GCSE Revision 101
Guide to Biology
AQA Specification A
Unit Biology B3 BLY3H
Daniel Holloway
Contents
1 Exchange of Materials
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2 Transporting Substances Around the Body
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3 Microbiology
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End of Unit Questions
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Copyright © 2009 Daniel Holloway
Significant contribution Nelson Thornes AQA Science [GCSE Biology]
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Active Transport
Two of the main ways in which diffused substances are transported across cells are osmosis
and diffusion. Diffusion is the movement of a fluid across a concentration gradient useful to
the cells; osmosis is the movement of water across a semi-permeable membrane. However,
sometimes substances need to be transported against a concentration gradient or
membrane, which is when active transport takes place.
By active transport, cells are able to move substances from an area of low concentration to
an area of high concentration. This is what is meant by moving against the gradient. Because
the substances are being transported against a gradient, energy is required for an active
transport system to carry a molecule across the membrane and then return to its original
position (see below diagram).
The energy required for active transport to take place comes from cellular respiration. The
rate of active transport and rate of respiration in cells are closely linked. The process of
respiration releases energy – so in other words, the more respiration happening, the more
active transport is taking place. This is why cells involved in active transport (e.g. root hair
cells and gut lining cells) usually have a lot of mitochondria to provide the energy needed
from respiration.
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Active transport is an important process in plants. The uptake of mineral ions through the
soil requires active transport because the ions are found in very dilute solutions, whereas
the solution inside the plant cells is a lot stronger. This means the ions have to be taken in
against the gradient (from dilute to concentrated). Glucose is moved out of the gut and
kidney into your blood, even though that is against a gradient. Active transport is also used
in marine birds and reptiles, because they consume large amounts of salt when they drink
water, and as the kidneys cannot get rid of it all, they have salt glands which use active
transport. Without the ability for the cells to do active transport, these marine animals
would die, so active transport is essential to their lives.
Gaseous Exchange
We require a constant supply of oxygen to allow for respiration. Breathing in and out takes
in oxygen as a supply for the cells and removes the waste carbon dioxide produced by the
cells. The lungs (found in the thorax) are protected by the rib cage. The lungs are separated
from the digestive organs, found in the abdomen, by the diaphragm.
When you breathe in…
your ribs move UP and OUT
 your diaphragm flattens
 air is pulled INTO the lungs

When you breathe out…
your ribs move DOWN and IN
 your diaphragm returns to its
domed shape
 air is forced OUT OF the lungs

The lungs have been adapted especially for
making gas exchange more efficient. They are
made up of clusters of alveoli, which are tiny air
sacs with large surface areas, and are kept
moist. They also have a rich blood supply, which
maintains a concentration gradient in both
directions. Oxygen constantly being removed
from the blood and carbon dioxide constantly
entering the lungs means that gas exchange
happens at the highest concentration gradients
to make it rapid and effective.
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The diagram here shows an alveolus (singular of alveoli). It has been adapted to make
gaseous exchange as efficient as possible in the following ways:




spherical shape gives large surface area
moist surface makes diffusion easy as gases can dissolve
thin walls make diffusion easy
good bloody supply (explained before)
The following table shows the effectiveness of the gas exchange in the lungs:
Nitrogen, N
Oxygen, O
Carbon Dioxide, CO2
Air Breathed In
Approx 80%
20%
0.04%
Air Breathed Out
Approx 80%
16%
4%
The Human Gut
The food we eat is broken down in the gut. It forms simple sugars, such as glucose, amino
acids, fatty acids and glycerol. These products are of no use in the gut, and if they stayed
there would simply be removed with faeces. This is why, via a combination of active
transport and diffusion, the molecules from food enter the bloodstream. This is why food is
broken down during the digestion process. After being broken down, the food molecules are
small enough to pass through the walls of the small intestine and into the blood vessels.
They can move this way because there is a very high concentration of food molecules in the
gut, and a very low concentration in the blood, so the process here is diffusion. They move
along a very steep concentration gradient.
