multicellular organisms

• In multicellular organisms, diffusion across the outer membrane is too slow, for three reasons:

  1. Some cells are deep within the body - there's a big distance between them and the outside environment.
  2. 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.
  3. Multicellular organisms have a higher metabolic rate than single-celled organisms, so they use up oxygen and glucose faster.
• multicellular organisms need a specialised exchange surfaces due to a large diffusion distance, small surface area to volume ratio and a high metabolic rate