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GCSE Edexcel Biology Self-Studying Textbook by Ken Tu

GCSE Edexcel Biology
Self-Studying Textbook
Thank you to these people who helped make this resource possible
Aimua Igbenehi
Damilola Olatunji
Faran Ahmad
James San
Ohm Joshi
Sulaiman Galaria
Unit 1 – Key Concepts Biology…………………………………….………….3
Unit 2 – Cells and Control …………………………………………………...15
Unit 3 – Genetics ……………………………………………………...……...21
Unit 4 – Natural Selection and Genetic Modification ……………………...31
Unit 5 – Health, Disease, and the Development of Medicine ………...……41
Unit 6 – Plant Structures and their Functions ……………………...……...49
Unit 7 – Animal Coordination and Homeostasis …………………………...59
Unit 8 – Exchange and Transport in Animals ……………………………...69
Unit 9 – Ecosystems ………………………………………………………….77
Although I was pleased with my results, the lockdowns had shut out education for many
students. Tens of thousands of schoolboys and schoolgirls had fragmented knowledge with
regards to their syllabus. They had internet issues. The recommended school textbooks were
expensive. It was not their fault.
Earlier in the February of 2021, my laptop had broken down. The solder that connects the
power button to the motherboard had snapped off, so I had some minor problems with online
school. Fortunately, I was able to use my phone to hear what the teacher was saying and still
learn during my lessons.
For others, they are not so fortunate. Fuelled with a sense of responsibility, I decided to
embark on a project alongside the best students at my school.
A team of seven of the top students at WCGS helped to synthesise this free ‘GCSE Edexcel
Self-Studying Physics Textbook’ and the ‘GCSE Edexcel Self-Studying Biology Textbook
over the duration of summer and we began working the week after our exams had finished.
We had read through all the specification points and have completed a myriad of past paper
questions and mark schemes to ensure that the knowledge delivered to you is correct.
Please use this book in supplement to practice questions. I hope that you will be inspired to
do the best for yourself in education because it is a powerful tool that will leverage you
through the difficulties and to the heights.
Warm wishes,
Ken Tu
(L6 2021-2022)
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Introduction to Biology
1.1) Prokaryotic vs Eukaryotic
Cells can be prokaryotic or eukaryotic. Eukaryotic cells are bigger and complex (they also
have a nucleus) and include all animal and plant cells. Prokaryotic cells are smaller and simpler
(have no nucleus), e.g. bacteria. Both eukaryotic and prokaryotic cells have organelles.
Nucleus: contains the genetic material which controls the activities of the cell.
Cytoplasm: a jelly-like substance where most chemical reactions happen. It contains enzymes
which control these chemical reactions.
Cell membrane: this holds the cell together and controls what goes in and out of the cell.
Mitochondria: these are where most of the reactions for aerobic respiration take place. Aerobic
respiration produces ATP, which releases energy that the cell needs to work.
Ribosomes: these are involved in protein synthesis, where new proteins are made for the cell.
Plant cells contain the same organelles as animal cells and these:
Cell wall: this is a rigid layer made of cellulose. It supports and strengthens the cell.
Vacuole: this large structure contains cell sap – a weak solution of sugar and salts. It maintains
the internal pressure to support the cell, keeps the cell turgid and prevents the cell from
undergoing lysis.
Chloroplasts: these contain a green pigment called chlorophyll. This absorbs the light needed
for photosynthesis. Photosynthesis creates glucose and oxygen, which are needed by the plant.
Note: you can remember the above for plant cells only organelles as CCV,
Chloroplast, Cell wall, and Vacuole.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Specialised Cells
1.2) Specialised Cells
A specialised cell is a cell that performs a specific function. Most cells in an organism are
specialised. A cell’s structure (e.g. the shape and the parts it contains) helps it to carry out its
Sperm cells transport the male DNA to the female DNA in the egg via sexual reproduction to
create a zygote. As a result, sperm cells have:
long tails to swim to the egg
lots of mitochondria in their middle pieces to
provide the energy they need to swim to the
enzymes in the acrosome of their heads that
digest through the membrane of the egg
haploid nuclei (23 chromosomes in the
nuclei; every healthy human has 23 pairs of
chromosomes i.e 46 total) to ensure that the
zygote has the correct number of
The main functions of an egg (ovum) are to carry the
female DNA and to nourish the developing embryo
in the early stages. To enable it to do this, egg cells contain nutrients in the cytoplasm to feed the
embryo. Like sperm cells, egg cells have haploid nuclei. Straight after fertilisation, its membrane
hardens to stop any more sperm getting in. This makes sure the offspring end up with the correct
amount of DNA.
Epithelial cells line the surfaces of organs. Some of them have cilia (hair-like structures) on
the top surface of the cell. The function of these ciliated epithelial cells is to move substances –
the cilia beat to move substances in one direction, along the surface of the tissue. The lining of
the airways contains lots of ciliated epithelial cells. These help to move mucus (and all of the
particles from the air that it has trapped) up to the throat so it can be swallowed and doesn’t
reach the lungs.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
1.3 - 1.5) Microscopy
Light (optical) microscopes: These were invented in
the 1590s. They use light and lenses to form an
image of a specimen and magnify it. They let us see
individual cells and larger organelles, like nuclei and
Electron microscopes: These were invented in the
1930s. They use beams of electrons instead of light,
and so they have a higher magnification and
resolution than light microscopes. Electron
microscopes allow us to see organelles in much more
detail, e.g. the internal structure of mitochondria and
chloroplasts, as well as smaller organelles like
Magnification is the process of enlarging the
physical appearance or image of something.
If you know the power of the microscope lenses used to view a specimen, you can work out the
total magnification of the image using this formula:
Total magnification = eyepiece lens magnification × objective lens magnification
If you don’t know the power of the microscope lenses, but know the actual size of the specimen
and the measurement of the image, you can use this formula:
Magnification = image size ÷ actual size
Or remember the mnemonic AIM, a formula triangle. Actual size (bottom left), Image size (top),
Magnification (bottom right).
Microscopes are used at very small objects, and so the actual sizes of the specimen may be in
standard form – so you need to know how to calculate with them.
Resolution is the ability to distinguish separate structures at close points. If resolution is low, the
image is ‘blurry’ and unclear, but if resolution is high, then the image is clearer and generally
more saturated.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
You need to be able to convert between units of length. Here’s a guide:
Metres = m
1 m = 1000 mm
Millimetres = mm
1 mm = 1000 μm
Micrometres = μm
1 μm = 1000 nm
Nanometres = nm
1 nm = 1000 pm.
Picometres = pm
1.6) Microscopy Core Practical
To carry out microscopy, you need: a specimen, a
light microscope, a slide, a coverslip, a mounted needle, a stain, a pipette, a paper towel,
tweezers / cotton buds, distilled water, a knife and disinfectant.
Using a pipette, add a drop of distilled water to the middle of a clean slide.
If examining onion cells, cut up an onion with a knife and separate it into layers. Use tweezers to
pull off some epidermal tissue from one of the layers. If examining cheek cells, stroke the inside
of your cheek gently with a cotton bud, collecting only the loose cells. Use the end of the cotton
bud to stir the distilled water on the slide. Put the used cotton bud in disinfectant.
Use a different pipette to add a stain. For onion cells, add iodine solution. For cheek cells,
add methylene blue solution.
Place a coverslip onto the slide at a 45° angle on one edge of the drop of distilled water.
Gently lower the coverslip down onto the drop with a mounted needle, without trapping
air bubbles.
Dab on excess liquid from under the coverslip with a paper towel.
Examining the specimen
Start by clipping the slide you’ve prepared onto the stage.
Select the lowest-power objective lens (i.e., the objective lens with the lowest
Use the coarse adjustment knob to move the stage upwards to just below the objective
Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards
until the image is roughly in focus.
Adjust the focus with the fine adjustment knob, until you get a clear image of the
specimen on the slide.
If you need to see the slide with greater magnification, swap to a higher-power objective
lens and refocus.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
The Purpose of Enzymes
1.7 - 1.9) Enzyme Activity
Enzymes are biological catalysts – they increase the speed of a reaction, without being
changed or used up in the reaction. Enzymes are proteins with different structures.
Since they are proteins, they are made up of chains of amino acids. These chains are folded into
unique shapes, which enzymes need to perform their functions.
The lock-and-key model is used to show how enzymes work.
The enzyme has an active site which is
specific to a substrate. When this substrate
joins with the enzyme, this is an enzymesubstrate complex. Their shapes can fit
because they have complementary shapes to
each other. The enzyme is also to slightly
change its shape to be fully complementary to
the substrate. This is called induced fit.
When the reaction finishes, products are made.
These products are from two types of enzymesubstrate complexes: catalysis and synthesis.
Catalysis is when a complex substance (e.g., a
lipid) is separated or broken down into simpler substances (e.g. fatty acids and glycerol). The
diagram above demonstrates an example of catalysis.
Synthesis is when simpler substances (e.g., glucose) are joined to make a complex substance
(e.g. starch).
If the active site changes shape so that it is no longer specific to the substrate, the enzyme is
Enzyme Activity
Temperature is a major factor to the rate of enzyme activity. At first, an increase in temperature
increases the rate of reaction as the enzymes have more kinetic energy, and so they move more
quickly and collisions with substrates are more likely. However, if the temperature goes past the
optimum temperature – the temperature the enzyme works best at – the bonds holding the amino
acids in the enzyme break, causing the enzyme to denature and lose its specificity. Enzymes in
the human body generally work best at around 37 °C – this is also the normal body temperature
for a human.
The pH can also affect the rate of enzyme activity. If the pH is too high or too low for an
enzyme, it can interfere with the bonds holding the enzyme together. Therefore, the active site
can change and so denatures the enzyme. All enzymes have an optimum pH in which they work
best at.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Another major factor of enzyme activity is substrate concentration. If the substrate
concentration is initially increased, the rate of an enzymatic reaction also increases. This is
because since there are more substrates, there is a more likely chance that an enzyme can form an
enzyme-substrate complex with it. However, if the substrates are too concentrated, then the
active sites of enzymes will be full, and no more substrates can be catalysed or synthesised
with them, therefore there is no increase in the rate.
1.10 - 1.11) Core Practical Effect of pH on Enzyme Activity
This practical investigates how pH affects the enzymatic activity amylase, a carbohydrase
enzyme. You will need: iodine solution, a spotting tile, a water bath / Bunsen burner with heatproof mat, tripod and gauze, a beaker, distilled water, syringes, amylase solution, buffer
solutions of different pH, a boiling tube, a stop clock, and a dropping pipette.
Put a drop of iodine solution into every well of a spotting tile.
Place a Bunsen burner on a heat-proof mat, and a tripod and gauze over the Bunsen
Put a beaker of distilled water on top of the tripod and heat the water until it reaches the
optimum temperature of the amylase you are using. Alternatively, use a water bath.
Use a syringe to add 3 cm of amylase solution and 1 cm of a buffer solution with a pH of
5 to a boiling tube.
Next, use a different syringe to add 3 cm of a starch solution to the boiling tube.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Immediately mix the contents of the boiling tube and put into a water bath (or in a beaker
if using a Bunsen burner) and start a stop clock.
• Every 30 secs take a sample from the beaker and place it in the wells. When it stays
brown, orange, no starch is present
Calculate rate of reaction by doing the amount of substance / time taken!
Carbohydrates, proteins, and lipids are big and complex biological molecules, which are
essential for life. Organisms need to be able to break them down into smaller and simple
molecules, so that they can be used for growth and other life processes. These breakdown
reactions are catalysed by enzymes. For example, the molecules in the food we eat are too big to
pass through the walls of our digestive system, and so digestive enzymes break them down into
smaller, soluble molecules. These can now pass through the walls and be absorbed into the
Organisms also need to be able to synthesise carbohydrates, proteins and lipids from their
smaller components (monomers) to their complex molecules (polymers). Synthesis enzymes are
used for these reactions.
Carbohydrates can be synthesised from simple sugars.
Proteins can be synthesised from joining amino acids.
Lipids can be synthesised from fatty acids and glycerol.
Different types of digestive enzymes (hydrolases) catalyse the breakdown of carbohydrates,
proteins, and lipids.
Carbohydrases convert carbohydrates into simple sugars. Examples are amylase with starch,
maltase with maltose and lactase with lactose.
Proteases convert proteins into individual amino acids. Examples are pepsin, trypsin and
Lipases convert lipids (or more specifically, triglycerides) into fatty acids and glycerol. An
example is human pancreatic lipase (HPL) which breaks down dietary fats
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Testing for substances
Food can be tested for specific substances e.g reducing sugars like glucose or proteins. This is
how it can be done!
Benedict’s Test
Benedict’s test: Sugars are classed into reducing sugars and non-reducing sugars. You can use
Benedict's test for both.
For reducing sugars, you’ll need a food sample, a test tube, Benedict’s reagent and a water bath.
• Transfer some of the food samples to a test tube.
Prepare a water bath to about 75 °C.
Add some Benedict’s reagent to the
food sample and put into the water
After a few minutes, take the test tube
and examine its colour.
Benedict’s test is semi-quantitative, so there
is a range of colours depending on how much
reducing sugar is present. It goes from blue
(nothing) to green (little)
to orange (some) to red (lots).
For non-reducing sugars, you can do the
same method as above, but you need to hydrolyse the sugar first. Add dilute hydrochloric acid,
put it into the water bath, take it out and then add sodium hydrogen carbonate. This hydrolyses
the sugar and then you can use Benedict’s as normal. The results should be the same colours as
the reducing sugars
Starch is a non-reducing sugar, however it reacts with iodine.
For the iodine test, you’ll need a food sample, a spotting tile, distilled water and iodine solution.
• Place some of the food samples on a spotting tile.
• Add some distilled water to the food sample to create liquid.
• Add some iodine solution to the liquid.
If starch is present in the food sample, it will become blue-black. If not, it will remain brownorange.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Biuret Test
Proteins can be tested using the Biuret test. For the Biuret test, you’ll need a food sample, a test
tube, potassium hydroxide solution and copper(II) sulfate solution.
• Transfer some of the food
sample to a test tube.
• Add a few drops of potassium
hydroxide solution to make the
solution alkaline.
• Add a few drops of copper(II)
sulfate solution.
If proteins are present, then the food
sample will change from blue to pink /
purple. Otherwise, the food sample
will stay blue.
Ethanol Emulsion Test
You can test for the presence of
lipids (fats and oils) using the
emulsion test. For the emulsion test,
you’ll need a food sample, test tubes,
ethanol and distilled water.
Transfer some of the food samples to a test tube.
Add some ethanol to the test tube.
Shake the test tube well
for 1 minute until the
test substance
Pour the solution into
another test tube
containing some
distilled water.