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The lining of the small intestine is folded into thousands of tiny villi (above). These greatly
increase the uptake of digested food by diffusion. This is because only a certain amount of
digested food molecules can diffuse over a certain surface area at a given time – so the villi
increase the surface area dramatically.
Diffusion is very rapid and efficient in the gut, because, as with the lungs, it has a rich blood
supply – so digested food molecules are carried away the second it diffuses from one side to
the other. Therefore, a steep concentration gradient is constantly maintained.
Material Exchange in Other Organisms
Fish have protective scales all over their bodies
which prevent them from directly taking in oxygen
from the water – so they have gills, made of very
thin layers of tissue with a rich blood supply. The
gills are thin so there is less surface area for the
gas to diffuse across. Fish do not need to worry
about keeping the gills moist, considering the live
in water.
Gills do not work in air, and “suffocate” out of water – because if they are not kept moist
constantly the gills stick together and there isn’t enough surface area for the fish to get
enough oxygen from the air to survive.
Frogs begin their life cycles as tadpoles. All tadpoles
begin with a set of gills to help them survive, seeing
as they spend their entire lives in water – but when
they become adult frogs, they can live on land and
yet breathe in water at the same time. Tadpoles get
their oxygen from diffusion through their gills, in the
same way they lose carbon dioxide along the
concentration gradient. But as the creature grows
into an adult frog, the gills are reabsorbed back into the body – and the gills aren’t there
anymore. We say the tadpole has undergone metamorphosis. An adult frog has moist, thick
skin with a rich blood supply. The majority of its gaseous exchange takes place through the
skin. However, gas exchange can be done through the mouth also, using the frog’s simple
lungs system. In water, all respiration is done through the skin.
Insects’ muscles require a lot of oxygen because they are so active. But extremely little, or
no, gaseous exchange can take place through the tough outer covering of an insect, so they,
like us, have an internal respiratory system which supplies oxygen to all the cells which need
it and removes carbon dioxide. Insects have many spiracles, which are tiny openings – they
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open when oxygen is needed and close when it is not. This also prevents water loss, much
like plant stomata. Spiracles lead to a tube system which delivers the oxygen straight to the
tissues where it is needed. Most of the gas exchange takes place in the tracheoles, tiny
tubes which are freely permeable to gases. They are moist and constantly have air pumped
in and out of them by the insect to maintain a steady concentration gradient.
Material Exchange in Plants
All plants require carbon dioxide and water for photosynthesis. The carbon dioxide is
obtained via diffusion through the leaves. The flattened shape of the leaves increases the
surface area for diffusion to take place across. Leaves are usually flat, too, so that the
distance between the air and the photosynthesising cells is as short as possible.
A problem is that water is always
being lost by evaporation. So
allowing carbon dioxide in will
also lose water vapour. However,
the plant does not need carbon
dioxide all the time, because at
night there is no sunlight – so
photosynthesis cannot take place.
So they have openings known as
stomata which can open and close
at specific times to allow carbon
dioxide in and out. Another
adaptation is that they have a waxy cuticle covering them, which is both gas-proof and
waterproof.
Roots have been adapted for uptake of water and mineral ions. Water is vital for shaping
cells and for photosynthesis. Minerals are needed to make proteins and other chemicals.
The roots themselves are thin and have a large surface area. The root hair cells have also
adapted to increase surface area and increase efficiency of water uptake. The cell
membranes of root hair cells have microvilli which further increase surface area for
diffusion and osmosis. The distance between here and the xylem (transport tissue for the
water) is minimal, also.
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Transpiration
The loss of water vapour through the surface of
the leaves is called transpiration. As water is lost
through the opening in stomata, more water is
pulled up through the xylem to take its place. This
constant movement of water around the plant is
known as the transpiration stream. Because it is
all caused by evaporation, anything affecting
evaporation on a plant will also affect
transpiration. Factors which increase evaporation
will also increase transpiration. Sunny and warm
conditions increase rate of photosynthesis, which
means more carbon dioxide is needed, which
means stomata are opened, which means water is lost – so these conditions also increase
transpiration rate: hot, dry and windy.
