Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like human
Answer:
A cell of a unicellular organism is in direct contact with the environment and intake of oxygen occurs directly by diffusion. Not all cells of multi-cellular organisms like human are in contact with the external environment. So diffusion becomes insufficient for them to meet the oxygen requirement. If our body relies only on diffusion, it will take three days for oxygen to reach our legs. As a result animal including human has developed respiratory organs and the circulatory system where the exchange of gases takes place, and oxygen is transported to various parts of the body respectively.
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