Question

why is diffusion insufficient to meet the oxygen requirements of multi cellular oraganisms like humans?

Answer 1

Multicellular organisms such as humans possess complex body designs. 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. ....
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Answer 2

they have specialised cells and tissues for performing various necessary function of the body such as intake of food and oxygen. unlike unicellular organisms multicellular are not in direct contact with environment therefore diffusion can't meet their oxygen requirements