If lipids are present, a milkywhite emulsion will form on
top of the liquid. If there are
lots of lipids in the food
sample, the milky whiteness of
the emulsion will be more
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Food can be burnt to see how much energy it contains. The energy is released as heat. This is
called calorimetry. Calorimetry can be done in the lab. You’ll need dry food, a mounted needle
or tongs, a boiling tube, distilled water, a clamp stand, a thermometer, a Bunsen burner and a
Pour cold distilled water in a boiling tube.
Record the starting temperature of the water with a thermometer.
Record the mass of the food sample with a scale.
Hold the food sample with a mounted needle and heat it with a Bunsen burner until it
catches fire.
Heat the cold water using the flame from the food sample.
Record the final temperature of the water.
Use the equation below to calculate the energy transferred:
Energy transferred = mass of water x temperature change x 4.2
Use the equation below to calculate the energy per gram of the food burnt:
Energy per gram = energy transferred / mass of food
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Transporting Substances
Diffusion is the spreading out of particles from an area of higher concentration to an area of
lower concentration, down or along a concentration gradient. It happens in both liquids and
gases – this is because the particles are free to move about
randomly. Diffusion is a passive process – it doesn’t require
energy to work.
Cell membranes hold the cell together but also control the
transport of molecules in and out of the cell. They are
partially permeable – small molecules can diffuse through
the membranes (such as oxygen, glucose, and water), but
large molecules cannot (such as proteins and starch).
Just like diffusion in air, particles flow through the cell
membrane from a higher concentration to a lower
concentration. As they move randomly, particles can flow in
both horizontal directions, but if the concentration gradient
is steep, a net movement of particles in one direction will be
more likely.
Osmosis is a special type of diffusion involving water molecules. It is the net movement of water
molecules from a higher water concentration to a lower water concentration, across a partially
permeable membrane. Like diffusion, osmosis is a passive process. The water molecules pass
both ways through the membrane in osmosis, as they move randomly. But if the concentration
gradient is steep, a net movement of water
molecules is more likely.
A solution with a high-water concentration will
have a low solute concentration, and a solution with
a low water concentration will have a high solute
concentration. The net movement of particles in
osmosis will cause a solute concentrated solution to
be more dilute.
The solution surrounding a cell will usually have a
different concentration to the fluid inside the cell.
This means that water will either move into the cell
from the surrounding solution, or out of the cell, by
osmosis. If a cell is short of water, the solution inside it will become quite concentrated with
water. This usually means the solution outside the cell is more dilute and so water will move into
the cell by osmosis. If a cell has lots of water, the solution inside it will be less dilute, and water
will be drawn out of the cell and into the fluid outside by osmosis.
Biology: Unit 1 - Key Concepts
written by Faran Ahmad, edited by Ken Tu
Active transport is the movement of particles against a concentration gradient (i.e. from an
area of lower concentration to an area of higher concentration) using energy transferred during
respiration (ATP).
Active transport is different from
diffusion because particles are moved
against or up a concentration gradient
(i.e. from an area of low concentration
to an area of high concentration), and
the process requires energy, so it is an
active process. It allows cells to
absorb substances from very dilute
Active transport is also useful in
plants, e.g. to absorb mineral ions
from the soil.
Core Practical - Osmosis in potatoes:
Plant cells gain and lose water by osmosis. You can investigate how changing the concentration
of the solution surrounding the cell affects osmosis. To investigate osmosis, you’ll need: a
potato, a cork borer, a scale, 6 beakers, a waterproof pen, 6 salt solutions from 0% to 5%, forceps
and paper towels.
Using the waterproof pen, label each beaker with the name of one of the solutions.
Cut 6 potato strips from a potato with a cork borer.
Dry each potato strip carefully by blotting it with a paper towel. Measure its mass on the
Place the potato strip into one of the beakers. Record the label on the beaker and the mass
of the strip in your results table.
Repeat steps 3 and 4 until all strips have been measured and placed in beakers.
Carefully fill each beaker with the appropriate solution, so that the potato strip is fully
covered. Leave the potato strips in the salt solutions for at least 15 minutes.
For each potato strip, use the forceps to remove it from its solution, blot dry on a paper
towel and measure its mass again. Record all the masses in the results table.
Edexcel GCSE Biology: Unit 2 - Cells and Control
written by Ohm Joshi, edited by Ken Tu
Mitosis is one of the processes of cell duplication, where one cell and all of its properties divide
into two genetically identical diploid daughter cells. (Diploid cells are cells with 2 set of
chromosomes and almost all cells are diploid apart from gametes/sex cells which are haploid).
First interphase occurs, then mitosis which includes 5 stages:
Interphase: Subcellular parts of a cell (mitochondria, chromosomes, etc) in the nucleus
are duplicated.
Prophase: The membrane of the nucleus begins to break down and spindle fibres appear
on either side of the cell.
Metaphase: The nucleus has fully broken down and the chromosome copies line up on
either side of the cell along the spindle fibres.
Anaphase: The chromosome copies are then separated and move to either side of the cell
along the spindle fibres.
Telophase: A membrane begins to form around each pair of chromosomes on either side
of the cell to form nuclei.
Cytokinesis: The cell surface then forms a membrane between the two halves and
separates it, causing the one cell to now become two genetically identical daughter cells.
Cancer is when a cell begins to divide uncontrollably as a result of a change in the cell, which
over time can create lumps of cells called tumours which damage the body and result in death
if not able to be treated.
Edexcel GCSE Biology: Unit 2 - Cells and Control
written by Ohm Joshi, edited by Ken Tu
Mitosis is used in growth of organisms, defined as ‘an increase in size as a result of increase
in number of cells’ which would be as a result of mitosis.
Growth in animals occurs by cell division and then cell differentiation/specialisation, where
cells change their shape and parts according to their functions in the body.
Growth in plants occurs by cell division, differentiation and then elongation at the
meristems, where the cells in a plant continually increase in length throughout their lives.
Asexual reproduction is when an organism produces genetically identical offspring also
known as clones without the requirement of a partner, meaning asexual reproduction relies on
Percentile charts are used to monitor growth of new-born babies for the first 12 months of life
to compare their growth rates alongside the rest of the new-born population and place them on a
percentile. To be on a specific percentile means to have that percentage of babies as smaller than
them (25th percentile means 25% of babies are less heavy, 70th percentile means 70% of babies
are less heavy).
Stem cells are cells which divide continuously
and can differentiate into specialised cells.
They are initially undifferentiated. These are
found in the meristems of plants and can
continually specialise and elongate for the rest of
the plant’s life. Embryonic stem cells (ESCs)
are the stem cells in an embryo, formed after the
female egg cell is fertilised with the male sperm
cell. ESCs can differentiate into any type of
specialised cell.
Adult stem cells are limited specialised cells
which can only differentiate to specific cells
which are in the surrounding tissues, only for
replacing damaged cells. ESCs could possibly be cultured or extracted from embryos (which
is seen as unethical as you would have to kill the embryo) to treat many diseases which damage
and destroy cells. However, ESCs could continue to divide inside a new body and lead to
cancer, as well as the body seeing the foreign ESCs as a threat and be killed off by the immune
system, called rejection.
Edexcel GCSE Biology: Unit 2 - Cells and Control
written by Ohm Joshi, edited by Ken Tu
The Brain and the CNS
The brain is made up of millions of nerve
cells/neurons which run through the entire
body to send signals and information up and
down. 3 main parts of the brain include:
The cerebral cortex at the front,
separated into 2 cerebral hemispheres
controlling the body. The left
hemisphere controls the right side of
the body while the right hemisphere
controls the left side. It is responsible
for controlling most of our senses,
language, memory, behaviour and
The cerebellum at the bottom of the brain is also separated into two halves, each
controlling balance and posture, as well as fine tuning muscle activity and making
movements smoother.
The medulla oblongata connects the spinal cord to the brain and controls heart rate,
breathing and reflexes such as sneezing, swallowing and vomiting. The spinal cord is a
large bundle of nerves which carry information across the body to the brain.
Brain and Spinal Cord Problems
Scans allow for the brain to be remotely observed without having to
perform surgery.
CT scans show shape and structures of the brain by moving an X-ray
beam around the head. Detectors on the opposite side measure the
absorption of X-rays by the brain to create a clear image which is easily
PET scans show brain activity and what parts of the brain are working
during certain activities. It is done by injecting radioactive glucose into
the patient. Glucose is used in cellular respiration for energy, so the
part of the brain which is more active takes in the glucose. The
radioactive atoms in the glucose cause gamma rays to be emitted which
scanners can detect and the more radiation from one area of the brain, the
higher the activity is there for a certain task.
Brain and spinal cord damage can cause paralysis (loss of use or feeling in parts of the body)
and there is very little that can be done to cure this. Brain tumours are caused by cancer in the
brain. This lump of cells can cause parts of the brain to not function as it is being obstructed.
Tumours can be killed using radiotherapy (focusing high energy X-ray beams to kill the cells)
and chemotherapy (injecting drugs to kill the tumour directly). However, they can harm the
Edexcel GCSE Biology: Unit 2 - Cells and Control
written by Ohm Joshi, edited by Ken Tu
body as well, and chemotherapy could be ineffective due to the blood-brain barrier which only
allows certain substances to get from the blood into the brain.
Nervous System
The Central Nervous System (CNS) is made up of the brain, spinal cord and neurons which
allow the body to send information around in the form of electrical impulses. These impulses
are created by the detection of a stimulus by the receptor cells in the sensory organs which
travel to the brain through neurotransmission along the nerves for the brain to formulate a
response. Neurons meet each other at synapses which are tiny gaps and when an impulse
reaches the synapses, a chemical substance called a neurotransmitter is released and detected
by the next neuron and repeats along the CNS. This is done to slow down neurotransmission and
to allow for impulses to travel in the correct direction.
The different types of neurons have different functions.
The sensory neuron carries impulses
to and from the receptor cells and
brain. Impulses enter the neuron by
the dendrites, travel along the
dendron, through the axon and out
to the next neuron by the axon
terminals. They are surrounded by a
layer called the myelin sheath which
allows the impulse to travel along
one neuron and prevent it from
jumping to others, as well as
insulating it to maintain speed of
Motor neurons carry impulses towards
effectors (after the brain creates a response to
a stimulus, it sends it to an effector to carry
out that response).
Relay neurons found in the spinal cord link
up the motor and sensory neurons, as well as
play an integral part in the reflex arc.
In the diagram on the right, it refers to the relay neuron as an interneuron.
Reflexes are actions which are taken by the body without having to think about it. It may be used
to avoid harm, or it may be a natural bodily function (sneezing/swallowing). Reflexes do not
need to be thought about so the impulses bypass the route to the brain. The impulse travels
along the sensory neuron, onto the relay neuron at the spinal cord and then to the motor
neuron to the effector to carry out what must be done to prevent harm to the body (example:
pulling your hand away from a fire. Receptor cells feel pain, send an impulse, travel along the
reflex arc, travel to the effector, muscle moves, pulling the arm away).
Edexcel GCSE Biology: Unit 2 - Cells and Control
written by Ohm Joshi, edited by Ken Tu
The Eyes
The eyes are sense organs which allow for sight. Two
types of receptor cells called rods (which detect light
intensity) and cones (which detect colour of light)
are found in the eye in a layer called the retina. Light
enters the eye through the pupil, found as the dark
area in the centre of the eye. The iris controls the
amount of light entering the eye. Constricting the
pupil (make it smaller) allows less light through,
while dilating it (making it bigger) allows more light
through. Light rays need to be focused on the retina
in order to create a clear image and is done by the
cornea which refracts light rays to bring them
together. Then the lens fine tunes the focus which are
controlled by ciliary muscles which fatten the lens to
focus on nearer objects and make them thinner for further objects.
If the lens does not change shape correctly, the light doesn’t converge at the retina and
vision becomes blurry. Short sightedness (myopia) is when distant objects are blurry
and light rays meet before the retina. This can be because the eyeball is too long.
Long sightedness (hyperopia) is the opposite, where close objects are blurry and light
rays meet after the retina. This can be because the eyeball is too long. This can be fixed
by corrective lenses. Short sightedness is fixed by a diverging/concave lens which
bends light outward to make light rays meet at the retina.
Long sightedness is fixed by converging/convex lenses which bend light towards each other to
meet at the retina, and not after it.
Cataracts are an eye problem where protein build up makes vision cloudy and can be fixed by
replacing the lens with a clear, plastic one in surgery.
Edexcel GCSE Biology: Unit 2 - Cells and Control
Colour blindness is a sex-linked
disorder and causes some cones in
the retina to function incorrectly. It
is a genetic disorder and cannot be
written by Ohm Joshi, edited by Ken Tu
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
Sexual and Asexual Reproduction
Sexual reproduction is the most common method of reproduction involving a the fertilisation of
a female gamete (female sex cell). Whilst asexual reproduction produces clones which are
genetically identical to the parent without needed another gamete.
As with any biological process, there are pros and cons.
Asexual Reproduction - Producing new organisms from one parent only. These organisms are
genetically identical to the parent.
Advantages of Asexual Reproduction
Disadvantages of Asexual Reproduction
Asexual reproduction can produce lots
of offspring very quickly because it
does not require another gamete to
begin reproduction.
There is no genetic variation between offspring in
the population. So, change in the environment (like a
disease) makes conditions unfavourable leading to a
whole population being affected.
Only one parent is needed. This
means organisms can reproduce when
conditions are favourable, without
having to look for a mate.
Sexual Reproduction - This is where genetic information from two organisms ( a male and
female parent) is combined to produce offspring which are genetically different from both
Advantages of Sexual Reproduction
Disadvantages of Sexual Reproduction
Sexual reproduction creates genetic variation
within the population which means that
everyone has different characteristics.
• This means that they are less prone to
being entirely wiped out by a change
in the environment.
Two parents are needed for sexual
reproduction. This means that organisms
must look for and find a mate in order to
produce offspring.
Involves meiosis hence it produces genetically Sexual reproduction takes more time and
different haploid gametes, which fuse to form energy than asexual reproduction, hence they
a diploid cell at fertilisation
produce less offspring.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
Meiosis is a form of cell division in which one parent cell produces four haploid daughter
Produces 4 cells
Produces genetically varied cells
Produces haploid cells
Produces gametes/sex cells
Gamete Production
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
DNA strands are polymers made up of nucleotides. Nucleotides consist of a sugar molecule, a
phosphate backbone and one “base”. The sugar-phosphate groups form a backbone to DNA
strands. The bases are A(adenine), T(thymine), C(cytosine), G(guanine).
On the right, shows a diagram of the structure
of DNA where the dotted lines in between
represent the weak hydrogen bonds. The sugar
molecule that connects the phosphate backbone
to the bases is called deoxyribose although you
don’t need to know this for GCSE.
Double Helix
A DNA molecule is coiled together in the
shape of a double helix.
