AQA GCSE Biology – Unit 3 summary notes
AQA
GCSE Biology
Unit 3
Summary Notes
Page 1
AQA GCSE Biology – Unit 3 summary notes
13.1 How do dissolved materials get into and out of animals and plants?
Absorption of substances by cells

Dissolved substances move by diffusion, osmosis and active transport.
Diffusion (covered in more detail in unit 2):
 The movement of particles in a liquid or gas from an area where they are in higher
concentration to an area where they are in lower concentration.
 The greater the difference in concentration, the faster the rate of diffusion.
 2 examples of diffusion through the cell membrane:
o Oxygen required for respiration diffuses into cells.
o Carbon dioxide produced by respiration diffuses out of cells.
Membrane
Osmosis (covered in more detail in unit 2):
 The diffusion of water from a dilute to a more concentrated solution through a partially
permeable membrane that allows the passage of water molecules.
 2 examples:
o Absorption of water by the root hair cells in roots in plants.
o Absorption of water by epithelial cells in the small intestine.
Active Transport:
 The process by which cells absorb substances against a concentration gradient.
 The substance being transported attaches to large protein molecules in the cell
membrane.
 Energy released through respiration is used to change the shape of the protein.
 This releases the substance on the other side of the membrane.
 This process enables cells to absorb substances from very dilute solutions.
 2 examples:
o Absorption of mineral ions by the root hair cells in roots in plants.
o Absorption of glucose by epithelial cells in the small intestine.
Membrane
Carrier
protein
Page 2
Energy
AQA GCSE Biology – Unit 3 summary notes
Gas Exchange in the lungs



The lungs are in the upper part of the body (thorax).
They are protected by the ribcage.
They are separated from the lower part of the body (abdomen) by the diaphragm.




The breathing system takes air into and out of the body.
Oxygen from the air diffuses into the bloodstream.
Carbon dioxide can diffuse out of the bloodstream into the air.
The alveoli provide a very large, moist surface, richly supplied with blood capillaries so
that gases can readily diffuse into and out of the blood.
Page 3
AQA GCSE Biology – Unit 3 summary notes




Many small spherical alveoli provide a large surface area.
The moist surface helps oxygen to dissolve so that it can diffuse into the cells.
The capillaries are very close to the alveoli so that the gases have a short distance to
diffuse.
Concentration gradients are maintained by:
o The blood removing oxygen from and bringing carbon dioxide to the lungs.
o Breathing, which replenishes oxygen and removes carbon dioxide.
 Muscles in the diaphragm and between the ribs are used to increase
and decrease the volume in the thorax.
Absorption of nutrients in the small intestine








Food in the small intestine has been digested into:
o Glucose
o Amino acids
o Fatty acids
o Glycerol
These nutrients need to be absorbed into the blood across the wall of the small
intestine.
They are absorbed by diffusion and active transport.
The wall of the small intestine is covered with finger-like extensions called villi.
Many villi provide a large surface area.
The wall is only one cell thick so the diffusion pathway is short.
Villi have an extensive network of capillaries to absorb the products of digestion.
Concentration gradients are maintained because the blood removes the absorbed
nutrients.
Page 4
AQA GCSE Biology – Unit 3 summary notes
Absorption of water by roots




The surface area of the roots is increased by root hairs.
Most of the water and mineral ions are absorbed by root hair cells.
Water is absorbed by osmosis.
Most of the mineral ions are absorbed by active transport.
Gas Exchange in the leaf

The surface area of leaves is increased by the flattened shape and internal air spaces.




Most photosynthesis takes place in the palisade cells.
Carbon dioxide needs to reach the palisade cells.
Plants have stomata to obtain carbon dioxide from the atmosphere.
This carbon dioxide is used in photosynthesis.
Page 5
AQA GCSE Biology – Unit 3 summary notes


The size of stomata is controlled by guard cells, which surround them.
Stomata open during daylight hours, to enable carbon dioxide to diffuse in.
Transpiration


The process by which plants lose water vapour from the surface of their leaves.
It evaporates into the air spaces in the leaf, and then diffuses out through the stomata.
Page 6
AQA GCSE Biology – Unit 3 summary notes





Transpiration is more rapid in hot, dry and windy conditions:
o Heat causes the water to evaporate quicker.
o Dry conditions increases the water vapour concentration gradient.
o Wind moves the water vapour away from the leaf, maintaining the concentration
gradient.
Most of the water lost by transpiration leaves through the stomata.
Stomata close when it is dark, when carbon dioxide is not required.
This reduces the amount of water lost by the plant at a time when it is not needed for
photosynthesis.
If plants lose water faster than it is replaced by the roots, the stomata can close to
prevent wilting.
Page 7
AQA GCSE Biology – Unit 3 summary notes
13.2 How are dissolved materials transported around the body?




