Biology Chapter 2: Chemistry of Life

AP Biology Mrs. Douma Water Potential

Water Potential and Osmosis

The net uptake of loss of water by a cell occurs by osmosis, the passive transport of water across a membrane. How can we predict the direction of osmosis when a cell is surrounded by a particular solution? In the case of an animal cell, it is enough to know whether the extracellular solution is hypotonic or hypertonic (low or high solute concentration) to the cell. Water will move by osmosis from the hypotonic to hypertonic direction. But in the case of a plant cell, the presence of a cell wall adds a second factor affecting osmosis: physical pressure. The combined effects of these two factors –solute concentration and pressure – are incorporated into a single measurement called water potential, abbreviated by the Greek letter psi (). The most important thing for you to learn about water potential is that water will move across a membrane from the solution with the higher water potential to the solution with the lower water potential. For example, if a plant cell is immersed in a solution having a higher water potential than the cell, osmotic uptake of the water will cause the cell to swell. By moving, water can perform work. The potential in water potential refers to this potential energy, the capacity to perform work when water moves from a region of higher  to a region of lower .

Plant biologists measure  in units of pressure called megapascals (MPa). An MPa is equal to about 10 atmospheres of pressure (pressure exerted at sea level by an imaginary column of air – 1 kg of pressure per cm2). A couple of nonbiological examples will give you some idea of the magnitude of a megapascal: a car tire is usually inflated to a pressure of about 0.2 MPa; the water pressure in home plumbing is about 0.25 MPa

Let’s see how solute concentration and pressure affect water potential. For purposes of comparison, the water potential of pure water in a container open to the atmosphere is defined as zero MPA, therefore = 0 MPa. The addition of solutes lowers the water potential. And since  is standardized as 0 MPa for pure water, any solution at atmospheric pressure has a negative water potential due to the presence of solutes. If this solution is separated from pure water by a selectively permeable membrane, water will move by osmosis into the solution, from the region of higher . So far, this is just another way of saying that the water is moving in the hypotonic to hypertonic direction. But we have not yet factored in the influence of physical pressure on.

In contrast to the inverse relationship of to solute concentration, water potential is directly proportional to pressure; increasing pressure raises. Physical pressure-pressing the plunger of a syringe filled with water, for example – causes water to escape via any available exit. If a solution is separated from pure water by a selectively permeable membrane, external pressure on the solution can counter its tendency to take up water due to the presence of solutes. In fact, even greater pressure will force water across the membrane from the solution to the compartment containing pure water. It is also possible to create a negative pressure, or tension, on water or solutions. For example, if you pull up on the plunger of a syringe, the negative pressure within the syringe draws a solution through the needle. AP Biology Mrs. Douma Water Potential

The movement of water due to a pressure difference between two locations is called bulk flow. It is usually much faster than simple diffusion, the process responsible for water movement due to a difference of solute concentration.

The combined effects of pressure and solute concentration on water potential are incorporated into the following equation:

p + s

Where p is the pressure potential (physical pressure on a solution) and s is the solute potential

(which is proportional to the solute concentration of a solution; s is also called osmotic potential). Pressure on a solution (p) can be either a positive number or a negative number (tension or negative pressure). In contrast, a solution’s solute potential (s) is always a negative number, and the greater the solute concentration, the more negative the value of s. This makes sense if you think of s as the effect solutes have on a solution’s overall water potential (), which is always to lower  below the water potential of pure water.

Let’s see how this equation works: A 0.1M solution has a s of -0.23 MPa. Thus, in the absence of a physical pressure (p = 0) , water potential is -0.23 MPa for a 0.1M solution:

 = p + s = 0 + (-0.23) = -0.23

If we apply a physical pressure of +0.23 MPa to this solution, we raise its water potential from a negative value to 0. ( = 0.213 – 0.23). If this pressurized solution is separated from water by a selectively permeable membrane, there will be no net flow between the two compartments. If we increase p + 0.3 MPa, then the solution has a water potential of +0.07 MPa ( = 0.3 – 0.23), and this solution will lose water by bulk flow to a compartment containing pure water. In dissecting water potential to see these opposing effects of pressure and solutes, it is important to remember the key point: Water will move across a membrane in the direction of lower water potential.

Let’s now apply what we have learned about water potential to the uptake and loss of water by plant cells. First imagine a flaccid cell (that is p = 0) bathed in a solution of higher solute concentration than the cell itself. Since the external solution has the lower (more negative) water potential, water will leave the cell by osmosis, and the cell will plasmolyse, or shrink and pull away from its wall. Now lets’ place the same flaccid cell in pure water ( = 0). The cell has a lower water potential because of the presence of solutes, and water enters the cell by osmosis. The cell begins to swell and push against the wall, producing a turgor pressure. The partially elastic wall pushes back against the turgid cell. When this wall pressure is great enough to offset the tendency for water to enter because of the solutes in the cell, then p and s are equal in magnitude, and thus  = 0. This matches the water potential of the extracellular environment – in this example, 0 MPa. A dynamic equilibrium has been reached, and there is no further net movement of water, although an equal exchange of water across the membrane continues.