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Water PotentialWater potential is the tendency of water to move from one place to another. It is typically measured in units of atmospheric pressure: bars or dynes per square centimeter. It is derived from two potentials: the pressure potential and solute potential. The symbols for these, respectively, are ψp and ψs, so we can say that: - ψ = ψp + ψs
Pressure potential Pressure potential is increased as water enters a plant cell. As water seeps through a cell wall the insoluble protoplast is pushed against the cell wall. Protoplast is defined as all the living molecules within a cell – cytoplasm, mitochondria, choloroplast etc. These are pushed against the wall and thus force water against the inside of the cell wall. This is called pressure potential. The protoplast forcing water towards the outside of the cell naturally increases water potential as the water has pressure from within the cell trying to force it out. Solute potential Distilled water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes negative. This causes water potential to decrease also. Basically, as solute is added, the water potential of a solution drops, and water will tend to move into the solution.
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