Most single-celled organisms are very small. They have a large surface area compared to their volume. In these organisms, substances rapidly get in and out of the cell by simple diffusion to meet the cell requirements.
The multicellular organisms are usually big and hence have a small surface area compared to their volume. In addition, many of their cells are far from the outside environment which is a source of oxygen and nutrients or into which they release metabolic waste substances, which can be harmful if left to accumulate in the cells. These organisms therefore need a special transport system to efficiently move substances into and from the cells.
A transport system in living organisms is made up of specialised tissues and organ systems. The type of transport system contained in an organism depends on its complexity and whether it is a plant or an animal.