All cells need to take in substances from their environment, and get rid of unwanted substances.
In a single-celled organism, this can happen quickly enough by diffusion alone. This
- no point in the cell is very far from the surface, so it does not take long for gases to diffuse from the cell surface membrane to the centre of the cell, or vice versa;
- the surface area to volume ratio of the cell is relatively large - that is, it has a large amount of surface area compared to its total volume.
In a large organism, diffusion is no longer sufficient. This is because:
- the centre of the organism may be a long way from the surface, so it would take too long for substances to diffuse all that way;
- the surface area to volume ratio is much smaller - that Is, It has a small amount of surface area compared to its total volume.
- they have transport systems that carry substances by mass flow from one part of the body to another, rather than relying solely on diffusion;
- they increase the surface area of parts of the body involved in exchange with the environment, for example by having thin, flat leaves or by having a highly folded gas exchange surface.
(a) explain the need for transport systems in multicellular plants and animals in terms of size and surface area to volume ratios;