The need for exchange surfaces
Living entity, eg animals, plants or microorganisms. must take in food,Gaseous element making up about 20% of the air, which is needed by living organisms for respiration. and water, and other essential substances, from the environment. Plants also need A gaseous compound of carbon and oxygen, which is a by-product of respiration, and which is needed by plants for photosynthesis. for A chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis. Algae subsumed within plants and some bacteria are also photosynthetic.. Organisms also need to remove waste substances.
Small organisms exchange these essential and waste substances between themselves and the environment. They do this over their body surface. Simple chemical substances can When particles spread out from a region of higher concentration to a region of lower concentration. in and out of their bodies.
Inside their bodies, in small organisms, substances don鈥檛 have to move far.
The size of their surface, or surface area, defines how quickly they can absorb substances. The size of their volume defines how much of these substances they need.
Modelling cells
If we represent the cell of an organism by a cube:
This is what happens when the cube increases in size:
As the volume increases, surface area does not increase at the same rate.
As multicellular organisms increase in size, they face two problems:
| Problem | Solution to the problem | |
| Their surface area does not increase as fast as the volume | Insufficient surface area to meet their needs | Body systems that add additional absorbing area to exchange surfaces |
| Their volume increases | Diffusion is not quick enough to move substances to where they鈥檙e needed in the organism鈥檚 body | A transport system |
| Their surface area does not increase as fast as the volume | |
|---|---|
| Problem | Insufficient surface area to meet their needs |
| Solution to the problem | Body systems that add additional absorbing area to exchange surfaces |
| Their volume increases | |
|---|---|
| Problem | Diffusion is not quick enough to move substances to where they鈥檙e needed in the organism鈥檚 body |
| Solution to the problem | A transport system |