Substances are transported around the body by the circulation system:
o The heart
o The blood vessels
o The blood
They are transported from where they are taken into the body to the cells, or from the
cells to where they are removed from the body.
Substances are taken in:
o In the digestive system
o In the lungs
Substances are removed:
o From the lungs
o By the kidneys
The Heart





The heart pumps blood around the body.
Its walls are mainly made of muscle.
This muscle contracts to pump blood through the circulation systems.
It is a double pump, because there are two separate circulation systems:
o One to the lungs
o One to all the other organs of the body.
The blood passes through the heart twice in order to pass round both of these
circulation systems.


Blood is pumped to the lungs from the right ventricle through the pulmonary artery.
The blood returns from the lungs to the left atrium through the pulmonary vein.
Page 8
AQA GCSE Biology – Unit 3 summary notes


Blood is pumped to the organs of the body from the left ventricle through the aorta.
Blood returns to the heart from the organs into the right atrium through the vena
cava.
Blood vessels

Blood flows from the heart to the organs through arteries and returns through veins.

In the organs, blood flows through capillaries.
Page 9
AQA GCSE Biology – Unit 3 summary notes

Substances needed by cells in the body tissues pass out of the blood, and substances
produced by the cells pass into the blood through the walls of the capillaries.
Arteries
Carry blood away from the
heart to the organs.
High blood pressure.
Capillaries
Carry blood away from
arteries into organs then
back into veins.
Decreasing blood pressure.
Veins
Carry blood away from the
organs back to the heart.
Elastic to withstand high
blood pressure.
No elastic tissue – walls one Not elastic.
cell thick.
Low blood pressure.
Muscular walls, to create a No muscle – walls one cell
pulse, to maintain blood
thick.
pressure.
Very little muscle.
Rarely contain valves.
Contain valves to control
the direction of blood flow.
No valves.
Blood

Blood has two main components:
o Plasma
o Cells
Plasma:


The liquid part of the blood.
Blood plasma transports:
o Carbon dioxide from the organs to the lungs
o Soluble products of digestion from the small intestine to other organs:
 Glucose
 Amino acids
o Urea from the liver to the kidneys.
Blood cells:
Page 10
AQA GCSE Biology – Unit 3 summary notes

White blood cells – involved in the immune system (covered in Unit 1).

Platelets – fragments of cells; involved in blood clotting.

Red blood cells transport oxygen from the lungs to the organs.



Red blood cells have no nucleus.
They are packed with a red pigment called haemoglobin.
In the lungs:
o Oxygen diffuses into the red blood cells.
o Haemoglobin combines with oxygen to form oxyhaemoglobin.
In the other organs:
o Oxyhaemoglobin splits up into haemoglobin and oxygen.
o Oxygen diffuses out of the red blood cell.
They have a biconcave shape.
This increases their surface area to increase rate of diffusion across the cell membrane.



Page 11
AQA GCSE Biology – Unit 3 summary notes
13.3 How does exercise affect the exchanges taking place within the body?
The role of respiration during exercise


Muscles contract to move the bones in our bodies.
Respiration releases energy, which is used to contract the muscles:
(Refer to Unit 2 for more details on aerobic respiration.)
glucose + oxygen





carbon dioxide + water + energy released
When we exercise, our muscles contract more quickly and with more force.
This requires more energy.
This requires more glucose and oxygen.
Also, more carbon dioxide is created which needs to be removed.
The human body needs to react to the increased demand for energy during exercise.
Changes during exercise


During exercise a number of changes take place:
o The rate and depth of breathing increases.
 This increases the rate of gaseous exchange.
 More oxygen is taken into the blood.
 More carbon dioxide is removed from the blood.
o The heart rate increases.
 This increases rate of blood flow to the muscles
o The arteries supplying the muscles get wider (they dilate).
 This allows more blood to enter the capillaries in the muscles.
All of these changes increase the supply of sugar and oxygen to the muscles and
increase the rate of removal of carbon dioxide from the muscles.
Glycogen



Glucose is stored as glycogen is in the muscles (refer to unit 2).
During exercise, glycogen is broken down into glucose in the muscles.
This increases the amount of glucose that can be respired.
Anaerobic respiration