Other than having a waxy cuticle, another adaptation to help with the problem of water loss
is that a plant can wilt. This happens when water is being lost faster than it is being gained.
Wilting of leaves involves them collapsing and hanging downwards to prevent much water
loss by minimising the surface area.
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The Circulatory System
The blood circulation system we have is made up of three main
components: blood vessels, the heart and the blood. It is made up
of two different blood systems – a double circulation. The diagram
shows that one transports blood from the heart to the lungs and
back again, the other takes blood around the rest of the body.
Having a double circulation is vital in animals like ourselves because
we are constantly active and in need of a rich blood supply – and
with this system, we are constantly receiving oxygenated blood from
the lungs which is sent around the body in one cycle.
There are three main blood vessels in the system, which have all adapted to carry out
specific functions. The diagram below shows each of them…
The arteries (left) carry blood away from the heart to the organs in the body. This is usually
oxygenated blood, explaining the red tubes. When you feel your pulse, that is the arteries
stretching as blood is forced through them and returning back into their original shape.
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The veins carry blood towards the heart, usually low in oxygen and hence are deep purplered in colour. No pulse in veins, but they do contain valves usually which prevent the
backflow of blood.
The capillaries are found in junctions between the arteries and veins. These are found in
huge networks. The walls are a single cell thick so that substances which need to get out of
the blood and into body cells can easily via diffusion.
The Human Heart
Our hearts are made of two pumps, for the double circulation. These together beat around
seventy times a minute. The walls of the heart are made pretty much entirely from muscle,
which gets oxygen from the coronary blood vessels.
Blood Transport
The liquid part of our blood is called plasma. It transports red blood cells, white blood cells
and platelets. White blood cells (for immune system) and platelets (blood clotting) are not
involved in transporting materials around the blood – it is the red blood cells and plasma.
Blood plasma is a yellow liquid which transports all blood cells and other substances around
the body. Carbon dioxide produced in the organs is carried in plasma back to the lungs. It is
the red blood cells which give blood its red colour.
Urea, a waste product formed in the liver is carried in the plasma to the kidneys. In the
kidneys, urea is removed from the blood and transformed into urine. All the small, soluble
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products of digestion pass into the blood from the gut, which are carried around the body
by plasma to the individual cells which need the certain substances.
Red blood cells are the most common cell type in the
human body. There are around 5 million per each square
millimetre of blood. The function of red blood cells is to
pick up oxygen from the lungs and deliver it to cells and
tissues where it is needed. Their adaptations to improve
efficiency at their job include:
being shaped like biconcave discs (concave/pushed in on both sides), this increases
surface area : volume ratio over which diffusion takes place
 being packed full of haemoglobin, pigments which can carry oxygen
 having no nucleus, more room for haemoglobin and diffusion!

A haemoglobin is a large protein molecule folded around four iron atoms. In an area of high
oxygen concentration, haemoglobin can react with oxygen to form oxyhaemoglobin, which
is bright red in colour, ergo blood being the colour it is. Oxygen is delivered by red blood
cells, we know, to where it’s needed, but the way it is done is via the reverse reaction to the
formation of oxyhaemoglobin. It happens when the oxyhaemoglobin arrives at an area of
low oxygen concentration (i.e. where it is needed), and so the reaction reverses – the
oxygen splits and diffuses into the cells where it is required. Lone haemoglobin after is
purple/deep red, which explains the colour of veins.
Because haemoglobin are made from iron, a diet lacking iron can results in anaemia making
you pale and have no energy. This is because your body cannot make enough red blood cells
and you cannot carry enough oxygen around the body for your needs.
