chapter the plasma membrane, 2 membrane transport, and...

The Plasma Membrane,Membrane Transport, and the RestingMembrane PotentialStephen A. Kempson, Ph.D.

2C H A P T E R

2

The intracellular fluid of living cells, the cytosol, has acomposition very different from that of the extracellu-

lar fluid. For example, the concentrations of potassium andphosphate ions are higher inside cells than outside, whereassodium, calcium, and chloride ion concentrations are muchlower inside cells than outside. These differences are nec-essary for the proper functioning of many intracellular en-zymes; for instance, the synthesis of proteins by the ribo-somes requires a relatively high potassium concentration.The cell membrane or plasma membrane creates and main-tains these differences by establishing a permeability bar-rier around the cytosol. The ions and cell proteins neededfor normal cell function are prevented from leaking out;those not needed by the cell are unable to enter the cellfreely. The cell membrane also keeps metabolic intermedi-

ates near where they will be needed for further synthesis orprocessing and retains metabolically expensive proteins in-side the cell.

The plasma membrane is necessarily selectively perme-able. Cells must receive nutrients in order to function, andthey must dispose of metabolic waste products. To functionin coordination with the rest of the organism, cells receiveand send information in the form of hormones and neuro-transmitters. The plasma membrane has mechanisms thatallow specific molecules to cross the barrier around the cell.A selective barrier surrounds not only the cell but also everyintracellular organelle that requires an internal milieu dif-ferent from that of the cytosol. The cell nucleus, mito-chondria, endoplasmic reticulum, Golgi apparatus, andlysosomes are delimited by membranes similar in composi-

■ THE STRUCTURE OF THE PLASMA MEMBRANE

■ MECHANISMS OF SOLUTE TRANSPORT

■ THE MOVEMENT OF WATER ACROSS THE PLASMA

MEMBRANE

■ THE RESTING MEMBRANE POTENTIAL

C H A P T E R O U T L I N E

1. The two major components of the plasma membrane of a cell are proteins and lipids, present in about equalproportions.

2. Membrane proteins are responsible for most of the func-tions of the plasma membrane, including the transport ofwater and solutes across the membrane and providingspecific binding sites for extracellular signaling moleculessuch as hormones.

3. Carrier-mediated transport systems allow the rapid trans-port of polar molecules, reach a maximum rate at high sub-strate concentration, exhibit structural specificity, and arecompetitively inhibited by molecules of similar structure.

4. Voltage-gated channels are opened by a change in themembrane potential, and ligand-gated channels areopened by the binding of a specific agonist.

5. The Na�/K�-ATPase pump is an example of primary activetransport, and Na�-coupled glucose transport is an exam-ple of secondary active transport.

6. The polarized organization of epithelial cells produces a di-rectional movement of solutes and water across the ep-ithelium.

7. Many cells regulate their volume when exposed to osmoticstress by activating transport systems that allow the exit orentry of solute so that water will follow.

8. The Goldman equation gives the value of the mem-brane potential when all the permeable ions are ac-counted for.

9. In most cells, the resting membrane potential is close tothe Nernst potential for K�.

K E Y C O N C E P T S

19

20 PART I CELLULAR PHYSIOLOGY

tion to the plasma membrane. This chapter describes thespecific types of membrane transport mechanisms for ionsand other solutes, their relative contributions to the restingmembrane potential, and how their activities are coordi-nated to achieve directional transport from one side of acell layer to the other.

THE STRUCTURE OF THE PLASMA MEMBRANE

The first theory of membrane structure proposed thatcells are surrounded by a double layer of lipid molecules,a lipid bilayer. This theory was based on the known ten-dency of lipid molecules to form lipid bilayers with lowpermeability to water-soluble molecules. However, thelipid bilayer theory did not explain the selective move-ment of certain water-soluble compounds, such as glucoseand amino acids, across the plasma membrane. In 1972,Singer and Nicolson proposed the fluid mosaic model ofthe plasma membrane (Fig. 2.1). With minor modifica-tions, this model is still accepted as the correct picture ofthe structure of the plasma membrane.

The Plasma Membrane Has Proteins Inserted

in the Lipid Bilayer

Proteins and lipids are the two major components of theplasma membrane, present in about equal proportions byweight. The various lipids are arranged in a lipid bilayer,and two different types of proteins are associated with thisbilayer. Integral proteins (or intrinsic proteins) are embed-ded in the lipid bilayer; many span it completely, being ac-cessible from the inside and outside of the membrane. Thepolypeptide chain of these proteins may cross the lipid bi-layer once or may make multiple passes across it. Themembrane-spanning segments usually contain amino acids

with nonpolar side chains and are arranged in an ordered �-helical conformation. Peripheral proteins (or extrinsic pro-teins) do not penetrate the lipid bilayer. They are in con-tact with the outer side of only one of the lipidlayers—either the layer facing the cytoplasm or the layerfacing the extracellular fluid (see Fig. 2.1). Many membraneproteins have carbohydrate molecules, in the form of spe-cific sugars, attached to the parts of the proteins that areexposed to the extracellular fluid. These molecules areknown as glycoproteins. Some of the integral membraneproteins can move in the plane of the membrane, like smallboats floating in the “sea” formed by the bilayer arrange-ment of the lipids. Other membrane proteins are anchoredto the cytoskeleton inside the cell or to proteins of the ex-tracellular matrix.

The proteins in the plasma membrane play a variety ofroles. Many peripheral membrane proteins are enzymes,and many membrane-spanning integral proteins are carri-ers or channels for the movement of water-soluble mole-cules and ions into and out of the cell. Another importantrole of membrane proteins is structural; for example, cer-tain membrane proteins in the erythrocyte help maintainthe biconcave shape of the cell. Finally, some membraneproteins serve as highly specific recognition sites or recep-tors on the outside of the cell membrane to which extra-cellular molecules, such as hormones, can bind. If the re-ceptor is a membrane-spanning protein, it provides amechanism for converting an extracellular signal into anintracellular response.

There Are Different Types of Membrane Lipids

Lipids found in cell membranes can be classified into twobroad groups: those that contain fatty acids as part of thelipid molecule and those that do not. Phospholipids are anexample of the first group, and cholesterol is the most im-portant example of the second group.

Phospholipids. The fatty acids present in phospholipidsare molecules with a long hydrocarbon chain and a car-boxyl terminal group. The hydrocarbon chain can be satu-rated (no double bonds between the carbon atoms) or un-saturated (one or more double bonds present). Thecomposition of fatty acids gives them some peculiar char-acteristics. The long hydrocarbon chain tends to avoidcontact with water and is described as hydrophobic. Thecarboxyl group at the other end is compatible with waterand is termed hydrophilic. Fatty acids are said to be amphi-pathic because both hydrophobic and hydrophilic regionsare present in the same molecule.

Phospholipids are the most abundant complex lipidsfound in cell membranes. They are amphipathic moleculesformed by two fatty acids (normally, one saturated and oneunsaturated) and one phosphoric acid group substituted onthe backbone of a glycerol or sphingosine molecule. Thisarrangement produces a hydrophobic area formed by thetwo fatty acids and a polar hydrophilic head. When phos-pholipids are arranged in a bilayer, the polar heads are onthe outside and the hydrophobic fatty acids on the inside.It is difficult for water-soluble molecules and ions to pass di-rectly through the hydrophobic interior of the lipid bilayer.

