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MASS TRANSPORT SYSTEMS Living cells require regular supplies of oxygen and nutrients for metabolic reactions such as respiration. These important materials such as glucose need to get to living cells and the waste products such as ammonia and urea need to be removed. This diffusion of materials into cells and waste products out of cells (which may need to happen via active transport if it is against the concentration gradient) can easily happen in small, single cellular organisms as they have a large surface area : volume ratio and also have a short diffusion path to all cells so enough material can get to cells sufficiently quickly. But as the organism gets larger the surface area : volume ratio decreases and also the diffusion path increases making it difficult to get substances to cells in the centre of the organisms and so there is a need to adapt and so have internal...

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