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Once a certain amount of water enters a plant cell, the entry of more water is opposed by a force exerted by rigidity of the cell wall. This counterbalancing force is known as turgor pressure.

D. Water potential and effect of pressure

36.3, 36.4

The tendency of water to leave a solution when put on the other side of a membrane from pure water is known as the water potential(y). Water potential (y) = osmotic or solute potential (ys) + Pressure potential (yp) yis expressed in units of pressure, Mpa (megapascal) ysis set at 0 for pure water, the addition of solutes lowers(makes negative) ypis set at 0 for atmospheric pressure (1 atm) ypis positive for pressure above 1 atm, negative below 1 atm (tension) Water always moves towardsthe the region of lower or more negativey Fig 36.3 (a) A 0.1 M solution has aysof -0.23, which is more negativethan pure water, so the water moves to the right. (b) If there is a piston supplying additional pressure of 0.23 atm on the right, it counterbalances the tendency of water to move.

(c) If the piston pushes harder,ypis greater thanys greaterthan 0, so water moves to the left.

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The total (0.3-0.23) is

(d) If, on the other hand, there is a piston on the left exerting negative pressure (pulling up), the water potential on the left is more negativethan the water potential on the right, and water moves to the left. Figure 36.4 Similar equations can be written for the movement of water into and out of cells. If a cell is placed in a solution with a higher solute concentration (more negative ys), it will loose water and eventually