EXERCISE 1C: Determining the Water Potential of Potato Cells
Introduction: Water potential was defined above as an expression of the tendency of water to diffuse from one region to another. Water potential is a numerical value that must be determined for a given temperature and pressure. In this exercise you will determine the water potential of potato cells at room temperature and ambient pressure by placing cores of potato tissue in sucrose solutions of different concentrations and measuring the net movement of water in each case. In animal cells, movement of H2O into and out of a cell is influenced by the relative concentration of solute on either side of the cell membrane. If water moves out of the cell, the cell will shrink or crenate. If water moves into the cell it will swell and may even burst or cytolyze. In plant cells, the presence of a rigid cell wall prevents cells from bursting as water enters the cells, but pressure eventually builds up inside the cell and affects the process of osmosis. When the pressure inside the cell becomes large enough, no additional water will accumulate in the cell even though the cell still has a higher solute concentration than does water. So movement of water through plant tissue cannot be predicted simply through knowing the relative solute concentrations on either side of the plant cell wall. Instead, the numerical value of the water potential of plant cells is used to predict the direction in which water will diffuse through living plant tissues. The water potential of a given plant cell is affected both by solute concentration and pressure. Water potential is the chemical potential of water, which is the free energy per mole of water. It is expressed in bars; a metric unit of pressure equal to about 1 atmosphere and measured with a barometer. Water potential is abbreviated by the Greek letter psi (() and has two major components: osmotic potential (((), which is dependent on solute concentration, and pressure potential ((p), which results from the exertion of pressure—either positive or negative—on a solution. We express this as:
This expression makes sense when you think back to the fact that the tendency of water to diffuse into plant cells is affected by both solute concentration and pressure. The osmotic potential portion of ( ((() represents the contribution to water potential from solute concentration. The pressure potential portion of ( ((p) represents the contribution to water potential from pressure. Thus total water potential ( is a combination of the contributions of solute concentration and pressure. The water potential of pure water in a beaker open to the atmosphere is zero (( = 0) because both the osmotic and pressure potentials are zero ((( = 0; (p = 0). An increase in pressure raises the pressure potential (makes (p, more positive) and therefore raises water potential ((). Think about this example. Suppose you had a dialysis bag filled with distilled water in a beaker of distilled water, with everything at ambient pressure. There would be no net movement of water; the water potential of both systems would be 0. Now imagine you could apply extra pressure to the water in the dialysis bag—say, by forcing in extra air as in a balloon. It is easy to picture that water would then move from the dialysis bag into the beaker. The additional pressure on the water in the bag raised its water potential, so water moved into the beaker. The addition of solute to water lowers the osmotic potential (makes (( ~ more negative) and therefore lowers the water potential ((). Therefore, a solution at atmospheric pressure ((p = 0) will always have a negative water potential due to the solute. For instance, a 0.1-M solution of sucrose at atmospheric pressure ((p = 0) has an osmotic potential ((( ) of -2.3 bars due to the solute, and thus a water potential (() of -2.3 bars....
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