The Need for Specialised Exchange Surfaces

Oxygen, glucose and fatty acids are needed for respiration and carbon dioxide needs to be removed. Exchange can be passive (diffusion, osmosis) or active (active transport, endocytosis, exocytosis).The rate and amount of substance exchanged depends on the organism's requirements and rate of waste production will depend on the size and metabolic rate of the organism. The exchange surface is therefore adapted to meet the requirements of each organism.Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.In small, unicellular or multicellular organisms such as bacteria and protoctists with a high surface-area to volume ratio, the gas exchange surface is the cell membrane.Some small multicellular organisms such as flatworms, are able to perform gas exchange across the skin or cuticle that surrounds their bodies.