can vary. Pepsin is an enzyme

found in the stomach’s acidic conditions and therefore made to
work best in a pH of approximately 2. Amylase on the other
hand, is found in saliva (more neutral conditions) and therefore
has an optimum pH of 7. Very high or very low pH’s can
denature these enzymes if it deviates too much from their
optimum.
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Topic 6
Plant Nutrition
Photosynthesis
Background

Photosynthesis is the process by which plants manufacture

carbohydrates from raw materials using energy from light:

Chlorophyll is where photosynthesis happens. It transfers light

energy into chemical energy for the synthesis of carbohydrates
(i.e. glucose).

The glucose is then either converted to sucrose for transport

around the plant or starch for storage.

Glucose is too reactive to be transported around the plant on its

own. There it has to to be converted to sucrose first.

It is the same story with storage. Glucose cannot be stored due to its
reactivity, so therefore it must be converted to starch first.

Limiting factor

The term limiting factor is something present in the

environment in such a short supply that it restricts life
processes.
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For instance, if there was a shortage of carbon dioxide but all

other raw materials were in abundance, then carbon dioxide
would be the limiting factor for photosynthesis. If there was a
shortage of chlorophyll, then that would become the limiting
factor instead.

Investigations
Necessity of chlorophyll, light, and carbon dioxide

The equation for photosynthesis shows that chlorophyll, light,

and carbon dioxide (and water) are required, otherwise it won’t
work… But can we prove it? We sure can.

In a nutshell, what we need to do is get a functioning plant and

deprive it of each of these factors individually and prove that
the plant stops photosynthesizing when these factors are
absent.

The way to do that is pretty simple. First of all we are going to

de-starch all our plants by leaving the plants in the dark for 48
hours. During this period the plants will be unable to
photosynthesize and therefore use up all its starch for
respiration. This means that in the beginning of our
experiments, all plants (test and control) will have absolutely no
starch in them.

This means that after the experiment, if we do a starch test

(iodine test) and we find starch is present, it would indicate that
photosynthesis had occurred.

It is to note however that we can’t just add iodine onto a fresh

leaf and expect results. First of all, we need to break the leaf so
that iodine can seep in to begin with. Moreover, we need to
remove the chlorophyll to decolourize the leaf so that the colour
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change from iodine is easier to see. So here are the steps we

need to take:

 Boil the leaf in water

 Kills the leaf to make it permeable
 Boil the leaf in ethanol
 Chlorophyll dissolves and the leaf decolours
 Rinse the leaf in water
 Spread the leaf out on a white tile
 Add iodine solution

To investigate the effect of light on photosynthesis, we need

to partially cover the leaves of the plant and leave it under
sunlight. The covered areas will be deprived of light whereas
the rest will be exposed. A starch test is then carried out after a
few hours. The results should show that the covered areas have
a negative starch test (i.e. no photosynthesis) whereas the
exposed areas have a positive starch test.

To investigate the effect of carbon dioxide on

photosynthesis, we place a test plant in a container with a
carbon dioxide absorber (i.e. sodium hydroxide) and a control
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plant without the absorber. The absorber will remove the

carbon dioxide.

A starch test is then carried out after several hours. The results
should show that the test plant has a negative result whereas
the control plant has a positive one.

To investigate the effect of chlorophyll on photosynthesis,

we need to use a plant with variegated leaves. This means
some parts of the leaf have chlorophyll whereas other parts do
not (and are whiter thus).

After several hours, a starch test is carried out. The results

should show that parts of the leaf without chlorophyll will show
negative results whereas the parts that do have chlorophyll will
show a positive result.
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Effect of light intensity, carbon dioxide concentration,

and temperature on rate of photosynthesis

You need to be aware of a couple of different graphs. They are

quite simple so don’t worry.

 Graph 1: Rate of photosynthesis increases with light

intensity until it plateaus. The graph plateaus because
something else becomes the limiting factor (i.e. carbon
dioxide). This means that even with a stronger light
intensity, there may not be enough carbon dioxide to
make the rate of photosynthesis even faster.
 Graph 2: Rate of photosynthesis increases with increasing
carbon dioxide concentration. Again, at a certain point
the graph will plateau. In this case, the light may become
the limiting factor.
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 Graph 3: The rate of photosynthesis increases with

temperature until the graph reverses and eventually
drops down to zero. This is because high temperatures
will denature enzymes that are required for
photosynthesis.

Leaf structure
You need to know the structure of a leaf, and how this structure
is adapted for photosynthesis.

We will go through the functions of each of the structures in the

diagram above:

 Cuticle – Made of wax which waterproofs the leaf

 Upper epidermis – A barrier against disease organism. The
cells are thin and transparent to allow light to enter the
leaf.
 Palisade mesophyll – Main site of photosynthesis. Cells are
long and packed with chloroplasts to trap light energy.
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They receive carbon dioxide via diffusion from air spaces

in the spongy mesophyll
 Spongy mesophyll – Cells are spherical and loosely packed.
They contain chloroplasts but not as many as the
palisade layer. Loose packing creates air spaces and this
allows gas exchange (i.e. carbon dioxide to the cells, and
oxygen from the cells)
 Vascular bundle – Contains xylem and phloem. Xylem
vessels bring water and minerals to the leaf. Phloem
vessels transport sugars and amino acids away from the
leaf to the rest of the plant (translocation)
 Lower epidermis – Acts as a protective layer. It contains
the stomata
 Stomata – These are gaps in the underside of the leaf,
surrounding by a pair of guard cells. The guard cells
control whether the stoma is open or closed. It is through
stomata that carbon dioxide diffuses into the leaf and
oxygen diffuses out. Water vapour is also lost through
this structure in transpiration.

Mineral requirements
There are two important mineral requirements for plants that
you need to be aware of.

Firstly, nitrate ions are important for plants as they are used in
building amino acids (which eventually become proteins). A
nitrate ion deficiency would slow down the growth of the plant,
the stem would weaken. Lower leaves will turn yellow-ish and
the upper leaves will become pale green as they die off.

Secondly, magnesium ions are required to make chlorophyll. If a

plant has a magnesium ion deficiency then they will lack
chlorophyll. Leaves turn yellow from the bottom of the stem
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

upwards and plant growth will slow down due to reduced

photosynthesis.

Topic 7
Human Nutrition
ByFree Exam AcademyJune 25, 2019
,

Diet
Balanced diet

A balanced human diet contains all essential ingredients in the

correct proportions

There are certain factors that affect diet such as:

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Age – Children require more protein per kg of body weight

than adults
 Gender – Males generally use more energy than females
 Activity – Higher levels of physical activity will increase
demand for nutrients
 Pregnancy – Higher demands for nutrients in order to
supply fetus with energy for development
 Breast feeding mother – Higher requirements for
vitamin and water

Malnutrition

Malnutrition is the lack of a balanced diet such as deficiencies,

excesses or imbalances in a person’s intake of nutrients.

Malnutrition can lead to consequences such as:

 Obesity (excessive nutrients)

 Starvation (insufficient nutrients)
 Coronary heart disease (excessive saturated fat and
cholesterol)
 Kwashiorkor (too much carbohydrates, too little protein)
 Constipation (lack of fibre)

Essential nutrients
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Alimentary canal
Useful definitions

 Ingestion – Taking in of substances e.g. food and drink

into the body through the mouth
 Egestion – Passing out of food (as feces) that has not
been digested or absorbed via the anus
 Mechanical digestion – Break down of food into smaller
pieces without chemical change to food molecules
 Chemical digestion – Break down of large insoluble
molecules into small, soluble molecules
 Absorption – Movement of chemically digested food
molecules through the small intestine walls into the
blood
 Assimilation – Movement of digested food molecules into
the cells of the body where they are used and become a
part of the cells
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Structure and function

Mouth

This is the location of ingestion. Mechanical digestion of food

occurs as we chew on it with our teeth. Chemical digestion
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

occurs due to amylase enzymes in our saliva which break down

starch into maltose.

Esophagus

Round clumps of food (boluses) are passed down the esophagus

via peristalsis from the mouth to the stomach.

Peristalsis is the contraction and relaxation of the esophagus

wall muscles which creates a wave-like motion that pushes the
food down the canal.

Stomach

Mechanical digestion occurs as the stomach walls squeeze the

food to liquefy it.

Gastric juices contain pepsin (a protease) which chemically

digests proteins. It also contains hydrochloric acid which kill
bacteria, but also maintains an optimum acidic pH for pepsin.

Pancreas

The pancreas secretes pancreatic juice into the duodenum.

The juice contains a large variety of different enzymes involved

in the chemical digestion of food.

 Proteases break down proteins

 Lipases break down lipids
 Amylases break down carbohydrates

Most enzymes in the pancreatic juice have an optimum pH of

around 7. The pH of pancreatic juice is slightly alkaline, and this
is to neutralize the acidity of the food coming from the
stomach.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Duodenum

This is the first part of the small intestine. It receives pancreatic

juice which contains enzymes for the chemical digestion of
food.

Ileum

This is the second part of the small intestine. The inner walls
have finger-like extensions called villi which massively
increases the surface area for nutrient absorption.

Liver

The liver produces a substance called bile which is stored in the

gal bladder.

Bile has the function of emulsifying fat into droplets to increase

the surface area for lipases to come and digest them.

Bile is also basic which assists in neutralizing the acidity of the

food coming from the stomach.

Colon

This is the second part of the large intestine. The main function
of the colon is to reabsorb water from undigested food and also
bile salts to return back to the liver.

Rectum

The rectum stores feces until it is egested

Anus

Muscles control egestion of feces

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Cholera infection

Cholera bacteria releases toxins which causes chloride ions to

be secreted into the small intestine. This causes the osmotic
movement of water into the gut, and leads to diarrhea.

NOTE: Diarrhea is defined as the loss of watery feces

Diarrhea can be treated using oral rehydration therapy. It

involves drinking water with modest amounts of sugar and
salts, specifically sodium and potassium.

Mechanical digestion
Types of human teeth

Mechanical digestion involves physically breaking food into

smaller pieces, and our teeth serve exactly that function!

There are four types of teeth:

 Molar
Located at back of mouth

 4 or 5 cusps
 2 or 3 roots
 Used for chewing and grinding food
 Used for chewing and grinding food
 Pre-molar
 Behind canines
 2 cusps
 1 or 2 roots
 Used to tear and grind food
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Canine
 On either side of incisors
 More pointy than incisors
 Used to bite pieces of food
 Incisor
 In front of the mouth
 Chisel shaped
 Used to bite off food pieces

Structure of human teeth

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Dental decay

 Dental decay is caused by bacteria which is present on the

surface of our teeth.
 The bacteria and food deposits form a layer
called plaque.
 Bacteria in plaque feed on sugars which produce acid that
dissolves the enamel, resulting in a hole.
 As the hole deepens it may eventually reach the nerves
which result in pain i.e. tooth ache.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Proper dental care

 Avoid sugary food so that bacteria cannot make acid

 Regular cleaning to remove plaque
 Use floss to remove trapped food
 Visit the dentist regularly for treating early decay and
removal of thick plaque

Chemical digestion
Chemical digestion involves breaking down large, insoluble food
into smaller soluble nutrients that can be absorbed and used by
the cells.