Integral proteinsGlycoprotein

Extracellular fluid

Cytoplasm

Glycolipid

Channel

Phospholipid Cholesterol

Peripheral protein

The fluid mosaic model of the plasma

membrane. Lipids are arranged in a bilayer. In-tegral proteins are embedded in the bilayer and often span it.Some membrane-spanning proteins form channels. Peripheralproteins do not penetrate the bilayer.

FIGURE 2.1

The phospholipids, with a backbone of sphingosine (along amino alcohol), are usually called sphingolipids andare present in all plasma membranes in small amounts. Theyare especially abundant in brain and nerve cells.

Glycolipids are lipid molecules that contain sugars andsugar derivatives (instead of phosphoric acid) in the po-lar head. They are located mainly in the outer half of thelipid bilayer, with the sugar molecules facing the extra-cellular fluid.

Cholesterol. Cholesterol is an important component ofmammalian plasma membranes. The proportion of cho-lesterol in plasma membranes varies from 10% to 50% oftotal lipids. Cholesterol has a rigid structure that stabi-lizes the cell membrane and reduces the natural mobilityof the complex lipids in the plane of the membrane. In-creasing amounts of cholesterol make it more difficult forlipids and proteins to move in the membrane. Some cellfunctions, such as the response of immune system cells tothe presence of an antigen, depend on the ability ofmembrane proteins to move in the plane of the mem-brane to bind the antigen. A decrease in membrane fluid-ity resulting from an increase in cholesterol will impairthese functions.

MECHANISMS OF SOLUTE TRANSPORT

All cells need to import oxygen, sugars, amino acids, andsome small ions and to export carbon dioxide, metabolicwastes, and secretions. At the same time, specialized cellsrequire mechanisms to transport molecules such as en-zymes, hormones, and neurotransmitters. The movementof large molecules is carried out by endocytosis and exocy-

tosis, the transfer of substances into or out of the cell, re-spectively, by vesicle formation and vesicle fusion with theplasma membrane. Cells also have mechanisms for therapid movement of ions and solute molecules across theplasma membrane. These mechanisms are of two generaltypes: passive movement, which requires no direct expen-diture of metabolic energy, and active movement, whichuses metabolic energy to drive solute transport.

Macromolecules Cross the Plasma Membrane by

Vesicle Fusion

Phagocytosis and Endocytosis. Phagocytosis is the in-gestion of large particles or microorganisms, usually occur-ring only in specialized cells such as macrophages (Fig.2.2). An important function of macrophages in humans is toremove invading bacteria. The phagocytic vesicle (1 to 2�m in diameter) is almost as large as the phagocytic cell it-self. Phagocytosis requires a specific stimulus. It occursonly after the extracellular particle has bound to the extra-cellular surface. The particle is then enveloped by expan-sion of the cell membrane around it.

Endocytosis is a general term for the process in which aregion of the plasma membrane is pinched off to form anendocytic vesicle inside the cell. During vesicle formation,some fluid, dissolved solutes, and particulate material fromthe extracellular medium are trapped inside the vesicle andinternalized by the cell. Endocytosis produces muchsmaller endocytic vesicles (0.1 to 0.2 �m in diameter) thanphagocytosis. It occurs in almost all cells and is termed aconstitutive process because it occurs continually and spe-cific stimuli are not required. In further contrast to phago-cytosis, endocytosis originates with the formation of de-pressions in the cell membrane. The depressions pinch off

CHAPTER 2 The Plasma Membrane, Membrane Transport, and the Resting Membrane Potential 21

Coated pit

Exocytosis

Receptor-mediatedendocytosis

Fluid-phaseendocytosis

Phagocytosis

Extracellularfluid

Cytoplasm

Endocytosis

Ligand

Receptor

Plasmamembrane

The transport of macromolecules across the

plasma membrane by the formation of vesi-

cles. Particulate matter in the extracellular fluid is engulfed andinternalized by phagocytosis. During fluid-phase endocytosis, ex-tracellular fluid and dissolved macromolecules enter the cell inendocytic vesicles that pinch off at depressions in the plasmamembrane. Receptor-mediated endocytosis uses membrane recep-

FIGURE 2.2 tors at coated pits to bind and internalize specific solutes (lig-ands). Exocytosis is the release of macromolecules destined for ex-port from the cell. These are packed inside secretory vesicles thatfuse with the plasma membrane and release their contents outsidethe cell. (Modified from Dautry-Varsat A, Lodish HF. How recep-tors bring proteins and particles into cells. Sci Am1984;250(5):52–58.)


within a few minutes after they form and give rise to endo-cytic vesicles inside the cell.

Two main types of endocytosis can be distinguished (seeFig. 2.2). Fluid-phase endocytosis is the nonspecific up-take of the extracellular fluid and all its dissolved solutes.The material is trapped inside the endocytic vesicle as it ispinched off inside the cell. The amount of extracellular ma-terial internalized by this process is directly proportional toits concentration in the extracellular solution. Receptor-mediated endocytosis is a more efficient process that usesreceptors on the cell surface to bind specific molecules.These receptors accumulate at specific depressions knownas coated pits, so named because the cytosolic surface ofthe membrane at this site is covered with a coat of severalproteins. The coated pits pinch off continually to form en-docytic vesicles, providing the cell with a mechanism forrapid internalization of a large amount of a specific mole-cule without the need to endocytose large volumes of ex-tracellular fluid. The receptors also aid the cellular uptakeof molecules present at low concentrations outside the cell.Receptor-mediated endocytosis is the mechanism by whichcells take up a variety of important molecules, includinghormones; growth factors; and serum transport proteins,such as transferrin (an iron carrier). Foreign substances,such as diphtheria toxin and certain viruses, also enter cellsby this pathway.

Exocytosis. Many cells synthesize important macromol-ecules that are destined for exocytosis or export from thecell. These molecules are synthesized in the endoplasmicreticulum, modified in the Golgi apparatus, and packed in-side transport vesicles. The vesicles move to the cell sur-face, fuse with the cell membrane, and release their con-tents outside the cell (see Fig. 2.2).

There are two exocytic pathways—constitutive and reg-ulated. Some proteins are secreted continuously by thecells that make them. Secretion of mucus by goblet cells inthe small intestine is a specific example. In this case, exo-cytosis follows the constitutive pathway, which is presentin all cells. In other cells, macromolecules are stored insidethe cell in secretory vesicles. These vesicles fuse with thecell membrane and release their contents only when a spe-cific extracellular stimulus arrives at the cell membrane.This pathway, known as the regulated pathway, is respon-sible for the rapid “on-demand” secretion of many specifichormones, neurotransmitters, and digestive enzymes.

The Passive Movement of Solutes Tends

to Equilibrate Concentrations

Simple Diffusion. Any solute will tend to uniformly oc-cupy the entire space available to it. This movement,known as diffusion, is due to the spontaneous Brownian(random) movement that all molecules experience and thatexplains many everyday observations. Sugar diffuses in cof-fee, lemon diffuses in tea, and a drop of ink placed in a glassof water will diffuse and slowly color all the water. The netresult of diffusion is the movement of substances accordingto their difference in concentrations, from regions of highconcentration to regions of low concentration. Diffusion isan effective way for substances to move short distances.

