Answer:
In the case of a
single-celled organism, no specific organs for exchange of gases may be needed
because the entire surface of the organism is in contact with the environment.
But in multicellular organism, all the cells may not be in direct contact with
the surrounding environment.
Thus, diffusion is insufficient to meet the oxygen
requirements of large multicellular organisms (human beings) because the volume
of human body is so big that oxygen cannot diffuse into all the cells of the
human body quickly.
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