1. Gas Exchange

2. Aims Understand the relationship between the size of an organism and its surface area:volume. Understand the relationship between the surface area:volume and the need for a specialised gas exchange surface

3. Objectives by the end of the lesson you should be able to Explain the process of diffusion Explain why small unicellular organisms do not need exchange surfaces whilst larger multicellular organisms do Explain the relationship between size, surface area to volume ratio and rate of diffusion in a model cell.

4. Gas exchange surfaces This is a photomicrograph of an Amoeba Amoeba is an aquatic unicellular organism about 1mm in diameter The exchange surface is the cell membrane

5. Hydra Hydra are multicellular aquatic organisms about 10-13mm long They carry out gas exchange through their body surface

6. Earthworms Are terrestrial organisms although mainly subterranean About 80-120mm long They have a damp body surface and so need to live in damp earth The whole body surface is the gas exchange surface

7. Rabbits Rabbits are mammals They vary in size but are approximately 300mm long Terrestrial organisms with an internal specialised gas exchange surface-the lungs

8. Small or large organisms Small and unicellular organisms do not need a specialised gas exchange system Their body surface is large enough to supply all the oxygen they need Larger, multicellular organisms have a larger surface area but they also have a larger volume The size of surface is not enough to supply the needs of the volume so special exchange surfaces like lungs or gills are needed

9. Surface area to volume ratio The size of an organisms surface divided by the size of its volume gives the surface area to volume ratio The larger the organism, the smaller the surface area to volume ratio