Each base links to the corresponding base from
the opposite direction strand in the helix.
Complementary base pairing
A pairs with T
C pairs with G
The bases are joined by weak hydrogen
A gene is a section of DNA on a chromosome that codes for a particular protein.
All an organism's DNA makes up its genome.
DNA Extraction
For the GCSE Biology specification, you are required to know how to extract
DNA from a fruit using the following method:
Mix a solution of detergent and salt.
o The detergent breaks down the cell membranes to release DNA
o The salt will make the DNA stick together
Add some strawberries that have been mashed to the solution and mix
Filter the mixture to get froth and remove the insoluble bits of the cell out
Add some ice-cold ethanol to the filtered solution
The DNA will come out of the solution as it is not soluble in cold alcohol. It will appear
as a stringy white precipitate. This can be taken out with a glass rod.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
DNA Composition
DNA controls the production of proteins in a cell. Proteins are made up of a chain called amino
acids. They are the monomers of proteins. The chains fold up to give the protein a different
specific shape - hence they each have a different function. A gene is a section of DNA on a
chromosome that codes for a particular protein. All an organism's DNA makes up its genome.
The amino acids join to make proteins and the different sequences of bases allow it to code for
specific proteins.
Protein Synthesis
This is the process whereby long polypeptide chains (proteins) are formed. Proteins are made in
two stages, transcription, and translation.
In transcription, the DNA bases are used to form a strand of RNA (ribonucleic acid).
1. RNA polymerase, an enzyme, binds to regions of non-coding DNA in front of a gene.
2. The two DNA strands unzip and the RNA polymerase moves along one of the strands of
the DNA.
3. It uses the coding DNA in the gene to make the mRNA (messenger RNA). As the RNA
polymerase moves along the template DNA strand, it adds complementary nucleotides
(bases) to the mRNA. Except the T (thymine) base is replaced by another base - a U
(uracil) base
4. The mRNA leaves the nucleus through the nuclear pore entering cytoplasm and joins
with a ribosome.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
1. Amino acids are brought to the ribosome by another molecule called tRNA (transfer
2. The order of the amino acids matches the base triplets in mRNA (which can be known as
3. The tRNA have anticodons which are complementary to the codons.
4. The amino acids are joined together by the ribosome. This makes a polypeptide chain.
Overall, the process can be described as the following:
a) RNA polymerase binds to non-coding DNA located in front of a gene.
b) RNA polymerase produces a complementary mRNA strand from the coding DNA of the
c) The mRNA strand swims into the cytoplasm attaching itself to the ribosome.
d) The coding by triplets of bases (codons) in the mRNA for specific amino acids.
e) The transfer of amino acids to the ribosome by tRNA.
f) The linking of amino acids to form polypeptides.
When the protein is formed, its functionality depends on the original gene being correct. If the
gene is faulty or experienced a mutation, the mRNA produced will be attached to different
codons and therefore a different amino acid will be in the protein causing it to fold up differently.
Consequently, the protein formed will be a different shape thus it may not perform its function as
effectively as it should. A similar result would reduce the quantity of protein produced if the
length of the two non-coding sections of DNA were mutated to be shorter.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
Gregor Mendel's (1822-1884) research formed a foundation for the genetics we know today.
Mendel crossed plants with different characteristics. He observed the characteristics of the
offspring and drew conclusions about the inheritance of characteristics.
Mendel's Pea Experiment
1. Mendel crossed two pea plants. One tall pea plant and one short pea plant. The offspring
were all tall.
2. He then crossed the offspring with each other and found that they were produced at ratio
of tall: short at 3:1
The differences in the offspring are down to the dominant and recessive alleles.
Mendel's conclusions
1. Characteristics in plants are determined by “hereditary units”
2. Hereditary characteristics are passed on from generations.
3. These characteristics can be dominant or recessive. In the case that the person has both
the characteristics the dominant characteristic is expressed.
Below is the terminology used to describe genotypes and phenotypes in the genetics an organism
may receive.
Chromosome - A thread-like structure found in the nuclei of cells. Each chromosome contains
one enormously long DNA molecule packed with proteins.
Gene - A section of the long strand of DNA found in a chromosome that often contains
instructions for a specific protein.
Allele - Most genes come in different versions called alleles.
Dominant - Describes an allele that will always affect a phenotype as opposed to a recessive
allele, whose effect will not be seen if a dominant allele is present.
Recessive - Describes an allele that will only affect the phenotype if the other allele is also
recessive. It has no effect if the other allele is dominant.
Homozygous - When both alleles for a gene are the same in an organism.
Heterozygous - When both the alleles for a gene are different in an organism.
Genotype - The alleles for a certain characteristic that are found in an organism.
Phenotype - The characteristics produced by a certain set of alleles.
Gamete - A haploid cell produced by meiosis used for sexual reproduction.
Zygote - A fertilised egg cell.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
A monohybrid (single gene) cross can show the probability of having offspring with certain
traits with a Punnett square. Uppercase letters are for dominant alleles and lowercase letters for
recessive alleles.
The Punnett square demonstrates the probability that an offspring will receive a certain
Family pedigrees show how alleles are passed down generations.
Squares represent males, circles represent females, black shows affected and white shows
The sex of a child can be represented using a punnet’s square too. Females have two X
chromosomes hence they are XX. Males have an X and Y chromosome hence they are XY.
As shown below, when the two gametes fuse, there is a 50% chance that they are male or female.
Using the on the right, diagram we can see that the XY (probability of a male) is 50% as it only
appears 2 out of 4 times.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
Multiple and Missing Alleles
1. There are multiple alleles that determine blood
2. There are 4 different blood types O, A, B, and
3. The gene for the blood types in humans has only
3 alleles: I I I
4. I I are codominant hence the blood type
AB. I is recessive
Sex-linked Genetic Disorders
1. A characteristic is sex-linked if the allele is on a
sex chromosome (X or Y). To is linked to the
2. The Y chromosome is smaller, so it carries
fewer genes than the X chromosome.
3. As men only have one X chromosome, they
often only require one allele for sex-linked
genetic disorder.
4. Men inherit the X chromosome from mother, and the Y chromosome from father.
5. Women inherit the X chromosome from mother, and the X chromosome from father.
Hence for a man to receive a sex-linked disorder e.g., colour-blindness, they only require one
allele to be colour-blind. The Punnett’s square for colour-blindness may look like the following.
Let B = the dominant allele for non-colour blindness and b = recessive allele for colour
In the example above, the homozygous woman with colour blindness is crossed over with a man
who has the dominant allele for non-colour blindness. The probability of a girl being born with
colour blindness is 0% whilst the probability of a boy being born with colour blindness is 100%
as the Y chromosome has no allele for colour blindness.
Edexcel GCSE Biology: Unit 3 - Genetics written by Sulaiman Galaria, edited by Ken Tu
Although some characteristics are determined by two alleles, some phenotypic features like eye
colour are determined by as many as 16 genes. Hence single gene inheritance is not always the
There are different factors that can influence phenotype such as the following:
Genetic Variation – different characteristics as a result of mutation and sexual reproduction
Environmental Variation – different characteristics caused by an organism’s environment for
example if you grew up in a poor country with food insecurity, you would lack the nutrients to
grow properly. These characteristics created as a consequence of their environment are called
acquired characteristics.
The Human Genome Project
Thousands of scientists have collaborated on the Human Genome Project and was created to
find every single human gene. It started in 1990 and ended in 2003 and was able to map 20,500
genes. The project helped to identify 1800 genes related to disease to benefit medicine.
Medical Applications
• Prediction and prevention of disease
o If doctors could identify which gene predisposed people to what diseases, we
could get tailored advice on diet and lifestyle and give early treatment.
• Testing and treatment of inherited disorder
o Due to the Human Genome project doctors can identify faulty alleles in a person's
genome. This can help to develop better treatment and cures.
• New and better medicines
o Scientists can design new drugs specifically tailored to a particular genetic
• Increased stress
o People can panic if they know they are susceptible to a certain disease
• Gene-ism
o People with genetic problems could come under pressure to never have children.
• Discrimination by employers and insurers
o Genetic likelihood of certain diseases can prevent people from getting jobs and
insurance (or it will be very expensive)
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Natural Selection
As time goes on, species of animals can randomly mutate. This is caused by the radiation
absorbed from a wide variety of sources such as the background radiation from small amounts of
Uranium in rocks to the Sun projecting its UVA (ultraviolet) and UVB rays. These mutations
cause minute differences (genetic variation) in the characteristics of that species. Some are more
advantageous than others. The species that receive an advantageous genetic variation is more
competitive in seeking food/hunting prey or resistant to a particular pathogen. So the weaker
species dies out leaving the species with the advantageous genome higher in population. When
that species then breeds, the genes are passed on from offspring to offspring resulting in the large
majority of that species having the superior gene. Hence the species is naturally selected i.e
higher in population by the weaker species dying out.
During the 18th Century, Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913)
came up with this idea. In essence you can think of this in four stages:
Genetic Variation: the characteristics of individuals in the species alter due to random changes
in DNA.
Competition/Environmental Changes: changes in the environment may have allowed one
genetically varied species to compete more effectively against other organisms.
Natural Selection: by chance, the variations made on species better at surviving whilst the
weaker species with inferior adaptations did not survive AKA “survival of the fittest”.
Inheritance: the survivors will breed and pass on their heredity (genetic information).
Evolution: this whole process occurs over and over again over millions of years where species
evolve with more advantageous traits pertaining to whatever difficult environment the species
migrate to.
Modern scientists now use this idea of evolution to demonstrate how a species shared a common
Resistance Organisms
In the 1960s, scientists discovered bacteria which evolved to have a strain of genetically
resistant DNA, meaning the bacteria with this DNA would not be affected by antibiotics. This
genetic variation would increase the number of species of those bacteria with that trait. As a
result, this increased the number of resistant bacteria.
The genetically resistant bacteria continue to pass this gene on rendering antibiotics ineffective
as they reproduce and spread, most likely causing more infections if pathogenic.
The emergence of resistance organisms supports Darwin's idea of Natural Evolution as by
chance, the random mutation in DNA led to the strongest (the species most likely to survive)
bacteria increasing in population and evolving.
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Evidence for Human Evolution
As evolution is the gradual change in the characteristics of an organism over time, scientists
discovered fossils which indicate how the human has evolved over eons. Although the fossil
record is complete. There are species missing which would indicate more of the ancestry of
humans. This could be because they are yet to be found or the environmental conditions for
fossil formation may not be right.
You are required to know the following ancestors of homo sapiens (human species).
Name of Fossil
4.4 million
years ago
1.2m tall; leg bones show she could walk
upright; long big toes allowing her to
climb trees.
3.2 million
years ago
1.07m tall; she could walk upright - her
toes were curved similar to modern
Leakey’s discovery of the homo
habilis (2nd most recent human
million years
Short with long arms but walked upright.
Ardi and Lucy were the names given to the fossils found from the Scientist's discovery.
In the 1960s, Mary Leaky and Louis Leaky found the homo habilis fossil.
Australopithecus africanus – Lucy
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Homo habilis was just before the time period of the homo erectus.
Ardi existed between the period of Lucy and the Ape (Sahelanthropus tchadensis).
It is also believed that humans evolved from Apes. As both species have tails of some sort. The
human coccyx resembles the longer tail of an Ape.
As humans evolved from Apes, we became more intelligent. The skull volume increased from
the original Ardi (350cm ) to the Homo sapiens (1450cm ). Subsequently the way humans hunted
for prey evolved. We began to use stone tools. Minecraft, I know right!
As tools in the diagram progress to the left, the stone
tools become sharper and sharper. This is also
indicative of the time. Older ancestors of humans had
more blunt tools whilst the closer ancestors used tools
which were sharper. When the tools were often used
for hunting prey. The blunt rock could be used to
crush animals whilst the sharper stones would be used
as an arrowhead.
Development of Darwin’s Theory
Context: in 1838, Darwin read an essay by Thomas Malthus (1766-1834), an English
Economist, which argued that if people gave birth to many children, this could result in food
shortages as more children needed more food,
that the supply of food does not increase of course. This would result in a struggle for survival
which only the genetically strongest could survive. This gave Darwin the idea that when
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
organisms produce many offspring, only those individuals best suited to the environment would
survive thus reproducing and passing on their genes.
Many people during Darwin’s time (19th Century) believed that a God created all of Earth’s
species, including their characteristics and that they could not evolve. However, in 1835, Darwin
visited the Galapagos Islands where he noticed different characteristics of the mockingbirds on
the different islands. They were close geographically and held similar characteristics but minute
differences e.g., beak shape.
Long story short Darwin published a book called On the Origin of Species and towards the end
he considers the evolution of vertebrates (animals with a spine) who all have five fingers - a
pentadactyl limb.
Essentially the idea behind the existence of the pentadactyl limb is to support how all these
vertebrates have similarities: a Radius and Ulna connecting to a Humerus with five fingers
(Phalanges) which must mean that at one point, they shared a common ancestor and evolved
into their various species from then.
Classification in Genetic Analysis
In 1735, a Swedish Zoologist named Carl Linnaeus published his classification system - how he
thought different animals/species should be divided. He classified the species based solely on the
physical characteristics of each animal. NOT considering their genetics. He would create only
two kingdoms of classification whilst in the modern days, we now have five kingdoms: animals,
plants, fungi, protists, and prokaryotes (cells with no nuclei).
Of course, with Linnaeus’ original model there were problems that arose where although two
species had evolved similar characteristics e.g., an elephant and a polar bear, they both have four
legs, they were not closely related in terms of their genetic information.
Here is a table to demonstrate the main characteristics of each kingdom:
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Main Characteristics
Multicellular organisms where cells are arranged to form tissue and organs, cells
have nuclei but no cell walls.
Multicellular organisms where cells are arranged to form tissue and organs,
chloroplasts for photosynthesis, cells have nuclei with cell wall made from
Multicellular, live on dead matter which they feed off of, cells have nuclei, cell
walls contain chitin not cellulose
Unicellular, the cells have nuclei and cell walls.
Prokaryotes Unicellular, no nuclei and flexible cell walls.
However, after microscopes were invented, scientists discovered the genes of plant and animal
cells and found that Archaea genes (unused section of DNA - most DNA is used to produce
proteins, but the “unused” section doesn’t help with this) existed. This led a scientist named Carl
Woese (1928-2012) to propose the system of classification into three domains:
Archaea (prokaryotes but distinct from bacteria); cells with no nucleus, genes contain
unused sections of DNA.
Bacteria: cells with no nucleus, no unused sections of DNA, no Archaea genes).
Eukarya: cells with nucleus, had unused sections of DNA, and had Archaea genes.
Selective Breeding
Some species adapt to the environment to form characteristics which, by chance, benefit them.
Artificial selection is where humans will choose certain organisms to breed with each other.