If muscles are subjected to long periods of vigorous activity they become fatigued,
o Oxygen supply gets too low.
o They stop contracting efficiently.
If insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain
energy.
Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.
As the breakdown of glucose is incomplete, much less energy is released than during
aerobic respiration.
glucose


lactic acid + small amount of energy released
However, lactic acid is poisonous. We can only tolerate small amounts in our body.
During and after exercise, we breathe heavily to take in extra oxygen to oxidise the lactic
acid.
Page 12
AQA GCSE Biology – Unit 3 summary notes

The extra oxygen is called the oxygen debt
lactic acid + oxygen




carbon dioxide + water
The heart continues to pump faster.
The breathing rate remains high.
This delivers the extra oxygen to the muscles.
This pays back the oxygen debt.
Page 13
AQA GCSE Biology – Unit 3 summary notes
13.4 How do exchanges in the kidney help us to maintain the internal environment in
mammals and how has biology helped us to treat kidney disease?








Most humans have two kidneys.
Kidneys have an important role in homeostasis.
(More detail on homeostasis in Unit 1 and 2.)
They control the water content of the blood.
They control the ion (salt) content of the blood.
They remove the urea from the blood.
o Urea is a toxin produced when the liver breaks down excess amino acids.
Each kidney receives blood from the aorta (via a renal artery).
The artery branches into millions of capillaries inside each kidney.
Each kidney also contains about one million microscopic tubules, which are responsible
for forming urine.
Page 14
AQA GCSE Biology – Unit 3 summary notes

A healthy kidney produces urine by:
1.




Filtering the blood
Blood enters the capillaries under high pressure.
Most of the liquid leaves the blood and enters a tubule.
The cells and large molecule remain in the blood.
The blood becomes very concentrated.
2. Reabsorbing all the sugar
 All of the sugar is reabsorbed from the tubule, back into the blood by active transport.
 It moves against a concentration gradient.
3. Reabsorbing the dissolved ions needed by the body
 Some of the ions are also reabsorbed back into the blood by active transport.
 Some salt is left behind to balance what the body needs.
4. Reabsorbing as much water as the body needs
 Water is reabsorbed into the blood by osmosis.
 This occurs because there is a much higher solute concentration (lower water
concentration) in the blood than in the tubule.
 The body balances how much water it needs by changing how much water is lost in
the urine.
5.



Releasing urine
Urine is released from the kidney.
It contains urea, excess ions and water.
Urine is stored in the bladder, before being expelled from the body.
Treating Kidney Failure


People whose kidneys do not function properly can die because toxic substances
accumulate in their blood.
Their lives can be saved by using dialysis machines or by having a healthy kidney
transplanted.
Dialysis
 The blood vessels in the arm are connected to a dialysis machine.
 In the dialysis machine a person’s blood flows between partially permeable membranes.
 The dialysis fluid contains the same concentration of useful substances as the blood.
 This ensures that glucose and useful mineral ions are not lost.
 There is a high concentration of urea in the person’s blood, and no urea in the dialysis
fluid.
 There is a concentration gradient of urea between the blood and the dialysis fluid.
 Urea passes out from the blood into the dialysis fluid by diffusion.
 Treatment by dialysis restores the concentrations of dissolved substances in the blood
to normal levels.
 It has to be carried out at regular intervals.
Page 15
AQA GCSE Biology – Unit 3 summary notes
Kidney transplants
 A kidney transplant enables a diseased kidney to be replaced with a healthy one from a
donor.
 However, the donor kidney may be rejected by the immune system:
o The antigens on the cells of the donated kidney may be different to the recipient’s
antigens.
o His white blood cells will be stimulated and the immune system will attempt to
destroy the new kidney.
 To prevent rejection of the transplanted kidney:
o A donor kidney with a tissue-type similar to that of the recipient is used.
o The recipient is treated with drugs that suppress the immune system.
Dialysis
Advantages
No major surgery needed.
Disadvantages
Diet needs to be controlled carefully.
Kidney transplant
No waiting lists – it prevents
people dying.
No need for dialysis.
Restricts normal life – it takes about 8
hours, several times a week.
Major surgery has many risks.
The recipient can lead a
relatively normal life.
Possibility of rejection.
Diet does not need to be
controlled.
Immunosuppresant drugs need to be
taken for life.
The donated kidney only lasts for
about 9 years.
Some religious groups do not agree
with organ donation.
Page 16
AQA GCSE Biology – Unit 3 summary notes
13.5 How are microorganisms used to make food and drink?
Biogenesis
 The accepted scientific principle is that life can only be passed on by living things.
o Biogenesis is the process of life forms producing other life forms.
 Until the 19th Century, people believed in spontaneous generation (abiogenesis):
o That life can arise from non-living things.
o Eg the appearance of maggots or mould in food.