The speed with which the diffusion of a solute in wateroccurs depends on the difference of concentration, the sizeof the molecules, and the possible interactions of the dif-fusible substance with water. These different factors appearin Fick’s law, which describes the diffusion of any solute inwater. In its simplest formulation, Fick’s law can be written as:

J � DA (C1 � C2)/�X (1)

where J is the flow of solute from region 1 to region 2 in thesolution, D is the diffusion coefficient of the solute andtakes into consideration such factors as solute molecularsize and interactions of the solute with water, A is the cross-sectional area through which the flow of solute is measured,C is the concentration of the solute at regions 1 and 2, and�X is the distance between regions 1 and 2. J is expressedin units of amount of substance per unit area per unit time,for example, mol/cm2 per hour, and is also referred to as thesolute flux.

Diffusive Membrane Transport. Solutes can enter orleave a cell by diffusing passively across the plasma mem-brane. The principal force driving the diffusion of an un-charged solute is the difference of concentration betweenthe inside and the outside of the cell (Fig. 2.3). In the caseof an electrically charged solute, such as an ion, diffusion isalso driven by the membrane potential, which is the elec-trical gradient across the membrane. The membrane po-tential of most living cells is negative inside the cell relativeto the outside.

The diffusion of gases and lipid-soluble

molecules through the lipid bilayer. In thisexample, the diffusion of a solute across a plasma membrane isdriven by the difference in concentration on the two sides of themembrane. The solute molecules move randomly by Brownianmovement. Initially, random movement from left to right acrossthe membrane is more frequent than movement in the oppositedirection because there are more molecules on the left side. Thisresults in a net movement of solute from left to right across themembrane until the concentration of solute is the same on bothsides. At this point, equilibrium (no net movement) is reached be-cause solute movement from left to right is balanced by equalmovement from right to left.

FIGURE 2.3

Diffusion across a membrane has no preferential direc-tion; it can occur from the outside of the cell toward the in-side or from the inside of the cell toward the outside. Forany substance, it is possible to measure the permeabilitycoefficient (P), which gives the speed of the diffusionacross a unit area of plasma membrane for a defined drivingforce. Fick’s law for the diffusion of an uncharged soluteacross a membrane can be written as:

J � PA (C1 � C2) (2)

which is similar to equation 1. P includes the membranethickness, diffusion coefficient of the solute within themembrane, and solubility of the solute in the membrane.Dissolved gases, such as oxygen and carbon dioxide, havehigh permeability coefficients and diffuse across the cellmembrane rapidly. Since diffusion across the plasmamembrane usually implies that the diffusing solute entersthe lipid bilayer to cross it, the solute’s solubility in a lipidsolvent (e.g., olive oil or chloroform) compared with itssolubility in water is important in determining its perme-ability coefficient.

A substance’s solubility in oil compared with its solu-bility in water is its partition coefficient. Lipophilic sub-stances that mix well with the lipids in the plasma mem-brane have high partition coefficients and, as a result,high permeability coefficients; they tend to cross theplasma membrane easily. Hydrophilic substances, such asions and sugars, do not interact well with the lipid com-ponent of the membrane, have low partition coefficientsand low permeability coefficients, and diffuse across themembrane more slowly.

For solutes that diffuse across the lipid part of the plasmamembrane, the relationship between the rate of movement

and the difference in concentration between the two sidesof the membrane is linear (Fig. 2.4). The higher the differ-ence in concentration (C1 � C2), the greater the amount ofsubstance crossing the membrane per unit time.

Facilitated Diffusion via Carrier Proteins. For manysolutes of physiological importance, such as sugars andamino acids, the relationship between transport rate andconcentration difference follows a curve that reaches aplateau (Fig. 2.5). Furthermore, the rate of transport ofthese hydrophilic substances across the cell membrane ismuch faster than expected for simple diffusion through alipid bilayer. Membrane transport with these characteris-tics is often called carrier-mediated transport because anintegral membrane protein, the carrier, binds the trans-ported solute on one side of the membrane and releases itat the other side. Although the details of this transportmechanism are unknown, it is hypothesized that the bind-ing of the solute causes a conformational change in the car-rier protein, which results in translocation of the solute(Fig. 2.6). Because there are limited numbers of these carri-ers in any cell membrane, increasing the concentration ofthe solute initially uses the existing “spare” carriers to trans-port the solute at a higher rate than by simple diffusion. Asthe concentration of the solute increases further and moresolute molecules bind to carriers, the transport systemeventually reaches saturation, when all the carriers are in-volved in translocating molecules of solute. At this point,additional increases in solute concentration do not increasethe rate of solute transport (see Fig. 2.5).

The types of carrier-mediated transport mechanismsconsidered here can transport a solute along its concentra-tion gradient only, as in simple diffusion. Net movement


1 2

Solute concentration (mmol/L)outside cell

3

Simple diffusion

5

10

Rat

e of

sol

ute

entr

y (m

mol

/min

)

A graph of solute transport across a plasma

membrane by simple diffusion. The rate ofsolute entry increases linearly with extracellular concentration ofthe solute. Assuming no change in intracellular concentration, in-creasing the extracellular concentration increases the gradientthat drives solute entry.

FIGURE 2.4

32

Solute concentration (mmol/L)outside cell

Carrier-mediated transport

1

10

5

Vmax

Rat

e of

sol

ute

entr

y (m

mol

/min

)

A graph of solute transport across a plasma

membrane by carrier-mediated transport.

The rate of transport is much faster than that of simple diffusion(see Fig. 2.4) and increases linearly as the extracellular solute con-centration increases. The increase in transport is limited, how-ever, by the availability of carriers. Once all are occupied bysolute, further increases in extracellular concentration have no ef-fect on the rate of transport. A maximum rate of transport (Vmax)is achieved that cannot be exceeded.

FIGURE 2.5


stops when the concentration of the solute has the samevalue on both sides of the membrane. At this point, withreference to equation 2, C1 � C2 and the value of J is 0. Thetransport systems function until the solute concentrationshave equilibrated. However, equilibrium is attained muchfaster than with simple diffusion.

Equilibrating carrier-mediated transport systems haveseveral characteristics:• They allow the transport of polar (hydrophilic) mole-

cules at rates much higher than expected from the parti-tion coefficient of these molecules.

• They eventually reach saturation at high substrate con-centration.

• They have structural specificity, meaning each carriersystem recognizes and binds specific chemical structures(a carrier for D-glucose will not bind or transport L-glu-cose).

• They show competitive inhibition by molecules withsimilar chemical structure. For example, carrier-medi-ated transport of D-glucose occurs at a slower rate whenmolecules of D-galactose also are present. This is be-cause galactose, structurally similar to glucose, competeswith glucose for the available glucose carrier proteins.A specific example of this type of carrier-mediated trans-

port is the movement of glucose from the blood to the in-terior of cells. Most mammalian cells use blood glucose as amajor source of cellular energy, and glucose is transportedinto cells down its concentration gradient. The transportprocess in many cells, such as erythrocytes and the cells of

fat, liver, and muscle tissues, involves a plasma membraneprotein called GLUT 1 (glucose transporter 1). The ery-throcyte GLUT 1 has an affinity for D-glucose that is about2,000-fold greater than the affinity for L-glucose. It is an in-tegral membrane protein that contains 12 membrane-span-ning �-helical segments.

