All cells need nutrients, oxygen and food, and also need an efficient waste removal system. These useful products are available in the environment and the waste products also have to be sent back to the environment. Single celled organisms or unicellular organisms are in constant contact with their environment. Due to their large surface area to volume ratio, they can use the process of diffusion (and osmosis) for exchanging material with their immediate environment. In comparison, multicellular organisms are larger in size and may contain a number of cells that are too far away from the environment and thus are unable to exchange material with the environment. Without an efficient transport system, these interior cells will not be able to function and the organism will not be able to grow in size or complexity. A transport system is thus a necessity for multicellular organisms, while being a luxury for unicellular organisms.
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