Chemical digestion is carried out by enzymes. A lot about

chemical digestion has already been covered above in regards
to the alimentary canal, and what enzymes are found where.

Summary of enzymes that you should be aware of:

Remember, the acidic pH of the stomach is maintained by

hydrochloric acid which also kills bacteria via denaturing their
enzymes.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The alkaline conditions of the small intestine is maintained by

pancreatic juice and bile. The bile also has the function of
emulsifying fats into droplets to increase the surface area for
digestion

Absorption
After large food molecules get digested by enzymes, the small
soluble nutrients diffuse into the small intestine walls and then
into the blood.

Sometimes the concentration of nutrients i.e. glucose may be

higher in the blood than in the small intestine. In such cases
diffusion cannot be relied upon. Instead, active transport is
used for absorption.

Absorption is defined as the movement of small food molecules

and ions through the wall of the intestine into the blood. It can
either be done through diffusion or active transport.

The inner walls of the small intestine have finger-like structures

called villi which greatly increases the surface area of
absorption.

Structure of a villus
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Blood vessels/capillaries
 Absorbs glucose and amino acids by diffusion
 Lacteal
 Absorbs fatty acids and glycerol
 Epithelial lining
 One cell thick to increase diffusion rate
 Microvilli increase surface area even further
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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Plant Transport
ByFree Exam AcademyJuly 2, 2019
Xylem and phloem
Function

The transport system in plants is mainly made of the xylem and

phloem tissue.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Xylem tissue transports water, mineral ions, and solutes

from the roots to the leaves.
 Phloem tissue transports nutrients from the leaves to the
rest of the plant.

Structure

You need to be able to recognize the xylem & phloem in three

different parts of the plant: leaf, stem, and root

Leaf

Stem

Root
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Water uptake in roots

Root structure

Water, mineral ions, and other nutrients are absorbed into the
plants via the roots.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Root hair cells are cellular extensions which drastically increase

the surface area for absorption.

Waters moves from the soil into the xylem via osmosis.

Pathway of water through a plant

Movement in the root

Water in the soil is absorbed into the root hair cells via osmosis.
It then makes it way through the root cortex, across the
endodermis, and into finally into the xylem.

Movement in the xylem

Water moves up the xylem due to a combination of factors that

include: transpirational pool, root pressure, and cohesion.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Transpirational pool is when water leaves the leaf via

transpiration and therefore results in negative pressure at the
top of the plant.

Root pressure is when water is absorbed in the roots via

osmosis resulting to positive pressure at the bottom of the
plant.

A column of water is drawn up the plant from low to high

pressure. Water molecules ‘stick together’ due
to cohesion allowing the entire column of water to be drawn.

Water movement in the xylem is much like water movement in a

straw. When you stuck on the top end of the straw it reduces
pressure compared to the bottom of the straw. A column of liquid is
therefore drawn from low to high pressure. The liquid column sticks
together due to cohesion.

Movement in the leaf

Water enters the leaf through the vascular bundle and can
move directly into palisade or spongy mesophyll cells to be
used for photosynthesis.

Otherwise water evaportates into the air spaces in the spongy

mesophyll layer, where they diffuse out via the stomata. We call
this transpiration.

Transpiration
Transpiration is the loss of water vapour from leaves via the
stomata.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Once water enters the leaf via the xylem vessels of the vascular
bundle, it travels to the mesophyll cells to be used in
photosynthesis.

Some water remain on the surfaces of these mesophyll cells

and evaporate into the air spaces of the spongy mesophyll
layer. Water vapour diffuses out of the leave via the stomata.

Factors that affect transpiration rate are:

 Temperature
 Higher temperatures result in faster moving
molecules and therefore increases diffusion rate,
which in turn, increases transpiration rate
 Humidity
 Higher humidity results in a lower concentration
gradient and thus reduces diffusion rate, which in
turn, reduces transpiration rate

Translocation
Translocation is the movement of sucrose and amino acids in
the phloem from the source to the sink.

 Source is the region of production

 Sink is the region of storage OR where they are used in
respiration or growth

Some parts of the plant may act as a source and sink at

different times during the plant life.

 Leaves are the source most of the time but can become a
sink in periods of growth i.e. after autumn when plants
growth their leaves again
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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Animal Transport
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

ByFree Exam AcademyJuly 2, 2019

Circulatory system
The circulatory system is a system of blood vessels with a pump
and valves to ensure one way flow of blood.

 Singlecirculation means blood passes through the heart

only once i.e. fish
 Double circulation means blood passes through the heart
twice i.e. mammals

Double circulation is advantageous because it maintains a

higher blood pressure compared to a single circulation system.

Blood flow around the body

All mammals (including humans) have a double circulatory
system of blood flow.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

1. Deoxygenated blood enters the right atrium (RA)

2. Deoxygenated blood enters the right ventricle (RV)
3. Deoxygenated blood is pumped by the RV to the lungs to
become oxygenated
4. Oxygenated blood enters the left atrium (LA)
5. Oxygenated blood enters the left ventricle (LV)
6. Oxygenated blood is pumped by the LV to the rest of the
body
7. Body cells use the oxygen and cause the blood to
become deoxygenated
8. Deoxygenated blood returns to the heart and the cycle
repeats (step 1)

It is very important to understand these steps now so that it makes

life easier when we dive deeper into the heart structure later.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Heart structure and function

Structure of the heart

It is extremely important you familiarize yourself with the different

structures of the heart and the direction of blood flow through it

Vena cava → Right atrium → Atrioventricular valve → Right

ventricle → Semilunar valve → Pulmonary artery → Lungs
→Pulmonary vein → Left atrium → Atrioventricular valve →
Left ventricle → Semi-lunar valve → Aorta → Body → Vena
cava

Functions of the different structures

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Atrium

The right and left atrium contracts to pump blood into the right
and left ventricles respectively

Ventricles

The right ventricle contracts to pump blood to the lungs (to

become oxygenated).

The left ventricle contracts to pump blood to the rest of the

body. It has a thicker wall than the right ventricle because it
needs to pump blood further, and therefore needs more force.

Atrioventricular valves

The aterioventricular valves separate the atrium and ventricles

on both sides of the heart.

These valves prevent the backflow of blood, thus ensuring a one

way flow of blood from the atria to the ventricles.

Semilunar valves

Semilunar valves are found within the pulmonary arteries and

the aorta. They prevent the backflow of blood and ensures
unidirectional blood flow in the arteries.

Pulmonary artery

The pulmonary artery carries blood from the right ventricle to

the lungs

Pulmonary vein

The pulmonary vein carries blood from the lungs to the left
atrium
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Aorta

The aorta is a large artery which carries blood from the left
ventricle to the rest of the body

Vena cava

The vena cava is a large vein which carries deoxygenated blood

from the body back to the heart (right atrium)

Septum

The septum is a thick muscular wall which separates the right

and left side of the heart. This separation is important to ensure
that oxygenated and deoxygenated blood does not mix.

Activity of the heart

Physical activity increases heart rate

Heart rate is the rate at which the heart beats. The most
common way to measure heart rate is by measuring the pulse
rate.

The pulse rate is exactly equal to the heart rate, as the contractions
of the heart cause the increases in blood pressure in the arteries
that lead to a noticeable pulse.

Physical activity increases the energy demand in muscles such

as the arms and legs. With an increased rate of respiration,
blood must be travel quicker to the muscles to supply them of
oxygen/nutrients whilst also removing waste products such as
carbon dioxide. The heart rate therefore increases to meet
these demands.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Electrocardiogram (ECG)

An electrocardiogram is a device which can track heart activity.

It can accurately measure pulse rates via the opening and
closing of heart valves.

Details regarding the electrocardiogram is not required

Coronary heart disease

What is coronary heart disease

So far we know that the heart functions as a pump which

delivers blood to the rest of the body. However, the heart
muscles themselves also need a blood supply because they too,
are respiring muscles.

The coronary artery is the very important artery which provides

the heart muscles with blood.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Coronary heart disease is when the coronary artery becomes

blocked, leading to blood (and oxygen) starvation in the heart
muscles. This leads to a heart attack.

Causes

Blockage of the coronary artery begins by the narrowing of the

artery due to cholesterol build up on the inner walls.

Total blockage can occur when a blood clot gets ‘stuck’ in these
narrow arteries.

Risk factors

There are certain factors that increase the risk of an individual

developing coronary heart disease:

 Poor diet
 Stress
 Smoking
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Genetics
 Age
 Gender

Treatment with medications

Blood thinning medications are used to reduce the chances of a

blood clot forming

Treatment with surgery

 Stents are a tube shaped device which is placed inside the

coronary arteries to physically hold it open

 Angeioplasty is a stent with a balloon which can be inflated

once the stent is inserted to even further increase the
diameter of the artery
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Bypass is the process of making a separate ‘new’ artery to

allow for an alternative blood path that the heart can use
to receive blood (rather than relying only on the coronary
artery)
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Blood vessels
Blood vessels are a tubular structures carrying blood through
the tissues and organs. Starting from the heart, the pathway of
blood is as follows:

[HEART] → Artery → Arteriole → Capillary → Venule → Vein →

[HEART]
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

You do not need to know all the details of this diagram. Just
concentrate on the arrangement of the vessels i.e. how the artery
branches out into arterioles which branch further into capillaries.
Also notice how capillaries join to form venules which join further to
become the vein.

Arteries

Arteries take blood away from the heart. They have several
important structural features:

 Thick muscular walls to withstand blood being carried at

high pressures
 Narrow lumen which expands as blood pulsates through to
maintain blood pressure
 Valves absent since high blood pressures prevent back
flow
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Arterioles

Arterioles are smaller branches of an artery. They eventually

branch further to form capillaries.

 Arterioles have muscular/elastic walls that can constrict &

dilate in order to regulate blood flow.

Capillaries

Capillaries are fine branching blood vessels that form a network

between the arterioles and venule.

They allow for the nutrient & waste exchange between the
blood and the tissues of the body. The features of capillaries are
as follows:

 Walls are one cell thick to allow for quick diffusion of

diffuse rates of nutrients/wastes
 Luman has a diameter of just one RBC to allow blood cells
to pass closely to the walls for faster diffusion rates
 Valves are absent since the narrow capillary lumen
ensures unidirectional blood flow

Venules

Venules are small vessels formed from the joining of the

capillaries. Venules combine to establish a vein.

Veins

Veins take the blood towards the heart. Their structural features
are as follows:

 Thin walls with little muscle & elastic fibres (thick muscles
not required since blood is carried at low pressure)
 Large lumen to reduce blood flow resistance
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Valves present to prevent blood back flow

Shunt vessels

Shunt vessels are blood vessels that connect blood directly from
the arterioles to the venules. This allows for an alternative route
for blood flow (i.e. blood bypasses the capillaries).

