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Smaller organisms, such as unicellular organisms, rely on diffusion and osmosis for transport of substrate, nutrients, water, metabolites and excrement. For such a transport mechanism to effectively work, cells have to have a large contact surface with the environment. However, as the organisms grow larger and more complex, most of the constituent cells have hardly any contact with the external environment. In such a scenario an extensive (and complex) transport mechanism is a necessity for growth and maintenance.
In human beings and other large animals, we have heart acting as a pump to move nutrients around, using a liquid stream (called blood). We have passageways for this transport, such as arteries and veins, etc. Plants have a different mechanism and they use xylem and phloem tissues for transport of nutrients to various parts of the plants. In absence of such an elaborate transport systems, higher organisms would never be able to grow in size or sustain themselves.
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