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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

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the following ways:
spherical shape gives large surface area

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…

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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 open when oxygen is
needed and close when it is not. This also prevents water loss, much like plant stomata. Spiracles lead…

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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.…

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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,…

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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…

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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…

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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…

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adding a culture of the right type of bacteria to warm milk
keeping the mixture warm so the bacteria grow, reproduce and ferment
as the bacteria break down the lactose, lactic acid is produces (this gives yoghurt the sharp,
tangy taste) ­ this process is lactic fermentation
the lactic acid…

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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, lowfat meat substitute. This means it is good for dieters and
Antibiotic Production
In 1928, Alexander Fleming left some bacteria culture he had…


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