Like arterioles, shunt vessels have walls that can construct &
dilate in order to regulate blood flow.

Tissue fluid

Tissue fluid is the fluid which bathes most body tissues.

The fluid is the mode of nutrient & waste exchange between the
blood and respiring tissues.

For example:

 Waste products (such as carbon dioxide) from cells diffuse

into the tissue fluid first before diffusing in the blood
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Nutrients (such as glucose) diffuse into the tissue fluid first

before diffusing into the cells

Tissue fluid is produced by leakage of certain substances from

blood capillaries, and drained out by the lymphatic vessels of
the lymphatic system.

Details of tissue fluid production and drainage are NOT required. Just
understand that blood capillaries LEAK tissue fluid and the tissue
fluid is DRAINED by lymphatic vessels. The tissue fluid inside lymph
vessels are called LYMPH.

The lymphatic system is composed of lymphatic vessels which

carry “lymph” and lymph nodes which produce lymphocytes for
immunity.

Blood
Blood is a mixture of several components such as:

 Red blood cells (RBCs)

 White blood cells (WBCs)
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Platelets
 Plasma

Plasma

Blood plasma makes up about 50% of the blood. It is a yellowish

liquid that carries the other blood components such as
RBCs/WBCs/platelets.

Red blood cells

Red blood cells contain hemoglobin which binds to oxygen for

transportation around the body.

White blood cells

White blood cells are part of the immune system that helps to
destroy foreign organisms such as bacteria.

There are two types of white blood cells that you need to be
aware of:

 Phagocytes are types of WBCs which engulf and digest

pathogens via phagocytosis
 Lymphocytes are WBCs which produce antibodies

More will be learnt about phagocytosis and antibodies in the next

chapter

Platelets

Platelets are substances that form blood clots which is a

protective mechanism to prevent blood loss during an injury.

At the site of damage, platelets immediately stick together and

release chemical signals which attract other nearby cells and
clump them together.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

A series of of chemical reactions take place. Fibrinogen is

converted into fibrin and this forms a thread which traps RBCs
to establish a thick clot. The clot seals off the site of damage.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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Diseases And Immunity
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

ByFree Exam AcademyJuly 3, 2019

Pathogen and disease
A pathogen is defined as a disease causing organism. Each
pathogen has a uniquely shaped ‘marker’ on the surface of their
membranes called antigens.

It is very important to understand that antigens are very specific in

shape. Every pathogen is unique because they have differently
shaped antigens on their cell surfaces!

A transmissible disease is a disease in which the pathogen can

be passed from one host to another. The pathogen can be
transmitted through direct contact (i.e. blood/body fluids) or
indirect contact (i.e. contaminated food/air/animals/etc.)

Body defenses
In order to protect ourselves from pathogens and disease, our
body has several protective mechanisms in place.

Mechanical barriers
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Mechanical barriers act as a physical obstruction in order to

prevent pathogens from entering our body. For example:

 Skin
 Nose hairs

Chemical barriers

Chemical barriers are chemical substances in the body which

help to trap or destroy pathogens. For example:

 Mucus
Traps bacteria that enters the respiratory system

via air. The mucus is then beat upwards by the
cilia and pushed up to the mouth.
 Stomach acid
 The acidity kills pathogens that enter our
digestive system via the food we eat.

Cellular barriers

Cellular barriers are the protection offered by the cells of our

immune system, which take part in combating against
pathogens and disease. For example:

 Lymphocytes (which produce antibodies)

 Phagocytes

Antibodies and phagocytes will be covered in detail below

Cellular barriers
Phagocytosis
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Phagocytosis is the mechanism by which phagocytes digest

pathogens.

The steps of phagocytosis is as follows:

 Phagocyte extends itself and engulfs the pathogen

 Pathogen becomes trapped in a vesicle called phagosome
 Other vesicles containing digestive enzymes fuse with the
phagosome
 Enzymes digest the pathogen

Antibodies

Function

Antibodies are proteins produced by cells called lymphocytes.

They bind to the antigens on pathogens and can either:

 Destroy the pathogen directly

OR
 Alert phagocytes for destruction via phagocytosis

Remember, each pathogen has a uniquely shaped antigen on their

cell surfaces. Therefore, for a particular antibody to aid the
destruction of a certain pathogen, the shape of the antigen must be
‘fit’ or be ‘complementary’ to the shape of that particular antigen!
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

In the example below, only antibody A can ‘lock’ onto antigen A and
therefore aid its destruction. Antibody B cannot due to the fact that
it does not have a complementary shape to antigen A.

Production

Antibodies are produced by lymphocytes. Lymphocytes have

receptors in their cell membranes that have a complementary
shape to a certain antigen.

Lets consider lymphocyte A which have receptors that have a

complementary shape to antigen A.

When a lymphocyte A comes across a pathogen with antigen A

on its cell surface, their receptors can recognize the antigen
and activate lymphocyte A.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The activated lymphocyte A then produces antibodies that have

the same shape as their receptors, meaning that these
antibodies can lock onto antigen A and destroy them.

Activated lymphocytes also produce memory cells, which are

long-lived cells that remain in the blood even after the infection
is over. They have the ability to rapidly produce the original
antibodies if it were to come across the same pathogen/antigen
again.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Vaccination
Vaccination is a treatment with a vaccine to evoke immunity to
a certain antigen.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The vaccination process

The process of vaccination is as follows:

 A harmless variant on a certain antigen is introduced to

the body i.e. via needle
 Immune response is triggered and activate lymphocytes
produce antibodies and memory cells to fight against the
antigen variant
 Therefore if the real antigen/pathogen eventually makes its
way inside the body, there are pre-established
antibodies/memory cells which can rapidly fight against
it.

Controlling the spread of disease

Herd immunity is the idea that if most people of a population is

immune to a disease, then the population becomes significantly
resistant to the spread of that disease.

In other words, if majority of the population is immunized, it

protects the people that are not.

Other consideration for controlling the spread of disease is as

follows:

 Personalhygiene
 Food hygiene
 Waste disposal
 Sewage treatment

Passive immunity
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Passive immunity is a short-term defense again a pathogen by

antibodies transferred from one individual to another (rather
than making their own).

For example, children benefit from the antibodies present in the

mother’s breast milk.

Autoimmune disease
Some diseases are caused by the immune system targeting and
destroying friendly body cells.

Type 1 diabetes is an example of an autoimmune disease

whereby immune cells attack insulin producing beta cells in the
pancreas.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

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Gas Exchange In Humans
ByFree Exam AcademyJuly 5, 2019
Respiratory system
The respiratory system is a biological system consisting of
specific organs and structures used for gas exchange.

Gas exchange (in humans) is the delivery of oxygen from the

lungs to the bloodstream, and the elimination of carbon dioxide
from the bloodstream to the lungs.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The air enters the nasal cavity and makes it’s way down the
trachea and into the lungs. The trachea branches into bronchi,
which branches further into bronchioles.

The trachea is surrounded by cartilage which strengthens and

supports the trachea.

The rib cage (not shown in the diagram) surrounds the lungs and its
contents. It acts as a protective barrier and plays a vital role in
inhalation and exhalation.

At the end of the bronchioles we find alveoli which is the gas

exchange surface of the human respiratory system. The
alveoli is the site at which oxygen is exchanged for carbon
dioxide.

The alveoli have an extremely large surface area for gas

exchange, and are surrounded by numerous capillaries to allow
good ventilation with air.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Inhalation and exhalation

Inhalation allows the body to obtain oxygen from air, and
exhalation allows the excretion of carbon dioxide.

To understand how inhalation and exhalation works, it is

important to understand the concept of thoracic volume i.e. the
volume of the thorax.

Thoracic volume and air pressure

Air will always move from higher air pressure to lower air
pressure.

 When the thorax volume increases it reduces the air

pressure inside the thorax (below atmospheric pressure).
Air therefore travels into the body i.e. inhalation
 When the thorax volume decreases it increases the air
pressure inside the thorax (above atmospheric pressure).
Air therefore travels out of the body i.e. exhalation

Thorax is the part of the body of a mammal between the neck and
the abdomen

The volume of the thorax is dependent on two factors:

 Position of the rib cage

 Position of the diaphragm
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Inhalation

During inhalation, the thoracic volume increases to make the air

pressure in the thorax to fall below atmospheric levels, and thus
forcing air into the body.

This increase in thoracic volume is due to:

 Contraction of external intercostal muscles which cause

the ribs to move up and out
 Contract of diaphragm which causes the dome-shape to
flatten

Exhalation

During exhalation, the thoracic volume decreases to make the

air pressure in the thorax above atmospheric levels, and thus
forcing air out of the body.

The decrease in thoracic volume is due to:

 Relaxation
of intercoastal muscles causing the rib cage to
move down and in
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Relaxationof the diaphragm resulting in the original dome-

shaped appearance

Air composition
The air composition of inspired and expired air is different.
Inspired air has more oxygen, whilst expired air has more
carbon dioxide (for obvious reasons of respiration). Nitrogen
remains constant as our bodies cannot make use of nitrogen
gas.

The limewater test can confirm the elevated presence of carbon

dioxide in expired air. It is a clear liquid which turns milky when
it reacts with carbon dioxide. Blowing into limewater with a
straw will indeed cause it become milky.

Physical activity on breathing

With physical activity both the rate and depth of breathing
increases.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Exercise means body cells respire more rapidly. More carbon

dioxide is produced as a result and the increased carbon
dioxide concentration in the blood leads to an increase in blood
acidity i.e. decreased blood pH.

The brain detects the increased blood acidity and signals for the
increase in breathing rate and depth to allow carbon dioxide to
be excreted quicker. It also of course allows oxygen to get to
respiring cells faster as well.

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Respiration
ByFree Exam AcademyJuly 5, 2019
Aerobic respiration
Aerobic respiration is defined as chemical reactions in cells that
use oxygen to break down nutrient molecules to release
energy.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The energy released from respiration are used for various

things in the body:

 Muscle contraction
 Protein synthesis
 Cell division
 Active transport
 Growth
 Passage of nerve impulses
 Maintenance of constant body temperature

Investigation: Oxygen uptake by respiring

organisms

A simple respirometer is used. An organism is placed in the

boiling tube on top of the wire gauze. A capillary tube
connected to the boiling tube is measured against a ruler, and a
colored dye is introduced into it.

With the starting point of the dye measured, the dye will move
towards the boiling tube as the organism uses up the oxygen in
the air.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The carbon dioxide produced by the organism is

eliminated/absorbed by the soda lime, and therefore does will affect
the dye position.

We can measure the rate of respiration by calculating the

distance traveled by the dye and dividing it by the time it took.

Investigation: The effect on temperature on

rate of respiration of germinating seeds

The experimental set up for this experiment is virtually the

same before.

Germinating seeds are placed in a boiling tube on a wire gauze.

Soda lime is placed at the bottom (to absorb CO2) and the
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

boiling tube is placed in a water bath of a known temperature

(i.e. 30 degrees)

A capillary tube is connected to the boiling tube, with colored

dye introduced. The initial position of the dye is measured
against a ruler and as the seeds respire, the dye will move
towards the boiling tube.