This causes the offspring to have characteristics which humans may desire in a species e.g they
bred a white thick-wooled sheep with a black sheep to produce an offspring to be black and
thick-wooled. By repeating this process over and over, over the course of years, they will end up
with the species with the desired characteristics. Hence by breeding animals this way, it is called
selective breeding.
A strong real-life example of selectively bred animals are hybrid dogs. The dog breeder will
breed two different dogs to produce a puppy with qualities from the mother and from the father.
The same selective breeding technique can be implemented in the plants where breeders will
cross breed crops to produce plantlets which will grow and have qualities of both parents. This
can be useful if one parent plant has a stronger disease resistance system than the other.
Plants and animals may be selectively bred to produce the subsequent results:
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Crops with a higher disease resistance defence system against pathogens/pests.
Yield, selectively bred fish may produce offspring which grow more than normal.
To have higher survival rates within their given environment.
However, selectively breeding animals may not always be a good thing. In the
case of dogs, many pugs (a species of dog with a flat face) suffer from health
problems such as brachycephalic obstructive airway syndrome (BOAS) breathing problems caused by their cute but squished noses or a range of eye
problems such as dry eyes. These are at the detriment of the pug’s life, yet
humans desire these cosmetic features as they’re deemed cute. This can be
argued as unethical.
Tissue Culture
Tissue culture is the cultivation of tissue. Essentially you are
growing/reproducing the group of cells in a controlled environment. Often the new tissue is
placed in a liquid solution which contains all the nutrients to allow the new tissue to grow.
Sometimes this source of nutrients can be solid medium such as nutrient agar. When the cells
divide and grow into many identical cells, this can form a callus (a clump of undifferentiated
cells). Hormones or different forms of treatment can be introduced to the cells to cause them to
differentiate and specialise.
The photo on the right shows many plant
clones produced. Clones of the same cells can
be used to produce new plants which are at
risk of extinction. It is also used to produce
plants which may be difficult to have grown
only from a seed, such as orchids.
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
The following steps describe how a tissue culture is grown.
1. A piece of the plant such as the root, is cut off. The root contains the meristem cells
which are undifferentiated.
2. The cells are placed into a nutrient medium to grow. These are aseptic conditions
(sterile) to prevent the growth of unwanted microorganisms as they may compete for
3. Although sometimes only a few cells are cut off and placed into the nutrient medium to
grow into a callus which will then be treated with hormones, similar to the description in
point 2, so they develop into roots and shoots.
4. When the plantlet grows large enough, they can be moved into soil/compost to grow.
Tissue culture also has its applications in medicine. By culturing a thin layer of cells onto a solid
medium agar, scientists can more easily experiment with how cells communicate with each
other. For context, Nobel Prize winners for Medicine in 2019 were given to three Scientists who
discovered how cells sense and adapt to the availability of oxygen. This was using tissue
cultures. Cell structures may also be used to study viruses, which require a host i.e cannot
replicate outside of a cell, whereby scientists can investigate how infected cells will respond to
new medicines without harming/risking harm to real animals or humans.
Tissue culture may also be developed into real functioning organs if correctly supported by the
right conditions, e.g., tissue-engineering a synthetic windpipe.
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Genetic Engineering
Genetic engineering involves directly altering the DNA of an organism by inserting or taking
out genes from one species to another. This organism with its DNA genetically modified is
called a genetically modified organism (GMO). Often this is done to produce an organism with
more desirable attributes such as higher yield, stronger defence system against pathogens/pests
or possibly for the organism to produce extractable insulin that can be used to treat diabetic
How an organism(bacteria) is genetically
engineered can be described as the
1. A bacteria’s DNA is one large loop
(chromosomal DNA). Some of its DNA is
in smaller loops called plasmids.
2. A section of the chromosomal DNA or
plasmid is cut by using restriction
3. Which leaves a few unpaired bases at the
end of the chromosomal DNA to which a
new DNA strand can be inserted into and
bond to the unpaired bases.
4. Once the new DNA strand is inserted the
complementary bases from the new DNA
and the chromosomal DNA pair up and an
enzyme called a ligase is used to join the strands of DNA together again.
5.The new DNA is called the recombinant DNA which is inserted back into the bacteria cell.
The insertion of the recombinant plasmid DNA can also be referred to as a vector because it
carries new information into the cell.
Selective Breeding Risks and Genetic Engineering Issues
As with any process, there are the advantages and disadvantages.
Selective breeding may reduce genetic diversity. As genes are divided into alleles, with selective
breeding there is a higher prevalence of one particular allele whilst others may become scarce
and rare. These genes/alleles which would have been useful in the future are no longer available.
This can impact the biodiversity of different species so if conditions change e.g., a new pathogen
is introduced, then entire species will die as the selective breeding has led to a species with a
major, single weakness that affects every organism.
Edexcel GCSE Biology: Unit 4 - Natural Selection and Genetic Modification
written by Ken Tu
Genetic Engineering and Agriculture
GM crops have been genetically modified to be resistant to insects therefore less insecticide is
required to kill the insects as they no longer affect the yield of crops. Other crops may be
genetically modified to be resistant to weed killing herbicides which then kills weeds but not the
crops. So, less herbicide is required for maintenance. In the 1900s, a soil bacterium (Bacillus
thuringiensis) was discovered to produce a natural insecticide called Bt toxin. What’s great
about genetically modifying crops to contain this gene producing Bt toxin, is that crops will
begin to naturally secrete this. The toxin was only released if pests/insects ate the crop as the
toxin is only released when cells are broken. This differs from insecticide because that kills a
whole range of insects including ones which do not feed on the crop. However, this can lead to
problems with an increase in insects which are resistant to the Bt toxin. Luckily, the Bt toxin has
many different strains (varieties) which all produce slightly different toxins allowing the toxin to
still impact the insects.
GM bacteria can be genetically modified to insert an insulin producing gene which can allow
bacteria to produce insulin available for extraction and use on diabetic patients. This can be great
because it's cheaper and suitable for ethical/religious reasons as insulin, previously, used to be
extracted from cows/pigs.
Fertilisers and Biological Control
As mentioned before, sometimes insecticide is used to control the number of pests feeding on
crops. So, to solve this problem farmers may introduce organisms to control pests. This is an
example of biological control whereby new organisms are introduced to control populations
levels of others. Biological control can reduce the level of weeds by introducing a species of
insect that can consume weeds.
Fertilisers are of supplementary nutrients to crops. This may include compounds which have
nitrogen, phosphorus, and potassium in them as they help a plant grow therefore increasing yield.
However, a negative effect this can have is that if excess fertiliser is used, and not all the
nutrients are absorbed by the crop, the nutrients can be absorbed by algae in the river as the
excess nutrients wash up into the river. See Unit 9, Ecosystems for this in further detail.
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
What is Disease?
At the beginning of this topic, the most important thing to understand is the definition of health.
Heath is a state of complete physical, mental and social wellbeing and not just merely the
absence of disease.
Communicable and Non-Communicable Diseases
There are two types of diseases: non-communicable and communicable diseases. Noncommunicable diseases are diseases that cannot be passed from person to person, often inherited
via genetics or one's lifestyle, while communicable diseases can be spread through microorganisms called pathogens.
The types of pathogens are viruses, protists, bacteria and fungi.
Diseases can be correlated, meaning having one disease can lead to having another. This is
• One disease damages the immune system, making it easier for other pathogens to cause
• Diseases also damage the body’s natural barriers and defences, allowing pathogens to
get into the body more easily
• Diseases stop organ systems from working effectively, making other diseases likely to
The idea that a person can be more susceptible to another disease as they contract one can be
seen in examples such as HIV (human immunodeficiency virus) leading to AIDS (acquired
immunodeficiency syndrome) where the viruses will infect and attack the immune system. The
progressive failure of the immune system allows life-threatening, opportunistic infections to
There are several types of non-communicable diseases. One type is genetic disorder which is
caused by faulty alleles in the genes. These are non-communicable diseases as they can only be
passed onto offspring.
Non-communicable diseases can also be caused by a poor lifestyle such as malnutrition,
exercise amount or obesity. An example of this is cirrhosis, caused by large amounts of alcohol
consumption impairing the liver’s functionality.
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
Cardiovascular disease
Another example of a non-communicable disease
is cardiovascular disease. This is caused by the
circulatory system functioning poorly due to large
amounts of fat in the body.
To measure the amount of fat in the body you can use
either the body mass index (BMI) or the waist-to-hip ratio.
For body mass index, you use the equation:
If a person’s BMI is above the value of 30 they are considered
obese. Obese people are more likely to develop cardiovascular
disease as they have a disproportional amount of fat in
comparison to their height.
For waist-to-hip ratio you divide the measurement around the waist by the measurement around
the hip in order to receive a better measurement of how much fat is in the abdominal area of the
patient. Waist-to-hip ratio is a more accurate measurement of a person’s health as BMI does not
account the amount of muscle mass in the body and assumes the mass is all fat. In addition
waist-to-hip ratio indicates that the fat is not located around the vital organs such as the liver.
Cardiovascular disease can also be
caused by smoking. Tobacco
smoke contains carcinogens that
can damage the lungs when
breathed in. The smoke damages
the arteries, causing cholesterol
(fat) to build up at the damage site
which makes the arteries
narrower. This eventually leads to
blood clots and blockages, causing heart attacks or strokes.
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
Treatment to Cardiovascular Diseases
Treatments to cardiovascular
disease include:
• More exercise
• Give up smoking
• Inserting a stent (A
small mesh tube that
wides arteries in order
to help blood flow)
• Inserting new blood
vessels in order to
create more paths for
Communicable Diseases
For the GCSE Biology, you need to be able to describe certain diseases, what they’re caused by
and how they are spread, such as:
• Cholera, caused by bacteria, causes diarrhoea, and spreads in water.
• Tuberculosis, caused by bacteria, causes lung damage, and spreads through air.
• Chalara die ash back, caused by fungi, causes leaf loss and bark lesions, and spread
through air.
• Malaria, caused by protists, causes damage to the T-lymphocytes and liver, and spread
by animal vectors. In this case, the vector is a mosquito.
• Stomach ulcers, caused by bacteria (Helicobacter), spread through oral transmission.
• Ebola, caused by viruses, causes haemorrhagic fever (basically imminent death lol
[vascular system fails and the body cannot repair itself] ), and spreads through exchange
of bodily fluids.
Viruses are not true organisms as they have no cellular structure. They multiply, infecting cells
with their genetic material in order to create new viruses and kill cells.
Bacteria release toxins and rapidly multiply within the body, paralysing cells with the toxins and
overwhelming cells and tissues with their vast population.
Protists bind onto the lining of the small intestine, preventing the host from absorbing nutrients.
Fungi cause plant cells stress and/or kill plant cells.
Virus Life Cycles
Viruses have many different forms but have a few features in common. All viruses have one or
more strands of genetic material surrounded by a protein
coat (capsid).
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
Viruses have two different ways of
infecting the body: the lytic and
lysogenic cycle.
The lytic cycle has 4 steps:
1. The virus hijacks the cell,
injecting their genetic material
into the cell.
2. The viral DNA inserts itself into
the cell's DNA.
3. The cells begin to create viral
genetic material and assemble it.
4. The new virus lyses(bursts) out
of the cell, killing it.
The lysogenic cycle has 3 steps:
1. Virus hijacks the cell, injecting their genetic material into a bacterium.
2. Viral DNA inserts itself into the bacterial chromosome.
3. Bacteria reproduce, replicating viral genetic material
Physical and Chemical Barriers
The body has various ways of defending itself against diseases through means of chemical
barriers or physical barriers. For example the body’s skin acts as a physical barrier to prevent
diseases from entering our organs and the gastric (hydrochloric) acid in our stomach to kill any
pathogens we may consume.
Plants have a waxy cuticle which acts as a physical barrier, making it difficult for pathogens to
enter the cells beneath. Other plants have chemical defences such as poisons or insect repellents
in order to stop insects from coming near the plant.
These chemical barriers have been used in medicines such as aspirin to control certain symptoms
of pain or fever.
Plant Diseases
Plants show signs of stress when conditions are not good for growth, such as when there is an
incorrect amount of water, when the soil lacks nutrients or when plants are attacked by pets or
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
First you must look at the visible symptoms on the plants. This includes
changes in growth, changes in colour/blotching of leaves and/or lesions on
Next you need to do a distribution analysis to see where the damaged plants
are occurring. This allows farmers to see what might be causing disease. For
example, flooding or lack of soil nutrients causes similar symptoms in a certain
area. Wind will cause widespread symptoms. Soil pathogens will only affect a
very small area.
After this, the farmer will do a final analysis in the lab using equipment in order
to look for pathogens in the plants and send off these reports and soil samples to labs to decide
what the issue is.
Physical Barriers
The bodies physical barriers include the skin, which is very thick, meaning pathogens can only
cross it through wounds or animal vectors that pierce it
Chemical Barriers
Chemical barriers include lysozymes on the skin’s surface which break down the walls of
bacteria in order to make them inactive. We also have acid in our stomach (HCl) that has a very
low pH meaning some pathogens can’t survive in the acid. Mucus is sticky secretion produced
by cells in order to catch pathogens that enter the body. Finally, the body has ciliated cells and
goblet cells which catch and sweep pathogens out of the body.
The Immune System
Most cell surfaces have proteins called antigens that can be used by
the immune system to find and kill pathogens. White blood cells
called lymphocytes have antibodies on the surfaces of their cells
that match the shape of certain antigens. Once they find the
complementary antigen, they become activated, either releasing
more antibodies or dividing to produce more antigens with the
antibodies necessary to kill the pathogens containing the
complementary antigens. After the initial infection, memory
lymphocytes are created which stay dormant in the body. If the
same kind of pathogen re-enters the body the secondary response
will be faster, meaning the pathogens can be killed much quicker.
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
As shown in the diagram on the right, the first infection
(first antigenic challenge) shows a smaller amount of
antibodies produced (antibody titre[IgM & IgM]) whilst
during the second response, many more antibodies are
produced to overwhelm the pathogen.
Vaccines contain weakened or dead pathogens with the
pathogen’s antigens. This gets injected into the person’s
body and causes the corresponding antibodies and memory
lymphocytes to be produced which will stay in the body and
protect it from the disease for years.
There is also herd immunity, where most of the population is immunised. This means that the
few people who aren't immunised have a very low chance of catching the disease as they have a
low chance of encountering someone else who has the virus.
Sexually transmitted diseases
STIs (sexually transmitted infections) such as chlamydia are spread through contact with bodily
fluids such as semen or vaginal fluid. This type of transmission is prevented by stopping fluids
from being exchanged using condoms or other artificial barriers. Screening can also be used in
order to check if you have a sexually transmitted disease.
Antibodies are proteins that either kill bacteria or inhibit their cell processes, which stops them
from growing or reproducing. They don't influence viruses however as they only affect the cell
processes of bacteria.
