Science Biology

Two of the main ways in which diffused substances are transported across cells are




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 tak

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Biology 1 Diffusion

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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