The rate of respiration for that particular temperature (i.e. 30

degrees) is calculated:

The experiment can be repeated multiple times to calculate and

compare the rate of respiration at different temperatures i.e.
40°C , 50°C, etc.

Anaerobic respiration
Anaerobic respiration is defined as chemical reactions in cells
that break down nutrient molecules to release energy without
oxygen.

Anaerobic respiration releases less energy per glucose than

aerobic respiration, so it is less efficient.

The equation for anaerobic respiration is different between our

muscles and yeast:

 In muscles:
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 In yeast

Lactic acid

Oxygen debt

During vigorous exercise muscles may not be receiving oxygen

quick enough for aerobic respiration. In such instances,
anaerobic respiration occurs in order to supply the muscles with
energy despite the lack of oxygen.

As we see in the equation above, anaerobic respiration in

muscles produces lactic acid, which is a toxic waste product.

Oxygen is required to remove lactic acid once the exercise

stops, and therefore we call this ‘oxygen debt’ i.e. the body
‘borrowed’ oxygen during anaerobic respiration, and must pay it
back to remove the lactic acid.

Lactic acid removal

Lactic acid is aerobically respired in the liver.

Even after exercise stops, the heart rate remains quick and the
breathing rate remains deep.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Fastheart rate allows lactic acid to be transported quickly

from the muscles to the liver
 Deep breathing rate allows sufficient supply of oxygen to
aerobically respire/remove the lactic acid

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Excretion
ByFree Exam AcademyJuly 7, 2019
Excretion
Excretion is defined as the removal of toxic materials, the waste
products of metabolism, and substances in excess of
requirements in the body.

For example:

 Kidneys excrete urea and excess water and salts to form

urine
 Lungs excrete carbon dioxide

Urea is the waste product formed in the liver, urine is made in the
kidneys and is the combination of urea with water and other salts
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Urea is formed by deamination. Deamination is the removal of the

nitrogen containing part of excess amino acids to form urea.

Urinary system
Production and removal of urine

The urinary system is composed of the kidneys, ureter, the

bladder, and the urethra.

1. Urea is made in the liver, and enters the kidneys.

2. Urea, water and other substances make urine
3. Urine is passed onto the bladder to be stored (via the
ureter)
4. The urine is eventually excreted from the body through
the urethra

Volume and concentration of urine

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The volume and concentration of urine may depend on certain

factors:

 Hydration
More water intake leads to more excess water.

This means the volume of urine will increase and
the concentration will decrease
 Hot-temperatures (dehydration)
 Dehydration results in less excess water. This
means the volume of urine will decrease and the
concentration will increase

Kidney
Structure of the kidney
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Kidney tubules (nephrons)

The medulla of the kidneys are made of kidney tubules called

nephrons.

The glomerulus (beginning of the tubule) filters water, glucose,

urea and salts from the blood. The rest of the tubule reabsorbs
most (but not all) of the glucose, water and salts back. The urea
remains in the tubule.

The urea along with the other remaining components in the

tubule is what we call urine. The urine leaves the kidney via the
ureter and eventually leaves the body through the urethra.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Kidney machines
As detailed above, the kidneys play a major role is removing the
urea from our blood.

If a patient has kidney dysfunction, then other methods must be

used to remove the urea instead.

Kidney dialysis

Kidney dialysis is the method of removingurea from the blood

via diffusion.

The blood is extracted and made to pass through tubes in the

dialysis machine (see diagram above).

The tubes have a semi-permeable membrane, and the fluid

outside of the tube is called the bathing liquid.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The bathing liquid is made to have a similar concentration of

substances as the blood except urea.

The low urea concentrations in the bathing liquid means that

urea will diffuse out of the blood as it passes through the tube
in the machine.

Other important substances such as glucose, proteins, etc. will

remain in the blood because there is no concentration gradient
between the bathing fluid & the blood.

Kidney transplant

The main advantage of a kidney transplant as opposed to

kidney machines is that they can return to a normalized lifestyle
(i.e. no need for regular visits to use the machine). Moreover, it
would save the cost of using the kidney machines themselves.

However, for a kidney transplant to work a suitable donor is

required which may often be difficult to find. Moreover, there is
always the chance of a tissue rejection.

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Coordination And Response
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

ByFree Exam AcademyJuly 18, 2019

Nervous control in humans
Human nervous system is composed of the central nervous
system (CNS) and the peripheral nervous system.

The CNS is the brain and the spinal cord, whereas the
peripheral nervous system is the nerves and neurons which
connect the CNS to the rest of the body.

Nervous impulses are electrical signals that pass along neurons

to transmit information to and from the brain.

The general nervous pathway

We have various sense organs in our body. Sense organs have

the ability to detect certain sensory stimuli.

 Skin = touch i.e. pain, pressure, temperature etc.

 Eyes = light
 Nose = smell
 Ears = noise
 Tongue = taste

When these sense organs detect stimuli, they send raw

information to the brain (CNS) via nervous impulses in the
sensory neurons.

The brain then interprets the information. If an action is

required, then the brain will send impulses to the relevant
muscles via motor neurons to carry out this action. These
muscles are called effectors.

Sensory receptor -> Sensory neuron -> CNS -> Motor neuron ->
Effector
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Example 1

Imagine putting a piece of food in your mouth. You find it to be

overly disgusting and you therefore decide to spit it out.

 Your tongue is a sensory organ and it detects the taste of

the food that you put into the mouth.
 Nerve impulses from the tongue is transmitted to the brain
(via sensory neurons)
 The brain then interprets this information. It is at this level
that you feel that the food is disgusting.
 The brain transmits nerve impulses (via motor neurons) to
the tongue and mouth muscles, instructing it to spit the
food out

Tongue -> Sensory neuron -> CNS -> Motor neuron ->
Tongue/mouth muscles

This is an example of a voluntary action i.e. actions which are a

result of a conscious decision by the brain.

Example 2

“Imagine touching a very hot pan, causing you to IMMEDIATELY

take your hands off it.”

 The skin (sense organ) detects high heat

 Instead of nerve impulses being sent to the brain and
awaiting it’s response, the information is transmitted
straight from the sensory neurons to the motor neurons
 Motor neurons then transmit impulses to the hand/arm
muscles
 The muscles immediately take the hand off the pan
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

This is a prime example of a reflex arc. If we had awaited the

brain to tell the muscles to take our hands off the high heat,
then it would take too long.

Instead, we are instinctively able to carry out this action

WITHOUT the brain telling us to do so because of this reflex arc.

Tongue -> Sensory neuron -> Motor neuron -> Tongue/mouth

muscles

This is an example of an involuntary action i.e. an action which

is instinctive and not by choice.

Neurons

There are three different types of neurons that you need to be

aware of.

Sensory neurons

Sensory neurons carry impulses from sense organ to the brain

(CNS).

Motor neurons

Motor neurons carry information from the brain to the effector

Relay neurons

Relay neurons connect a sensory neuron to a motor neuron in a

reflex arc
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Reflex arc

As we discussed above, a reflex arc allows an immediate

response to certain stimuli by bypassing the brain (CNS).

Synapses

A synapse is a junction between two neurons.

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The function of a synapse is to ensure a single direction of

neuronal transmission.

Consider impulses passing from neuron A to neuron B. The

space between the two neurons is the synapse.

 Neuron A = pre-synaptic neuron

 Neuron B = post-synaptic neuron

The presynaptic neuron (neuron A) has vesicles which contain

neurotransmitters. When electrical impulses reach the pre-
synaptic neuron (neuron B), these vesicles release the
neurotransmitters into the synapse.

The neurotransmitters then diffuse across the synapse and bind

to specific, corresponding receptors on the post-synaptic
neuron.

This allows the electrical impulses to be transmitted to, and

carried along neuron B (post-synaptic neuron).
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The human eye

Structure and function

It is important to understand how the eyes work.

1. Light rays hit the cornea

2. The cornea refracts (bends) light
3. Light passes through the pupils
4. The lens refracts light further to focus it onto the fovea
5. The fovea contains light receptors that transmit electrical
impulses to the brain
6. The brain interprets these impulses and gives us what we
call ‘vision’

 Cornea – Refracts light

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Lens – Focuses light onto the retina

 Iris – Controls pupil size
 Pupil – Controls the amount of light entering the eye
 Optic nerves – Carries impulses to the brainn
 Retina – Contains light receptors called rods (responsible
for night vision) and cones (responsible for colour vision)
 Fovea – An area of the retina where most of the light is
focused on, and has a very high density of cones

Pupil reflex

Light enters the light via the pupils. While too much light can
damage the retina, too little light makes it very difficult to see.

The pupil reflex is designed to appropriately control the level of

light that entering the eyes. They become smaller in bright
conditions and larger in dim conditions.

There are two muscles of the iris that control the diameter of
the pupils.

1. Circular muscles
2. Radial muscles
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Accommodation

Accommodation is how the lens changes shape in order to fine

focus the light directly onto the fovea.

It does so by bending/refracting incoming light rays so that it

converges onto the fovea.

 Light rays from a distant object are parallel so it takes less

effort for the lens to focus it
 Light rays from a close object are divergent so it takes
more effort for the lens to focus it
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

For a distant object, ciliary muscles are relaxed. This allows

suspensory ligaments to pull tightly on each side of the lens.
The lens therefore becomes stretched and thin. A thin lens has
less power, and therefore bends light less.

For a near object, ciliary muscles are contracted. This allows the
suspensory ligaments to loosen. The lens thus becomes
thicker. A a thicker lens has more power, and therefore bends
light more.

Hormones in humans
Hormones are defined as a chemical substances produced by
an endocrine gland and carried by blood which alters the
activity of one or more specific target organs.

Examples

 Adrenal gland secretes adrenalin

 Pancreas secretes insulin & glucagon
 Testes secrete testosterone
 Ovaries secrete oestrogen
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Adrenaline

Adrenaline is a hormone secreted in fight or flight situations.

It increases blood supply to the necessary parts of the body to

cope with a dangerous situation.

Adrenaline has multiple effects on the body:

 Increased breathing rate

 Increased heart rate
 Secretion of glucose from the liver
 Pupil dilation

Nervous system vs hormonal system

There are some key differences between the nervous system

and the hormonal system.

Hormonal control:

 Chemical transmission
 Blood pathway
 Slow speeds of transmission
 Long-term effect

Nervous control

 Electrical
transmission
 Nervous pathway
 Fast speeds
 Short-term effect

Homeostasis
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The definition of homeostasis is the maintenance of a constant

internal environment.

Positive and negative feedback loops control internal conditions

within set limits.

 For example, our optimum body temperature is

approximately 37°C – This is the SET VALUE/LIMIT
 If temperature goes beyond 37 then negative
feedback brings it back down
 If temperature falls below 37 then positive
feedback brings it back up

Control of sugar levels

Blood sugar levels are controlled by two hormones that are

secreted by the pancreas

1. Insulin – Causes liver to use blood glucose and covert it

to glycogen for storage inside liver cells
2. Glucagon – Causes the liver to convert glycogen to
glucose and release it into the blood

Essentially, insulin causes blood sugar levels to fall and

glucagon causes blood sugar levels to rise.