Development of medicines
For medicines to be industrially used, it must be safe to use. Here are the following steps a
medicine supplier will do to produce medicine. The discovery of penicillin - the first antibiotic was found by Alexander Fleming.
Stage 1: Pre - Clinical trial - Medicine is tested on tissue samples and cells in agar plates in
order to test their side effects to see if any unintended or harmful effects are produced. It is then
tested on animals to see how it works in a whole body without risks to humans.
Stage 2: Clinical trial - Medicine is now tested on a small number of healthy people to see if it
is safe and any other side effects not shown by the previous test.
Stage 3: Large - Clinical trial - Medicine is now tested on many people who have the disease to
work out the dosage as well as a final check for side effects the medicine has.
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
Antibiotics Practical
1. Use aseptic techniques to pour an agar plate.
2. Carefully open the bacterial culture bottle and pass the neck through a flame.
3. Draw some culture using a sterile pipette before passing the neck of the bottle through the
flame and replacing the lid.
4. Lift the lid of the petri dish by a small margin and add the drops of culture, replacing the lid
and disinfecting the pipette after.
5. Spread culture over agar and replace lid.
6. Mark sections on the dish and use sterile forceps to place each antibiotic disc on the plate,
sterilising after every disc.
7. Tape lid on and incubate.
8. Now measure radius of each zone of inhibition, the radius in which no bacteria formed from
the antibiotic discs, to work out the effectiveness of each antibiotic
Aseptic techniques are techniques used during practicals to make sure that all equipment used is
still sanitary and clean, reducing chances of infection. Examples of this include:
Using an autoclave (uses steam under pressure to kill bacteria)
Working in the environment of a Bunsen burner
Wearing gloves
Using disinfectant on all items after use
Using sterile equipment
Closing and taping agar plate to prevent pathogens escaping
Monoclonal Antibodies
Monoclonal antibodies are artificial antibodies created in the lab and used to target specific
antigens. They have been used in many different areas, such as pregnancy tests where they bind
onto specific proteins found in pregnant women’s urine or in cancer treatment where they bind
onto and kill cancer cells.
The process to create antibodies has 2 steps:
An animal is injected with a
pathogen, causing the mouse to
produce lymphocytes that make
antibodies against the pathogen
These are then extracted and fused
with cancer cells to create
hybridoma cells that can both
divide and make antibodies
(Monoclonal Antibodies)
These can be very useful instead of
chemotherapy and radiotherapy as cancer
drugs can be attached to only the hybridoma
Edexcel GCSE Biology: Unit 5 - Health, Disease and the Development of Medicines
written by Oluwadamilola Olatunji, edited by Ken Tu
cells meaning that dosage does not harm any surrounding healthy cells during the process. They
can also be used in diagnosis as if antibodies are made slightly radioactive, when they attach to
cancer cells the radioactivity can be detected, making locating cancer cells easier.
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
All living organisms whether plant, mammal, or bacteria, they need energy to survive. Plants use
photosynthesis to use the energy from light to be used as a store of energy in their own bodies.
This energy comes in the form of ATP (adenosine triphosphate is what it stands for and it's a
molecule which provides energy). This energy can be used to grow biomass and because all of
its energy initially came from the sun, photosynthetic organisms - plants - can be described as the
producers or autotrophs (an organism which produces its own food) of the food chain.
The word equation for photosynthesis is the following:
Energy from sunlight gets transferred
Carbon Dioxide + Water → Glucose + Oxygen + (Energy [ATP])
Via Chlorophyll
The balanced formula equation is below:
6CO + 6H O → C H O + 6O
Note: The reverse reaction, oxygen + glucose → water + carbon dioxide, is the equation for
aerobic respiration.
As glucose is formed as a product, it is called a monomer. You may learn this in Chemistry
where glucose acts as a single unit for a polymer. Many of those same units chemically bonded
together are called a polymer. When many glucose monomers are joined together, it is called
starch. Plants can also convert this glucose into a “double unit” called sucrose. An important
difference to note between glucose and starch is that starch is insoluble (does not dissolve in
water) whilst glucose is soluble (dissolves in water).
Curiosity: As glucose is a sugar molecule, these are called saccharides, another term for sugar.
Starch is called a polysaccharide; glucose is called a monosaccharide and sucrose being a
“double unit” is called a disaccharide. You are not required to remember this terminology for
GCSE Biology.
The site for photosynthesis to occur in, is the chloroplast which contains chlorophyll. As
sunlight hits the plant cells and light energy is transferred to the chloroplasts, the surrounding
energy is entering into the reaction therefore we call this an endothermic reaction. Endothermic
reactions are where heat from the surrounding area is absorbed into the reaction. An exothermic
reaction would be the opposite, so heat being released to the surroundings.
Factors of Photosynthesis
As with any chemical reactions, there are factors at play which may speed up or slow down the
rate of reaction.
Energy from sunlight gets transferred
Carbon Dioxide + Water → Glucose + Oxygen + (Energy [ATP])
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
With carbon dioxide and water being a reactant of photosynthesis, it is evident that more carbon
dioxide and more water can cause more energy being released, a faster rate of reaction.
The diagram on the right shows the x-axis as the factor affecting the rate of photosynthesis whilst
the y-axis represents the rate itself. When the concentration of carbon dioxide increases, the rate
of photosynthesis increases
until the rate of reaction
The rate of reaction no longer
increases as carbon dioxide
increases. So, carbon dioxide is
not the limiting factor and
another factor e.g., light
Factors that affect the rate of the
following three factors:
Light intensity: the more light energy received by the chloroplast, the more
photosynthetic reactions occur.
Temperature: the higher the temperature, the faster gas molecules, such as water vapour
and carbon dioxide - reactants of photosynthesis, move. Therefore, there are more
collisions between the reactants with the chloroplasts and therefore more successful
collisions to cause a reaction resulting in a higher rate of reaction.
Carbon dioxide concentration: there are more reactants.
During photosynthesis, if one factor is not fully optimised to produce the highest rate of
photosynthesis, then that factor is called the limiting factor and it is restricting the rate of
reaction. The maximum rate of photosynthesis can be determined by the factor in the lowest
supply e.g if carbon dioxide concentration and temperature are very high but there’s little light
being received, the rate of photosynthesis will be slower. It is only until you increase the light
that then, the rate of photosynthesis increases.
Inverse Square Law
The relationship between light intensity and the rate of photosynthesis are directly proportional
however if the distance were to increase, the rate of photosynthesis would decrease. The
relationship between the rate of photosynthesis can be described as an example of the inverse
square law.
Link to a video explain how the inverse square law works:
If the distance between the light source increases by a factor of 2, the rate of photosynthesis
would decrease by a factor of 2 so 4.
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
The equation used to calculate the new Intensity of light (Inew) where the distance of a light
source changes from the original distance (dorig) to the new distance (dnew), you use:
Inew =
( 𝐈𝐨𝐫𝐢𝐠 × 𝐝^𝟐𝐨𝐫𝐢𝐠 )
So, with a given original light intensity of constant k, and that the distance doubles, the
only a fourth of the original light intensity has
reached the plant. In English when the distance
doubled, the light intensity decreased fourfold.
Leaf Adaptations for Photosynthesis
Leaves are often broad (wide) and flat giving a
large surface area for light to be absorbed by the
chloroplasts in the palisade cells.
Leaves are also thin. This reduces the distance gas
molecules have to travel in order to reach the
chloroplasts for photosynthesis.
The microscopic pores in the bottom of the leaf,
called the stoma/stomata, allow for water vapour and carbon dioxide to diffuse into the leaf
when there’s a high presence of water vapour. The amount of water vapour and carbon dioxide
entering the leaf can be determined by whether the guard
cells are open or closed. The specialised guard cells will
become turgid/rigid, swollen if water diffuses into them
- when there’s a high presence of water - opening the
stomata, or flaccid (shrivelled up) if it lacks water closing the stomata. This can cause the stomata to open
and close allowing more gas in or less. At night, when
temperatures are lower and there’s no sunlight therefore
less water vapour, the stoma closes to prevent water loss.
Also, because no sunlight means there’s no photosynthesis occurring unless artificial lights are
Plant Adaptations
As mentioned before, plants have adaptations which help them to survive in a given climate.
Many deciduous (broad-leafed) plants have leaves which have the following characteristics.
The reason for their adaptations is as follows:
Irregularly shaped spongy mesophyll layer: to create air gaps within the cells to allow
the gases to diffuse to the chloroplasts easily
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
Waxy cuticle: to prevent pathogens from easily entering the cellular structure, like
human skin and to prevent water loss.
Upper epidermis layer (outermost layers) or the plant; they are transparent to allow lots
of light in for photosynthesis.
Many plants in hot climates e.g., cacti, have the following adaptations:
Needles/spines instead of leaves; this minimises the surface area from which water loss
can occur. It can act as a deterrent to herbivores from eating the plant’s stem.
Some cacti plants have hair. This is to trap escaping water vapour so reduces water loss
from transpiration.
Thick cuticle: this is to reduce water loss from evaporation via transpiration.
Transpiration and Translocation
All plants need a system where they move nutrients around the body. Transpiration is the
movement of water and mineral ions through the leaves often from the root hair cells up through
the xylem vessel. Translocation is the movement of sugar/glucose from the companion cells to
the phloem vessels bidirectionally, up or down, the body of the plant. Wow, that was a lot of
terminology. Let’s break it down!
The water absorbed by a plant can be used for the following reasons:
They carry mineral ions e.g., nitrates which are used for producing amino acids, the
monomer for proteins.
To keep the cells turgid otherwise the leaves would wilt - droop down losing its structure.
To cool the leaves when water evaporates from the leaves.
For photosynthesis obviously.
The movement of water initially occurs in the root hair
cells - see diagram on the right. The extended hair
increased the surface area which allowed for more water
and mineral ions to be absorbed more quickly. The cells
walls are also thin to reduce the distance that the water
molecules have to diffuse into the cells.
The water molecules enter the cell membrane via
osmosis, from a high concentration of water to a lower
concentration of water via the partially permeable
membrane of the cell. Some water molecules can diffuse
into the plant via the cell walls. The water travels
through the cytoplasm in the cell membrane until it
reaches the xylem vessel. This is a passive process.
When mineral ions enter the root hair cell, they do so by
active transport from an area of low concentration outside the root cell hair, to an area of high
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
concentration inside the cell where proteins in the cell membrane actively pump the mineral ions,
such as nitrates - again, into the cell. This is an active process.
Xylem Vessel
The diagram on the right, shows the flow of
water going up the vessel. Essentially the
xylem vessel consists of cells which have
died. They are stacked on top of each other
with the top and bottom of their cell walls
disintegrated creating a hollow tube. They
are strong and rigid because the sides of the
walls contain hard lignin. We refer to these
as the lignified dead cells. This rigid system
supports the high-pressure water evaporating
up the column may create so that the xylem
vessel does not burst. Hence as more water
evaporates from the plant, the more water
diffuses up the leaves.
Water is able to continually go up the vessel
as there are weak forces of attraction between the molecules, similar to the effects of water
tension hence as water molecules from the top of the column travel up the xylem vessel, water
molecules below are being pulled upward by that weak force of attraction. Transpiration only
occurs in one direction - up.
As transpiration is the movement of water out of the plant, a faster evaporation rate may cause
faster transpiration. The following factors can affect the rate of transpiration:
Temperature: higher temperatures mean water particles move faster and diffuse faster
therefore the rate of transpiration increases.
Greater light intensity: this can cause the stomata to open wider, so more water
evaporates out of the stoma hence higher transpiration rate.
Wind: higher wind, the more water molecules move away from the stomata cause other
water molecules to follow.
Low humidity: when there’s little water vapour in the air, there’s a higher concentration
of water vapour in the stomata than outside therefore diffusion of water vapour from
inside to outside is less likely, hence lower rate of transpiration.
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
The rate of transpiration can be investigated by using a potometer.
The plant would transpire, and the air bubble would move to the
left. The place in which the air bubble travels is called the capillary
tube with scale. So as water diffuses out of the stomata during
transpiration, the air bubble moves at a rate dependent on its
environment - the factors mentioned earlier. You can measure the
rate of transpiration by the distance travelled by the air bubble in a
certain time frame e.g., (mm/min).
Phloem Vessel
During photosynthesis, plants produce glucose, and from this, they
can produce sucrose as a sugar which can be transported up or down
the plant. Sucrose is translocated (transported) via the sieve tubes
of the phloem tissue bidirectionally (up or down) when the companion cell pumps sucrose into
or out of the sieve cells. This occurs via active transport. The movement of sucrose can go up if
the companion cells pump more sucrose
into the vessel. This increases the pressure
therefore the flow of the solution goes up
and vice versa.
The structure of the sieve cells is stacked on
top of each other, similar to the xylem
vessel, however, the top and bottom of each
sieve cell has perforations (holes) allowing
the movement of the larger sugar molecules
up or down the phloem vessel. An
additional adaptation of the sieve cells is
that they contain very little cytoplasm on
the edges. This means there’s more space
for the sucrose solution to move around the
plant. The diagram on the right illustrates
how the sucrose can be pumped into the
sieve tube alongside a diagram of the xylem
vessel. Water from the xylem can enter via
osmosis into the phloem tissue to create a
sucrose-water solution where the sucrose
dissolves into the water.
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
Plant Hormones
Hormones are present throughout the body of a plant. When there’s a change in stimulus, a
corresponding response occurs.
When a light source hits a plant, the plant begins to grow in the direction of the light. This is an
example of phototropism in action. A tropism is where a plant will react to a stimulus by
growing towards or away when given a stimulus. The two examples of tropism required for
GCSE Edexcel are phototropism and gravitropism.
In plants, there’s a plant hormone called an auxin which
causes a plant cell to elongate more. Auxins move
towards areas of shade so when a light hits a particular
area of the plant, the auxins move to the opposite side the shaded side of the plant - causing more cell
elongation. As a result, the plant’s stem bends towards
the light. This helps the plant reach more sunlight for
In the diagram on the right, you can see how the auxin
causes the cells to become longer therefore causing the
plant shape to bend closer to the light source.
To find the location of the auxins, in the 19th Century,
Charles Darwin devised an experiment followed by
another scientist named Frits Warmolt Went (early 20th
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
The diagram on the top shows the experiments conducted by the scientists.
The location of the auxins is at the tip of the shoot. This is because no bending occurs when the
tip of the shoot was either cut off, detached from the stem, or not exposed to light.
Auxins can also be found in the roots of a plant; however, the effects are the opposite than the
effect of auxins in the shoot.
Auxins present in the roots inhibits cell elongation so a high concentration of auxins on one
side of the root causes the cells to become shorter and as an overall result, the root bends
downwards. This is because the force of gravity pulls most auxin hormones to the lower side of
the plant, and when in the root of a plant, causing the root to bend into the soil to absorb nutrients
like water and mineral ions.
