Topic 3: Exchanges Text book – section 6&7

Exchanges between organisms and their environment Organisms need to transfer materials between their external and internal environment The bulk of an organisms body = its cells = its volume The exchange happens at an ‘exchange surface’

Surface Area to Volume Ratio Surface area has to be large enough to cope with its volume Heat loss is also affected by surface area to volume ratio Size and metabolic rate affect the amount of exchange that occurs

Surface Area to Volume Ratio Organisms with a high metabolic rate need more exchange to occur and so need a larger surface area to volume ratio Surface area can be increased at exchange surfaces – e.g. villi in intestines, air spaces in leaves, alveoli in lungs or gills in fish. These surfaces are also thin to reduce the diffusion distance. Because they are thin they easily dry out so are protected inside the organism

Mass Transport Systems Most cells are too far away from exchange surfaces in large organisms and so ‘Mass transport systems’ are needed to move materials Mass transport systems also maintain diffusion gradients to allow exchange to continue Small flat animals have a large enough surface area to volume ratio to cope with exchange and so don’t need a mass transport system

Calculating surface area to volume ratio Cubes, spheres and cylinders