Figure 617

The addition of solute molecules to pure water lowers the water concentration in the solution.

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Pure water (high water concentration)

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Solution (low water concentration)

Solution (low water concentration)

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Movement of Molecules Across Cell Membranes CHAPTER SIX

For example, 1 mol of glucose in 1 L of solution decreases the water concentration to approximately the same extent as does 1 mol of an amino acid, or 1 mol of urea, or 1 mol of any other molecule that exists as a single particle in solution. On the other hand, a molecule that ionizes in solution decreases the water concentration in proportion to the number of ions formed. Hence, 1 mol of sodium chloride in solution gives rise to 1 mol of sodium ions and 1 mol of chloride ions, producing 2 mol of solute particles, which lowers the water concentration twice as much as 1 mol of glucose. By the same reasoning, a 1 M MgCl2 solution lowers the water concentration three times as much as a 1 M glucose solution.

Since the water concentration in a solution depends upon the number of solute particles, it is useful to have a concentration term that refers to the total concentration of solute particles in a solution, regardless of their chemical composition. The total solute concentration of a solution is known as its osmolarity. One osmol is equal to 1 mol of solute particles. Thus, a 1 M solution of glucose has a concentration of 1 Osm (1 osmol per liter), whereas a 1 M solution of sodium chloride contains 2 osmol of solute per liter of solution. A liter of solution containing 1 mol of glucose and 1 mol of sodium chloride has an osmolarity of 3 Osm. A solution with an osmolarity of 3 Osm may contain 1 mol of glucose and 1 mol of sodium chloride, or 3 mol of glucose, or 1.5 mol of sodium chloride, or any other combination of solutes as long as the total solute concentration is equal to 3 Osm.

Although osmolarity refers to the concentration of solute particles, it is essential to realize that it also determines the water concentration in the solution since the higher the osmolarity, the lower the water concentration. The concentration of water in any two solutions having the same osmolarity is the same since the total number of solute particles per unit volume is the same.

Let us now apply these principles governing water concentration to the diffusion of water across membranes. Figure 6-18 shows two 1-L compartments separated by a membrane permeable to both solute and water. Initially the concentration of solute is 2 Osm in compartment 1 and 4 Osm in compartment 2. This difference in solute concentration means there is also a difference in water concentration across the membrane: 53.5 M in compartment 1 and 51.5 M in compartment 2. Therefore, there will be a net diffusion of water from the higher concentration in 1 to the lower concentration in 2, and of solute in the opposite direction, from 2 to 1. When diffusion equilibrium is reached, the two compartments will have identical solute and water concentrations, 3 Osm and 52.5 M, respectively. One mol of water will have diffused from

Initial

Initial

Penetrating And Nonpenetrating Solutes

Q> Solute 2 Osm Water 53.5 M Volume 1 L

Q> Solute 2 Osm Water 53.5 M Volume 1 L

Equilibrium

Equilibrium

FIGURE 6-18

Between two compartments of equal volume, the net diffusion of water and solute across a membrane permeable to both leads to diffusion equilibrium of both, with no change in the volume of either compartment.

FIGURE 6-18

Between two compartments of equal volume, the net diffusion of water and solute across a membrane permeable to both leads to diffusion equilibrium of both, with no change in the volume of either compartment.

compartment 1 to compartment 2, and 1 mol of solute will have diffused from 2 to 1. Since 1 mol of solute has replaced 1 mol of water in compartment 1, and vice versa in compartment 2, there is no change in the volume of either compartment.

If the membrane is now replaced by one that is permeable to water but impermeable to solute (Figure 6-19), the same concentrations of water and solute will be reached at equilibrium as before, but there will now be a change in the volumes of the compartments. Water will diffuse from 1 to 2, but there will be no solute diffusion in the opposite direction because the membrane is impermeable to solute. Water will continue to diffuse into compartment 2, therefore, until the water concentrations on the two sides become equal. The solute concentration in compartment 2 decreases as it is diluted by the incoming water, and the solute in compartment 1 becomes more concentrated as water moves out. When the water reaches diffusion equilibrium, the osmolarities of the compartments will be equal, and thus the solute concentrations must also be equal. To reach this state of equilibrium, enough

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

I. Basic Cell Functions

6. Movement of Molecules Across Cell Membranes

© The McGraw-Hill Companies, 2001

Water o 9

O Solute 2 Osm Water 53.5 M Volume 1 L

Equilibrium

O Solute 2 Osm Water 53.5 M Volume 1 L

Equilibrium

O Solute O Water Volume

FIGURE 6-19

The movement of water across a membrane that is permeable to water but not permeable to solute leads to an equilibrium state in which there is a change in the volumes of the two compartments due to the net diffusion of water (0.33 L in this case) from compartment 1 to 2. (We will assume that the membrane in this example stretches as the volume of compartment 2 increases so that no significant change in compartment pressure occurs.) [Q]

O Solute O Water Volume

FIGURE 6-19

The movement of water across a membrane that is permeable to water but not permeable to solute leads to an equilibrium state in which there is a change in the volumes of the two compartments due to the net diffusion of water (0.33 L in this case) from compartment 1 to 2. (We will assume that the membrane in this example stretches as the volume of compartment 2 increases so that no significant change in compartment pressure occurs.) [Q]

water must pass from compartment 1 to 2 to increase the volume of compartment 2 by one-third and decrease the volume of compartment 1 by an equal amount. Note that it is the presence of a membrane impermeable to solute that leads to the volume changes associated with osmosis.

We have treated the two compartments in our example as if they were infinitely expandable, so that the net transfer of water does not create a pressure difference across the membrane. This is essentially the situation that occurs across plasma membranes. In contrast, if the walls of compartment 2 could not expand, the movement of water into compartment 2 would raise the pressure in compartment 2, which would oppose further net water entry. Thus the movement of water into compartment 2 can be prevented by the application of a pressure to compartment 2. This leads to a crucial definition: When a solution containing non-penetrating solutes is separated from pure water by a membrane, the pressure that must be applied to the solution to prevent the net flow of water into the solution is termed the osmotic pressure of the solution. The greater the osmolarity of a solution, the greater its osmotic pressure. It is important to recognize that the osmotic pressure of a solution does not push water molecules into the solution. Rather it is the amount of pressure that would have to be applied to the solution to prevent the net flow of water into the solution. Like osmolarity, the osmotic pressure of a solution is a measure of the solution's water concentration—the lower the water concentration, the higher the osmotic pressure.

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Essentials of Human Physiology

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