- in multicellular organisms , diffusion across the outer membrane is too slow, for two reasons:

some cells are deep within the body - there's a big distance between them and the outside environment.
larger animals have a low surface area to volume ratio - it's difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface.

- so rather than using straightforward diffusion to absorb and excrete substances, multicellular organisms need specialised exchange organs.
- they also need an efficient system to carry substances to and from their individual cells - this is mass transport. in mammals, mass transport normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body. it also carries hormones, antibodies and waste like carbon dioxide. mass transport in plants involves the transport of water and solutes in the xylem and phloem.

	Created by izzy smith about 7 years ago

(very brief) notes on size and surface area for the aqa biology spec.