Unit 2.2

• At high surface area to volume ratios, skin can be used as a diffusion surface.
• Every cell needs an oxygen supply, glucose, water and the elimination of waste (eg CO2 or nitrogenous water)
• The larger the size of the organism, the larger the volume and so the more cells which require oxygen. As organisms increase in size, their surface area to volume ratios decrease.
• Gas exchange surfaces must be moist for gasses to be dissolved. Gasses can only diffuse once they are dissolved.
• An earth worm is a multicellular terrestrial organism. It has a circulatory system which Maintains the concentration gradients at its gas exchange surface. 5 psuedohearts push the blood around the dorsal and central blood vessles. Blood is quite close to the surface, so diffusion pathways are short. The blood has heamoglobin with a high affinity for oxygen.
• Flatwork are relatively inactive, and so require less oxygen. They are flat and so have a high surface area to volume ratio, so its gas exchange surface is its external skin. It is flat, so diffusion pathways are short.
• The concentration gradient affects the rate of diffusion. The steeper the gradient, the faster the rate.
• Surface area will affect the diffusion rate. The higher the surface area, the more area available for diffusion to occur.
• The shorter the diffusion pathways, the faster the rate of diffusion.
• Unicellular organisms have a high surface area to volume ratio, so it's external membrane is its gas exchange surface. There is a small distance to the centre, so the diffusion pathways are short. If the CO2 concentration in the cytoplasm becomes too high, it becomes too acidic for enzymes to function.