Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?
In multicellular organisms like humans, diffusion is insufficient because not all cells are in direct contact with the surrounding environment. As the body size increases and the body design becomes more complex, simple diffusion cannot meet the oxygen requirements of all the cells.
If diffusion were to move oxygen in our body, it is estimated that it would take 3 years for a molecule of oxygen to reach our toes from our lungs. This is far too slow to sustain life processes. Therefore, multicellular organisms require specialized respiratory pigments like haemoglobin to efficiently transport oxygen to all body parts.
Explanation
The textbook explains that single-celled organisms can rely on diffusion because their entire surface is in contact with the environment. However, in multicellular organisms, the complexity of body design means cells are not in direct contact with the environment. The textbook provides a striking example: diffusion alone would take 3 years for oxygen to travel from lungs to toes. This illustrates why specialized transport mechanisms (respiratory pigments like haemoglobin) are essential for larger organisms.