Explain why multicellular organisms needs to develop a specialised exchange system and transport system, yet unicellular organisms do not

Unicellular organisms have a large surface area to volume that is one cell thick, therefore they have a short diffusion distance and are metabolically inactive. Therefore, they can obtain sufficient oxygen and nutrients via the cell surface membrane through simple diffusion and remove carbon dioxide and other waste products through the cell surface membrane in the same way, so a specialised transport system is not required.As the complexity of the organism increases, the surface area to volume ratio decreases. As the size of the organism increases, the metabolic activity of the organism increases. So the exchange surfaces from the body cells to the outer environment is deeper. To supply the necessary materials, such as oxygen and nutrients and to remove waste products such as carbon dioxide and water, there must be a specialised exchange system and transport system.