Commercial Uses of Plant Hormone
Plant hormones can be used or a variety of different purposes. Here are the few uses you are
required to know for GCSE.
Auxin Use
Artificial auxins make plants cell elongate and grow. By using too much artificial auxins, this
can cause the plant to grow uncontrollably and die. Selective weedkillers may contain artificial
auxin specifically designed to kill a certain species of plant e.g., those with broad leaves,
Edexcel GCSE Biology: Unit 6 - Plant Structures and their Functions
written by Ken Tu
therefore farmers can use them on their crops as the selective weedkiller will not target the crops
they grow e.g., wheat.
Rooting powder also contains artificial auxins. By dipping a plant stem in the rooting powder,
roots will quickly grow from the plant. These roots can then be cut off - referred to as the
cuttings - and planted to form genetic clones of the plant. This can allow for the same plant to be
grown faster as opposed to planting a seed.
When a seed begins to germinate, essentially it is the reactivation of the metabolic machinery of
the seed resulting in the emergence of the roots and stem. For this to happen, a plant will
naturally release a plant hormone called gibberellins. Some seeds require a period of darkness
before they germinate however by artificially adding gibberellins, you can commence the
germination process.
Plants use photoperiodism as the response of an organism to the number of daylight exposure
they receive. Plants can use these stimuli to bloom, releasing their pollen. Plants grower can
override this by adding gibberellins. This can cause pollination to occur earlier.
Additionally, seeds are produced after successful pollination i.e., the male and female gamete
fuse causing the egg cell to be fertilised. However spraying gibberellins onto some plants can
cause fruit, the zygote of a fruit to be formed, without the seed. This gives us a seedless fruit.
Ethene Gas
During the transportation of fruits, they are often transported not ripe. This means they are often
harder as when they’re ripe, they tend to be softer and more susceptible to damage. So unripe
fruit can be ripened when needed by using ethene gas/ethylene.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
What are Hormones?
Hormones are chemical messengers produced in endocrine glands and are carried by the blood
to the target organ which they affect. When there is a change in the condition of the body e.g. a
change in temperature or chemical imbalance within your body, hormones are released by the
endocrine glands to cause a change in the body’s system allowing it to maintain homeostasis
effectively. Simply put, homeostasis is the body’s method of maintaining the optimum, physical,
or chemical conditions for survival.
The different endocrine glands include the following:
Pituitary Gland:
The pituitary gland is in the brain and releases many hormones including FSH (follicle
stimulating hormone), LH (luteinising hormone) and growth hormone.
Thyroid Gland:
The thyroid gland is in the neck and produces
many hormones, including thyroxine.
Adrenal Glands:
The adrenal glands are positioned above the
kidneys, and release many hormones, including
The pancreas contains cells which produce
insulin, and others that produce glucagon.
The testes release the hormone testosterone.
The ovaries produce oestrogen and progesterone.
The functions of the following hormones will be mentioned later on in the notes.
Hormonal Control of Metabolic Rate
The body’s metabolic rate is the rate at which energy stored in your food/body (fat) is
transferred by all the reactions that take place in your body to keep you alive. When one is
exercising or participating in an activity that brings about chemical/physical changes e.g., eating
food, the metabolic rate responds to this by releasing specific hormones. The basal metabolic
rate (BMR or resting metabolic rate) is measured at rest when the temperature is room
temperature and given a long time before their last meal.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
Adrenaline is produced by the adrenal glands and
prepares the body for a “fight or flight” response.
The effects of this are the following:
An increased heart rate as the heart cells
contract more rapidly.
Increased blood pressure.
Increased blood flow to the muscles for
increased aerobic respiration.
Raised blood sugar levels; the liver is
stimulated to convert glycogen (polymer) to
glucose (monomer).
The role of thyroxine serves to increase the
concentration of itself if concentration levels of
thyroxine is low in the bloodstream and vice versa
(if there’s high levels of thyroxine, thyroxine will
serve to decrease the release of thyroxine.) This is
called a negative feedback loop the increase in
thyroxine concentration results in the decrease of thyroxine being released.
Thyroxine causes heart muscles to contract more rapidly similar to adrenaline and it also
increases the rate at which proteins and carbohydrates are broken down inside cells,
consequently resulting in more aerobic respiration.
Thyroxine Mechanism
If levels of thyroxine are low, the hypothalamus is stimulated to produce TRH (thyrotropinreleasing hormone). This
production of TRH in the
hypothalamus causes the release of
TSH (thyroid-stimulating
hormone) in the pituitary gland.
TSH acts on the thyroid gland,
causing it to produce thyroxine.
The increase in levels of thyroxine
inhibits the production and release
of TSH & TRH. The inverse would
happen if the initial levels of
thyroxine were high.
The thyroxine mechanism can be
represented using the following
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
The Menstrual Cycle
The menstrual cycle represents the cycle of changes that takes place during a woman’s
reproductive system. The cycle continues from puberty (ages 8-14) to menopause (ages 45-55).
In summary, the menstrual cycle demonstrates how a woman’s body prepares for the
fertilisation of her egg as she begins menstruation. This is a woman’s body’s system to prepare
for a pregnancy.
The four hormones involved in the menstrual cycle are the following:
• Oestrogen, released in ovaries, thickens the uterus lining, and stimulates the release of
LH and FSH.
• Progesterone, released by the corpus luteum - egg follicle after ovulation, inhibits
release of FSH and LH and maintains the uterus lining.
• Luteinising Hormone (LH), released from the pituitary gland, surge in LH triggers
ovulation, the egg to be released from the ovary.
Follicle Stimulating Hormone (FSH), released from the pituitary gland, stimulates
growth and maturation of the egg follicle.
1. At the start of the cycle, FSH released by
the pituitary gland, causes the egg to
mature in the follicle.
2. The levels of oestrogen released in the ovaries
increases, which causes the lining of the uterus to
3. These increasing levels of oestrogen cause
the levels of FSH & LH released by the
pituitary gland to surge, causing ovulation. This
results in an egg being released.
4. Once the egg is released, the egg follicle becomes a
corpus luteum releasing progesterone,
which maintains the lining of the uterus.
5. If the egg is fertilised, the levels of oestrogen
and progesterone remain high in order to maintain the
lining of the uterus.
6. If the egg is not fertilised then the levels of
progesterone will decrease as the corpus luteum
shrinks therefore resulting in the breaking of the uterus lining, called menstruation (a
period) which comes out as blood.
In the diagram above, day 14 is typically the day where ovulation occurs.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
Hormonal and Physical Contraception
Contraception are measures to prevent a pregnancy from occurring.
The pill - oral contraception, also known as the pill, contains oestrogen, progesterone, or a
combination of both these hormones. These hormones inhibit the production of FSH, so the eggs
do not mature.
Condoms/Diaphragm - they both act as a physical barrier to prevent the sperm from entering
the uterus.
Hormonal methods of contraception are more effective than barrier methods; barrier methods
rely on the barrier remaining intact, whereas hormonal methods make the chance of
pregnancy as low as possible via chemical methods.
Assisted Reproductive Technology (ART)
Assisted Reproductive Technology (ART) is used to aid those who have trouble conceiving.
ART uses hormones to increase the chance of pregnancy.
Clomifene therapy is where women who rarely release an egg are given a drug, called
clomifene, that increases the levels of FSH and LH in the blood. This encourages egg follicle
Another technique is called IVF (in vitro fertilisation). This is where sperm cells are taken from
man. Hormones are given to the woman to cause ovulation releasing the egg. The egg is taken
from the ovary to combine with the sperm for fertilisation to take place. Then the one or two
healthy embryos are placed in the uterus.
Maintaining a normal environment internally allows for processes such as enzyme activity and
metabolism to occur at a constant rate. Homeostasis are the self-regulating physiological
processes allowing humans to perform a task at the optimum rate given the environmental
Homeostasis is important; it regulates body temperature to allow for optimum rate of enzyme
activity; the body's enzymes function best at 37°C. Osmoregulation is important as it allows for
the water potential of the body’s cells to remain optimal.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
If the hypothalamus detects blood or brain
temperatures below 37°C, a number of changes
will occur.
The body will begin to shiver, where muscles start
to contract and relax rapidly. Some of the
energy released from respiration in the cells
increases the body’s temperature. The hair
erector muscles in the dermis of the skin causes
body hairs to stand upright. In humans this has
little effect, but in animals it traps air next to the
skin for insulation.
Narrowing of the capillaries near the skin
causes reduced blood flow near the skin, keeping
blood deeper inside the body. This reduces the
rate of heat energy transfer to the air as more
warm blood flows through the shunt of the
capillaries. This is called vasoconstriction.
If the hypothalamus detects body temperatures above 37°C, a number of changes will occur.
The body will begin to sweat; sweat spreads out as a thin layer over the epidermis, where it
evaporates. As sweat evaporates, it transfers energy to the surroundings as heat, so the skin
cools down. The hypothalamus also stimulates the capillaries near the skin to widen,
increasing the blood flow to the skin. This makes it easier for blood to transfer energy to the
surroundings as heat. This is known as vasodilation.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
Controlling Glucose Levels
Blood glucose concentration is controlled by the levels
of the hormone insulin. Insulin is a hormone that causes
cells in the liver and other organs to take in glucose from
the blood therefore it serves to reduce the blood glucose
concentration in the body.
If blood glucose levels are too high (hyperglycaemia),
receptors in the pancreas detect the rise in blood glucose
concentration, causing insulin to be released from the
The release of insulin causes excess glucose to be stored
as glycogen in the liver, which causes blood glucose
levels to fall.
If blood glucose levels are too low (hypoglycaemia),
receptors in the pancreas detect this and the hormone
glucagon is released. Glucagon causes cells in the liver
to convert glycogen back into glucose.
Type 1 diabetes occurs when the pancreas of the patient is faulty, causing it to produce not
enough/ no insulin. This is controlled by insulin injections and lifestyle changes such as diet
Type 2 diabetes occurs when the body no longer responds to insulin, and is caused by lifestyle
choices such as lack of exercise and a poor diet. It is controlled by having an improved diet, and
engaging in exercise.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
Urinary System and Osmoregulation
The urinary system is the
body’s method of removing
the waste material from the
bloodstream. This waste
material (urea) is naturally
produced when the liver
breaks from proteins. This
process occurs in the kidneys
where thousands of
microscopic tubes called
nephrons filter the blood
contents to produce urine.
The high pressure of the
blood entering the
glomerulus from the renal
arteries causes an aggressive
form of filtration to take
place, where water, urea, sugars and mineral ions are filtered out of the bloodstream and into
the Bowman's capsule. Then selective reabsorption takes place in the proximal convoluted
tubule, where the glucose and amino acids are actively transported (via active transport)
back into the bloodstream using energy from respiration. Some water and mineral ions are
allowed to re-enter the bloodstream via osmosis to then circulate filtered blood back to the heart
via the renal veins. The reabsorption of water occurs in the loop of Henle. The amount of water
returning into the blood can also be controlled by levels of antidiuretic hormone in the
collecting duct. Waste products then enter the bladder to be stored as urine, where it exits
through the ureter and then the urethra during excretion as urine.
Wow fam that was bare words uno!
How am I gonna remember all that? - Ken
Let’s break it down into 5 main steps.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
1. Blood flows through the network of
capillaries called the glomerulus
under pressure. This is within the
Bowman’s capsule.
2. The Bowman’s capsule is adapted to
allow small molecules such as water,
urea and glucose into the nephron but
large molecules such as protein,
lymphocytes stay in the blood.
3. The filtration fluid flows through the
nephron where selective
reabsorption of mineral ions and
glucose occurs in the 1st convoluted
tube (AKA proximal convoluted
4. The water is reabsorbed by osmosis
depending on how dehydrated the
body is. This occurs in the loop of
Henle and the collecting duct.
5. The remaining fluid will exit the
body through the ureter and urethra as urine.
And of course, the nephron has a large surface area (microvilli) to increase the reabsorption of
substances whilst the cells in their cell membrane have many mitochondria to allow for active
Antidiuretic Hormone
Osmoregulation is also controlled by the levels of ADH - antidiuretic hormone. This makes the
kidney tubules more permeable, allowing more water to re-enter the bloodstream. For example,
if blood water levels are too high, less anti diuretic hormone will be released by the pituitary
gland, making the kidney tubules less permeable and resulting in more water being urinated, and
the levels of water in the blood decreasing as water is being used for other bodily functions.
If the water levels are too low, the pituitary gland will release more antidiuretic hormone, so that
the kidney tubules are more permeable and hence more water will re-enter the bloodstream,
increasing water levels.
Kidney Treatments
Your body can manage with only one kidney however problems arise with the removal of waste
products from the body if both kidneys fail to function properly. The person’s blood will contain
too high of a concentration of toxins such as urea.
To solve this, a kidney transplant may occur where the kidneys of the patient are replaced with
an organ donor's kidney.
Edexcel GCSE Biology: Unit 7 - Animal Coordination, Control and Homeostasis
written by Aimua Igbinehi, edited by Ken Tu
Another solution is to have
frequent kidney dialysis
each couple days to remove
the level of toxic substances
in the body. The diagram on
the right explains how
kidney dialysis works.
Essentially the urea diffuses
into the dialysis fluid to
reduce the concentration of
urea in the blood.
The advantages and
disadvantages of dialysis can
be shown in a table like so.
Kidney Dialysis
Kidney Transplant
Keeps the
patient with
kidney failure
Permanent cure
No need to frequently visit the hospital
Very time
consuming as
you have done
dialysis very
Patient’s diet
must be
controlled to
include low
levels of
protein, water,
and salts.
Can cost a lot of
money over
Possibility of rejection where the organ
donor’s organ is attacked by the patient's
immune system since they do not recognize
the different antigens on the donor’s organ.
Therefore immunosuppressants (drugs
which suppress the immune system) must
be taken for the rest of your life
Because of the immunosuppressants, it’s
easier to catch other diseases
Few supply of kidney
FYI: Urea is the breakdown of excess amino acids in the liver. This process is also called
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
What is the purpose of exchange?
All living things must have metabolism to survive. To carry out metabolic processes, we need to
take in substances from the environment and excrete waste substances to the environment.
• Cells need glucose and oxygen for aerobic respiration, a metabolic process that produces
carbon dioxide. They also need a sufficient volume of water which prevents crenation
(shrinking of cells due to water loss by osmosis) and cytolysis (bursting of cells due to
water gain by osmosis).
• Urea is a waste substance that is produced by the breakdown of excess amino acids
(deamination) in the liver. Urea, as well as carbon dioxide produced by aerobic
respiration and excess amounts of water and mineral ions, must be excreted.
