Why transport systems are needed
Simple, unicellular organisms
A single-celled organism such as Amoeba or a bacterium relies entirely on diffusion to move oxygen, carbon dioxide, food molecules and waste in and out of the cell. No heart, no blood, no vessels — and it works fine. Two reasons:
- Large surface area to volume ratio. The whole cell surface membrane is available for exchange, and there is only a tiny volume of cytoplasm to supply.
- Short diffusion distance. No part of the cell is far from the surface, so substances reach the centre quickly enough to keep the cell alive.
Add to that a low metabolic rate overall (a small organism has small total demands) and diffusion alone meets its needs.
Multicellular organisms
As an organism gets bigger, its volume increases faster than its surface area, so the surface area to volume ratio falls. That causes two problems at once:
| Problem | Consequence |
|---|---|
| Small surface area compared with volume | Not enough surface for all the oxygen and food the organism needs to diffuse in |
| Long diffusion distance to inner cells | Diffusion is far too slow to supply cells deep inside the body |
| Higher metabolic demand (many active cells) | Oxygen and glucose used up quickly; waste such as carbon dioxide and urea builds up |
So multicellular organisms need:
- specialised exchange surfaces (lungs, small intestine, root hairs, leaves) to get substances into the organism, and
- a transport system to carry substances between the exchange surface and every cell — the blood circulatory system in humans, and xylem and phloem in flowering plants.
Common mistake
Don't just say "it's too big". State the biology: surface area : volume ratio is too small and the diffusion distance is too great, so diffusion alone would be too slow to supply the cells.
Examiner tip. Watch the command word. Describe = say what happens; Explain = give the reason why. If a question says "Explain why a large organism needs a transport system", a description of the blood system scores nothing — you must link small surface area:volume ratio and long diffusion distance to diffusion being too slow.