Equilibrating carrier-mediated transport, like simplediffusion, does not have directional preferences. It func-tions equally well in bringing its specific solutes into orout of the cell, depending on the concentration gradient.Net movement by equilibrating carrier-mediated trans-port ceases once the concentrations inside and outside thecell become equal.

The anion exchange protein (AE1), the predominantintegral protein in the mammalian erythrocyte mem-brane, provides a good example of the reversibility oftransporter action. AE1 is folded into at least 12 trans-membrane �-helices and normally permits the one-for-one exchange of Cl� and HCO3

� ions across the plasmamembrane. The direction of ion movement is dependentonly on the concentration gradients of the transportedions. AE1 has an important role in transporting CO2 fromthe tissues to the lungs. The erythrocytes in systemiccapillaries pick up CO2 from tissues and convert it toHCO3

�, which exits the cells via AE1. When the ery-throcytes enter pulmonary capillaries, the AE1 allowsplasma HCO3

� to enter erythrocytes, where it is con-verted back to CO2 for expiration by the lungs (seeChapter 21).

A BThe role of a carrier protein in facilitated

diffusion of solute molecules across a

plasma membrane. In this example, solute transport into the cellis driven by the high solute concentration outside compared toinside. A, Binding of extracellular solute to the carrier, a mem-brane-spanning integral protein, may trigger a change in protein

FIGURE 2.6 conformation that exposes the bound solute to the interior of thecell. B, Bound solute readily dissociates from the carrier because ofthe low intracellular concentration of solute. The release of solutemay allow the carrier to revert to its original conformation (A) tobegin the cycle again.

Facilitated Diffusion Through Ion Channels. Small ions,such as Na�, K�, Cl�, and Ca2�, also cross the plasmamembrane faster than would be expected based on theirpartition coefficients in the lipid bilayer. An ion’s electricalcharge makes it difficult for the ion to move across the lipidbilayer. The rapid movement of ions across the membrane,however, is an aspect of many cell functions. The nerve ac-tion potential, the contraction of muscle, the pacemakerfunction of the heart, and many other physiological eventsare possible because of the ability of small ions to enter orleave the cell rapidly. This movement occurs through se-lective ion channels.

Ion channels are integral proteins spanning the width ofthe plasma membrane and are normally composed of sev-eral polypeptide subunits. Certain specific stimuli cause theprotein subunits to open a gate, creating an aqueous chan-nel through which the ions can move (Fig. 2.7). In this way,ions do not need to enter the lipid bilayer to cross the mem-brane; they are always in an aqueous medium. When thechannels are open, the ions move rapidly from one side ofthe membrane to the other by facilitated diffusion. Specificinteractions between the ions and the sides of the channelproduce an extremely rapid rate of ion movement; in fact,ion channels permit a much faster rate of solute transport(about 108 ions/sec) than carrier-mediated systems.

Ion channels are often selective. For example, somechannels are selective for Na�, for K�, for Ca2�, for Cl�,and for other anions and cations. It is generally assumedthat some kind of ionic selectivity filter must be built intothe structure of the channel (see Fig. 2.7). No clear relationbetween the amino acid composition of the channel pro-tein and ion selectivity of the channel has been established.

A great deal of information about the characteristic be-havior of channels for different ions has been revealed bythe patch clamp technique. The small electrical currentcaused by ion movement when a channel is open can be de-tected with this technique, which is so sensitive that theopening and closing of a single ion channel can be ob-

served (Fig. 2.8). In general, ion channels exist either fullyopen or completely closed, and they open and close veryrapidly. The frequency with which a channel opens is vari-able, and the time the channel remains open (usually a fewmilliseconds) is also variable. The overall rate of ion trans-port across a membrane can be controlled by changing thefrequency of a channel opening or by changing the time achannel remains open.

Most ion channels usually open in response to a specificstimulus. Ion channels can be classified according to theirgating mechanisms, the signals that make them open orclose. There are voltage-gated channels and ligand-gatedchannels. Some ion channels are always open and these arereferred to as nongated channels (see Chapter 3).

Voltage-gated ion channels open when the membranepotential changes beyond a certain threshold value. Chan-nels of this type are involved in the conduction of actionpotentials along nerve axons and they include sodium andpotassium channels (see Chapter 3). Voltage-gated ionchannels are found in many cell types. It is thought thatsome charged amino acids located in a membrane-spanning�-helical segment of the channel protein are sensitive tothe transmembrane potential. Changes in the membranepotential cause these amino acids to move and induce aconformational change of the protein that opens the wayfor the ions.

Ligand-gated (or, chemically gated) ion channels cannotopen unless they first bind to a specific agonist. The openingof the gate is produced by a conformational change in theprotein induced by the ligand binding. The ligand can be aneurotransmitter arriving from the extracellular medium. Italso can be an intracellular second messenger, produced in re-sponse to some cell activity or hormone action, that reachesthe ion channel from the inside of the cell. The nicotinicacetylcholine receptor channel found in the postsynapticneuromuscular junction (see Chapters 3 and 9) is a ligand-gated ion channel that is opened by an extracellular ligand(acetylcholine). Examples of ion channels gated by intracel-


An ion channel. Ion channels are formed be-tween the polypeptide subunits of integral pro-

teins that span the plasma membrane, providing an aqueous porethrough which ions can cross the membrane. Different types ofgating mechanisms are used to open and close channels. Ionchannels are often selective for a specific ion.

FIGURE 2.7

A patch clamp recording from a frog mus-

cle fiber. Ions flow through the channel whenit opens, generating a current. The current in this experiment isabout 3 pA and is detected as a downward deflection in therecording. When more than one channel opens, the current andthe downward deflection increase in direct proportion to thenumber of open channels. This record shows that up to threechannels are open at any instant. (Modified from Kandel ER,Schwartz JH, Jessell TM. Principles of Neural Science. 3rd Ed.New York: Elsevier, 1991.)

FIGURE 2.8


lular messengers also abound in nature. This type of gatingmechanism allows the channel to open or close in response toevents that occur at other locations in the cell. For example, asodium channel gated by intracellular cyclic GMP is involvedin the process of vision (see Chapter 4). This channel is lo-cated in the rod cells of the retina and it opens in the presenceof cyclic GMP. The generalized structure of one subunit of anion channel gated by cyclic nucleotides is shown in Figure2.9. There are six membrane-spanning regions and a cyclicnucleotide-binding site is exposed to the cytosol. The func-tional protein is a tetramer of four identical subunits. Othercell membranes have potassium channels that open when theintracellular concentration of calcium ions increases. Severalknown channels respond to inositol 1,4,5-trisphosphate, theactivated part of G proteins, or ATP. The gating of the ep-ithelial chloride channel by ATP is described in the ClinicalFocus Box 2.1 in this chapter.

Solutes Are Moved Against Gradients

by Active Transport Systems

The passive transport mechanisms discussed all tend tobring the cell into equilibrium with the extracellular fluid.Cells must oppose these equilibrating systems and preserveintracellular concentrations of solutes, particularly ions,that are compatible with life.

