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BY03 - Gaseous Exchange Small, simple organisms (single cell) have a relatively small oxygen demand. All the oxygen they need diffuses into their body through it's surface. This strategy of diffusion would not work through the surface of a larger organism. As organisms get larger, their volume increases at a faster rate than their surface area; resulting in the need to exchange materials quickly outstripping their ability to do so (The material an organism can exchange is proportional to it's surface area). Therefore, larger organisms can only survive if they evolve strategies to increase their surface area:volume ratio. These strategies involve specialist exchange organs called respiratory organs. Respiratory Organs Gases pass into and out of the body of an organism by diffusion. Respiratory organs simply increase the efficiency of diffusion by having the following features in common: * A large surface area. The greater the area of tissue in contact with the environment, the greater...

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