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 multicellular organisms, the body size is large and diffusion pressure alone is not enough to carry food, exchange gases or remove wastes. If diffusion were to move oxygen in our body, it is estimated that it would take 3 years for a molecule of oxygen to get to our toes from our lungs. Thus, multicellular organisms have developed various organs/organ systems for transportation of gases, wastes, food etc., like blood in animals or xylem and phloem in plants etc.