Exercise and Your Body
Muscles in our bodies need a lot of energy. They are made of protein fibres which contract
when they receive energy from respiration. They contain many mitochondria to supply this
energy. Muscles also contain glycogen stores – glycogen is a carbohydrate which can turn
into glucose. This supplies the fuel needed for cellular respiration when muscles contract:
glucose + oxygen → carbon dioxide + water (+ energy)
When resting, your muscles are using up a certain amount of oxygen, but when exercising
your muscles contract harder and faster, so need more glucose and oxygen to supply their
energy needs. More carbon dioxide is obviously produced – which has to be removed to
keep muscles working efficiently. So during exercise…
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heart rate increases and arteries dilate – these changes increase the blood flow to
exercising muscles, this in turn increases oxygen and glucose supply and increases the
rate of carbon dioxide removal
 breathing rate increases, and breaths become more deep – these changes mean you
breathe more often and draw more air into the lungs with each breath, again increasing
amount of oxygen being brought into the body and picked up by red blood cells – and
this oxygen is carried to the exercising muscles

Exercise is definitely very beneficial to us when done regularly. Regular exercise increases
the size of both the heart and the lungs, and they develop a bigger and more efficient blood
supply – meaning they will begin to function as effectively as possible, whether you are
exercising or not.
Anaerobic Respiration
When you are doing extremely vigorous exercise over a long period of time, the muscles
need so much oxygen that even an increase in breathing rate and heart rate does not supply
sufficient amounts. So respiration which does not involve oxygen must be done – anaerobic
respiration. Muscles only switch to anaerobic respiration when they have been exercising
for a long time and fatigue. This is because anaerobic respiration is not as efficient as
aerobic respiration, because the glucose molecules are not completely broken down and so
less energy is released. The end products of anaerobic respiration are lactic acid and water:
glucose → lactic acid (+ energy)
After finishing a lot of exercise, you are out of breath for quite some time – this length of
time depends on your fitness. The reason your body does this post-exercise is because your
body needs to get rid of this waste lactic acid, which would otherwise cause you problems –
but it cannot be breathed out unlike carbon dioxide. As a result, the lactic acid has to be
broken down into carbon dioxide and water (the products of aerobic respiration) which
requires oxygen. The amount of oxygen required to break down all of the lactic acid is called
the oxygen debt. Even though your muscles have stopped exercising, this is why your heart
rate and breathing rate remain high for a while after the exercise – to supply the oxygen
needed to repay the oxygen debt. Oxygen debt repayment:
lactic acid + oxygen → carbon dioxide + water
Human Kidneys
Your kidneys are vital in maintaining homeostasis (see B2). For example, they filter out urea
and remove it in urine because urea is poisonous. Another example is that water balance in
the body must be maintained because too much water (turgid) or too little water (flaccid) in
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cells can destroy them – so the kidneys can remove excess water and release it from the
body in urine. Similarly, the kidneys can remove excess salt from the body in the same way.
The kidneys filter the blood and then reabsorb (take back) everything your body needs. So:
sugar (glucose), amino acids, mineral salts and urea all move out of the blood and into the
kidneys along a concentration gradient. The blood cells are too big to pass through the
tubules and so are left behind. Next, ALL of the sugar is reabsorbed back into the blood by
active transport. But the amount of water and the dissolved mineral ions which are
reabsorbed vary. It depends on how much of each is needed by the body – this is selective
reabsorption.
Urine contains waste urea along with excess mineral ions and water not needed by the
body. The quantities vary depending on how much you have taken in and given out. For
example, on a hot day if you drink little and exercise a lot, you will produce very little urine,
which will be concentrated, a dark yellow. Whereas if you drink lots and do no exercise on a
cold day, you will produce a lot of dilute, pretty colourless urine.
The Dialysis Machine
The human kidneys are not immune from damage. And when they are damaged and stop
functioning, those toxins like urea stop being removed from the body, leading to death.
There are two ways we can deal with this problem, the first being dialysis. The machine
used in dialysis is called the dialysis machine, and relies on a process called dialysis to clean
the blood.
A person’s blood leaves their body and flows into the machine, through partially permeable
membranes. After the membranes comes the dialysis fluid, which contains a certain
concentration of substances to ensure diffusion of unwanted substances from the blood
into the fluid. However, glucose remains in the blood.
