The relationship between the size of an organism or structure and surface area to volume ratio. Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as the ratio reduces. Candidates should be able to explain the significance of the relationship between size and surface area to volume ratio for the exchange of substances and of heat.

The bigger the surface area (SA) in comparison to the volume, the faster material can diffuse across an object or organism.

So it is crucial in living things that rely on diffusion to deliver oxygen for respiration, that the SA is large in proportion to size.

In bigger organisms the ratio reduces so that the SA is not big enough to allow for diffusion to supply the whole organism with oxygen. In addition to this the diffusion distance is larger and so it would take a long time for gas exchange to take place via diffusion.

To overcome this large organisms develop systems for gas exchange, for example lungs and gills. These systems will have a large surface area allowing for quick diffusion.

Other animals change their shape to increase their surface area, for example Flat-worms have a flat shape.

jochemnet

The graph displays the fact that as size increases, the SA gets smaller (in proportion to the volume.) As a consequence of this, diffusion will be slower in larger objects meaning heat and substances will take longer to diffuse to the middle of an object.