- In single-celled organisms, no specific organs for exchange of gasses are needed because the entire surface of the organism is in contact with the environment.
- Multicellular organisms such as human beings have complex body designs and large body size. Because of that oxygen cannot diffuse into all the cells of the human body quickly.
- Therefore, diffusion is insufficient to meet the oxygen requirement of multicellular organisms like humans.
- Multicellular organisms (Human beings) bear specialized cells and tissues for performing various necessary functions of the body such as intake of food and oxygen, etc.
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