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The dialysis machine
prevents unwanted
substances from building up
and restores them to normal
levels, so the patient can live
a normal life – but they will
build up again after a couple
of days, which means regular
dialysis must be done, which
sometimes means dialysis
machines are fitted in the
patients’ homes. They have
to remain attached to the
machine for about eight
hours each time.
But it is essential the patient does not lose vital substances from the blood like glucose and
important mineral ions. The way this is done is by having the dialysis fluid at exactly the
right concentration so there is no net movement of glucose and mineral ions from blood
plasma out into the fluid. It also contains the normal content of mineral ions, so that any
excess mineral ions are lost by diffusion, but no more.
There is no urea in the dialysis fluid, so there is a very strong concentration gradient for the
urea – so it simply leaves the blood in its entirety.
Disadvantages of these machines include:
repeated use at 8 hours per use
 must also follow a strict, healthy diet
 after some years, the levels can be hard to maintain

Kidney Transplants
The other solution to the problem of kidney failure is a kidney transplant. A replacement
kidney is required for this, which must be healthy and donated by a donor. The patient
receiving the kidney is the recipient, and the kidney is attached to the normal blood vessels
of their groin. Hopefully, it would then fully function – job done.
Unfortunately, the majority of the time this is not the case. One issue is that the kidney the
recipient receives was not originally theirs, so the antigens on its surface will differ from the
antigens they already have. Of course, the problem with this is that the recipient’s immune
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system may reject the new kidney – which means your body will destroy it. During a
transplant, everything is done to prevent such a thing, but it is always a risk.
There are certain things that can be done to minimalise the risk of rejection. The main
method is to use a donor similar to the recipient in terms of body tissue, for example blood
type. This means they will share some of the same antigens. Another method, which sounds
more worrying, is using immunosuppressant drugs. These drugs are given to recipients
which suppress (shut down) their immune system – for the rest of their lives. However, as
these drugs are developed and get better, the need for matching tissue type is decreasing in
importance.
Of course receiving immunosuppressant drugs means you are prone to disease and your
body cannot deal well with any infection once caught. But most people think it is a price
worth paying since no kidney would mean certain death.
A downside to kidney transplants is the fact that they are not permanent. A borrowed
kidney will not last forever, in fact on average of about nine years, before shutting down, in
which case the patient must return to hospital dialysis until another kidney is found and
another transplant operation takes place.
A reason why transplant is favourable compared to dialysis machines is that once you have a
kidney via transplant, you can proceed to live a normal life and not worry about it – you can
eat what you want and don’t have to attend regular dialysis sessions. However, a
disadvantage is rejection – you will have to take your medicine everyday for the rest of your
life in case it is ever rejected by the immune system. A reason why dialysis is favoured is
because it is much more readily available. Waiting lists for kidney recipients can go on for
years. And, one of the main problems with kidney transplants is finding a suitable donor,
which often proves hard.
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Growing Microbes
The study of microorganisms is called microbiology. These include bacteria, viruses and
fungi, all too small to be seen by the naked eye. Many microorganisms can be grown in a
lab, where we can research them and find out what they need to survive – and learn which
are useful for us and which want to kill us.
Learning more about microorganisms requires culturing them (i.e. growing large numbers to
see their behaviour as a colony). For this to happen, you must provide them with everything
they need: a culture medium with carbohydrate(s) to act as an energy source, and
necessary nutrients (mineral ions, and sometimes proteins and vitamins – included). The
nutrients are usually contained in an agar medium – a substance which dissolves in hot
water and will set to form a jelly. Hot agar containing the nutrients is poured into a Petri
dish and left to cool before any microorganisms are added. Warmth and oxygen is usually
needed for growth too.
Safety in the Lab
It is essential that all microbe culture is done carefully, even when growing the safe
microorganisms. This is because they can be pathogenic, and the safe ones can mutate to
become harmful pathogens. The other problem is cross-contamination between
microorganisms, which can upset experiments – but more importantly is a health and safety
issue when they get onto human skin and you bring them everywhere with you.
