1. Topic 3: Exchanges Text book – section 6&7

2. 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’

3. 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

4. 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

5. 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

7. Calculating surface area to volume ratio
Cubes, spheres and cylinders