What is osmosis? How do plants take water from the soil into their roots? How do animals absorb digested food from the intestines into the blood vessels? After all, no holes can be seen in the roots of a plant or in the walls of animal intestines. The process by which this is done is called "osmosis." When you place any two gases side by side with nothing to separate them, they will mix quickly. The same is true of most liquids. For instance, a drop of ink will gradually give a faint and uniform color to a whole pint of water. In osmosis, this mixing goes on through a membrane, such as the thin walls of the tiny root hairs of a plant or the lining of the intestines. The membrane slows down the mixing process, but does not stop it. During osmosis in living things, the membranes allow certain substances to pass through while others are stopped. This is decided partly by the structure of a membrane and partly by the structure of the substance in contact with it. Scientists think that in osmosis dissolved substances pass through the spaces between the molecules making up the membrane. The substances in solution in contact with a membrane push against it and exert what is called "osmotic pressure." The side with more particles of dissolved substance has the greater osmotic pressure, and therefore the direction in which most of the traffic' goes is from the region of greater to lesser pressure. But the traffic moves in both directions; for anything that can go through an osmotic membrane can also come out. In our body, the membranes of the blood vessels, for example, are letting substances pass through in both directions constantly. In this way, digested food goes into the blood stream, and waste carbon dioxide comes out from the blood through the lungs. For Details
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