1 2 3 4 5 6

COOHH2N

In

Out

I

Bindingsite

A

B

III

III

IV

Structure of a cyclic nucleotide-gated ion

channel. A, The secondary structure of a singlesubunit has six membrane-spanning regions and a binding site forcyclic nucleotides on the cytosolic side of the membrane. B, Fouridentical subunits (I–IV) assemble together to form a functionalchannel that provides a hydrophilic pathway across the plasmamembrane.

FIGURE 2.9

K+

Na+

Lipid bilayer

Na+

Out

In

K+

Na +

Pi

Pi

K+

K+

ATP

ADP1

2

Pi

K+3

4

5

The possible se-

quence of events dur-

ing one cycle of the sodium-potassium

pump. The functional form may be atetramer of two large catalytic subunitsand two smaller subunits of unknownfunction. Binding of intracellular Na�

and phosphorylation by ATP inside thecell may induce a conformational changethat transfers Na� to the outside of thecell (steps 1 and 2). Subsequent bindingof extracellular K� and dephosphoryla-tion return the protein to its original formand transfer K� into the cell (steps 3, 4,and 5). There are thought to be threeNa� binding sites and two K� bindingsites. During one cycle, three Na� are ex-changed for two K�, and one ATP mole-cule is hydrolyzed.

FIGURE 2.10


CLINICAL FOCUS BOX 2.1

Cystic Fibrosis

Cystic fibrosis is one of the most common lethal geneticdiseases of Caucasians. In northern Europe and the UnitedStates, for example, about 1 child in 2,500 is born with thedisease. It was first recognized clinically in the 1930s, whenit appeared to be a gastrointestinal problem because pa-tients usually died from malnutrition during the first yearof life. Survival has improved as management has im-proved; afflicted newborns now have a life expectancy ofabout 40 years. Cystic fibrosis affects several organ sys-tems, with the severity varying enormously among indi-viduals. Clinical features can include deficient secretion ofdigestive enzymes by the pancreas; infertility in males; in-creased concentration of chloride ions in sweat; intestinaland liver disease; and airway disease, leading to progres-sive lung dysfunction. Involvement of the lungs deter-mines survival: 95% of cystic fibrosis patients die from res-piratory failure.

The basic defect in cystic fibrosis is a failure of chloridetransport across epithelial plasma membranes, particu-larly in the epithelial cells that line the airways. Much of theinformation about defective chloride transport was ob-tained by studying individual chloride channels using thepatch clamp technique. One hypothesis is that all thepathophysiology of cystic fibrosis is a direct result of chlo-ride transport failure. In the lungs, for example, reducedsecretion of chloride ion is usually accompanied by a re-duced secretion of sodium and bicarbonate ions. Thesechanges retard the secretion of water, so the mucus secre-tions that line airways become thick and sticky and thesmaller airways become blocked. The thick mucus alsotraps bacteria, which may lead to bacterial infection. Onceestablished, bacterial infection is difficult to eradicate fromthe lungs of a patient with cystic fibrosis.

It was predicted that the flawed gene in patients withcystic fibrosis would normally encode either a chloridechannel protein or a membrane protein that regulateschloride channels. The gene was identified in 1989 and en-codes a protein of 1,480 amino acids, the cystic fibrosistransmembrane conductance regulator (CFTR). Evi-dence indicates that CFTR contains both a chloride channeland a channel regulator. Although it functions as an ionchannel, it has structural similarities to adenosine triphos-

phate (ATP)-driven ion pumps that are integral membraneproteins. The CFTR protein is anchored in the plasmamembrane by 12 membrane-spanning segments that alsoform a channel. A large regulatory domain is exposed tothe cytosol and contains several sites that can be phos-phorylated by various protein kinases, such as cyclicadenosine monophosphate (AMP)-dependent protein ki-nase. Two nucleotide-binding domains (NBD) controlchannel activity through interactions with nucleotides,such as ATP, present in the cell cytosol. A two-step processcontrols the gating of CFTR: (1) phosphorylation of specificsites within the regulatory domain, and (2) binding andhydrolysis of ATP at the NBD. After initial phosphorylation,gating between the closed and open states is controlled byATP hydrolysis. It is believed that the channel is opened byATP hydrolysis at one NBD and closed by subsequent ATPhydrolysis at the other NBD.

A common mutation in CFTR, found in 70% of cystic fi-brosis patients, results in the loss of the amino acid pheny-lalanine from one of the NBD. This mutation produces se-vere symptoms because it results in defective targeting ofnewly synthesized CFTR proteins to the plasma mem-brane. The number of functional CFTR proteins at the cor-rect location is decreased to an inadequate level.

Increased understanding of the pathophysiology ofairway disease in cystic fibrosis has given rise to newtherapies, and a definitive solution may be close at hand.Two approaches are undergoing clinical trials. One ap-proach is to design pharmacological agents that will ei-ther regulate (open or close) defective CFTR chloridechannels or bypass CFTR and stimulate other membranechloride channels in the same cells. The other approachis the use of gene therapy to insert a normal gene forCFTR into affected airway epithelial cells. This has theadvantage of restoring both the known and unknownfunctions of the gene. The field of gene therapy is in itsinfancy, and although there have been no “cures” forcystic fibrosis, much has been learned about the prob-lems presented by the inefficient and short-lived transferof genes in vivo. The next phase of gene therapy will fo-cus on improving the technology for gene delivery. Genetherapy may become a reality for many lung diseasesduring this century.

Primary Active Transport. Integral membrane proteinsthat directly use metabolic energy to transport ions againsta gradient of concentration or electrical potential areknown as ion pumps. The direct use of metabolic energy tocarry out transport defines a primary active transportmechanism. The source of metabolic energy is ATP syn-thesized by mitochondria, and the different ion pumps hy-drolyze ATP to ADP and use the energy stored in the thirdphosphate bond to carry out transport. Because of this abil-ity to hydrolyze ATP, ion pumps also are called ATPases.

The most abundant ion pump in higher organisms is thesodium-potassium pump or Na�/K�-ATPase. It is foundin the plasma membrane of practically every eukaryotic celland is responsible for maintaining the low sodium and highpotassium concentrations in the cytoplasm. The sodium-potassium pump is an integral membrane protein consisting

of two subunits, one large and one small. Sodium ions aretransported out of the cell and potassium ions are broughtin. It is known as a P-type ATPase because the protein isphosphorylated during the transport cycle (Fig. 2.10). Thepump counterbalances the tendency of sodium ions to en-ter the cell passively and the tendency of potassium ions toleave passively. It maintains a high intracellular potassiumconcentration necessary for protein synthesis. It also playsa role in the resting membrane potential by maintaining iongradients. The sodium-potassium pump can be inhibited ei-ther by metabolic poisons that stop the synthesis and sup-ply of ATP or by specific pump blockers, such as the car-diac glycoside digitalis.

Calcium pumps, Ca2�-ATPases, are found in theplasma membrane, in the membrane of the endoplasmicreticulum, and, in muscle cells, in the sarcoplasmic reticu-


lum membrane. They are also P-type ATPases. They pumpcalcium ions from the cytosol of the cell either into the ex-tracellular space or into the lumen of these organelles. Theorganelles store calcium and, as a result, help maintain alow cytosolic concentration of this ion (see Chapter 1).

