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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 15.8Osmosis, Water Channels, and the Regulation of Cell Volume

In this section, we examine two types of transport phenomena that, at first glance, may seem unrelated: the regulation of cell volume in both plant and animal cells, and the bulk flow of water (the movement of water containing dissolved solutes) across one or more layers of cells. In humans, for example, water moves from the blood filtrate that will form urine across a layer of epithelial cells lining the kidney tubules and into the blood, thus concentrating the urine. (If this did not happen, one would excrete several liters of urine a day!) In higher plants, water and minerals are absorbed by the roots and move up the plant through conducting tubes (the xylem); water is lost from the plant mainly by evaporation from the leaves. What these processes have in common is osmosis — the movement of water from a region of lower solute concentration to a region of higher solute concentration. We begin with a consideration of some basic facts about osmosis, and then show how they explain several physiological properties of animals and plants.

Osmotic Pressure Causes Water to Move across Membranes

As noted early in this chapter, most biological membranes are relatively impermeable to ions and other solutes, but like all phospholipid bilayers, they are somewhat permeable to water (see Figure 15-1). Permeability to water is increased by water-channel proteins discussed below. Water tends to move across a membrane from a solution of low solute concentration to one of high. Or, in other words, since solutions with a high amount of dissolved solute have a lower concentration of water, water will move from a solution of high water concentration to one of lower. This process is known as osmotic flow.

Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions (Figure 15-30). In this context, the “membrane” may be a layer of cells or a plasma membrane. If the membrane is permeable to water but not to solutes, the osmotic pressure across the membrane is given by

Image ch15e22.jpg
where π is the osmotic pressure in atmospheres (atm) or millimeters of mercury (mmHg); R is the gas constant; T is the absolute temperature; and ΔC is the difference in total solute concentrations, CA and CB, on each side of the membrane. It is the total number of solute molecules that is important. For example, a 0.5 M NaCl solution is actually 0.5 M Na+ ions and 0.5 M Cl ions and has approximately the same osmotic pressure as a 1 M solution of glucose or lactose. From Equation 15-11 we can calculate that a hydrostatic pressure of 0.22 atm (167 mmHg) would just balance the water flow across a semipermeable membrane produced by a concentration gradient of 10 mM sucrose or 5 mM NaCl.

Figure 15-30. Experimental system for demonstrating osmotic pressure.

Figure 15-30

Experimental system for demonstrating osmotic pressure. Solutions A and B are separated by a membrane that is permeable to water but impermeable to all solutes. If CB (the total concentration of solutes in solution B) is greater than CA, water will tend (more...)

Different Cells Have Various Mechanisms for Controlling Cell Volume

Animal cells will swell when they are placed in a hypotonic solution (i.e., one in which the concentration of solutes is lower than it is in the cytosol). Some cells, such as erythrocytes, will actually burst as water enters them by osmotic flow. Rupture of the plasma membrane by a flow of water into the cytosol is termed osmotic lysis. Immersion of all animal cells in a hypertonic solution (i.e., one in which the concentration of solutes is higher than it is in the cytosol) causes them to shrink as water leaves them by osmotic flow. Consequently, it is essential that animal cells be maintained in an isotonic medium, which has a solute concentration close to that of the cell cytosol (see Figure 5-22).

Even in an isotonic environment, all animal cells face a problem in maintaining their cell volume. Cells contain a large number of charged macromolecules and small metabolites that attract ions of opposite charge (e.g., K+, Ca2+, PO43−). Also recall that there is a slow leakage of extracellular ions, particularly Na+ and Cl, into cells down their concentration gradient. As a result of these factors, in the absence of some countervailing mechanism, the cytosolic solute concentration would increase, causing an osmotic influx of water and eventually cell lysis. To prevent this, animal cells actively export inorganic ions as rapidly as they leak in. The export of Na+ by the ATP-powered Na+/K+ pump plays the major role in this mechanism for preventing cell swelling. If cultured cells are treated with an inhibitor that prevents production of ATP, they swell and eventually burst, demonstrating the importance of active transport in maintaining cell volume.