Yeast
Yeast is probably the most important microorganisms for us.
Yeasts are single-celled organisms with a nucleus, cytoplasm
and membrane surrounded by a cell wall. They reproduce by
asexual budding (splitting into two to form two new yeast
cells).
Provided with a lot of oxygen, yeast cells will respire
aerobically. They break down sugar as an energy source,
producing the waste products carbon dioxide and water. However, sometimes there is a
lack of oxygen, so they respire anaerobically, which produces ethanol and carbon dioxide.
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Ethanol is alcohol. And this process of anaerobic respiration in yeast cells is called
fermentation.
Like with humans, aerobic respiration is better for the cells because it produces more
energy. This energy allows them to grow and reproduce; however, when there is a large
number of yeast cells, they can survive longer without oxygen and so can respire
anaerobically, breaking down sugars into ethanol.
Yeast is used to produce bread and alcoholic drinks. In bread production, the yeast grows
and respires – producing carbon dioxide which causes the bread to rise. The gas bubbles
expand when baked due to the high temperatures, giving the bread its light, wafery texture.
All yeast cells are killed by the heat in the cooking process.
We can make beers and wines using yeast. Making beer relies on the process of malting,
where barley grains are soaked in water to keep them warm. Germination begins and
enzymes break down the starch in the grains into a sugary solution. This solution is
extracted and used as an energy source for the yeast. The yeast and sugar mixture is
fermented to produce alcohol, when hops are often added to give the drink its flavour. The
beer is then left to settle, clear and develop fully its flavour.
Making wines however, uses the natural sugars found in fruit, e.g. grapes, for the yeast’s
energy source. The grapes would be pressed for their juice, which is mixed with yeast and
water. The yeast is then left to respire anaerobically until all of the sugar is used up. The
wine is later filtered to remove the yeast, and stored in bottles, where it is left for some
time to mature.
Food Production Using Other Bacteria
Yoghurt always used to be made by fermenting whole milk, but nowadays semi-skimmed,
skimmed and even soya milk can be used. Yoghurt is formed by the bacteria action on the
lactose (milk sugar). You can make yoghurt by:
1 adding a culture of the right type of bacteria to warm milk
2 keeping the mixture warm so the bacteria grow, reproduce and ferment
3 as the bacteria break down the lactose, lactic acid is produces (this gives yoghurt the
sharp, tangy taste) – this process is lactic fermentation
4 the lactic acid causes the milk to clot and solidify to form a yoghurt
5 further bacterial action gives the yoghurt its creamy texture
The same bacteria used to make yoghurt will also keep it from going off – yoghurts tend to
last for around three weeks, whereas fresh milk lasts a couple of days. Colourings,
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flavourings and other additives can be added to the yoghurt to improve its taste,
appearance and texture after the above steps.
Cheese-making also depends on the bacterial action in milks. They change the texture, taste
and help preserve the milk. Some cheeses can last decades without decaying. The stages in
cheese production are fairly similar, although a different type of bacteria is added. This
bacteria still converts the lactose into lactic acid as before, but in makes far more lactic acid
– to the extent that the solid parts (the curds) have solidified almost completely, unlike with
yoghurts. Enzymes are also added to help separate the milk – when it has completely
curdled, you can see separate curds from the liquid whey. The curds can then be used to
make cheeses, whey generally goes on to be animal feed.
Next, the curds are mixed with other bacteria and moulds and left to dry out. The bacteria
and moulds added at this stage affect the end texture and flavour, depending on how much
they allow it to ripen – so the types and amounts are particularly important here. The
ripening stage may take months or even years depending on the type of cheese being made.
