They have specialised cells and tissues for performing various necessary functions of the body such as intake of food and oxygen. Unlike unicellular organisms, multicellular cells are not in direct contact with the outside environment. Therefore, diffusion cannot meet their oxygen requirements
■ In multi - cellular organisms, all the cells may not be in direct contact with the surrounding environment.
■thus,simple diffusion will not meet the requirements of all the cells.
■In case of a single - celled organism, no specific organs for taking in food, exchange of gases or removal of wastes may be needed because the entire surface of the organism is in contact with the surrounding environment.
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Diffusion, being a slow process will take a lot of time to circulate oxygen to all parts of body cell
In case of a single-celled organism, no specific organs are needed because the entire surface of organism is in contact with the environment whereas in the multicellular organisms, all the cells may not be in direct contact with the surrounding environment. This. the simple diffusion can not meet the requirements of all cells.
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i don't know pls explain it