CBSE Class 10 Answered

Multi-cellular organisms are quite big and not all of their cells have contact with the external environment.  All organisms need to exchange substances with their environment - take in needed molecules and get rid of waste. 

This is not a problem for ?uni-cellul?a?r? organis?ms who use? ?the processes of osmosis, diffusion and active transport to transport substances in and out of their single cell.

Large multicellular organisms have very small surface area to volume ratio. There is a lot of volume that forms the bulk of our bodies, but the surface area (i.e. our skin) cannot provide for the rapid transport of materials into and out of the deepest recesses in our bodies. Cells that are buried under layers of other cells cannot rely on simple diffusion, osmosis or active transport for the exchange of substances simply because this will take too long. Thus, to increase surface area to volume ratio, multicellular organisms develop a complex system of tubes and channels that delivers materials from their external environment to the cells buried deep inside them. This system of tubes and channels becomes what is known as the transport system in these organisms. This transport system helps to exchange substances between the internal environment and the external environment.