An organism needs to absorb and/or exchange substances with the environment and these substances have to be distributed within the organisms
Animal tissues require oxygen, the products of digestion and water. Carbon dioxide is released.
This tissues in flowering plants require oxygen (especially at night), carbon dioxide (during the day), inorganic ions and water.
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Absorptive or exchange surfaces
Absorption or exchange of substances occurs at moist permeable surfaces.
A small (aquatic) organism has a large surface area-to-volume ratio, so can supply sufficient oxygen via its surface to satisfy the metabolic demands of its body (volume).
An organism requires a specialised absorptive/exchange surface if it is terrestrial (with an impermeable surface), if it is large (with a small surface area-to-volume ratio) or if it has a high metabolic rate.
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Specialised absorptive surfaces
Specialised absorptive surfaces often involve outfoldings or infoldings, which increase surface area-to-volume ratio.
Thinness also increases the surface area-to-volume ratio and, in addition, reduces the diffusion path of substances
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Transport
A transport system is required in larger animals and plants to distribute substances from one site to another
Transport systems involve mass flow, the bulk movement of substances due to differences in pressure and a means of generating a pressure difference (often a pump).
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