Multicellular organisms are typically much larger in size and are more complex than unicellular organisms. For example, human beings, plants, cats, etc. are all multicellular life forms and are more complex than a unicellular life form such as bacteria, etc. Unicellular organisms, because of their small size, can transport all the materials through diffusion and osmosis. In case of multicellular organisms, a large number of cells are away from direct contact with the environment and are thus incapable of exchanging materials (such as nutrients, water, oxygen, carbon dioxide, etc.) by diffusion or osmosis. They, thus, need a more complex transport system to carry nutrients, oxygen, etc. from the environment and/or other parts of the body to the required cells. A lack of such a complex transport system will mean inability to grow large in size, etc.
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