The H�/K�-ATPase is another example of a P-type ATPase. It is present in the luminal membrane of the parietalcells in oxyntic (acid-secreting) glands of the stomach. Bypumping protons into the lumen of the stomach in exchangefor potassium ions, this pump maintains the low pH in thestomach that is necessary for proper digestion (see Chapter28). It is also found in the colon and in the collecting ductsof the kidney. Its role in the kidney is to secrete H� ions intothe urine and to reabsorb K� ions (see Chapter 25).

Proton pumps, H�-ATPases, are found in the mem-branes of the lysosomes and the Golgi apparatus. Theypump protons from the cytosol into these organelles, keep-ing the inside of the organelles more acidic (at a lower pH)than the rest of the cell. These pumps, classified as V-typeATPases because they were first discovered in intracellularvacuolar structures, have now been detected in plasmamembranes. For example, the proton pump in the luminalplasma membrane of kidney cells is characterized as a V-type ATPase. By secreting protons, it plays an importantrole in acidifying the tubular urine.

Mitochondria have F-type ATPases located in the innermitochondrial membrane. This type of proton pump nor-mally functions in reverse. Instead of using the energystored in ATP molecules to pump protons, its principalfunction is to synthesize ATP by using the energy stored ina gradient of protons. The proton gradient is generated bythe respiratory chain.

Secondary Active Transport. The net effect of ionpumps is maintenance of the various environments neededfor the proper functioning of organelles, cells, and organs.Metabolic energy is expended by the pumps to create andmaintain the differences in ion concentrations. Besides theimportance of local ion concentrations for cell function,differences in concentrations represent stored energy. Anion releases potential energy when it moves down an elec-trochemical gradient, just as a body releases energy whenfalling to a lower level. This energy can be used to performwork. Cells have developed several carrier mechanisms totransport one solute against its concentration gradient byusing the energy stored in the favorable gradient of an-other solute. In mammals, most of these mechanisms usesodium as the driver solute and use the energy of thesodium gradient to carry out the “uphill” transport of an-other important solute (Fig. 2.11). Because the sodium gra-

A possible mechanism of secondary active

transport. A solute is moved against its con-centration gradient by coupling it to Na� moving down a favor-able gradient. Binding of extracellular Na� to the carrier proteinmay increase the affinity of binding sites for solute, so that solutealso can bind to the carrier, even though its extracellular concen-

FIGURE 2.11 tration is low. A conformational change in the carrier protein mayexpose the binding sites to the cytosol, where Na� readily dissoci-ates because of the low intracellular Na� concentration. The re-lease of Na� decreases the affinity of the carrier for solute andforces the release of the solute inside the cell, where solute con-centration is already high.

dient is maintained by the action of the sodium-potassiumpump, the function of these transport systems also de-pends on the function of the pump. Although they do notdirectly use metabolic energy for transport, these systemsultimately depend on the proper supply of metabolic en-ergy to the sodium-potassium pump. They are called sec-ondary active transport mechanisms. Disabling the pumpwith metabolic inhibitors or pharmacological blockerscauses these transport systems to stop when the sodiumgradient has been dissipated.

Similar to passive carrier-mediated systems, secondaryactive transport systems are integral membrane proteins;they have specificity for the solute they transport and showsaturation kinetics and competitive inhibition. They differ,however, in two respects. First, they cannot function in theabsence of the driver ion, the ion that moves along its elec-trochemical gradient and supplies energy. Second, theytransport the solute against its own concentration or elec-trochemical gradient. Functionally, the different secondaryactive transport systems can be classified into two groups:symport (cotransport) systems, in which the solute beingtransported moves in the same direction as the sodium ion;and antiport (exchange) systems, in which sodium movesin one direction and the solute moves in the opposite di-rection (Fig. 2.12).

Examples of symport mechanisms are the sodium-cou-pled sugar transport system and the several sodium-coupledamino acid transport systems found in the small intestineand the renal tubule. The symport systems allow efficientabsorption of nutrients even when the nutrients are presentat very low concentrations. The Na�-glucose cotransporter

in the human intestine has been cloned and sequenced. It iscalled sodium-dependent glucose transporter (SGLT). Theprotein contains 664 amino acids, and the polypeptidechain is thought to contain 14 membrane-spanning seg-ments (Fig. 2.13). Another example of a symport system isthe family of sodium-coupled phosphate transporters(termed NaPi, types I and II) in the intestine and renal prox-imal tubule. These transporters have 6 to 8 membrane-spanning segments and contain 460 to 690 amino acids.Sodium-coupled chloride transporters in the kidney are tar-gets for inhibition by specific diuretics. The Na�-Cl� co-transporter in the distal tubule, known as NCC, is inhibitedby thiazide diuretics, and the Na�-K�-2Cl� cotransporterin the ascending limb of the loop of Henle, referred to asNKCC, is inhibited by bumetanide.

The most important examples of antiporters are theNa�/H� exchange and Na�/Ca2� exchange systems,found mainly in the plasma membrane of many cells. Thefirst uses the sodium gradient to remove protons from thecell, controlling the intracellular pH and counterbalancingthe production of protons in metabolic reactions. It is anelectroneutral system because there is no net movement ofcharge. One Na� enters the cell for each H� that leaves.The second antiporter removes calcium from the cell and,together with the different calcium pumps, helps maintaina low cytosolic calcium concentration. It is an electrogenicsystem because there is a net movement of charge. ThreeNa� enter the cell and one Ca2� leaves during each cycle.

The structures of the symport and antiport proteintransporters that have been characterized (see Fig. 2.13)share a common property with ion channels (see Fig. 2.9)and equilibrating carriers, namely the presence of multiplemembrane-spanning segments within the polypeptidechain. This supports the concept that, regardless of themechanism, the membrane-spanning regions of a transportprotein form a hydrophilic pathway for rapid transport ofions and solutes across the hydrophobic interior of themembrane lipid bilayer.

The Movement of Solutes Across Epithelial Cell Layers.Epithelial cells occur in layers or sheets that allow the di-rectional movement of solutes not only across the plasmamembrane but also from one side of the cell layer to theother. Such regulated movement is achieved because theplasma membranes of epithelial cells have two distinct re-gions with different morphology and different transportsystems. These regions are the apical membrane, facing thelumen, and the basolateral membrane, facing the bloodsupply (Fig. 2.14). The specialized or polarized organiza-tion of the cells is maintained by the presence of tight junc-tions at the areas of contact between adjacent cells. Tightjunctions prevent proteins on the apical membrane frommigrating to the basolateral membrane those on the baso-lateral membrane from migrating to the apical membrane.Thus, the entry and exit steps for solutes can be localizedto opposite sides of the cell. This is the key to transcellulartransport across epithelial cells.

An example is the absorption of glucose in the small in-testine. Glucose enters the intestinal epithelial cells by ac-tive transport using the electrogenic Na�-glucose cotrans-porter system (SGLT) in the apical membrane. This


Secondary active transport systems. In asymport system (top), the transported solute

(S) is moved in the same direction as the Na� ion. In an antiportsystem (bottom), the solute is moved in the opposite direction toNa�. Large and small type indicate high and low concentrations,respectively, of Na� ions and solute.

