Diffusion is insufficient to meet the oxygen requirements of multicellular organisms like humans because diffusion is a slow process and the volume and complexity of the human body are large. In multicellular organisms, many cells are not in direct contact with the external environment, so oxygen cannot diffuse quickly enough to reach all cells. The larger body volume and increased distance oxygen must travel mean diffusion alone is too slow to meet the high metabolic demands of all cells in time. Therefore, multicellular organisms require specialized respiratory and circulatory systems to efficiently transport oxygen to all cells.

Key Reasons for Insufficiency of Diffusion in Multicellular Organisms

• Diffusion is slow and works effectively only over short distances.
• In multicellular organisms, many cells are located deep inside the body, far from the external environment.
• The surface area to volume ratio decreases as the organism size increases, limiting diffusion efficiency.
• Oxygen needs to be supplied quickly enough to support cell metabolism; diffusion alone cannot achieve this in large bodies.
• Complex body designs require specialized transport systems like blood circulation to ensure oxygen delivery to all cells.

Thus, diffusion's limitations in speed and distance make it inadequate for oxygen transport in multicellular organisms, necessitating efficient respiratory and circulatory adaptations.