Higher organisms use a transport system and not diffusion only to distribute nutrients because

Higher organisms use a transport system and not diffusion only to distribute nutrients because the ratio of their surface area to volume is small. Diffusion is the movement of particles from a region of high concentration to a region of low concentration, and it is an effective way to distribute nutrients in small organisms with a large surface area-to-volume ratio. However, larger organisms have a smaller surface area-to-volume ratio, which means that the diffusion of nutrients is slower and less efficient. Therefore, they need a specialized transport system, such as the circulatory system in animals or the vascular system in plants, to deliver nutrients and other essential substances to their cells. These transport systems have specialized structures, such as blood vessels or plant tissues, that can efficiently transport nutrients over longer distances and to specific locations within the organism. In conclusion, the small surface area-to-volume ratio of larger organisms is the main reason why they require a specialized transport system to distribute nutrients.