Osmosis is the net diffusion of water (the solvent) across the membrane. For osmosis to occur, the membrane must be selectively permeable; that is, it must be more permeable to water molecules than to at least one species of solute. There are thus two requirements for osmosis: (1) there must be a difference in the concentration of a solute on the two sides of a selectively permeable membrane; and (2) the membrane must be relatively impermeable to the solute. Solutes that cannot freely pass through the membrane are said to be osmotically active.
Like the diffusion of solute molecules, the diffusion of water occurs when the water is more concentrated on one side of the membrane than on the other side; that is, when one solution is more dilute than the other (fig. 6.5). The more dilute solution has a higher concentration of water molecules and a lower concentration of solute. Although the terminology associated with osmosis can be awkward (because we are describing water instead of solute), the principles of osmosis are the same as those governing the diffusion of solute molecules through a membrane. Remember that, during osmosis, there is a net movement of water molecules from the side of higher water concentration to the side of lower water concentration.
Imagine a cylinder divided into two equal compartments by an artificial membrane partition that can freely move. One compartment initially contains 180 g/L (grams per liter) of glucose and the other compartment contains 360 g/L of glucose. If the membrane is permeable to glucose, glucose will diffuse from the 360-g/L compartment to the 180-g/L compartment until both compartments contain 270 g/L of glucose. If the membrane is not permeable to glucose but is permeable to water, the same result (270-g/L solutions on both sides of the membrane) will be achieved by the diffusion of water. As water diffuses from the 180-g/L compartment to the 360-g/L compartment (from the higher to the lower water concentration), the for-
■ Figure 6.5 A model of osmosis. The diagram illustrates the net movement of water from the solution of lesser solute concentration (higher water concentration) to the solution of greater solute concentration (lower water concentration).
mer solution becomes more concentrated while the latter becomes more dilute. This is accompanied by volume changes, as illustrated in figure 6.6. Osmosis ceases when the concentrations become equal on both sides of the membrane.
Cell membranes behave in a similar manner because water is able to move to some degree through the lipid component of most cell membranes. The membranes of some cells, however, have special water channels that allow water to move through more rapidly. These channels are known as aquaporins. In some cells, the plasma membrane always has aquaporin channels; in others, the aquaporin channels are inserted into the plasma membrane in response to regulatory molecules. Such regulation is particularly important in the functioning of the kidneys, as will be described in chapter 17.
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