osmosis



osmosis defined in 1951 year

osmosis - osmosis;
osmosis - When a solution of, say, sugar in water is separated from pure water by a membrane permeable to water but not to sugar (semi-permeable membrane), water passes across the membrane into the sugar solution. This movement of water is osmosis. If external pressure is applied to the sugar solution, the movement of water into it will be opposed. The pressure required to stop the movement completely is called the osmotic pressure of the sugar solution. It is greater the more concentrated the solution. Water will similarly pass by osmosis from any solution having a weaker osmotic pressure to any having a stronger (involving the same or different dissolved substance) provided the membrane separating them is of the appropriate semi-permeable kind, until the two solutions attain equal osmotic pressures. And if the membrane is merely more permeable to water than to the dissolved substance, water will still move from weak solution to strong, while the dissolved substance moves more slowly in the opposite direction, until the two solutions have equal osmotic pressures. The amount of osmotic pressure produced by solutions (in standard conditions) depends on the number of dissolved particles (molecules or ions), whatever their size, in a given amount of water. Proteins, and other substances with large molecules, develop therefore a small osmotic pressure relative to the weight of them which is dissolved. More or less semi-permeable membranes are very widespread in living organisms, e.g. they surround all cells; and so osmosis plays a great part in controlling the distribution of water in living organisms.

near osmosis in Knolik


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