Spallanzi (1765):
o Boiled flasks of meat broth to kill microorganisms.
o The flasks were sealed.
o No microorganisms grew in the broth.
o Scientists criticised his work – they asserted that spontaneous generation
required fresh air in the flask.

Scwhann (1836):
o Used sulphuric acid and heated glass tubes to sterilise air.
o He showed that the amount of oxygen remained the same.
o This was passed through boiled broth in a flask.
o No microorganisms grew in the broth.
o Then, when exposed to normal air, microorganisms grew.
o Scientists criticised this work too – they asserted that something had been
destroyed in the air that enabled life to form.

Pasteur (1864):
o Developed a swan-neck flask.
o Air could enter but microbes were trapped in curved neck of the flask.
o He boiled the broth inside the flask.
o The broth remained clear.
o When the neck was broken off, the broth went cloudy.
o Scientists accepted this as proof that spontaneous generation does not occur.
Page 17
AQA GCSE Biology – Unit 3 summary notes
Use of microorganisms to make food and drink

People from many different cultures have known for thousands of years how to use
microorganisms to make various types of food and drink.
Use of bacteria:
 Yoghurt production
1.
Take milk and standardise its fat content – mix skimmed and full fat milk.
2.
Pasteurise the milk - kills most contaminating microorganisms.
3.
Add the bacterial starter culture.
4.
Incubate at warm temperature.
5.
The sugar lactose is converted into lactic acid.
6.
This causes the milk to clot and solidify into yoghurt.

Cheese production
1.
Standardise the fat content of the milk.
2.
Pasteurise the milk.
3.
Add the bacterial starter culture.
4.
This creates lactic acid.
5.
Add an enzyme that separates the curds (solid) from the whey (liquid).
6.
Remove solid curds, squash them and allow the cheese to ripen.
Use of yeast:
 Yeast is a single-celled organism.
 The cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall.

Anaerobic respiration in yeast:
 Yeast can respire without oxygen.
 This produces carbon dioxide and ethanol (alcohol).
 This is called fermentation.

Aerobic respiration in yeast:
 In the presence of oxygen yeast carries out aerobic respiration and produces carbon
dioxide and water.
 Aerobic respiration provides more energy and is necessary for the yeast to grow and
reproduce.
Page 18
AQA GCSE Biology – Unit 3 summary notes

Alcoholic drinks
 In brewing beer and wine-making, carbohydrates are used as an energy source for
yeast to respire.
 Brewing beer:
o Malting:
 The barley grains are soaked in warm water.
 They start to germinate.
 Enzymes in barley grains break down starch into a sugary solution
(maltose).
 This sugary solution is extracted.
o Fermention:
 Yeast and the sugar are mixed in a fermenter.
 It is kept warm.
 An air lock prevents oxygen getting in, ensuring anaerobic respiration.
 Alcohol is produced.
o Flavour development:
 Hops are then added to give the beer flavour.
 The beer is left for a while for the flavour to develop.
 The yeast settles to the bottom.
 The beer is bottled or put into barrels.

Making wine:
o Pressing:
 The grapes are pressed to release their juice.
o Fermentation;
 The juice is mixed with yeast and water in a fermenter.
 The yeast uses the natural sugars in the grapes as its energy source.
 An air lock is used to ensure anaerobic respiration.
 Alcohol is produced.
 The yeast settles to the bottom.
o Maturing.
 The wine is put into bottles.
 It is stored for the wine to mature.
Page 19
AQA GCSE Biology – Unit 3 summary notes
13.6 What other useful substances can we make using microorganisms?
Fermenters
 Microorganisms can be grown in large vessels called fermenters to produce useful
products such as antibiotics.
 Industrial fermenters usually have:
o An air supply - to provide oxygen for respiration of the microorganisms
o A stirrer - to keep the microorganisms in suspension and maintain an even
temperature.
o A water-cooled jacket - to remove heat produced by the respiring
microorganisms.
o Instruments to monitor factors such as pH and temperature.
Antibiotic production
 The antibiotic, penicillin, is made by growing the mould Penicillium in a fermenter.
 The medium contains:
o Sugar
o Other nutrients eg a source of nitrogen.
 The Penicillium only starts to make penicillin after using up most of the nutrients for
growth:
Page 20
AQA GCSE Biology – Unit 3 summary notes
Mycoprotein production




