Question

Why is diffusion insufficient to meet the oxygen requirements of large multicellular organisms such as humans?

Answer 1

Hii there pal!!
The answer to your question goes like this -------------------------------------------------

=> By the process of diffusion oxygen requirements can be fulfilled in unicellular organisms and in some other lowly organized living organisms where cells are arranged only in one or two layers. Human being is multicellular and cells or tissues are arranged in several layers. So, diffusion is quite insufficient to meet the oxygen requirements in multicellular organisms like human.

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Answer 2

In single celled organism no specific organism is needed for taking in food, exchange of gases or removal of waste because the entire body of the organism is in contact with the environment. 
But in case of multi cellular organisms, body size becomes bigger and body design becomes more complex . All cells are not in direct contact with the surrounding environment . Thus simple diffusion is insufficient to meet the requirements of the cell