Unicellular organisms can simply use diffusion to exchange enough substances to supply their
entire volume, but multicellular organisms cannot. This is because unicellular organisms have a
very high surface area to volume (SA:V) ratio - the amount of surface area per unit volume. As
you will read below, having a higher surface area increases the rate of diffusion and so
unicellular organisms can more easily diffuse a large number of substances. However,
multicellular organisms have a smaller surface area to volume ratio because their volumes are
substantially greater. Therefore, multicellular organisms require transport systems to move
substances. For humans, the main two transport systems are the respiratory system (involving the
lungs) and the circulatory system (involving the heart).
Fick’s law and factors affecting the rate of diffusion
The rate of diffusion is affected by three main factors:
• Surface area: The greater the surface area, the faster that molecules can move from one
side of the membrane to the other. Therefore, a higher surface area increases the rate of
• Concentration gradient: The steeper the concentration gradient, the faster the net
movement of molecules from the area of higher concentration to the area of lower
concentration. Therefore, a steeper concentration gradient increases the rate of diffusion.
• Membrane thickness: The thicker the membrane, the longer the diffusion distance that
molecules must travel, which slows down the net movement of molecules. Therefore, a
thicker membrane decreases the rate of diffusion.
Fick’s law neatly combines the rate of diffusion with these three factors into one equation:
rate of diffusion =
𝐬𝐮𝐫𝐟𝐚𝐜𝐞 𝐚𝐫𝐞𝐚 × 𝐜𝐨𝐧𝐜𝐞𝐧𝐭𝐫𝐚𝐭𝐢𝐨𝐧 𝐠𝐫𝐚𝐝𝐢𝐞𝐧𝐭
𝐦𝐞𝐦𝐛𝐫𝐚𝐧𝐞 𝐭𝐡𝐢𝐜𝐤𝐧𝐞𝐬𝐬
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
Gas exchange
The function of the lungs is to transfer oxygen from the air into the blood and carbon dioxide
from the blood to the air - this is gas exchange. To increase their efficiency to exchange these
gases, the lungs contain millions of air sacs called alveoli.
The alveoli are surrounded by a network of capillaries in which blood flows. Deoxygenated
blood from the rest of the body arrives at the alveoli in the capillaries, with a high concentration
of carbon dioxide and a low concentration of oxygen. Since there is a higher concentration of
oxygen in the alveoli than in the blood, oxygen diffuses from the alveoli to the blood. In the
same way, since there is a higher concentration of carbon dioxide in the blood than in the alveoli,
carbon dioxide diffuses from the blood to the alveoli. Now oxygenated, the blood leaves the
alveoli and returns to the heart, ready to be pumped across the rest of the body.
The alveoli and capillaries have several adaptations to maximise the efficiency of gas exchange:
• The epithelial cells that surround the alveoli are very thin and flattened. This reduces the
distance that gases need to move, which increases the rate of diffusion.
• Alveoli are folded, which increases the surface area for a faster rate of diffusion.
• Blood flows constantly in the capillaries to maintain a steep concentration gradient.
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
Components of the blood
Blood is a tissue, since it is made of groups of similar cells which work together for a specific
function. The function of the blood is to transport substances around the body.
One specialised cell in the blood is the erythrocyte, more commonly known as a red blood cell.
Erythrocytes specifically transport oxygen around the body. When erythrocytes in capillaries
reach the alveoli, oxygen diffuses into them and bonds with the red pigment haemoglobin to
form oxyhaemoglobin. Once the erythrocytes travel from the lungs to the rest of the body, the
oxyhaemoglobin is broken down into haemoglobin and oxygen, and the oxygen diffuses out of
the erythrocytes into the body cells.
Erythrocytes have the shape of a biconcave
disc, which increases their surface area to
volume ratio and so diffusion of oxygen is
faster. They also do not have a nucleus,
allowing more space for haemoglobin and
so more oxygen can be carried.
Another specialised cell in the blood is the
leukocyte, more commonly known as a
white blood cell. Leukocytes are very
important in the immune response, where
the body defends itself against pathogens.
When you have an infection, leukocytes
divide by mitosis to destroy the pathogens.
Therefore, when blood containing
pathogens is analysed with a
haemocytometer, there will be a high leukocyte count.
There are two main types of leukocytes that you need to know:
• Phagocytes engulf pathogens in a process called phagocytosis. To do this, their
cytoplasm flows around the pathogen and contains enzymes to break it down.
• Lymphocytes are a group of leukocytes that have different functions depending on their
type. For example, B lymphocytes produce antibodies that bind to pathogens, causing
them to agglutinate. This makes it easier for phagocytes to engulf the pathogens.
Platelets are small fragments of cells that do not have a nucleus. Their function is to aid blood
clotting at the site of a wound, which helps seal the wound. This prevents excessive blood loss
and pathogens in the air entering the wound.
Finally, plasma is the liquid that carries all the components in the blood. This includes the ones
described above, as well as glucose, amino acids, carbon dioxide, urea, hormones, proteins and
antibodies produced by B lymphocytes.
Blood vessels
When the heart pumps blood to the lungs and the rest of the body, the blood flows through
vessels called arteries. These arteries then branch out into smaller capillaries, which allow
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
gases to be exchanged in the lungs and supplies body cells with the glucose and oxygen needed
for aerobic respiration. These capillaries then converge again into veins, which carry blood back
to the heart.
Arteries have thick muscular walls with many elastic fibres. When blood is pumped into the
arteries at a fast rate and high pressure, the elastic fibres allow the arterial walls to stretch. Once
the blood has been pumped, the arterial walls spring back into their original state, maintaining
the blood pressure. This is called elastic recoil. In addition, the lumen (the hollow column where
blood flows) in arteries is relatively narrow, which keeps the blood flow at a high pressure.
Veins have relatively thinner muscular walls than arteries because blood flows through them at a
much lower pressure. The lumen is also relatively larger than arteries so that blood flows more
slowly. However, the most distinctive feature of veins are the valves. When skeletal muscle
contracts, it presses onto the veins and causes blood to move. Some of this blood can flow back
down the veins instead of up to the heart. Therefore, valves open and close to prevent the
backflow of blood, ensuring that it moves up to the heart.
Capillaries have a very small lumen so that erythrocytes only move across alveoli and body cells
one-at-a-time while increasing their surface area to volume ratio. Their walls are also very thin to
reduce the distance for substances to diffuse across, increasing the rate of diffusion.
Single and double circulatory systems
Fish have a single circulatory system. Deoxygenated blood from the fish’s body travels to the
heart, which then pumps it right round the body again in a single circuit. The blood goes via the
gills to absorb oxygen. This oxygenated blood then travels to the body capillaries to release the
oxygen. In single circulatory systems, only deoxygenated blood goes through the heart.
Mammals (including humans) have a double circulatory system. This means that the heart
pumps blood around the body in two circuits.
• In the first circuit, the heart pumps
deoxygenated blood to the lungs to absorb
oxygen. The oxygenated blood then
returns to the heart.
• In the second circuit, the heart pumps
oxygenated blood around all the other
organs in the body. The blood releases the
oxygen at the body capillaries and the
deoxygenated blood returns to the heart to
be pumped out to the lungs again.
The heart and the cardiac cycle
The heart is at the centre of the circulatory system, where blood flows in and out. The pathway in
which blood flows through the heart and around the body is called the cardiac cycle.
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
1. Deoxygenated blood from the rest of the body flows into the right atrium through the
vena cava. At the same time, oxygenated blood from the lungs flows into the left atrium
through the pulmonary vein.
2. The atria then contract, forcing blood to flow into the left and right ventricles. This stage
is called atrial systole. As the blood flows into the ventricles, it opens the
atrioventricular valves. The right atrioventricular valve is called the tricuspid valve,
while the left atrioventricular valve is called the bicuspid valve.
3. The ventricles then contract, forcing blood to flow into the pulmonary artery and aorta.
This stage is called ventricular systole. The contraction of the ventricles relaxes the atria
and closes the atrioventricular valves, which prevents any blood flowing back to the atria.
At the same time, the blood opens semilunar valves (which close after to prevent any
blood flowing back to the ventricles).
4. Once the blood flows through the pulmonary artery and aorta, the ventricles relax. This
stage is called diastole.
5. Blood flowing through the pulmonary artery goes to the lungs and becomes oxygenated
at the alveoli, while blood flowing through the aorta goes to the rest of the body and
becomes deoxygenated at the body cells (when oxyhaemoglobin breaks down and
oxygen is released from the erythrocytes into the cells).
Note that the atria and ventricles contract without being stimulated by impulses from motor
neurones, as would be the case for skeletal muscle. For this reason, cardiac muscle is described
as myogenic.
You might have noticed from the above diagram that the wall of the left ventricle is thicker than
the wall of the right ventricle. This is because the left ventricle must pump blood to the rest of
the body through the aorta at high pressure, whereas the right ventricle pumps blood at a much
shorter distance to the lungs. Ventricular walls are in general thicker than atrial walls since they
have to pump blood at longer distances away from the heart, whereas the atria only pump blood
down to the ventricles.
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
You might have also noticed that there is a wall in the heart which separates the right atrium and
ventricle from the left atrium and ventricle. This wall is called the septum and ensures that
deoxygenated and oxygenated blood flowing through opposite sides of the heart do not mix.
Cardiac output
Cardiac output is the total volume of blood pumped by a ventricle to an artery every minute.
You can calculate it using this formula:
cardiac output = heart rate × stroke volume, where cardiac output is in litres min ,
heart rate is in beats per minute (bpm) and stroke volume is in litres.
The heart rate is the number of beats per minute, and the stroke volume is the volume of blood
pumped by one ventricle each time it contracts.
Respiration is the process of releasing energy from the breakdown of organic compounds,
usually glucose (it is the main respiratory substrate). It is an exothermic reaction and the energy
released can be used for metabolic processes. Some of these metabolic processes include:
• synthesis of organic polymers from their monomers, e.g., proteins are made by the
joining of amino acids
• contraction of muscles which allows animals to move
• thermoregulation in mammals and birds
• active transport of substances through plasma membranes
• maintenance, repair and division of cells and cell organelles.
There are two different types of respiration - aerobic and anaerobic. Aerobic respiration uses
oxygen and is the most efficient type of respiration. It happens all the time in animals and plants,
inside the mitochondria.
• glucose + oxygen → carbon dioxide + water + energy
During exercise, muscles require more energy to contract and therefore the rate of aerobic
respiration must increase. To do this, the heart pumps blood faster at a high pressure to increase
blood flow. This allows more glucose and oxygen travelling in the blood to reach the muscles.
However, there is a limit to the amount of blood that the heart can pump. If not, enough oxygen
can reach the muscles, then anaerobic respiration takes place. Anaerobic respiration takes place
in the cytoplasm and only partially breaks down glucose into lactic acid, releasing much less
energy than aerobic respiration.
• glucose → lactic acid + energy
A build-up of lactic acid produced by anaerobic respiration can cause an oxygen debt, which is
the amount of oxygen that the body needs to oxidise the lactic acid to form carbon dioxide and
water (as would be produced by aerobic respiration).
Edexcel GCSE Biology: Unit 8 (Exchange and Transport in Animals) written by James San
Anaerobic respiration in plants and fungi is different than in animals:
• glucose → ethanol + carbon dioxide + energy
In yeast, anaerobic respiration is a fermentation reaction, which is often used during brewing and
bread-making. Ethanol is the alcohol found in alcoholic drinks like beer and wine. In breadmaking, bubbles of carbon dioxide gas expand the dough and help the bread rise.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
Introduction into Ecosystems
Organisms need nutrients and resources to survive. Animals such mammals, fish, and
microorganisms require oxygen to respire, food to maintain/grow their health and water to act as
a medium for metabolic processes e.g., carry oxygen, to occur. Plants will need light, carbon
dioxide and water to photosynthesise as well as mineral ions. Additionally, they may demand
shelter from the harsher climates or to avoid predation.
All organisms are continually interacting with the environment thus forming an
The diagram on the right shows water in the soil entering the root hair cells of the
plant via osmosis and mineral ions entering via active transport. The soil
(environment) interacts with the plant (organism)
9.1) The individual organism refers to a living structure which can reproduce,
respire, react to stimuli, adapt, grow, maintain homeostasis. The population refers to the volume
in number of different species which exist. The communities consist of the interactions between
the different species which live in different habitats. When the populations of species e.g., a fox
depends on a rabbit for food and the rabbit avoids the fox’s predation, we say they’re
interdependent. The state of the habitat can also impact the population levels of a species e.g.,
deforestation in a tropical rainforest may reduce levels of a certain species.
In the analogy mentioned, we have a fox and a rabbit. Both organisms are called biotic factors.
Ecosystem Interactivity
Biotic Factor - refers to the organisms within the ecosystem which can affect each other in terms
of health, population.
As you can imagine in the rabbit-fox analogy, higher levels of fox lead to higher predation and
lower population levels of rabbits.
Rabbits are herbivores so if there were another animal which ate the same food as the rabbit, they
would be competing for food. In Yellowstone the elks and beavers competed for food from trees.
This led to the overgrazing of many tree species within the area. So more grey wolves (a predator
of elks) were introduced into the region to reduce the consumption of trees. The aim was to
increase the predation of elks, lowering their population and as a consequence of the lower
population of elks, the beavers had less competition for their food, so the populations of beavers
rose. This demonstrates how predators and prey biotically affect each other.
This is quite a large chain. In a smaller prey-predator cycle the population levels over time may
look like this.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
As the population levels of the beavers rose,
of course, their activity levels rose, and they began to change the
habitat by building dams, increasing the biodiversity.
The construction of the dam allows
new adaptation of different plant species to form.
Abiotic Factors - refers to the effect a change in environment has on the distribution of
organisms within an ecosystem.
These can be physical as well as chemical changes e.g temperature, rainfall to soil nutrient
concentration and pollution. Each species has certain adaptations to their particular environment
so a change in environment may result in a different distribution of that species.
Temperature - the long-term rise in global warming caused by increased greenhouse gas
emissions can cause the decline in the distribution of animals adapted to living in a cooler
climate. E.g., polar bear populations will decline.
Water - if a flood occurs, most plants will be waterlogged (oversaturated with water) and likely
die followed by the animals which have fed on the plants
Light - very low light intensity may prompt an increase in a distribution of fungi as they do not
photosynthesis; they have no chlorophyll. In dense forests, the top layer will receive most
sunlight therefore prompting those trees to grow whilst at the bottom layer, very few plant
species grow large as there’s limited light for limited growth there.
Pollutants - can naturally harm the environment by introducing harmful chemicals which could
be consumed by animals and plants and accordingly, reduce biodiversity (variety of species
within a community)
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
9.3) Food webs show the feeding relationship
between producers, predators, and prey as they go
up the consumer layers.
