The Need for Specialised Exchange Surfaces

Single celled organisms

• exchange gases, nutrients and waste products across outer surfaces because they have a larger SA:V (surface area to volume ratio).
• low metabolic activity—diffusion via outer surface meets demands.

Multicellular organisms

• smaller SA:V
• higher metabolic demand
• outer surface not large enough for diffusion to meet demands
• substances have to travel further to reach the centre of the organism
• require specialized exchange surfaces
• specialized exchange surfaces: surfaces over which materials are exchanged for another material.

Features of good exchange surfaces

• increased SA—overcomes small SA:V of organism
• thin layers reduce distance for diffusion
• good blood supply creates a steep concentration gradient
• ventilation—maintains steep concentration gradient for movement of gases

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