FIGURE 2.12


increases the intracellular glucose concentration above theblood glucose concentration, and the glucose moleculesmove passively out of the cell and into the blood via anequilibrating carrier mechanism (GLUT 2) in the basolat-eral membrane (see Fig. 2.14). The intestinal GLUT 2, likethe erythrocyte GLUT 1, is a sodium-independent trans-porter that moves glucose down its concentration gradient.Unlike GLUT 1, the GLUT 2 transporter can accept othersugars, such as galactose and fructose, that are also ab-sorbed in the intestine. The sodium ions that enter the cellwith the glucose molecules on SGLT are pumped out bythe Na�/K�-ATPase that is located in the basolateral mem-brane only. The polarized organization of the epithelialcells and the integrated functions of the plasma membranetransporters form the basis by which cells accomplish trans-cellular movement of both glucose and sodium ions.

THE MOVEMENT OF WATER ACROSS

THE PLASMA MEMBRANE

Since the lipid part of the plasma membrane is very hy-drophobic, the movement of water across it is too slow toexplain the speed at which water can move in and out of thecells. The partition coefficient of water into lipids is verylow; therefore, the permeability of the lipid bilayer for wa-ter is also very low. Specific membrane proteins that func-tion as water channels explain the rapid movement of wa-ter across the plasma membrane. These water channels aresmall (molecular weight about 30 kDa) integral membraneproteins known as aquaporins. Ten different forms havebeen discovered so far in mammals. At least six forms areexpressed in cells in the kidney and seven forms in the gas-

1 2 3 4 5 6 7 8 9 10 11 12 13 14

COOH

NH2

In

Out

A model of the secondary structure of the

Na�-glucose cotransporter protein (SGLT)

in the human intestine. The polypeptide chain of 664 aminoacids passes back and forth across the membrane 14 times. Eachmembrane-spanning segment consists of 21 amino acids arrangedin an �-helical conformation. Both the NH2 and the COOH endsare located on the extracellular side of the plasma membrane. Inthe functional protein, it is likely that the membrane-spanning

FIGURE 2.13 segments are clustered together to provide a hydrophilic pathwayacross the plasma membrane. The N-terminal portion of the pro-tein, including helices 1 to 9, is required to couple Na� binding toglucose transport. The five helices (10 to 14) at the C-terminusmay form the transport pathway for glucose. (Modified fromPanayotova-Heiermann M, Eskandari S, Turk E, et al. Five trans-membrane helices form the sugar pathway through the Na�-glu-cose cotransporter. J Biol Chem 1997;272:20324–20327.)

Apical (luminal) side

Basolateral side

Lumen

Celllayer

BloodGlucose Intercellular

spaces

GlucoseTight junctions Amino

acid

Aminoacid

K+

K+

Na+

Na+

Na+ Na+

SGLT

GLUT 2

The localization of transport systems to dif-

ferent regions of the plasma membrane in

epithelial cells of the small intestine. A polarized cell is pro-duced, in which entry and exit of solutes, such as glucose, aminoacids, and Na�, occur at opposite sides of the cell. Active entry ofglucose and amino acids is restricted to the apical membrane andexit requires equilibrating carriers located only in the basolateralmembrane. For example, glucose enters on SGLT and exits onGLUT 2. Na� that enters via the apical symporters is pumped outby the Na�/K�-ATPase on the basolateral membrane. The resultis a net movement of solutes from the luminal side of the cell tothe basolateral side, ensuring efficient absorption of glucose,amino acids, and Na� from the intestinal lumen.

FIGURE 2.14

trointestinal tract, tissues where water movement acrossplasma membranes is particularly rapid.

In the kidney, aquaporin-2 (AQP2) is abundant in thecollecting duct and is the target of the hormone argininevasopressin, also known as antidiuretic hormone. This hor-mone increases water transport in the collecting duct bystimulating the insertion of AQP2 proteins into the apicalplasma membrane. Several studies have shown that AQP2has a critical role in inherited and acquired disorders of wa-ter reabsorption by the kidney. For example, diabetes in-sipidus is a condition in which the kidney loses its abilityto reabsorb water properly, resulting in excessive loss ofwater and excretion of a large volume of very dilute urine(polyuria). Although inherited forms of diabetes insipidusare relatively rare, it can develop in patients receivingchronic lithium therapy for psychiatric disorders, givingrise to the term lithium-induced polyuria. Both of theseconditions are associated with a decrease in the number ofAQP2 proteins in the collecting ducts of the kidney.

The Movement of Water Across the

Plasma Membrane Is Driven by

Differences in Osmotic Pressure

The spontaneous movement of water across a membranedriven by a gradient of water concentration is the processknown as osmosis. The water moves from an area of highconcentration of water to an area of low concentration.Since concentration is defined by the number of particlesper unit of volume, a solution with a high concentration ofsolutes has a low concentration of water, and vice versa.Osmosis can, therefore, be viewed as the movement of wa-ter from a solution of high water concentration (low con-centration of solute) toward a solution with a lower con-centration of water (high solute concentration). Osmosis isa passive transport mechanism that tends to equalize the to-tal solute concentrations of the solutions on both sides ofevery membrane.

If a cell that is normally in osmotic equilibrium is trans-ferred to a more dilute solution, water will enter the cell,the cell volume will increase, and the solute concentrationof the cytoplasm will be reduced. If the cell is transferred toa more concentrated solution, water will leave the cell, thecell volume will decrease, and the solute concentration ofthe cytoplasm will increase. As we will see below, manycells have regulatory mechanisms that keep cell volumewithin a certain range. Other cells, such as mammalian ery-throcytes, do not have volume regulatory mechanisms andlarge volume changes occur when the solute concentrationof the extracellular fluid is changed.

The driving force for the movement of water across theplasma membrane is the difference in water concentrationbetween the two sides of the membrane. For historical rea-sons, this driving force is not called the chemical gradientof water but the difference in osmotic pressure. The os-motic pressure of a solution is defined as the pressure nec-essary to stop the net movement of water across a selec-tively permeable membrane that separates the solutionfrom pure water. When a membrane separates two solu-tions of different osmotic pressure, water will move fromthe solution with low osmotic pressure (high water con-

centration) to the solution of high osmotic pressure (lowwater concentration). In this context, the term selectively per-meable means that the membrane is permeable to water butnot solutes. In reality, most biological membranes containmembrane transport proteins that permit solute movement.

The osmotic pressure of a solution depends on the num-ber of particles dissolved in it, the total concentration of allsolutes. Many solutes, such as salts, acids, and bases, disso-ciate in water, so the number of particles is greater than themolar concentration. For example, NaCl dissociates in wa-ter to give Na� and Cl�, so one molecule of NaCl will pro-duce two osmotically active particles. In the case of CaCl2,there are three particles per molecule. The equation givingthe osmotic pressure of a solution is:

� � n R T C (3)

where � is the osmotic pressure of the solution, n is thenumber of particles produced by the dissociation of onemolecule of solute (2 for NaCl, 3 for CaCl2), R is the uni-versal gas constant (0.0821 L�atm/mol�K), T is the absolutetemperature, and C is the concentration of the solute inmol/L. Osmotic pressure can be expressed in atmospheres(atm). Solutions with the same osmotic pressure are calledisosmotic. A solution is hyperosmotic with respect to an-other solution if it has a higher osmotic pressure and hy-poosmotic if it has a lower osmotic pressure.

