Chapter 4: Cell membranes and transport
partially permeable membrane
a
b
solute molecule water molecule A
B
A
B
Figure 4.10 Two solutions separated by a partially permeable membrane. a Before osmosis. The solute molecules are too large to pass through the pores in the membrane, but the water molecules are small enough. b As the arrows show, more water molecules moved from A to B than from B to A, so the net movement has been from A to B, raising the level of solution in B and lowering it in A.
solute molecules than solution A. Solution B is described as more concentrated than solution A, and solution A as more dilute than solution B. First, imagine what would happen if the membrane was not present. Both solute molecules and water molecules are free to move anywhere within the solutions. As they move randomly, both water molecules and solute molecules will tend to spread themselves evenly throughout the space available, by diffusion. Now consider the situation where a partially permeable membrane is present, as shown in Figure 4.10. The solute molecules are too large to get through the membrane. Only water molecules can pass through. The solute molecules move about randomly, but as they hit the membrane they simply bounce back. The numbers of solute molecules each side of the membrane stay the same. The water molecules also move about randomly, but they are able to move both from A to B and from B to A. Over time, the water molecules will tend to spread themselves out more evenly between A and B. This means that A will end up with fewer water molecules, so that the solution becomes more concentrated with solute. B will end up with more water molecules, so that it becomes more dilute. We will also find that the volume of liquid in B will increase, because it now contains the same number of solute molecules, but more water molecules. This movement of water molecules from a dilute solution to a concentrated solution, through a partially permeable membrane, is called osmosis.
Water potential
The term water potential is very useful when considering osmosis. The Greek letter psi, ψ, can be used to mean water potential. You can think of water potential as being the tendency of water to move out of a solution. This depends on two factors: ■■ how much water the solution contains in relation to solutes, and ■■ how much pressure is being applied to it. Water always moves from a region of high water potential to a region of low water potential. We say water always moves down a water potential gradient. This will happen until the water potential is the same throughout the system, at which point we can say that equilibrium has been reached. For example, a solution containing a lot of water (a dilute solution) has a higher water potential than a solution containing only a little water (a concentrated solution). In Figure 4.10a, solution A has a higher water potential than solution B, because solution A is more dilute than solution B. This is why the net movement of water is from A to B. Now look again at Figure 4.10b. What would happen if we could press down very hard on side B (Figure 4.11)?
A
B
Figure 4.11 Applying pressure to a solution increases the tendency of water to move out of it – that is, it increases its water potential. Here, water molecules move from B to A.
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