In multicellular organisms like humans there are complex body structure with specialised cells and tissues since these cells and tissues are not in direct contact with the environment simple diffusion is insufficient to meet the oxygen requirement of the body.

Introduction:
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. It is a passive process that does not require energy. However, diffusion alone cannot meet the oxygen requirement of multicellular organisms like us.

Limited Surface Area:
The main reason diffusion alone cannot meet the oxygen requirement of multicellular organisms is because of limited surface area. Diffusion occurs faster over shorter distances, and therefore, the surface area of a cell limits the amount of diffusion that can occur. A larger organism, like a human, has more cells and requires more oxygen. The surface area of the body cannot keep up with the oxygen demand of all the cells.

Longer Diffusion Pathways:
Another reason diffusion is insufficient is that it becomes less efficient over longer distances. The oxygen molecule has to travel through several layers of cells to reach the innermost cells. As the oxygen molecule moves farther away from the source, the concentration gradient decreases, and the rate of diffusion slows down.

Inefficient Transport System:
Multicellular organisms have developed an efficient transport system to overcome the limitations of diffusion. In humans, oxygen is transported from the lungs to the body's cells by the circulatory system. The heart pumps blood that carries oxygen to the tissues. This system is much more efficient than relying on diffusion alone.

Conclusion:
In conclusion, while diffusion is an essential process for the survival of organisms, it is insufficient to meet the oxygen requirement of multicellular organisms like us. The limited surface area, longer diffusion pathways, and inefficient transport system make it impossible for diffusion alone to meet the oxygen demand of all the cells in the body. Instead, organisms have developed an efficient transport system to overcome these limitations.