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Exchange and Transport in Protozoa The exchange of gases between the environment and cells occurs via the process of diffusion. Diffusion depends on: * The amount of surface area available for diffusion. The larger the surface area the greater the rate of diffusion. * The concentration gradient. An organism which respires very quickly will have a much lower concentration of oxygen in the cells and a higher than normal concentration of Carbon Dioxide. So the greater the concentration gradient across the respiratory surface the quicker the rate of diffusion. * The thickness or length of the diffusion path. The greater the thickness of the path the slower the rate of diffusion because the gases will have to travel a larger distance. So the respiratory surface must be as thin as possible. The relation of the three points above can be summed up by Fick's Law: However, the amount of gas which an organism needs to exchange...

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