Image plant.jpgUnlike animal cells, plant, algal, fungal, and bacterial cells are surrounded by a rigid cell wall. Because of the cell wall, the osmotic influx of water that occurs when such cells are placed in a hypotonic solution (even pure water) leads to an increase in intracellular pressure but not in cell volume. In plant cells, the concentration of solutes (e.g., sugars and salts) usually is higher in the vacuole than in the cytosol, which in turn has a higher solute concentration than the extracellular space. The osmotic pressure, called turgor pressure, generated from the entry of water into the cytosol and then into the vacuole pushes the cytosol and the plasma membrane against the resistant cell wall. Cell elongation during growth occurs by a hormone-induced localized loosening of a region of the cell wall, followed by influx of water into the vacuole, increasing its size (see Figure 22-33).

Although most protozoans (like animal cells) do not have a rigid cell wall, many contain a contractile vacuole that permits them to avoid osmotic lysis. A contractile vacuole takes up water from the cytosol and, unlike a plant vacuole, periodically discharges its contents through fusion with the plasma membrane (Figure 15-31). Thus, even though water continuously enters the protozoan cell by osmotic flow, the contractile vacuole prevents too much water from accumulating in the cell and swelling it to the bursting point.

Figure 15-31. The contractile vacuole in Paramecium caudatum, a typical ciliated protozoan, as revealed by Nomarski microscopy of a live organism.

Figure 15-31

The contractile vacuole in Paramecium caudatum, a typical ciliated protozoan, as revealed by Nomarski microscopy of a live organism. The vacuole is filled by radiating canals that collect fluid from the cytosol. When the vacuole is full, it fuses for (more...)

Water Channels Are Necessary for Bulk Flow of Water across Cell Membranes

Even though a pure phospholipid bilayer is only slightly permeable to water, small changes in extracellular osmotic strength cause most animal cells to swell or shrink rapidly. In contrast, frog oocytes and eggs, which have an internal salt concentration comparable to other cells (≈150 mM), do not swell when placed in pond water of very low osmotic strength. These observations led investigators to suspect that the plasma membranes of erythrocytes and other cell types contain water-channel proteins that accelerate the osmotic flow of water. The absence of these water channels in frog oocytes and eggs protects them from osmotic lysis.

Microinjection experiments with mRNA encoding aquaporin, an erythrocyte membrane protein, provided convincing evidence that this protein increases the permeability of cells to water (Figure 15-32). In its functional form, aquaporin is a tetramer of identical 28-kDa subunits, each of which contains six transmembrane α helices that form three pairs of homologs in an unusual orientation (Figure 15-33a). The channel through which water moves is thought to be lined by eight transmembrane α helices, two from each subunit (Figure 15-33b). Aquaporin or homologous proteins are expressed in abundance in erythrocytes and in other cells (e.g., the kidney cells that resorb water from the urine) that exhibit high permeability for water.

Figure 15-32. Experimental demonstration that aquaporin is a water-channel protein.

Figure 15-32

Experimental demonstration that aquaporin is a water-channel protein. Frog oocytes, which normally do not express aquaporin, were microinjected with erythrocyte mRNA encoding aquaporin. These photographs show control oocytes (bottom image in each panel) (more...)

Figure 15-33. The structure of aquaporin, a water-channel protein in the erythrocyte plasma membrane.

Figure 15-33

The structure of aquaporin, a water-channel protein in the erythrocyte plasma membrane. This tetrameric protein has four identical subunits. (a) Schematic model of an aquaporin subunit showing the three pairs of homologous transmembrane α helices, (more...)

Simple Rehydration Therapy Depends on Osmotic Gradient Created by Absorption of Glucose and Na+

Image med.jpgAn understanding of osmosis and the intestinal absorption of glucose forms the basis for a simple therapy that has saved millions of lives, particularly in less-developed countries. In these countries, diarrhea caused by cholera and other intestinal pathogens is a major cause of death of young children. A cure demands not only killing the bacteria with antibiotics, but also rehydration — replacement of the water that is lost from the blood and other tissues.

