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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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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 containingdissolved solutes) across one or more layers of cells. In humans, for example, watermoves from the blood filtrate that will form urine across a layer of epithelialcells 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 higherplants, water and minerals are absorbed by the roots and move up the plant throughconducting tubes (the xylem); water is lost from the plant mainlyby evaporation from the leaves. What these processes have in common is osmosis — the movement of water from aregion of lower solute concentration to a region of higher solute concentration. Webegin with a consideration of some basic facts about osmosis, and then show how theyexplain 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 relativelyimpermeable to ions and other solutes, but like all phospholipid bilayers, theyare somewhat permeable to water (see Figure15-1). Permeability to water is increased by water-channel proteinsdiscussed below. Water tends to move across a membrane from a solution of lowsolute concentration to one of high. Or, in other words, since solutions with ahigh amount of dissolved solute have a lower concentration of water, water willmove from a solution of high water concentration to one of lower. This processis known as osmotic flow.

Osmotic pressure is defined as thehydrostatic pressure required to stop the net flow of water across a membraneseparating 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 towater but not to solutes, the osmotic pressure across the membrane is givenby

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where π is the osmoticpressure in atmospheres (atm) or millimeters of mercury (mmHg);R is the gas constant; T is the absolutetemperature; and ΔC is the difference in total soluteconcentrations, CA andCB, on each side of the membrane. It is thetotal number of solute molecules that is important. Forexample, a 0.5 M NaCl solution is actually 0.5 M Na+ ionsand 0.5 M Cl− ions and has approximately the same osmoticpressure as a 1 M solution of glucose or lactose. From Equation 15-11 we cancalculate that a hydrostatic pressure of 0.22 atm (167 mmHg) would just balancethe water flow across a semipermeable membrane produced by a concentrationgradient of 10 mM sucrose or 5 mM NaCl.


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Figure 15-30

Experimental system for demonstrating osmotic pressure. Solutions A and B are separated by a membrane that is permeable towater but impermeable to all solutes. IfCB (the total concentration ofsolutes in solution B) is greater thanCA, 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 concentrationof 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 termedosmotic lysis. Immersion of all animal cells in a hypertonic solution (i.e., one inwhich the concentration of solutes is higher than it is in thecytosol) 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 soluteconcentration close to that of the cell cytosol (see Figure 5-22).

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

Unlike animal cells, plant, algal,fungal, and bacterial cells are surrounded by a rigid cell wall. Because of thecell wall, the osmotic influx of water that occurs when such cells are placed ina hypotonic solution (even pure water) leads to an increase in intracellularpressure 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 fromthe entry of water into the cytosol and then into the vacuole pushes the cytosoland the plasma membrane against the resistant cell wall. Cell elongation duringgrowth occurs by a hormone-induced localized loosening of a region of the cellwall, followed by influx of water into the vacuole, increasing its size (seeFigure 22-33).

Although most protozoans (like animal cells) do not have a rigid cell wall, manycontain a contractile vacuole that permits them to avoid osmotic lysis. Acontractile 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 thoughwater continuously enters the protozoan cell by osmotic flow, the contractilevacuole prevents too much water from accumulating in the cell and swelling it tothe bursting point.


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Figure 15-31

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


Water Channels Are Necessary for Bulk Flow of Water across CellMembranes

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

Microinjection experiments with mRNA encoding aquaporin, anerythrocyte membrane protein, provided convincing evidence that this proteinincreases the permeability of cells to water (Figure 15-32). In its functional form, aquaporin is a tetramer ofidentical 28-kDa subunits, each of which contains six transmembrane αhelices that form three pairs of homologs in an unusual orientation (Figure 15-33a). The channel through whichwater 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 andin other cells (e.g., the kidney cells that resorb water from the urine) thatexhibit high permeability for water.


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Figure 15-32

Experimental demonstration that aquaporin is a water-channelprotein. Frog oocytes, which normally do not express aquaporin, weremicroinjected with erythrocyte mRNA encoding aquaporin. Thesephotographs show control oocytes (bottom image in each panel) (more...)


Figure 15-33

The structure of aquaporin, a water-channel protein in theerythrocyte plasma membrane. This tetrameric protein has four identical subunits. (a) Schematicmodel of an aquaporin subunit showing the three pairs of homologoustransmembrane α helices, (more...)


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

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An understanding of osmosis and theintestinal absorption of glucose forms the basis for a simple therapy that hassaved millions of lives, particularly in less-developed countries. In thesecountries, diarrhea caused by cholera and other intestinal pathogens is a majorcause of death of young children. A cure demands not only killing the bacteriawith antibiotics, but alsorehydration — replacement ofthe water that is lost from the blood and other tissues.

Simply drinking water does not help, because it is excreted from thegastrointestinal tract almost as soon as it enters. To understand the simpletherapy that is used, recall that absorption of glucose by the small intestineinvolves the coordinated movement of Na+; one cannot betransported without the other (see Figure15-25). The movement of NaCl and glucose from the intestinal lumen,across the epithelial cells, and into the blood creates a transepithelialosmotic gradient, forcing movement of water from the intestinal lumen into theblood. Thus, giving affected children a solution of sugar and salt to drink (butnot sugar or salt alone) causes the bulk flow of water into the blood from theintestinal lumen and leads to rehydration.


Changes in Intracellular Osmotic Pressure Cause Leaf Stomata to Open

Although most plants cells do notchange their volume or shape because of the osmotic movement of water, theopening and closing ofstomata — the pores throughwhich CO2 enters a leaf — provides animportant exception. The external epidermal cells of a leaf are covered by awaxy cuticle that is largely impenetrable to water and to CO2, a gasrequired for photosynthesis by the chlorophyll-laden mesophyll cells in the leafinterior. As CO2 enters a leaf, water vapor is simultaneouslylost — a process that can be injurious to theplant. 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 vaporis lost.

Two guard cells surround each stomate (Figure15-34a). Changes in turgor pressure lead to changes in the shape ofthese guard cells, thereby opening or closing the pores. Stomatal opening iscaused by an increase in the concentration of ions or other solutes within theguard cells because of (1) opening of K+ andCl− channels and the subsequent influx ofK+ and Cl− ions from theenvironment, (2) the metabolism of stored sucrose to smaller compounds, or (3) acombination of these two processes. The resulting increase in the intracellularsolute concentration causes water to enter the guard cells osmotically,increasing their turgor pressure (Figure15-34b). Since the guard cells are connected to each other only attheir ends, the turgor pressure causes the cells to bulge outward, opening thestomatal pore between them. Stomatal closing is caused by the reverseprocess — a decrease in solute concentration andturgor pressure within the guard cells.


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, eachsurrounded by a pair of guard cells. (b) Opening ofK+ and Cl− channels inthe 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 theleaf interior so that photosynthesis can continue. When more water exits theleaf than enters it from the roots, the mesophyll cells produce the hormoneabscissic acid, which causes K+ efflux from the guard cells;water then exits the cells osmotically, and the stomata close, protecting theleaf from further dehydration.


 In response to the entry of water,protozoans maintain their normal cell volume by extruding water fromcontractile vacuoles.

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