The fungus Fusarium is used to make mycoprotein (Quorn).
This is a protein-rich food suitable for vegetarians.
The fungus is grown on starch in aerobic conditions.
The biomass is harvested and purified.
Fuel production
 Fuels can be made from natural products by fermentation.
 Biogas:
o Produced by anaerobic fermentation of a wide range of plant products or waste
material containing carbohydrates.
o It is mainly methane.
o On a large scale industrial waste can be used:
 From sugar factories.
 From sewage works.
o On a small scale, biogas generators can be used to supply the energy needs of
individual families or farms.
o Organic waste can be used:
 Plant waste from crops.
 Animal faeces and urine.
o Many different microorganisms are involved in the breakdown of materials in biogas
production.
o There are many environmental and economic advantages:
 It is a useful way to recycle waste material.
 Initial set up costs are expensive, but the raw materials for biogas production
are very cheap (or free).
 The fuel is readily available.
 This is particularly useful in developing countries.
 Waste products from the process can be used as fertilisers.
 It reduces the use of fossil fuels.
o There are various different designs:
Page 21
AQA GCSE Biology – Unit 3 summary notes
Floating dome design
Advantages
Floating dome stabilises the gas pressure
Straightforward to use.
Disadvantages
Metal construction:
Expensive to set up
Rusts easily
Fixed dome design
Advantages
Fewer metal parts:
Less expensive to set up
Does not rust
Disadvantages
Gas pressure fluctuates.
Less straightforward to control.
Gas leakage is more likely
 Ethanol-based fuels:
o Produced by the anaerobic fermentation of:
 Sugar cane juices, or
Page 22
AQA GCSE Biology – Unit 3 summary notes

o
o
o
o
Glucose derived from maize starch.
 The starch is broken down using carbohydrase enzymes.
The sugars are fermented using yeast.
The ethanol is distilled from the products of the fermentation.
This can be used in motor vehicle fuels.
It can also be mixed with petrol to make gasohol.
Advantages
Less fossil fuels used.
Disadvantages
Large quantities of plant material is required.
It is a renewable supply of energy.
This uses a lot of land to grow the crops.
No toxic gases produced with combustion.
Only certain countries have the climate and
the land available to grow them eg Brazil.
Carbon neutral - The carbon dioxide
released is balanced by the carbon dioxide
removed from the atmosphere by the plant.
Not all of the plant material can be used.
There are large amounts of plant waste
produced.
Page 23
AQA GCSE Biology – Unit 3 summary notes
13.7 How can we be sure we are using microorganisms safely?
 Microorganisms are organisms that can only be seen with a microscope.
 They are mainly single-celled:
o Bacteria
o Fungi
o Viruses
 Microorganisms need to be handled safely in laboratories.
 If they are contaminated, the other microorganisms that are present may be pathogens:
o They may produce harmful substances (toxins).
o They may cause disease.
 So it is only safe to use microorganisms if we have a pure culture containing only one
particular species of microorganism.
Culturing microoganisms
 To study microoganisms, they need to be cultured.
 This provides them with the conditions they need to reproduce quickly.
 A typical culture medium contains:
o Carbohydrates as an energy source.
o Mineral ions.
o In some cases supplementary protein and vitamins.
 These nutrients are often contained in an agar medium – a type of jelly made from
seaweed.
 When agar is heated up it is liquid.
 It can be poured into a Petri dish.
o A circular plastic or glass dish with a lid:
 The agar solidifies when left to cool.
Growing pure cultures
 Petri dishes and culture media must be sterilised before use to kill unwanted
microorganisms.
o An autoclave is used.
o This uses steam at high pressure to kill all microoganisms.
 Inoculating loops are used to transfer microorganisms to the media.
o These must be sterilised by passing them through a flame:
Page 24
AQA GCSE Biology – Unit 3 summary notes
 The lid of the Petri dish should be sealed with adhesive tape to prevent microorganisms
from the air contaminating the culture.
 It should not be sealed all the way round the edge, as this would prevent oxygen getting
in.
 In school and college laboratories, cultures should be incubated at a maximum
temperature of 25 °C.
o This greatly reduces the likelihood of pathogens growing that might be harmful to
humans.
o In industrial conditions higher temperatures can produce more rapid growth.
Page 25