When glucose levels are too high, negative feedback will reduce
it back down

 Pancreas secretes insulin

 Liver converts blood glucose to glycogen
 Blood sugar levels fall

When glucose levels fall too low, positive feedback will increase
it back up
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Pancreas halts insulin secretion

 Pancreas secretes glucagon instead
 Liver converts glycogen to blood glucose
 Blood sugar levels rise

Symptoms & treatment of type 1 diabetes

Type 1 diabetes is caused by insufficient insulin production.

As you may imagine, this leads to a very high blood glucose

level.

The symptoms of type 1 diabetes include

 Increased hunger
 Frequent urination
 Blurred vision
 Tiredness & fatigue
 Unexplained weight loss

The management of type 1 diabetes include

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Controlling sugar intakes

 Monitoring sugar levels
 Insulin injections

Maintenance of internal body temperature

Body temperature is maintained at 36 degrees. The body loses

heat when the environment is too hot, and retains heat when
the environment is too cold.

The brain has blood temperature receptors that can modulate

the temperature accordingly via certain mechanisms.

Skin structure

Mechanisms to retain heat

 Basic insulation
 The fat layer in the skin acts as an insulator
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Skin hair follicles stand up to trap a layer of air


around the skin which is also an insulator
 Shivering
 Increased metabolism in muscles increase heat
circulation
 Vasoconstriction
 Heat is carried in the blood
 If blood goes near the skin surface, then heat
radiates out of the body
 Constriction of the skin arterioles reduce the
amount of blood flowing near the skin surface to
retain heat in the blood

Mechanisms to lose heat

 Sweating
Sweat is a mixture of water, salt and urea

 Water evaporates from sweat which causes the
skin (and body) to cool down
 Vasodilation
 Dilation of skin arterioles increase the amount of
blood flowing near the skin surface to allow more
heat to radiate out of the body

Tropic responses
Auxins

Auxins are plant growth substances. They are produced by the

tips of root shoots and plants.

Auxins will always move away from sunlight and towards

gravity.

 In the shoots, auxins promote plant growth

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 In the roots, auxins inhibit plant growth

Gravitropism

Gravitropism is the response in which parts of the plant react to

gravity.

 Positivegeotropism = Growth towards gravity

 Negative geotripism = Growth away from gravity

Positive geotropism

Positive geotropism is when a plant grows towards gravity (i.e.

in roots)

In the absence of light, if a root is placed horizontally, then

auxins will accumulate on the lower side due to gravity.

Auxins inhibit cell growth in roots, so therefore the lower half of

the root will grow slower than the top. This uneven cell growth
causes the root to bend towards the ground.

Negative geotropism

Negative geotropism is when a plant grows away from gravity

(i.e. in shoots)

In the absence of light, if a shoot is placed horizontally, then

auxins will once again accumulate on the lower side due to
gravity.

In shoots, the auxins promote cell growth. Therefore the lower

side of the shoot will grow quicker the top. This results in the
shoot bending away from the ground.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Phototropism

Phototropism is the response in which plants react to light.

Positive phototropism

Positive phototropism is when a plant grows towards sunlight

(i.e. in shoots).

When light is exposed to one side of a shoot, auxins move away

and accumulate on the shaded side.

Auxins promote cell growth in shoots, so the shaded side grows

quicker than the exposed side. This results in the shoot bending
towards the sun.

Negative phototropism

Negative phototropism is when a plant grows away sunlight (i.e.

in roots).

When light is exposed to one side of a root, again, auxin once

again accumulate on the shaded side.

Auxins inhibit cell growth in roots, so therefore the shaded side

grows slower than the exposed side. This results in the root
bending away from the sun.

Synthetic hormone 2,4-D

This synthetic hormone is similar to auxins. Spraying these one

plants causes uncontrolled growth which can kill the plants.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

They are therefore used as weedkillers, and are also selective.

This means that they work better on some plant species than
others.

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Drugs
ByFree Exam AcademyJuly 7, 2019

Drugs
Drugs are defined as any substance taken into the body that
modifies or affects chemical reactions in the body.

Medicinal drugs

Medical drugs are drugs used to prevent or treat diseases.

Antibiotics
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Antibiotics are used in the treatment of bacterial

infections. They work by specifically targeting certain bacterial
features (such as cell walls) to kill them.

Viruses and other non-bacterial pathogens are unaffected by

antibiotics as they do not have bacterial features.

Antibiotic resistance

Thanks to genetic mutation, simply by chance, some bacteria

may be resistant to antibiotics.

If antibiotics are overused, then it will allow these resistant

bacteria to survive and reproduce, leading to the rise of an
entire strain with antibiotic resistance.

Antibiotic resistance is a good example of natural selection.

Antibiotics should therefore be used only when truly necessary,

and the course of antibiotics given to the patient should always
be fully completed before it is stopped.

Misused drugs

Some drugs are often misused, and some examples of this are:
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Alcohol
 Heroin
 Tobacco smoke

Alcohol and heroin misuse

Alcohol and heroin are depressants. When both of these drugs

are used in excess the effects are similar:

 Slower reactions times

 Loss of self control
 Addiction and withdrawal

Moreover, alcohol can lead to liver damage and heroin can lead
to STIs such as HIV due to needle sharing.

Tobacco smoke

Tobacco smoke contains a lot of bad chemicals and

substances.

 Carbon monoxide – Binds to hemoglobin in RBCs and

hinders oxygen transport
 Nicotine – Causes addiction
 Tar – Causes cancer

Smoking has been proven to be linked to chronic obstructive

pulmonary disease (COPD), lung cancer, and coronary heart
disease.

Testosterone in sports

Testosterone is an appealing drug for athletes because it

promotes muscle building and can give the athlete faster
recovery times.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Anabolic steroids are synthetic variations of testosterone.

In most sports, the use of testosterone is banned

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Reproduction
ByFree Exam AcademyJuly 22, 2019

Reproduction
Reproduction is the process of making more of the same kind of
organism.

There are two types of reproduction that you need to be aware

of:

 Asexual reproduction
 Sexual reproduction
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Asexual reproduction
Asexual reproduction is the process resulting in the production
of genetically identical offspring from a single parent i.e.
cloning

Advantages of asexual reproduction are:

 Quick
 Only single parent required
 Good genetic characteristics always passed on
 No dispersal so offspring will grow in the same favorable
environment

Disadvantages of asexual reproduction are:

 Littlevariation i.e. less adaptability to a changing

environment
 Unlikely to withstand disease if parent not resistant
 Lack of dispersal lead to increased competition for
nutrients

Sexual reproduction
Sexual reproduction is the process involving the fusion of two
gametes (one from each parent) to form a zygote. This results
in the production of offspring that is genetically different from
the parents.

Gametes are sex cells that have half of the normal chromosome
number (haploid).
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Therefore, when the male and female gamete fuse together

they form a new cell with the normal chromosome number
(diploid).

Advantages of sexual reproduction are:

 Variation among offspring, and therefore more adaptable

to a changing environment
 More likely to withstand disease
 In plants, seed dispersal reduces competition for nutrients
as offspring will grow in a different environment

Disadvantages of sexual reproduction are:

 Requires the fusion of two gametes

 Slower process

Plant reproduction
Plants mostly reproduce sexually, and therefore that will be the
focus of this section.

The sexual reproduction in plants involves the fusion of the

male gamete (pollen) and the female gamete (ovule).

A plant flower has both a ‘male part’ containing the pollen and
the ‘female part’ which contains the ovule.

 Malepart = Stamen
 Female part = Pistil/carpel

Sexual reproduction occurs when the pollen from the stamen of

one flower successfully reaches the ovule of either the same
flower, or a different flower.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

This transfer of pollen is called pollination, and we will look into

this in a bit more detail down below.

Important concepts to understand at this stage are:

 Transfer of pollen is pollination

 Successful pollination results in fertilization
 A flower can self pollinate i.e. pollen transfer within the
same flower or a different flower of same plant
 A flower can cross pollinate i.e. pollen transfer to a
different flower of a different plant

Structure of a flower

The term carpel & pistil has the same meaning, but I would suggest
using carpel in your examination as this is the official term for
IGCSE.

The carpel is the female reproductive part of the plant. It is

made of three important structures:

 Stigma = The sticky surface that catches pollen

 Style = Links stigma to ovary
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Ovary = Contains ovules which develop into seeds after

fertilization

The stamen is the male reproductive part of the plant. It is

made of two important structures:

 Anther = Contains pollen

 Filament = Supports the anther

Pollination

Pollination is the transfer of the pollen (male sex cell) from the
anther to the stigma.

Once the pollen lands on the stigma, it is transferred to the

ovule (female sex cell).

The fusion of the pollen and ovule leads to what we call

fertilization.

There are main two methods by which plants facilitate

pollination

 Insectpollination
 Wind pollination

Insect pollination

Insect pollination uses insects that land on the flower to carry

pollen.

As insects move around within the flower, some pollen become

caught onto the insect’s body. The insect therefore physically
carries pollen and successful pollination occurs when it rubs its
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

body against a stigma of the same flower (self pollination) or a

different flower (cross pollination).

Wind pollination

Wind pollination uses the wind to carry pollen.

Pollen that gets carried by the wind may end up on the stigma
of the same flower (self pollination) or a different flower (cross
pollination).

Insect pollinated vs wind pollinated flower

There are some key differences between an insect pollinated

flower and a wind pollinated flower.

Cross pollination vs self pollination

Self pollination is the transfer of pollen from the another of one

flower to the stigma of the same flower, or a different flower of
the same plant.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Advantages
Only one parent required so less reliance on

pollinators
 Less competition among offspring
 Disadvantages
 Less variation (since genes are all from the same
plant)
 Less adaptable to changing environment and
resistance to disease

Cross pollination is the transfer of pollen from the anther of one

flower to the stigma of another flower on a different plant (of
same species)

 Advantages
Increased variation

 Greater adaptability to changing environment and
more resistant to disease
 Disadvantages
 More reliance on pollinators

In the above diagram, A and B represent self pollination. C

represents cross pollination.

Fertilization

When a pollen grain lands on the stigma of the correct species,

a pollen tube will begin to grow.

It grows through the style, enters through a small gap in the

ovary called the micropyle, and eventually reaches the ovule.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The nucleus of the pollen then passes along the pollen tube and
fuses with the neuclus of the ovule, resulting in successful
fertilization.

The zygote eventually develops into a seed. The seed remains

dormant until the conditions are right, and germination occurs.

Germination

Germination is the development of a plant from a seed or spore

after a period of dormancy.

