- 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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- 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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