Why can't multicellular organisms rely on diffusion?
Answers
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As the size of an organism increases, its surface area to volume ratio decreases. ... Large multicellular organisms therefore cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products. Large multicellular organisms require specialised transport systems.
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In multicellular organisms like human beings diffusion is insufficient to meet the oxygen requirements of the body because of the following reasons :
• Diffusion is a very very slow process. Thus it can't fulfil the requirements of our body.
• The oxygen in human beings will have to travel last distances inside our body to reach each and every part of the cells.
• The volume of human body is so big that oxygen cannot diffuse into all the internal cells of the human body quickly.
• Also this is the reason for why the respiratory pigments are present in multicellular organisms also.