Mass Production of Microbes
We need microbes in large quantities for production of drugs, like antibiotics, and food. To
grow microbes on an industrial scale, large vessels called fermenters are used. These have
been developed to prevent occurrences which stop bacterial growth. They react to changes,
to try and maintain a stable environment (i.e. if external temperature increases, the
fermenter will decrease internal temperature to restore balance). Industrial fermenters
usually have:
1 An oxygen supply so the microorganisms can respire
2 A stirrer to keep the microbes in suspension – this maintains a constant temperature
and makes sure that the oxygen and food are evenly spread out throughout the
culture
3 A water-cooled jacket which removes excess heat produced from the respiration
4 Measuring devices for pH and temperature so changes can be made if necessary (i.e.
when a dependent change is caused)
The graph below shows bacterial growth for real-life conditions, not the suitable,
convenient conditions we give microbes in an industrial fermenter.
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Mycoprotein
A new substance was discovered not long ago, a food based on fungi, and it is called
mycoprotein. It is produced using the fungus Fusarium, which grows and reproduces very
rapidly based on a cheap energy supply (an inexpensive sugar syrup made from waste
carbohydrates) in a large fermenter. It does require aerobic conditions to grow. Its mass
doubles every 5 hours or so, and this biomass is harvested, purified and dried to leave
mycoprotein. On its own, it is pale yellow in colour and tastes faintly of mushrooms – but a
range of colours and flavours can be added to it to enhance it. Mycoprotein serves as a highprotein, low-fat meat substitute. This means it is good for dieters and vegetarians.
Antibiotic Production
In 1928, Alexander Fleming left some bacteria culture he had been growing on some plates
near an open window. When he returned to look at them, microbes had grown, but there
were patches of mould surrounding the agar and bacteria had stopped growing there.
Whatever blew in from the wind had killed the bacteria. He analysed the mould and found it
to be the fungus and Penicillium notatum extracted penicillin from it which he used to treat
a wound. However, it was hard to extract much penicillin without the available technology,
and even when extracted was very unstable, so he gave up on penicillin and returned to
other areas of research.
It was Howard Florey and Ernst Chain who returned to penicillin during the Second World
War and extracted enough to fully understand it. Firstly, they used it on animal tests, which
were successful, so they tried it on a policeman dying from a blood infection. He recovered,
but died later when the penicillin ran out. However, several months later they were
prepared and saved the life of a child. Unfortunately, Fleming’s original mould was hard to
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harvest on a mass scale, but a mould growing on a melon was discovered which yielded
more than 200 times the penicillin from before. It was grown in deep tanks industrially, so
that by 1945 enough was being produced to treat 7 million people a year.
Modern Penicillin Production
Nowadays we use modern strains of
the Penicillium mould which give
even higher yields. We grow the
mould in a sterilised medium which
contains sugar, amino acids,
mineral salts and other nutrients.
The mould grows rapidly in the first
forty hours of production, where
most of the nutrients are used up.
Only when most of these nutrients
have gone can penicillin begin to produce, which explains why there is a 40 hour lag
between the start of fermentation and the start of penicillin production. Over the next 140
hours, broth is removed and nutrients are repeatedly added. This allows us to get the
maximum yield of the drug.
Biogas
A flammable mixture of gases, known as biogas, forms when bacteria break down the waste
material of dead animals or plants in anaerobic conditions. The main component is
methane, although the contents varies. The methane content tends to be around 50 to 80
per cent of the gas, the rest is made of carbon dioxide, water, hydrogen and hydrogen
sulphide.
Animal waste, dead animal and plant material and garden waste all contain carbohydrates
which make them good energy sources for biogas generators. They tend to work best at
around 30°C so are usually in hot countries, although the reactions which take place are
exothermic, so if kept insulated, the generator can still be in a cold country.
On average, every 10kg of dry dung can produce 3 cubic metres of biogas. That 3m³ can be
three hours of cooking, three hours of lighting or 24 hours of running a refrigerator. Another
advantage of these generators is that the other product, the waste, can be used as a
fertiliser.
In China’s biodigesters, they will put anything in there – vegetables, human and animal
faeces and urine, etc – which produces low quality gas but high amounts of fertiliser.
