Biology 3 Revision Notes:Exchange of Materials

Notes on B3 Exchange of Materials

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REVISION NOTES: BIOLOGY 3
UNIT 1: 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 a fluid 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 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:
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 diluted 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.
Gas Exchange in the Lungs
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

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REVISION NOTES: BIOLOGY 3
UNIT 1: EXCHANGE OF MATERIALS
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.
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.…read more

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REVISION NOTES: BIOLOGY 3
UNIT 1: EXCHANGE OF MATERIALS
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.…read more

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REVISION NOTES: BIOLOGY 3
UNIT 1: EXCHANGE OF MATERIALS
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.…read more

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REVISION NOTES: BIOLOGY 3
UNIT 1: EXCHANGE OF MATERIALS
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.…read more

Comments

Swallowtail

A clear set of notes with colourful diagrams that can be used with AQA Biology Unit 3 on exchange surfaces.It is important to be able to label diagrams confidently. Try using the revision notes to make a mind map or a set of flash cards to  test your knowledge.

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