The conditions that must be met for germination to occur are as

follows:

 Water– For the activation of enzymes

 Oxygen – For respiration for growth
 Temperature – Optimum temperature for enzymes
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Investigation of germination

 Set up boiling tubes each containing 10 cress seeds on

cotton wool

 Leave tubes in set environmental conditions for a period of

time
 A, B and C placed in an incubator of 20°C
 D is placed in a fridge of 4°C
 Compare the growth/germination of each of the test tubes
 Results are as follows:
 A(no water) = No germination
 B (control) = Germination
 C (no oxygen) = No germination
 D (cold temperature) = No germination

Human reproduction
Human reproduction is focused on the male gamete (sperm)
fertilizing the female gamete (egg).

Male reproductive system

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Penis
 Can become firm and erect for insertion into the
vagina
 Testis
Sperm production
 Scrotal sac
 Holds testes outside of body to keep it colder
than body temperature (ideal for sperm
production)
 Epididymis
 Sperm storage
 Vas deferns/sperm duct
 Tube connecting testis to urethra
 Prostate gland
 Add nutrients & fluid to sperm to make it semen
 Urethra
 Passes semen or urine out through the penis (but
never at the same time)

Female reproductive system

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Vagina
 Entry point for penis
 Cervix
 Ring of muscle separating vagina from uterus
 Uterus
 Location of fetal development
 Ovary
Production of egg cells

 Oviduct/fallopian tubes
 Site of fertilization
 Movement of egg cell to uterus via cilia in the wall

Human pregnancy overview

1. Sperm enters vagina

2. Sperm passes through cervix
3. Sperm enters oviduct
4. If an egg is present, it becomes fertilized
5. Fertilization results in zygote formation
6. Zygote divides to make an embryo (ball of cells)
7. Embryo implants into the uterus walls
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

8. Embryo develops further to form a fetus

9. Fetus develops within uterus/womb
10. Birth of a baby

Fetal development & birth

Once an egg is fertilized by sperm, it forms a zygote. The

zygote undergoes cell division to form a ball of cells called the
embryo.

The emrbyo implants itself into the uterus lining, where it

begins to develop into a fetus

Fetal development

The placenta brings fetal blood supply close to the mother’s

blood without mixing. Nutrients diffuse from the mother’s blood
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

into the fetus’ blood. Oppositely, waste diffuses from the fetus’
blood into the mother’s blood.

The umbilical cord carries fetal blood to and from the placenta.
This cord is therefore essential for the nutrient/waste exchange
between the fetal blood and the mother’s blood.

The amniotic fluid protects the fetus from physical harm, and is
held inside the amniotic sac.

Umbilical cord

 Umbilical artery
 Carries blood from the fetus to the placenta
 Placenta
 Nutrient/waste exchange
 Nutrients pass from mother’s blood to fetal blood
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Waste products poss from fetal blood to mother’s


blood
 Umbilical vein
 Carries blood from the placenta to the fetus
 Some toxins such i.e. nicotine or pathogens i.e.
rubella, can cross the placenta from the mother’s
blood and affect the fetus

Ante-natal care of pregnant woman

Before birth, the baby obtains all dietary requirements from the
mother via the placenta.

It is therefore very important that the mother’s diet is well

balanced with:

 Amino acids/proteins for growth

 Calcium for bone development
 Iron for RBC formation

Process of childbirth

The process of childbirth is as follows:

 Labour is triggered by the oxytocin hormone

 Muscular walls of the uterus contract
 Pressure from contraction breaks the amniotic sac and
releases amniotic fluid
 Contractions become more violent and push the baby
down the cervix
 The cervix becomes dilated for the baby to pass through
 The vagina stretches in order to allow the baby to be born
 The Baby is still attached to the placenta via umbilical cord
post birtth
 The umbilical cord is therefore cut and tied
 The placenta breaks away from the uterus wall and passes
out
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Breast-milk vs formula

It is important to understand the ongoing debate about the

advantages and disadvantages of breast feeding compared to
formula.

Advantages:

 No risk of allergic reaction

 Natural antibodies in breast milk (passive immunity)
 Breast milk is at ideal temperature
 No additives
 Bonding between mother & baby

Disadvantages

 Difficult to do in public
 Painful
 Mother must be present

Puberty

Puberty is the name for the time when the body begins to
change as a child transitions into adulthood.

 Testosterone triggers the development of secondary

sexual characteristics in males
 Estrogen triggers the development of secondary sexual
characteristics in females

Menstrual cycle
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

There are four main hormones involved in the menstrual cycle:

1. Estrogen – Builds the uterus lining

2. Progesterone – Maintains the uterus lining
3. FSH – Causes a single follicle in the ovary to mature (a
mature follicle contains the egg, along with other
surrounding cells)
4. LH – Secreted from pituitary glands which stimulate
ovulation

The stages of the cycle can be summarized as follows:

1. Uterus lining breaks down due to the lack of

progesterone
2. Estrogen levels rise to build the uterus lining
3. FSH causes a single follicle to mature
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

4. On day 14, estrogen levels drop and LH levels spike to

cause ovulation
5. The ovum gets released into the oviduct, leaving behind
the remains of the mature follicle.
6. The cells that get left behind after ovulation become the
corpus luteum
7. The corpus luteum secretes progesterone which
maintains the thick uterus lining in preparation for
implantation

Human birth control

There are various methods of birth control that you must be

familiar with.

Common methods of birth prevention include:

 Natural
 Abstinence
 Withdrawal
 Rhythm
 Chemical
Contraception

 Spermicide
 Mechanical
 Condoms
 Femidom
 Diaphragm
 Surgical
 Vasectomy
 Laparotomy

Common methods of birth promotion include:

 Artificialinsemination
 Fertility drugs
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 In-vitro fertilization

Sexually transmitted disease

Sexually transmitted diseases (STIs) are infections that can be

transmitted via body fluids during sexual contact.

HIV (human immunodeficiency virus) is an example of an STI.

An HIV infection can lead to AIDS (acquired immunodeficiency
syndrome).

As the name suggests, the HIV virus destroys the body’s

immune system. The virus attacks lymphocytes in the blood
stream, and a decreased lymphocyte count leads to reduced
antibodies, and therefore compromised immunity.

Methods of transmission include:

 Unprotected sex
 Sharing needles
 Mother to fetus via the placenta

Methods to prevent the spread of HIV include:

 Condom usage
 Abstinence of sexual intercourse
 Using sterilized needles instead of sharing
 Bottled milk

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Inheritance
ByFree Exam AcademyJuly 24, 2019
Inheritance
Inheritance is the transmission of genetic information from
generation to generation.

Genetic information is stored in the the form of DNA within the

cell nucleus. Here are some key definitions that you must be
aware of:

 Chromosome is a thread like structure of DNA

 A gene is a length of DNA which codes for a specific
protein
 An allele is a version of a gene

For example, imagine a gene that codes for eye color. There are
variations in this gene called alleles. One allele for this gene may
code for brown eyes, whereas another allele may code for blue
eyes.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Sex inheritance
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Normal human cells have 23 pairs of chromosomes. These

called known as diploid cells.

One of the pairs of chromosomes codes for sex inheritance.

Sex inheritance depends on the presence of X and Y

chromosomes.

 Males have one X chromosome and one Y chromosome

(XY)
 Females have two X chromosomes (XX)

DNA structure and function

The function of DNA is to control cell function by controlling the

production of proteins i.e. enzymes, antibodies, cell receptors,
etc.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The DNA is made of a double helix backbone, which are

connected by pairs of bases.

 Adenine always pairs with thymine

 Cytosine always pairs with guanine

The sequence of bases along a particular length of a DNA strand

codes for the manufacture of a specific protein.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Consider the particular length of DNA (gene) in the diagram

above. The code within that length is ATTCGAA. This base
sequence within the gene codes for the specific types and order
of amino acids that become joined to form a protein.

All body cells contain the same genes, but not all genes are
expressed because the cell only makes certain proteins that it
really needs.

Protein manufacture

As we’ve discussed above, a gene is a particular length of a

DNA molecule containing a specific sequence of bases. This
base sequence codes for which amino acids should be joined in
what order, to build a particular protein.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

DNA held within the nucleus of the cell. Let us consider a

particular gene (gene A) that makes a certain protein (protein
A).

A molecule called mRNA ‘copy’ the DNA base sequences

found within gene A – This is called transcription
 mRNA molecule leaves the nucleus
 mRNA passes through the ribosomes in the cytoplasm
 Ribosomes ‘read’ the base sequences and assemble
various amino acids in a specific order based on the base
sequences – This is called translation

Diploid vs haploid
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

As mentioned before, a normal human cell contains 23 pairs of

chromosomes. This particular number is extremely important
for normal bodily function. Cells like this are called diploid cells,
and are majority of the cells in the human body.

Gametes i.e. sperm cells/egg cells, have only a single set of 23

chromosomes. This is half the normal amount in order to
maintain 46 chromosomes after fertilization.

In summary:

 Diploidcells have 23 pairs of chromosomes

 Haploid cells have a single set of 23 chromosomes

Cell division
Mitosis

Mitosis is the nuclear division giving rise to genetically identical

cells. This is very important for various bodily functions:

 Growth
 Repair
 Cell replacement

Imagine diploid cell A (46 chromosomes) undergoing mitosis. If

this cell were to simply divide in half, then it the resulting cells
will only have 23 chromosomes.

Therefore, the cell duplicate its chromosomes before mitosis

occurs (i.e. from 46 to 92) in order to maintain the chromosome
number after the cell division occurs.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Meiosis

Meiosis is a type of nuclear division giving rise to cells that are

genetically different. This is a reduction division to form haploid
cells which produce gametes.

Essentially, diploid cell A (46 chromosomes) will form haploid

cells with 23 chromosomes.

Monohybrid inheritance
Important definitions

 Genotype = The genetic make up of an organism in terms

of the alleles present
 Phenotype = Observable features of an organism

 Alleles = Variations of a given gene

 Homozygous = Two identical alleles of a
particular gene
 Heterozgous = Two different alleles of a particular
gene
 Dominant allele = Allele that is always expressed
if present
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Recessive allele = Allele that is only expressed if

the dominant allele is not present

Monohybrid crosses

A monohybrid cross is a genetic mix between two individuals

who have homozygous genotypes, or genotypes that have
completely dominant or completely recessive alleles, which
result in opposite phenotypes for a certain genetic trait.

Eye colour

There are different alleles that code for eye colour

 Blueeye allele (b) is recessive

 Brown eye allele is dominant (B)

A brown eyed individual can therefore have two possible genotypes:

BB or Bb

A blue eyed individual can only have one possible genotype: bb

Example 1 – Homozygous recessive (bb) X Heterozygous(Bb)

Consider a brown eyed person with genotype BB having a child

with another person with blue eyes genotype bb

You can use the punnet square to look at all the possible
genotypes of the child.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

In this scenario, the chance of a blue eyed child is 2/4 and the
chance of a brown eyed child is 2/4 , therefore the ratio is 1:1

Example 2 – Homozygous recessive (bb) vs Homozygous

dominant (Bb)

Consider a brown eyed person with genotype Bb having a child

with another person with blue eyes genotype bb

In this scnerario, the chance of a blue eyed child is 0/4, and the
chance of a brown eyed child is 4/4.

