In single celled organisms diffusion is sufficient to meet all their requirements of food, exchange of gases or removal of wastes but it is not in case of multicellular organisms. Explain the reason for this difference.

In single celled organisms surface area to volume ratio is optimum for diffusion/exchange of substances between the cell and its exterior and this rate of exchange suffices the requirement of a unicellular organism. However, in multicellular organisms the surface area to volume ratio is low, the exchange that takes place with the exterior or the environment occurs only through the body surface with the help of a specific structure/organ, example (Skin),However, this exchange does not compensate for millions of cells in a multicellular organism, which have distinct requirements based on their functions