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On the other hand, in India (where there are social taboos against using human waste), only
animal waste is fed in – which produces high quality gas, but very little amounts of fertiliser.
Ethanol
A crop like sugar cane can grow fast – roughly 4 or 5 metres a year, and has a juice rich in
carbohydrates (like sucrose). Maize (or sweet corn) can have its starches broken down into
glucose by carbohydrase enzymes. If the sugar-rich products from sugar cane or maize are
fermented with yeast anaerobically, the sugars break down to give ethanol and water.
Ethanol can be extracted by distillation and used as a car fuel. Many cars run on a mixture
of petrol and ethanol, which can prove cheaper than fully petrol.
Ethanol is an ideal fuel in the sense than it does not produce toxic gases when burned; it
does not pollute as much as other fuels which produce carbon monoxide and sulphur
dioxide; and it can be mixed with petrol to form gasohol (although this means there is still
half the problem with the petrol side of the mixture). Using a fuel like ethanol is called
carbon neutral because you are not contributing to carbon dioxide levels in the atmosphere
by using it. The original plants to make ethanol took carbon dioxide from the air, and you
are returning the same amount.
Gasohol is in common use in the USA, and is around 90% petrol, 10% ethanol, and most of
the ethanol comes from America’s own maize crops. But they don’t have that many, like us,
so making enough ethanol will always be a struggle for MEDCs (who don’t have the
resources) and LEDCs (who don’t have the money). This is the drawback to ethanol as a
biofuel.
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B3-1 : Exchange of Materials
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Explain the process of active transport
How does the rate of respiration affect the rate of active transport?
Where does the energy needed for active transport come from?
Explain why plant roots use active transport
What gaseous exchange takes place in the lungs?
When you breathe in/out, what happens to your ribs and diaphragm?
Explain the function of alveoli
Name the projections that line the small intestine wall
Explain the function of fish gills and how they have adapted to better perform at
their job
Name and explain the process undergone by tadpoles when becoming adult frogs
How does the respiratory system of an insect work?
Explain two adaptations of a plant to reduce water loss
What is the function of the guard cells in a leaf?
Explain the process of water and mineral ion uptake through the roots of a plant
What is meant by the term transpiration?
What is the constant movement of water called?
Name two environmental factors which affect transpiration
B3-2 : Transporting Substances Around the Body
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What three features are there to our circulatory system?
Why do we have double circulation?
Explain the features of arteries, veins and capillaries
What colour is normal blood plasma?
Name and explain three adaptations of the red blood cell which help it better
perform at its job
What is oxyhaemoglobin?
When you exercise, how does your body respond?
What are the benefits of regular exercise?
Write the word equation for anaerobic respiration
Why is aerobic respiration favourable?
How is lactic acid removed from the body?
Explain the term “oxygen debt”
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What is the function of the kidney?
Explain the process of selective reabsorption
Describe what is contained in human urine
What is kidney dialysis?
Why must air bubbles be removed in the dialysis machine?
Why might some people prefer a kidney transplant?
Explain the drawback(s) to a transplant
Why might it be hard to find donors for a kidney transplant?
B3-3 : Microbiology
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How can we grow simple microbes in a lab?
Why must safety precautions be taken when doing this?
What does fermentation mean?
Explain the function of yeast in bread production
Describe the process of beer production
Describe the process of wine production
How is yoghurt made?
How is cheese made?
Name the equipment used to industrially produce microbes
What features do these have to help grow microbes well?
What is mycoprotein?
Explain why it is suitable for vegetarians and dieters
How was penicillin discovered?
Which two scientists recovered the penicillin work after it was abandoned?
Explain the process of modern penicillin production
What is biogas, and what is it composed of?
Explain what a biogas generator is?
What are the two outputs of a biogas generator and what are they used for?
What can sugar cane and maize be used for?
Why is ethanol a good fuel to use?
What are the drawbacks to ethanol as a fuel?
Explain the term “carbon neutral”
Describe the advantages to using gasohol
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