Example 3 – Heterozygous (Bb) X heterozygous (Bb)

Consider a brown eyed person with genotype Bb having a child

with another person with brown eyes genotype Bb

In this case, the chance of a blue eyed child is 1/4, and the
chance of a brown eyed child is 3/4. The ratio of blue:brown is
1:3
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Co-dominance

Co-dominance is when a pair of alleles are neither dominant to

one another. Both alleles can therefore impact the phenotype.

Blood type is a good example of co-dominance. The alleles of

bloodtypes are: IA, IB and IO.

 IA and IB are co-dominant

 IA and IB are dominant over IO

Combination of these alleles can therefore results in blood

groups A, B, AB, and O.

Sex-linked characteristics

Sex linked characteristics are characteristics in which the gene

responsible is located on the sex chromosome, and therefore
making it more common in one sex than the other.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Red-green colour deficiency

The colour deficient gene is an abnormal gene found in the X

chromosome (Xc).

A male (with XY chromosomes) with these gene will inevitably

have colour deficiency because males only have one X
chromosome, so this abnormal gene will always be expressed.

Females on the other hand, may have the colour deficient gene
but have normal colour vision. This is because females have two
X chromosomes (XX).

The normal colour vision allele dominates the abnormal allele,

so as long as the female has one normal X chromosome then
they will have good colour vision.

To summarize:

 Male (XY) = Normal colour vision

 Male (XcY) = Colour deficient
 Female (XX) = Normal colour vision
 Female (XcX) = Normal colour vision i.e. carrier
 Female (XcXc) = Colour deficient

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Variation and selection
ByFree Exam AcademyJuly 26, 2019
Variation
Variation is defined as differences between individuals within
the same species.

Variation can be caused by various factors. The most important

ones that you need to be aware of is:

 Sexual reproduction
 The mixing of genes from the father & mother to
produce offspring with a completely unique
genetic make-up causes variation
 Mutation
 Random changes of the DNA of an organism

Genetic vs phenotypic variation

 Genetic variation – The differences in genetics among

individuals within the same species (mainly due to sexual
reproduction & mutation)
 Phenotypic variation – The variability of phenotypes within
a population (influenced by both the genetic &
environmental factors)

Continuous vs discontinuous variation

 Continuous variation – Results in a range of phenotypes

between two extremes i.e. height/weight/etc.
 Discontinuous variation – Results in limited number of set
phenotypes with no intermediates i.e. tongue rolling
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Sickle cell anemia

Sickle cell anemia is caused by the mutation in a gene that
codes for hemoglobin. The abnormal base sequence of the
hemoglobin gene causes sickle-shaped red blood cells.

Sickle celled RBCs are less efficient at carrying oxygen, and

more likely to become stuck in capillaries preventing blood
flow.

The faulty Hb gene is dominated by the normal Hb gene but it

can still have an effect in the heterozygous genotype.

Consider the three possible genotypes (N = normal, n =

abnormal)

 HN HN – 100% normal Hb
 HN Hn – Mostly normal Hb, but some abnormal Hb found
i.e. non-life threatening sickle cell trait
 HnHn – 100% abnormal Hb, life-threatening condition

The symptoms of sickle-cell anemia are as follows:

 Fatigue and anemia

 Pain crises
 Bacterial infections
 Lung & heart injury
 Leg ulcers
 Eye damage

Sickle-cell anemia and malaria

Malaria is a serious and sometimes fatal disease caused by a

parasite that commonly infects a certain type of mosquito which
feeds on humans.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Sickle-cell anemia provides protection against malaria, because

the parasites cannot penetrate sickle-celled RBCs.

Therefore, sickle-celled genotypes are favourable in countries

where malaria is highly prevalent.

A person with sickle cell anemia (HnHn) will be fully

protected against malaria, but likely to die from sickle-
cell disease
 A person that has no sickle cell anemia (HNHN) has no
protection from malaria, and is therefore likely to
contract the disease
 A person that is a sickle-cell carrier (HNHn) gains moderate
malarial protection, and will not die from sickle cell
disease

Adaptive features
Adaptive features are inherited functional features of an
organism that increases in fitness.

Fitness is defined as the probability of an organism surviving

and reproducing in the environment in which it is found.

Adaptive features of xerophytes

Xerophytes are plants that survive with very little water i.e.
cactus. These plants have adaptive features which allow it to
maximize water uptake and minimize water loss.

 Thick cuticle to minimize evaporation

 Small leaves to reduce surface area for evaporation
 Less stomata to reduce the surface area for diffusion
 Sunken stomata to maintain humdity around stomata (and
therefore reduce diffusion of water out of the plant)
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Rolledleaves to maintain humidity around the stomata

 Extensive roots to maximize water uptake

Adaptive features of hydrophytes

Hydrophytes are plants that grow on or in water.

 Wide flat leaves to increase surface area for floatation and

to be exposed to as much light as possible
 Reduced plant structure as there is less need for a rigid
structural support
 Thin waxy cuticle as water preservation isn’t as important
 Small roots as water can be obtained directly through the
leaf and stem
 Stomata are open on the upper side of teh leaf in order to
maximize gas exchange without worrying about too
much water

Natural selection and evolution

Natural selection

Natural selection describes the concept that for a given

environment, organisms with the most beneficial features are
‘selected’ to survive and pass on their genes to the next
generation.

The steps are as follows:

 There is variation within a population

 Many offspring are produced
 There are compeittion for resources among individuals
within the population
 There is struggle for survival
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 The most ‘fit’ individuals that are more adapted to the

environment will survive and reproduce
 Fitter individuals pass on their genes/alleles to their
offspring

Evolution

Natural selection ensures that only the most favourable genes

get passed onto the next generation.

Over time, populations therefore become more suited to their

environment. This is known as evolution.

Antibiotic resistance

Upon the use of antibiotics, most bacteria of a certain strain will

die. However, due to pure chance, there may be some bacteria
that have had a genetic mutation with gives them resistance to
the antibiotics used.

These resistant bacteria survive and reproduce rapidly. This

produces many more bacteria that also have the resistant gene.
Eventually, the antibiotic will be ineffective due to the
abundance of resistant bacteria.

Artificial selection
Artificial selection is the intentional reproduction of animals and
plants by humans that have desirable traits.

By using this method, the organisms can pass down favourable

traits to their offspring to produce more organisms with the
characteristics that we want.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Natural selection is natural because the environment provides

selective pressure for organisms that live within the
environment. Artificial selection is artificial because it is not the
environment which provides the pressure, it is human
manipulation instead.

Selective breeding

 Selection by humans of individuals with desirable traits

 Crossing these individuals to produce next generation
 Selection of offspring showing the desirable characteristics
 Over many generations this can improve the quality of
crop plants and domesticated animals

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Organisms And Their Environment
ByFree Exam AcademyJuly 26, 2019
The ecosystem
Many organisms live together in an ecosystem, and each of
them have an important role to play in maintaining the balance
of that ecosystem

Roles of organisms
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 Producer – An organism that makes its own organic

nutrients, usually from sunlight via photosynthesis
 Consumer – An organism that gains energy by feeding on
other organisms. They can be further classified into
primary, secondary, tertiary consumers
 Herbivores – Animals that gain energy by eating plants
 Carnivores – Animals that gain energy by eating other
consumers
 Decomposers – Organisms that gain energy by breaking
down dead, or organic waste material

Energy flow through an ecosystem

The principle source of energy input into the ecosystem is the

sun. Plants use photosynthesis to convert light energy (from
sunlight) into chemical energy in nutrients.

The chemical energy stored in food gets transferred to the

environment.

Trophic levels
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A trophic level is the position of an organism in a food chain,

food web, pyramid of numbers, or the pyramid of biomass.

All the things mentioned above demonstrate the direction of

energy transfer within organisms .

All feeding relationships begin with the producers. Producers

then are eaten by consumers.

The energy flow is as follows:

Primary producers -> Primary consumers -> Secondary

consumers -> Tertiary consumers -> Quaternary consumers
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Energy transfer along trophic levels

Starting from the producers, energy is transferred from one

trophic level to the next.

At each level, 90% of the original energy is lost due to various

factors:

 Respiration – Energy is used to respire

 Movement – Energy is used for movement
 Maintenance of body temperature – Energy is used in
homeostasis
 Indigestable material within an organism – Some parts of
eaten marterial cannot be digested or used by the
consumer

Therefore higher the trophic level, the smaller the amount of

energy that is available to them.

For this reason it is rare to see more than 5 trophic levels.

Organisms higher up would not be able to sustain themselves
due to the lack of energy.

Food chains & food webs

Food chains show the transfer of energy from one organism to

the next, beginning with producers.

A food web is a network of interconnected food chains

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Food pyramids

Food pyramids are graphical representations (in the form of

pyramids) that show feeding relationships of organisms at each
trophic level.

The pyramid of numbers simply show the number of

organisms in each trophic level. Unfortunately, this can at times
be somewhat misleading. It is misleading because it may not
depict the true amount of energy in each trophic level.

For instance, 10000 aphids may feed off a single large oak tree.
Using this information for the pyramid of numbers gives us a
fairly odd looking pyramid. It makes it feel as if there may not
be ‘enough’ producers to support the food chain.

Instead, the pyramid of biomass is a much more accurate

representation of the actual energy levels in each trophic level.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The oak tree has more than enough energy to support the food
chain.

Carbon, water, and nitrogen cycles

Carbon cycle
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Water cycle

Nitrogen cycle
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Population size
A population is a group of organisms of one species living in the
same area at the same time.

A community is a combination of all populations of different

species in an ecosystem.

An ecosystem is a unit containing the community of organisms

and their environment, interacting together.

Sigmoid population curve

There are four main phases of growth for a population which we

will look at in more detail below.

Some important factors that can influence population growth

are as follows:

 Food supply
 Predation
 Disease
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

1. Lag phase
 The population growth begins slowly from a few
individuals
2. Log phase
 Exponential growth due to ideal conditions and
maximum growth rate is achieved
3. Stationary phase
 The carrying capacity of the environment is
reached i.e. the maximum population size that
the environment can support
 This could be due to limitation of resources such
as food, space, etc.
4. Decline/death phase
 Sudden environmental change causes an inability
of the environment to support the population

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Biotechnology And Genetic
Engineering
ByFree Exam AcademyJuly 27, 2019

Biotechnology & genetic engineering

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Biotechnology is defined as the exploitation of biological

processes for industrial and other purposes, especially the
genetic manipulation of microorganisms for the production of
antibiotics, hormones, etc.

Genetic engineering is defined as the deliberate modification of

the characteristics of an organism by manipulating its genetic
material.

Examples of genetic engineering:

 Insertion of human genes into bacteria for the production

of insulin
 Insertion of genes into crop plants to confer resistance to
herbicides
 Insertion of genes into crop plants to confer resistance to
insect pests
 Insertion of genes into crop plants to provide additional
vitamins

Anaerobic respiration of yeast

The anaerobic respiration of yeast can be used for various

industrial processes such as biofuel production and bread
making.

