GCSE Biology Unit 3 Notes

Unit 3 notes

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B31: Exchange of Materials
Active Transport
Two of the main ways in which diffused substances are transported across cells are osmosis and
diffusion. Diffusion is the movement of particles across a concentration gradient useful to the cells
osmosis is the movement of water across a semipermeable 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 and
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
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.
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
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
havea rich blood supply, which maintains a concentration gradient in both directions. Carbon dioxide
constantly being removed from the blood and oxygen constantly entering the lungs means that gas
exchange happens at the highest concentration gradients to make it rapid and effective.
The diagram here shows an alveolus (singular of alveoli). It has
been adapted to make gaseous exchange as efficient as possible in

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Atmospheric Gas Air Breathed In Air Breathed Out
Nitrogen, N Approx 80% Approx 80%
Oxygen, O 20% 16%
Carbon Dioxide, CO2 0.04% 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.…read more

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Insects have many spiracles, which are tiny openings ­ they 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.…read more

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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...…read more

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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.…read more

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­ 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.…read more

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­ 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
But it is essential the patient does not lose vital substances from the blood like glucose and important
mineral ions.…read more

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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).…read more

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­ this process is lactic fermentation
the lactic acid causes the milk to clot and solidify to form a yoghurt
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, flavourings and other
additives can be added to the yoghurt to improve its taste, appearance and texture after the above
steps.…read more

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­ but a range of colours and flavours can be added to it to enhance it.
Mycoprotein serves as a highprotein, lowfat meat substitute. This means it is good for dieters and
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.…read more


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