osmosis

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Movement of water through a partially (selectively) permeable membrane separating solutions of different concentrations. Water passes by diffusion from a weak solution (high water concentration) to a strong solution (low water concentration) until the two concentrations are equal. A membrane is partially permeable if it lets water through but not the molecules or ions dissolved in the water (the solute; for example, sugar molecules). Many cell membranes behave in this way, and osmosis is a vital mechanism in the transport of fluids in living organisms. One example is in the transport of water from soil (weak solution) into the roots of plants (stronger solution of cell sap) via the root hair cells. Another is the uptake of water by the epithelium lining the gut in animals. There are also membranes that humans can manufacture that are partially permeable.

If solutions have different concentrations of dissolved substance, there will be a difference in the concentration of water as well. Where there is a lot of dissolved substance there is less water. Where there is less dissolved substance there is more water. Water behaves like any other substance in diffusion. It diffuses from an area where there is a high concentration of it to an area where there is a lower concentration of it. Water diffuses down the concentration gradient for water and across the membrane. This means that it will diffuse from the dilute solution into the concentrated solution.

Excessive flow of water into a cell by osmosis can burst the cell. Cells protect against this using processes of osmoregulation. If external pressure is applied to the stronger solution, osmosis is arrested. By this mechanism plant cells can osmoregulate, since the cell wall of a fully turgid cell exerts pressure on the solution within the cell. Animal cells such as the red blood cell cannot osmoregulate in this way since they have no cell wall. Instead, the correct concentration of plasma is maintained by the kidney. This is important because if the concentration changes significantly, osmosis will take place and a lot of water will either diffuse out of cells into the blood or into cells from the blood, thereby damaging them.

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