Ethanol used for biofuels

 Yeast is added to a source of sugar and kept in warm

conditions.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 In the absence of oxygen, ethanol is produced via

anaerobic respiration
 Ethanol can then be used as biofuels

Bread-making

 Yeast is activated by mixing it with sugar and water

 The mixture is then added to flour to form dough
 Dough is left in a warm place to rise
 The dough rises due to trapped carbon dioxide (a product
of anaerobic respiration)
 Appropriate temperature must be maintained as enzymes
control respiration
 When the bread is fully cooked, high temperatures kill the
active yeast and the ethanol evaporates

Fruit juice production

Pectinase is an enzyme that breaks down pectin – A protein
found within plant cell walls.

Pectinase is used commercially to break down pectin within fruit

cell walls in order to allow easier extraction of juice from the
fruit.

Biological washing powders

Biological washing powders are washing washing powders that
contain enzymes (mainly proteases & lipases) in order to break
down organic substances.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Hot water must be used with washing powders so it is important

that the containing enzymes have a high optimum temperature
– otherwise they would just become denatured.

The enzymes are often obtained from thermophillic bacteria

found in hot springs, and have a optimum temperature of 70
degrees celcius.

Lactose-free milk
For people that are lactose intolerant, it is important to produce
lactose-free milk. Lactase is the enzyme that breaks down
lactose.

Milk is passed down a series of fixed, immobile beads of lactase

which digest the lactose to produce lactose-free milk.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

The immobile set-up of the enzymes are advantageous for

various reasons:

 Enzymes can be conserved as they do not become

dissolved
 Separation of the product from the enzyme is easy since
the enzymes are attached on the surface, and do not get
mixed with the end product

Penicillin production
Penicillin is an antibiotic made by a fungus called penicillium. A
fermenter is used to mass produce penicillin.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Sugars, ammonium salt, and penicillium is added to the

fermenter.

 Sugar
is for respiration
 Ammonium is for protein and nucleic acid production

Here are some features about fermenters that you must be

aware of:

 Monitors pH and temperatures

 Air provides oxygen for respiration
 Has cooling mechanisms to maintain an optimum
temperature of approximately 24 degrees celcius
 Stirrer keeps the contents suspended

Bacterial production of human protein

Useful human proteins such as insulin (for management of type
1 diabetes) can be made by bacteria. This is an example of
genetic
engineering.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Here are the important steps of this process that you need to be
aware of:

1. Human cells with human insulin genes are selected

2. Chromosomes from those cells are extracted
3. Insulin gene from the chromosome is removed via an
enzyme called restriction endonuclease enzyme
4. A suitable bacterial cell is selected
5. Bacteria have loose DNA called plasmids
6. Plasmid is removed from the bacteria
7. Plasmids are cut open via the same restriction
endonuclease enzyme used to cut out the insulin gene
from the human chromosome
8. Human insulin gene is inserted into plasmids via ligase
enzymes
9. Plasmids are returned back to the bacterial cell
10. The bacteria is left in a fermenter to reproduce
11. Rapid aexual reproduction produces a huge colony of
bacteria that all have the human insulin gene, and hence
produces insulin
12. The insulin can be extracted from the fermenter and
used to treat diabetic patients

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Human influences on ecosystems
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

ByFree Exam AcademyJuly 27, 2019

Food supply
Modern technology has dramatically increased food supply:

 Agricultural machinery to use in larger land areas and

improve efficiency
 Chemical fertilizers to improve yields
 Insecticides to improve crop quality and yield
 Herbicides to reduce competition with weed
 Selective breeding to improve production by crop plants
and livestock

Monocultures and livestock production

Large scale mono-cultures of crop plants

Monoculture farming means that on a given agricultural land is

grown only one species of a crop at a time. If two or more
species are sown in the field together (for example beans and
corn), it is not a monoculture but a polyculture system.

The main benefits of monoculture farming are:

 Easier to manage
 More efficient
 Promotes technological advances in agriculture
 Offers higher earnings

The disadadvantages of monoculture farming are:

 Reduced diversity in ecosystem

 Increased pests
 Problems associated with insecticides
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Intensive live stock production

Livestock farming involves the rearing of animals for food and

other human uses, such as producing leather, wool and
even fertilizer. This type of farming primarily applies to cattle or
dairy cows, chickens, goats, pigs, horses and sheep.

The benefits of intensive livestock production are:

 Helps the economy and the agricultural industry

 Provides food security

The disadvantages of are:

 Easierspread of disease
 Welfare issues of livestock i.e. living in confined space

Providing sufficient food for the world

population

Famine is an extreme scarcity of food. There are various factors

as to why it exists today:

 Uneven distribution of food

 Drought/flooding
 Poverty
 Increasing population

Nearly 1 000 million people do not get enough to eat and over
400 million are chronically malnourished today.

It is therefore important to understand what it takes to find a

solution to this problem:

 Social implications
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Strategic plans that incorporate population


concerns such as population growth, distribution
and rural-urban migration patterns are crucial
 Community development strategies which
integrate essential social services as well as
production resources should be encouraged
 Economic implications
 Financial support must be given to research on
the integration of technologies for food
production.
 Environmental implications
 Reducing soil erosion and impoverishment,
deforestation, falling agricultural output, and poor
water management should be implemented,

Habitat destruction
Animal habitat destruction is a big issue that exists today.
Whilst habitats can be destroyed naturally i.e. tsunami,
earthquakes etc, a lot of it cause solely from human activity:

 Increased area for food crop growth, livestock production,

and housing
 Extraction of natural resources (such as trees)
 Marine pollution

Deforestation is the permanent destruction of forests in order to

make the land available for other uses.

There are many undesirable effects of deforestation:

 Loss of habitat for animals living in the forest

 Reduction of food resources and breeding grounds of
animals, potentially leading to extinction
 Soil erosion due to the lack of soil support from tree roots
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

 Lessphotosynthesis leading to increased atmospheric CO2

levels and thus contributing to the greenhouse ffect
 Reduction of rainfall due to less plants to transpire water in
the water cycle

Pollution
Pollution is the presence in or introduction into the environment
of a substance which has harmful or poisonous effects.

There are various causes of land and water pollution you need
to be aware of.

Land & water pollution

Insecticides

Insecticides are used to kill pests to enhance crop yields but

they can often also kill other harmless animals.

For example, DDT was used to kill mosquitots but it also

remained in the environment and was absorbed into food
chains resulting in bioaccumulation which killed a lot of other
good animals. DDT is now banned.

Herbicides

Herbicides are used to kill weeds and other unwanted crops to

reduce competition and increase crop yields.

Herbicides can get washed int water systems and kill aquatic
plants which result in a disruption of the food chain.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Nuclear fall-out

This is a leakage from a nuclear power station. Radioactive

particles can get carried into the environment and settle.
Organisms then absorb these particles which can result in
cancer.

Chemical waste

Inorganic waste waste (i.e. nickel, aluminium, lead, mercury)

that are dumped into water bodies can lead to the
bioaccumulation of these metals.

Plastics

Plastics can not only physically trap and harm animals, but they
can deteriorated into smaller pieces and be consumed by them
too. To make matters worse, toxic gases are produced when
plastics are burned.

Untreated sewage

Sewage is untreated organic waste produced along with

household and industrial waste material.

Dumping untreated sewage into the environment can lead to a

lot of problems such as:

 Eutrophication
 Death of aquatic animals due to pathogens in the sewage
 Spread of water borne diseases which can infect people by
drinking polluted water

Eutrophication has come up multiple times in previous examinations,

so make sure you learn this!
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Eutrophication is the excessive richness of nutrients in a lake or

other body of water, frequently due to run-off from the land,
which causes a dense growth of plant life.

The stages of eutrophication are as follows:

1. Sewage or fertilizers somehow leak into a water body i.e.

lake
2. Various things are present in sewage/fertilizers such as
phosphates, organic matter, bacteria
3. Phosphates promote algae growth while bacteria
reproduce by feeding on organic matter
4. Excessive algae forms a blanket on the water surface
5. Aquatic plants die due to the lack of light
6. As plants die, bacteria aerobically decompose dead
matter
7. Oxygen supply becomes depleted and aquatic animals
die
8. The entire food chain becomes disrupted

Now that we understand the damage that untreated sewage

can do, it is important to know how we actually treat it. The
treatment of sewage provides clean, drinkable water.

1. Large objects such as sticks are screened out

2. Suspended grit is allowed to settle in the grit settling
chamber’
3. Organic matter is digested via the ‘sludge digester’ and
allowed to settle in the ‘sludge settling tank’
4. The remaining liquid goes into an aeration tank which
contains stones that have microorganisms on the surface
to digest other remaining organic matter
5. Water passes out and may be chlorinated to kill any
remaining bacteria

Greenhouse effect (climate change)

IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Greenhouse gases are gases in the atmosphere that prevent

infrared radiation to pass through.

These gases are very important in maintaining the surface

temperature of the earth.

Examples of greenhouse gases are:

 Carbon dioxide
 Methane
 Chlorofluorocarbon (CFCs)
 Nitrogen oxides
 Water vapour

With industrial revolution we are now produced excessive

amounts of greenhouse gases. This leads to the enhanced
greenhouse effect, where more and more infra-red radiation is
becoming trapped. This is resulting unstable climate conditions
which is causing harm to the entire globe.
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Acid rain

Acid rain is rainfall made so acidic by atmospheric pollution that

it causes environmental harm.

Acid rain is caused by the release of nitrogen oxides and/or

sulfur dioxides into the atmosphere.

 Nitrogen oxides are made from the reaction of nitrogen +

oxygen in high temperatures of car engines
 Sulfur dioxide is release by coal factories, cars, and oil
refineries

These two gases can react with rain water to produce acid rain.

Acid rain can do a lot of damage to the environment:

 Damages leaf cuticles and kills plants

 Acidifies lakes and kills aquatic animals
 Damages buildings made of limestone
 Aluminium ions are leached out of the soil and washed into
watery bodies

Some solutions to reduce the incidence of acid rain may

include:

 Using oil instead of coal (since coal contains more sulfur)

 Switching to more renewable resources of energy
 Using catalytic converters in cars (which removes nitrogen
oxides from car exhausts)

Conservation
Extinction and endangerment of a species
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy

Extinction is the dying out or extermination of a species. Some

factors that may cause extinction are:

 Climate change
 Habitat destruction
 Hunting
 Pollution
 Introduced species i.e. species that are not native to a
particular location, and has the tendency to spread and
cause damage to the environment

An endangered species of animal or plant that is seriously at

risk of extinction.

Conservation of endangered species

It is important to conserve endangered species in order to

prevent extinction.

Conservation may involve:

 Monitoring and protecting habitats

 Education
 Captivebreeding programes
 Seed banks
 Conservation programmes
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
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IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy
IGCSE Biology Compliled by Mr. A.M.Masoko For Skylimit Academy