Simply drinking water does not help, because it is excreted from the gastrointestinal tract almost as soon as it enters. To understand the simple therapy that is used, recall that absorption of glucose by the small intestine involves the coordinated movement of Na+; one cannot be transported without the other (see Figure 15-25). The movement of NaCl and glucose from the intestinal lumen, across the epithelial cells, and into the blood creates a transepithelial osmotic gradient, forcing movement of water from the intestinal lumen into the blood. Thus, giving affected children a solution of sugar and salt to drink (but not sugar or salt alone) causes the bulk flow of water into the blood from the intestinal lumen and leads to rehydration.

Changes in Intracellular Osmotic Pressure Cause Leaf Stomata to Open

Image plant.jpgAlthough most plants cells do not change their volume or shape because of the osmotic movement of water, the opening and closing of stomata — the pores through which CO2 enters a leaf — provides an important exception. The external epidermal cells of a leaf are covered by a waxy cuticle that is largely impenetrable to water and to CO2, a gas required for photosynthesis by the chlorophyll-laden mesophyll cells in the leaf interior. As CO2 enters a leaf, water vapor is simultaneously lost — a process that can be injurious to the plant. Thus it is essential that the stomata open only during periods of light, when photosynthesis occurs; even then, they must close if too much water vapor is lost.

Two guard cells surround each stomate (Figure 15-34a). Changes in turgor pressure lead to changes in the shape of these guard cells, thereby opening or closing the pores. Stomatal opening is caused by an increase in the concentration of ions or other solutes within the guard cells because of (1) opening of K+ and Cl channels and the subsequent influx of K+ and Cl ions from the environment, (2) the metabolism of stored sucrose to smaller compounds, or (3) a combination of these two processes. The resulting increase in the intracellular solute concentration causes water to enter the guard cells osmotically, increasing their turgor pressure (Figure 15-34b). Since the guard cells are connected to each other only at their ends, the turgor pressure causes the cells to bulge outward, opening the stomatal pore between them. Stomatal closing is caused by the reverse process — a decrease in solute concentration and turgor pressure within the guard cells.

Figure 15-34. The opening and closing of stomata.

Figure 15-34

The opening and closing of stomata. (a) Light micrograph of a leaf of a wandering Jew (Tradescantia sp) plant shows two stomata, each surrounded by a pair of guard cells. (b) Opening of K+ and Cl channels in the plasma membrane of the guard cells (more...)

Stomatal opening is under tight physiological control by at least two mechanisms. A drop in CO2 within the leaf, resulting from active photosynthesis, causes the stomata to open, permitting additional CO2 to enter the leaf interior so that photosynthesis can continue. When more water exits the leaf than enters it from the roots, the mesophyll cells produce the hormone abscissic acid, which causes K+ efflux from the guard cells; water then exits the cells osmotically, and the stomata close, protecting the leaf from further dehydration.

SUMMARY

  •  Most biological membranes are more permeable to water than to ions or other solutes, and water moves across them by osmosis from a solution of lower solute concentration to one of higher solute concentration.
  •  Animal cells swell or shrink when placed in hypotonic or hypertonic solutions, respectively. To maintain their normal cytosolic osmolarity and hence cell volume, animal cells must export Na+ and other ions that leak or are transported from the extracellular space into the cytosol.
  •  The rigid cell wall surrounding plant cells prevents their swelling and leads to generation of turgor pressure in response to the osmotic influx of water.
  •  In response to the entry of water, protozoans maintain their normal cell volume by extruding water from contractile vacuoles.
  •  Aquaporin in the erythrocyte plasma membrane and other water-channel proteins increase the water permeability of biomembranes (see Figure 15-33).
  •  Opening and closing of K+ and Cl channels and the resulting changes in cytosolic solute concentrations of guard cells cause stomata in leaves to open and close (see Figure 15-34).
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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21739

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