In the diagram on the right, the fox and the eagle
are the top predators. They all rely on the high
populations of producers/autotrophs (an
organism who can produce its own food). Should
the population levels of the species in the trophic
level above decrease, then the population of the
trophic level below would decrease. Hence
maintaining a healthy even population is important
to continue the survival of its species.
9.4) In most biotic relationships the prey gets eaten
by the predator and the predator continues to find
new prey. In a parasitic relationship, the parasite,
no not the movie, one organism benefits by
leeching off another organism - its host organism - causing harm to the host.
The host may live for long provided that the parasite does minimal damage to the host.
Examples include tapeworms and head lice.
Tapeworm adaptations:
• Hooks and suckers to firmly attach itself to the wall of the intestine
Head lice adaptations:
• Sharp claws to grip onto scalp and hair
• Sharp teeth to pierce skin and suck blood
A mutualistic relationship is the opposite where both species
receive benefits from the relationship. E.g during pollination the
insect receives nectar from the flower as food and the flower
places it’s pollen onto the flower to increase its chances of
reproducing sexually.
Similarly in the relationship between a clownfish and the anemone,
the anemone’s stinging tentacle discourage large predators from
eating the clownfish if the clownfish swim into the anemone and
the clownfish deter predators of the anemone whilst providing
nutrients to the anemone via their faeces.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
Measuring Biodiversity
9.5 and 9.6)
The distribution of species within a given region can be measured using
quadrats and belt transects.
A quadrat (basically a uniform sized square) has been placed along this
field. Within the quadrats, the abundance is estimated by counting all the
samples using a quadrat. Additionally, the population size can be counted
using the formula below.
Population Size = number of organisms in all quadrats ×
total size of area where the organism lives
total area of quadrats
The belt transect can be used to measure the distribution of organisms affected by abiotic
factors. Quadrats are placed along a line where the change in environment can e.g. soil mineral
ion content can impact the growth of different plants. Changes in the abundance of a species can
demonstrate how the abiotic factors affect its growth as well as the abiotic factors that have the
greatest impact on the growth of the species.
Energy Transfer
The diagram on the right shows how much sunlight energy hits the
different regions of the Earth. Of course, closer to the equator,
more sunlight hits whilst further away and closer to the poles, less
light energy hits. As photosynthesis is used for growth, more
plant biomass (the mass of tissue) forms towards the equator and
consequently a greater amount of plant there, the more producers
there are to feed the trophic levels that follow. This can show how
the amount of sunlight (light energy) hitting a region can cause a
different distribution of organisms to form.
Producer (AKA autotroph) → primary consumer → secondary consumer.
Plant gets consumed by a rabbit and the rabbit gets consumed by the fox.
Plant → rabbit → fox
Reasons why not all the energy from the previous organism gets transferred:
Energy remained stored in plant biomass via the plant’s body e.g in the stem.
Chemical from the food gets dissipated to the surroundings as heat energy during an
organism’s metabolic processes.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
In the pyramid of biomass, as the trophic levels increase, the biomass decreases as not all the
energy from the prior level gets transferred to the surrounding therefore less biomass is
As shown in the diagram, as the trophic levels increase, the organisms have less mass.
The number of trophic levels is finite as the number of energy given by the sun then becomes too
little to support another level thus the new layer cannot be supported.
9.8B) energy efficiency = energy transferred to the new biomass/energy the prior organism had
Often it is measured from a scale of 0 to 1
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
Preserving Biodiversity
Fish Farming (9.9a)
Humans eat fish as a commodity. With around 17% of protein
consumed from fish globally, as the population in the world
increases, the demand for fish increases. This can cause the
overfishing of wild fish hence interrupting the biotic factors in the
natural world. This can damage ecosystems and their biodiversity
since fish also provide nutrients to the surrounding coral or surrounding species via its decay
from faeces and predation.
Fish farming may also cause problems as the fishes are relatively
close in proximity resulting in diseases being more easily transferred
as well as abiotic changes if uneaten food and faeces sink to the
bottom of their environment - possibly manifesting pathogenic
microorganisms if not cleaned properly.
Introducing species (9.9b)
By introducing a new species into the ecosystem, this can affect the distribution of the
native/indigenous species. A new species may be introduced with the intent of controlling the
population of another species however this could have an even greater negative impact that
exacerbates the problems. For example, cane toads from South America were introduced to
Australia as a foreign predator to control the high numbers of sugar beetles, which were eating
sugar cane crops. As the toads preyed on the sugar beetles, there was a low species population of
the sugar beetles, yet the toad’s population have now become a problem as they are poisonous so
kill the native animal therefore increasing the competition. This could reduce biodiversity if the
cane toads kept their feeding off the other indigenous creatures in the region.
Eutrophication (9.9c)
Eutrophication is the addition of more nutrients e.g., mineral ions like nitrates or phosphates,
into an ecosystem - more than it would usually have. An example of this can be using NPK
(nitrate, phosphate, potassium) fertilisers to help crops grow but as a side-effect, it causes the
growth of unwanted plants like algae.
The stages of eutrophication are the following:
1. The NPK fertilisers are added to the ecosystem. [1]
2. The nitrates and phosphates and potassium ions are dissolved into the soil. [1]
3. The stream of water takes in the additional nutrients providing the organisms in the water
with these. [1]
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
4. The high concentrations
of these new nutrients
such as nitrates and
phosphates encourage the
plants and algae to grow
more rapidly. [1]
5. As the algae grow, they
block more sunlight from
reaching the plants in the
water therefore less
photosynthesis occurs,
and less oxygen is
produced. [1]
6. As the oxygen concentration decreases, aquatic animals receive less oxygen therefore die
and decrease in population. [1]
Biodiversity is classified as the variety of species within an area. When ecosystems are
damaged, only a handful of species are adapted to survive in such regions and so there are fewer
complex food webs.
A reforestation is obviously where trees are planted in order to increase the range of habitats
and number of different species living there. By abiotically affecting the ecosystem, new species
can form and therefore result in more biodiversity.
Conservation is where efforts are made to protect the population of a certain species. Often that
species is endangered/rare.
Preserving the existence of a species is also more easily done if the habitat of that species is
maintained. Habitats are linked to the survival of said species.
Reasons why preserving biodiversity is important:
• Areas with greater biodiversity can recover faster from natural disasters e.g., flooding.
• We can use plants/animals for different sources of food.
• A variety of different animals/plants can be used as a source of medicine and other
Food Security
Food security indicates always having sufficient access to safe and healthy food. For context, an
area with good food security will include people who can easily go and buy food from the
supermarket whilst poor food security will include having to trek 10km just for potable water.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
Levels of food security could be affects by the following factors:
Rising human population increases the demand for food yet the quantity produced
remains the same. As a result, food shortages/famines could occur.
Increased meat consumption and fish consumption as opposed to agricultural inputs
(conventional farming of plants) could use too much land to create food.
If new pests/pathogens are found within a region, obviously this could reduce the growth
of crop hence resulting in a lower crop yield hence poorer food security
An environmental change caused by humans could increase the food yield if fertiliser is
used or make matter worse if carbon emissions increase as it can cause climate change
leading to more pests
An idea to reduce fossil fuels is to replace them with biofuels (plant fuel) therefore
decreasing the carbon emission released when mining for them
9.12) This specification point is mentioned in the teachings below.
Material Cycles: Carbon, Nitrogen and Water
Carbon Cycle
The Carbon Cycle refers to the movement of carbon through organisms and nature such as
through plants/animals to forms like fossil fuels for carbon dioxide in the air.
Painting a picture for you
Carbon dioxide in the air diffuses into the plant leaves; photosynthesis can occur
causing the carbon dioxide from the air to now be a part of the carbon in glucose
(C6H12O6 - all numbers should be subscripts of course).
If the glucose is used for respiration, the carbon is released back into the atmosphere as
carbon dioxide. Alternatively, it could be used to create more plant biomass.
The biomass of the plant consists of proteins, lipids (fat), and carbohydrates which all
contain carbon. It is via excretion that the plants and animals can release carbon
The waste material (excretion) gets decomposed by decomposers (microorganisms) as a
form of food.
When the microorganism/decomposers or fungi (same function) respire, they release
carbon dioxide into the atmosphere in the form of carbon dioxide.
Similarly, the plant biomass’s carbon can be transferred to animals if they directly
consume the plant.
If many plants/animals die at once, the decomposers cannot fully decompose them so
over millions of years, under the pressure of gravity and the material pushing down on
their dead remnants, they can change into peat/coal which can be burnt via combustion
to release carbon emissions into the atmosphere.
NB: the forms of respiration mentioned here are all AEROBIC
(glucose + oxygen → water + carbon dioxide + energy {ATP})
If you’re curious ATP stands for Adenosine Triphosphate.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
To reiterate, the movement of carbon via biotic and abiotic factors is called the carbon cycle.
2 Abiotic:
• Fossil fuels
• Carbon dioxide in the
3 Biotic:
• Decomposers
• Animals
• Plants
The diagram on the right is
part of Ken’s trusty flashcards
that got him a 9 in biology and
illustrates what the carbon
cycle essentially is.
By using your knowledge of
the biotic elements in the carbon cycle, you can infer that a reduction/increase in a particular part
of the carbon cycle can result in a decrease/increase in another.
Water Cycle
The water cycle refers to the movement of water as well as the dissolved mineral ions through its
abiotic factors - duh!
Let me paint you a picture
Living organisms need nutrients to survive and there are limited resources on Earth so they must
be recycled.
Water flows from lakes and rivers and into the sea.
In the ocean the water evaporates to form water vapour.
As the warm water vapour rises, it cools and condenses to form clouds.
Then the water droplets become too large and heavy thus they fall as water droplets (rain)
back into the lake to restart the cycle
Additionally, groundwater from the surround soils can diffuse into the mainstream of
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
Water is important as it provides a means for chemical
reactions/metabolic processes to occur e.g., cytoplasm in
the cells need water for chemical reactions to take place.
Furthermore, to make water potable (safe for
consumption), the water may be treated with chemicals
such as chlorine to kill bacteria or filtered to remove dirt.
Obtaining fresh water
Sea water is obviously salty, and humans cannot drink it duh. So, obtaining drinkable water from
salty seawater, the process is known as desalination. This is where the distilled water is obtained
by evaporating water then condensing it and collecting it afterwards.
Salty seawater enters the tube on the left and once it is heated past its boiling point. That leaves
distilled water out once it is condensed.
Nitrogen Cycle
The nitrogen cycle describes the movement of nitrogen via abiotic and biotic factors.
Nitrogen in plants
• Nitrogen is stored a as proteins/DNA in the plants
• Plants allow require nitrogen to help it grow however nitrogen in the air is unreactive (it’s
a diatomic molecule: inert)
• So, they absorb the nitrogen compounds from the nitrate ions via active transport in the
Bacteria and Nitrates
• The soil fertility is maintained by the decomposers (microorganisms) which release
nitrates as well are carbon when decomposing dead animals
• Additionally, farmers will add manure (animal waste e.g. faeces) to utilise the
decomposition to release the nitrates into the soil for the plants to absorb. Nitrates are
soluble and dissolve in water hence when plants absorb the water, they also absorb the
mineral ions.
• Farmers may also use artificial fertilisers to provide more nitrates to the plants
• Nitrogen-fixing bacteria in the soil converts the nitrogen in the air into nitrate ions
available to the plant.
• Lighting strikes can also cause unreactive nitrogen to form reactive nitrate ions
dissolved in the soil.
• Some plants such as peas and beans carry a mutualistic relationship with the nitrogenfixing bacteria; the bacteria is protected inside the nodules in the plant roots whilst the
nitrogen-fixing bacteria releases soluble nitrate ions for the plants growth.
• Farmers can use this to their advantage by adding a different crop after the peas have
been harvested so that the nitrogen-fixing bacteria will continue to release nitrate ions but
for a different crop. This is called crop rotation.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
A diagram of the nitrogen cycle can be shown above briefly summarizing what occurs.
The urea can be produced as a form of waste product.
Indicators of Pollution
9.16B) - HIGHER
As mentioned earlier, human activity such as fertilisers use, overfishing or introducing a nonindigenous species can affect the abiotic factors in the current environment. Especially when it
leads to pollution, this can cause a decline in the population of one species and the rise in the
other. The new species that rises in population will likely have adaptations which increase the
chances of its survival than the species declining.
Water Pollution
Some aquatic invertebrates can qualitatively indicate the level of pollutants in water qualitatively meaning whether the water is very highly polluted to very clean water (low levels
of pollution): this is called an indicator species. This pollution can often come in the form of
sewage from factories nearby. This can also release additional nutrients in the soil hence causing
unwanted eutrophication if the sewage pollute contains nitrate elements.
As less oxygen is in the habitat because of eutrophication, the species which were previously
adapted to high oxygen concentration levels, die out whilst there emerges a rising prevalence of
species adapted to surviving in low oxygen concentration levels.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
a) The presence of sludge worms and bloodworms = polluted
b) The presence of more complex organisms like freshwater shrimps and stoneflies
= clean water.
Freshwater Shrimp
Stone Fly
Air Pollution
Similarly, the presence of a particular plant/fungi indicates the quality of the substances in the
c) Presence of lichen - indicates higher levels of
sulphur-containing gases e.g., sulphur dioxide.
Blackspot fungus is a pathogen of roses and cannot grow in
high concentration levels of sulphur-containing gases. The
presence of it indicates low air pollution of sulphurcontaining gases
All indicator species show to a degree the quality of the air however they do
not quantify the concentration levels of these pollution levels. You can measure
both the air and water pollution levels by using a sensor. These give numerical,
quantitative data about the concentration of pollution in the region.
Edexcel GCSE Biology: Unit 9 - Ecosystems and Material Cycles
written by Ken Tu
Rates of Decomposition in Food Preservation
By recalling from the carbon cycle, you will know that microorganisms decompose the soft
tissues of an organism or its faeces once dead.
Temperature increase → increase the rate of decomposition
Increased water content (moisture) → increase the rate of decomposition
Increased oxygen → increase rate of decomposition as organisms will use this energy to respire
and speed up their metabolic processes to decompose more.
To preserve food, you want to restrict the activity of the decomposers hence why you can do the
opposite to preserve food:
Reduce the temperature e.g., in fridges/freezers
Reducing the water in the food e.g., salting meat causes water to exit the food via
osmosis therefore less microorganisms’ activity
By packaging foods in air-tight containers
Irradiation is using radiation to directly kill the bacteria/decomposers on your food to
preserve it and reduce chances of food poisoning
The same factors will increase the rate of decomposition in composting.
Composting is decayed, waste garden material. Think of it as a mixture of decomposed
organisms from the decomposers which releases nitrogen as a form of food for the garden you
place it in. This as you can imagine increases the soil fertility.
Rate is just the amount decayed (mass lost in the organism) / time elapsed.
rate of decomposition
mass lost in the organism
time elapsed