Water Potential

In thermodynamics, free energy represents the potential to do work. The potential energy of water is referred to as water potential.

Water will move by osmosis into and out of cells due to differences in water potential between the cell and its surroundings. Remember that water always moves from areas of high potential to areas of low water potential.

The scientific lab uses a simple technique to determine the water potential of potato cells. Solutions of varying osmotic potential (using sucrose molarity), from pure water to 1.0 molar sucrose, are used with potato cores. Six solutions are used, each increasing in molarity by 0.2. The potato samples are weighed before they are placed in the solutions and after 24 hours in each solution. The percent weight change is plotted on a graph and the point at which no weight change occurs is determined. The solution in which the potato cells do not change weight has the same water potential as the potato cell contents.

Water potential, w, is a 'measure of the ability of water molecules to move from one region to another'. The more water molecules there are per volume of the cell, the more likely it is by random movement that they will collide with the cell's plasma membrane and travel out of it. Pure water has a w of 0. As all solutions have less water molecules per volume than pure water, they have a lower w; therefore all solutions have negative water potentials.

The movement of water molecules is not totally random. The net movement of water molecules is always from a region of high water potential to one of lower water potential. They move down a water potential gradient until equilibrium is reached. Equilibrium is reached when the water potentials on both sides of the plasma membrane are the same.

Water potential, w, of any solution can be represented as individual components as per the following equation:
w = s + p +g

Each component, s, p, g denotes, the effects of solutes, pressure and gravity.

Solute potential is the amount the solute molecules reduce the w, by reducing the number of water molecules per volume. As an increase in solute reduces the w, the value for s is always negative.

Pressure potential is the amount added to water potential by pressure. By increasing the pressure in a cell, its water potential is increased, thus making it more likely for them to travel out of the cell. The pressure potential increases the w of a cell, making it closer to zero, therefore less negative and giving p a positive value.

Gravity potential is the effect of gravity on water potential. It depends on the height of water above the reference state of water, the density of water, and the acceleration due to gravity. If the vertical distances are small (less than 5 meters), the g is negligible and hence ignored.

In a plant cell, s and p only are important and thus, the above equation is simplified as:
w = s + p

Some Basic Principles

1. Water always moves from high water potential to low water potential.
2. Water potential is a measure of the tendency of water to move from high free energy to lower free energy.
3. Distilled water in an open beaker has a water potential of 0 (zero).
4. The addition of solute decreases water potential.
5. The addition of pressure increases water potential.
6. In cells, water moves by osmosis to areas where water potential is lower.
7. A hypertonic solution has lower water potential.
8. A hypotonic solution has higher water potential.