Diffusion is insufficient to meet the oxygen requirements of multicellular organisms like humans because their large size and complex structure mean oxygen cannot diffuse quickly enough from the external environment to all cells. The vast distances involved, coupled with the relatively slow rate of diffusion, make it impossible for oxygen to reach all cells within the body at a rate sufficient to support life.
Here’s a more detailed explanation:
Large Size and Complex Structure:
Multicellular organisms have a body volume that is significantly larger than their surface area, meaning that the distance oxygen needs to travel to reach internal cells is considerable.
Slow Diffusion Rate:
Diffusion is a slow process, and the distance oxygen needs to travel within the body makes it too slow to meet the demands of all cells.
Need for Specialized Systems:
To overcome these limitations, multicellular organisms have evolved specialized systems like the respiratory and circulatory systems to efficiently transport oxygen from the external environment to all cells. These systems use respiratory pigments like hemoglobin to carry oxygen to the tissues.