LABORATORY 1b: WATER POTENTIAL II
In this laboratory you will investigate the effect of solute concentration on water potential as it relates to living plant tissues.
Before you begin this lab you should understand: - the mechanisms of diffusion and osmosis and their importance to cells - the concept of water potential - the relationship between solute concentration, pressure potential and the water potential of a solution - the concept of molarity and its relationship to osmotic concentration **also read the section on water potential in the text book ** (Chapter 36 pp. 749-751 in 6th ed. of Biology by Campbell, Reece & Mitchell) At the completion of this lab you should be able to: - measure the water potential of a solution in a controlled experiment - determine the osmotic concentration of living tissue or an unknown solution from experimental data - describe the effects of water gain or loss in animal and plant cells - relate osmotic potential to solute concentration and water potential [NOTE: You may want to look over Lab 1a to refresh your memory!]
EXERCISE 1b-A: DETERMINING THE WATER POTENTIAL OF POTATO CELLS In this exercise you will use potato cores placed in different molar concentrations of sucrose in order to determine the water potential of potato cells. First, however, we will explore what is meant by the term "water potential." Water Potential. Water potential is abbreviated by the Greek letter psi (ψ). Water potential measures the tendency of water to leave one place in favor of another place. Water will always move from an area of higher water potential to an area of lower water potential. In animal cells, movement of water into and out of a cell is influenced by the relative concentration of solute on either side of the plasma membrane. Water will move by osmosis in the hypoosmotic → hyperosmotic direction. If water moves out of the cell, the cell will shrink. If water moves into the cell, it will swell and may even burst. In plant cells, the presence of a rigid cell wall adds a second factor affecting osmosis: pressure. Instead of cells bursting as water enters the cells, pressure will eventually build up inside the cell. The combined effects of these two factors--solute concentration and pressure--are incorporated into a single measurement called water potential. Added pressure causes the water potential of a solution to increase; it is measured by pressure potential (ψρ). As water enters a baggy or a cell with a cell wall, pressure will develop inside the bag or cell and will push on the walls of the bag or the cell. Addition of solute causes water potential of a solute to decrease. The effect of solute on water potential is the solute potential, also called osmotic potential, (ψs). The more solute, the greater the solute potential, and the lower the water potential.
-1HHS A.P. Biology - Laboratory Manual
LABORATORY 1b - Water Potential II
The equation which shows that water potential is affected by both solute concentration and pressure is:
ψ = ψρ + ψs
Water potential = Pressure potential + Solute potential
The water potential of pure water in a beaker open to the atmosphere is zero (ψ = 0) because both the osmotic and pressure potentials are zero (ψs = 0, ψρ = 0). A solution at atmospheric pressure (ψρ= 0) will always have a lower water potential than distilled water due to the presence of solute. For instance, a 0.1-M solution of sucrose at atmospheric pressure (ψρ= 0) has a water potential of -2.3 bars (-.23 mPa). A bar is a metric measure of pressure, measured with a barometer, that is about the same as 1 atmosphere. Another measure of pressure is the megapascal (mPa). 1 megapascal = 10 bars. Figure 1b.1: Potato Cell immersed in distilled water
(a) Before equilibrium
When a solution, such as that inside a potato cell, is separated from pure water by a selectively permeable membrane, water will move (via osmosis)...