Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans ?

Diffusion is a process where molecules move from an area of high concentration to an area of low concentration. While this process works well for single-celled organisms, it is insufficient for multicellular organisms like humans for several reasons:

1. Size: Multicellular organisms are much larger than single-celled organisms. This means that cells in the interior of the organism are too far away from the external environment for diffusion to effectively transport oxygen to them.

2. Complexity: Multicellular organisms have specialized cells and tissues that perform specific functions. These specialized cells often have higher energy needs and therefore require a more efficient method of oxygen delivery than diffusion can provide.

3. Metabolic rate: Multicellular organisms generally have a higher metabolic rate than single-celled organisms. This means they use energy (and therefore oxygen) more quickly. Diffusion is a slow process and cannot supply oxygen quickly enough to meet the demands of multicellular organisms.

4. Multicellularity: In multicellular organisms, not all cells are in direct contact with the external environment. Therefore, relying on diffusion from the external environment would not be sufficient to deliver oxygen to all cells.

Therefore, multicellular organisms like humans have developed specialized systems (like the circulatory system) to transport oxygen and nutrients to all cells in the body.