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water potential

Symbol Ïˆ; unit Pa. A measure of the tendency of a system or cell (in biological contexts) to give up water to another system. Water flows from a higher to a lower water potential, so any system with a positive Ïˆwill pass water into a system with a zero or negative Ïˆ.Pure water at standard temperature and pressure is assigned a water potential of zero. The presence of a solute in water lowers the water potential to give a solute potentialÏˆs,which is therefore always negative. Osmosis should be seen as the movement of water from an area of less negative Ïˆs to an area of more negative Ïˆs.The concept has particular significance for plant cells, in which hydrostatic pressure and the pressures associated with the cell wall generate the pressure potentialÏˆp required to produce support for structures such as young stems and leaves to prevent them from wilting. When the pressure potential is at a maximum, the water potential is zero and the cell is said to have maximum turgor (to be fully turgid) . The water potential Ïˆis related to the solute potential Ïˆ{TAG(tag=>sub)}s{TAG} and the pressure potential Î¨{TAG(tag=>sub)}p{TAG} by the equation Ïˆ = Ïˆ{TAG(tag=>sub)}s{TAG} + Ïˆ{TAG(tag=>sub)}p{TAG}.

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