8.1 - 8.2 - Exchange and transport needs
Learn why organisms move substances into and out of cells, and use surface area : volume ratios to explain why large organisms need specialised exchange surfaces and transport systems.
8.1 — Why organisms transport substances
A living organism has two transport problems. Substances must cross a boundary between the organism and its environment, and they must then reach, or leave, the cells where they are used or formed. In a single-celled organism the cell surface is also the organism's exchange boundary. In a large multicellular organism, most cells are internal, so exchange with the environment and transport inside the body are separate stages.
The route and direction depend on the substance and the organism:
| Substance | Representative route | Why transport is needed |
|---|---|---|
| Oxygen | External environment -> gas-exchange surface -> internal transport system in a large animal -> cells | Oxygen is used in aerobic respiration, which releases energy for metabolic processes. |
| Carbon dioxide | Respiring cells -> internal transport system -> gas-exchange surface -> external environment in a large animal; atmosphere -> photosynthesising cells in a plant | Carbon dioxide made during aerobic respiration must be removed from animals. Plants also take in carbon dioxide as a reactant for photosynthesis. |
| Water | Soil -> roots -> the rest of a land plant, with water vapour leaving through leaves; food and drink -> gut -> blood and body cells in an animal, with excess water -> kidneys -> urine | Water is needed for cell reactions, as a solvent and for transport. Excess water must leave to help maintain a suitable water balance. |
| Dissolved food molecules | Small intestine -> blood -> body cells in an animal | Molecules such as glucose and amino acids are transported to cells for respiration, growth, repair or storage. |
| Mineral ions | Soil solution -> roots -> other plant tissues | Mineral ions are needed to make cell materials and maintain cell functions. |
| Urea | Liver -> blood plasma -> kidneys -> urine -> outside the body | Urea is formed in the liver from the breakdown of excess amino acids and must be excreted. The kidneys filter urea from the blood; they do not make it. |
These are routes through whole organisms, not claims that every step occurs by diffusion. Diffusion moves some substances across exchange boundaries; a transport system carries substances over the much larger distances inside a multicellular organism.
8.2 — When diffusion is not enough
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Particles move randomly in both directions, but while there is a concentration difference, more move from high to low concentration than from low to high. At equilibrium the particles do not stop moving; only the net movement becomes zero.
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