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Multi-cellular organisms are too 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. A uni-cellular organism can use the processes of osmosis, diffusion and active transport to transport substances in and out of the organism, which is in this case just one cell. But as organisms increase in size, they need a specialized transport system to exchange substances between the internal environment and the external environment. Interestingly enough, osmosis and diffusion are still the important mechanisms at the cellular level.
A system of transport is found in most multicellular organisms. It allows the growth of these organisms to a substantial size in contrast with single-celled organisms. As the surface to volume ratio increases, those cells that are too far from the external environment would not be able to rely on osmosis and diffusion as these processes would not be able to get the needed nutrients, oxygen and other materials to the cell in time. Therefore, a system of transport allows for all cells in the body to get the necessary substances for life in a timely manner. In multicellular animals, you will a heart(pumping mechanism), a liquid tissue known as blood, arteries, veins and tiny capillaries. The capillaries are thin--walled and moist, allowing for transport of materials from blood to cells. In primitive multicellular animals, because they usually lack a transport system, their sizes remain small as they rely on diffusion and osmosis to get the substances they need. In plants, there are roots, stem, leaves and inside, xylem and phloem vascular tissues. These act as a transport system however, there is no heart to pump materials. Instead, capillary action and evapotranspiration drive materials up through the plant, against the pull of gravity.
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