Equation 3, called the van’t Hoff equation, is valid onlywhen applied to very dilute solutions, in which the particlesof solutes are so far away from each other that no interac-tions occur between them. Generally, this is not the case atphysiological concentrations. Interactions between dis-solved particles, mainly between ions, cause the solution tobehave as if the concentration of particles is less than thetheoretical value (nC). A correction coefficient, called theosmotic coefficient () of the solute, needs to be intro-duced in the equation. Therefore, the osmotic pressure of asolution can be written more accurately as:

� � n R T C (4)

The osmotic coefficient varies with the specific soluteand its concentration. It has values between 0 and 1. For ex-ample, the osmotic coefficient of NaCl is 1.00 in an infi-nitely dilute solution but changes to 0.93 at the physiolog-ical concentration of 0.15 mol/L.

At any given T, since R is constant, equation 4 showsthat the osmotic pressure of a solution is directly propor-tional to the term nC. This term is known as the osmolal-ity or osmotic concentration of a solution and is expressedin osm/kg H2O. Most physiological solutions, such asblood plasma, contain many different solutes, and eachcontributes to the total osmolality of the solution. The os-molality of a solution containing a complex mixture ofsolutes is usually measured by freezing point depression.The freezing point of an aqueous solution of solutes islower than that of pure water and depends on the totalnumber of solute particles. Compared with pure water,which freezes at 0C, a solution with an osmolality of 1osm/kg H2O will freeze at �1.86C. The ease with whichosmolality can be measured has led to the wide use of thisparameter for comparing the osmotic pressure of differentsolutions. The osmotic pressures of physiological solutions



are not trivial. Consider blood plasma, for example, whichusually has an osmolality of 0.28 osm/kg H2O, determinedby freezing point depression. Equation 4 shows that the os-motic pressure of plasma at 37oC is 7.1 atm, about 7 timesgreater than atmospheric pressure.

Many Cells Can Regulate Their Volume

Cell volume changes can occur in response to changes inthe osmolality of extracellular fluid in both normal andpathophysiological situations. Accumulation of solutes alsocan produce volume changes by increasing the intracellularosmolality. Many cells can correct these volume changes.

Volume regulation is particularly important in thebrain, for example, where cell swelling can have seriousconsequences because expansion is strictly limited by therigid skull.

Osmolality and Tonicity. A solution’s osmolality is de-termined by the total concentration of all the solutes pres-ent. In contrast, the solution’s tonicity is determined bythe concentrations of only those solutes that do not enter(“penetrate”) the cell. Tonicity determines cell volume, asillustrated in the following examples. Na� behaves as anonpenetrating solute because it is pumped out of cells bythe Na�/K�-ATPase at the same rate that it enters. A so-lution of NaCl at 0.2 osm/kg H2O is hypoosmotic com-pared to cell cytosol at 0.3 osm/kg H2O. The NaCl solu-tion is also hypotonic because cells will accumulate waterand swell when placed in this solution. A solution con-taining a mixture of NaCl (0.3 osm/kg H2O) and urea (0.1osm/kg H2O) has a total osmolality of 0.4 osm/kg H2Oand will be hyperosmotic compared to cell cytosol. Thesolution is isotonic, however, because it produces no per-manent change in cell volume. The reason is that cellsshrink initially as a result of loss of water but urea is a pen-etrating solute that rapidly enters the cells. Urea entry in-creases the intracellular osmolality so water also entersand increases the volume. Entry of water ceases when theurea concentration is the same inside and outside thecells. At this point, the total osmolality both inside andoutside the cells will be 0.4 osm/kg H2O and the cell vol-ume will be restored to normal.

Volume Regulation. When cell volume increases becauseof extracellular hypotonicity, the response of many cells israpid activation of transport mechanisms that tend to de-crease the cell volume (Fig. 2.15A). Different cells use dif-ferent regulatory volume decrease (RVD) mechanisms tomove solutes out of the cell and decrease the number ofparticles in the cytosol, causing water to leave the cell.Since cells have high intracellular concentrations of potas-sium, many RVD mechanisms involve an increased efflux ofK�, either by stimulating the opening of potassium chan-nels or by activating symport mechanisms for KCl. Othercells activate the efflux of some amino acids, such as taurineor proline. The net result is a decrease in intracellular solutecontent and a reduction of cell volume close to its originalvalue (see Fig. 2.15A).

When placed in a hypertonic solution, cells rapidly losewater and their volume decreases. In many cells, a de-

creased volume triggers regulatory volume increase (RVI)mechanisms, which increase the number of intracellularparticles, bringing water back into the cells. Because Na� isthe main extracellular ion, many RVI mechanisms involvean influx of sodium into the cell. Na�-Cl� symport, Na�-K�-2Cl� symport, and Na�/H� antiport are some of themechanisms activated to increase the intracellular concen-tration of Na� and increase the cell volume toward its orig-inal value (Fig. 2.15B).

Mechanisms based on an increased Na� influx are effec-tive for only a short time because, eventually, the sodiumpump will increase its activity and reduce intracellular Na�

A

B

The effect of tonicity changes on cell vol-

ume. Cell volume changes when a cell isplaced in either a hypotonic or a hypertonic solution. A, In a hy-potonic solution, the reversal of the initial increase in cell volumeis known as a regulatory volume decrease. Transport systems forsolute exit are activated, and water follows movement of soluteout of the cell. B, In a hypertonic solution, the reversal of the ini-tial decrease in cell volume is a regulatory volume increase. Trans-port systems for solute entry are activated, and water followssolute into the cell.

FIGURE 2.15

to its normal value. Cells that regularly encounter hyper-tonic extracellular fluids have developed additional mecha-nisms for maintaining normal volume. These cells can syn-thesize specific organic solutes, enabling them to increaseintracellular osmolality for a long time and avoiding alter-ing the concentrations of ions they must maintain within anarrow range of values. The organic solutes are usuallysmall molecules that do not interfere with normal cell func-tion when they accumulate inside the cell. For example,cells of the medulla of the mammalian kidney can increasethe level of the enzyme aldose reductase when subjected toelevated extracellular osmolality. This enzyme convertsglucose to an osmotically active solute, sorbitol. Brain cellscan synthesize and store inositol. Synthesis of sorbitol andinositol represents different answers to the problem of in-creasing the total intracellular osmolality, allowing normalcell volume to be maintained in the presence of hypertonicextracellular fluid.

Oral Rehydration Therapy

Oral administration of rehydration solutions has dramati-cally reduced the mortality resulting from cholera andother diseases that involve excessive losses of water andsolutes from the gastrointestinal tract. The main ingredi-ents of rehydration solutions are glucose, NaCl, and water.The glucose and Na� ions are reabsorbed by SGLT andother transporters in the epithelial cells lining the lumen ofthe small intestine (see Fig. 2.14). Deposition of thesesolutes on the basolateral side of the epithelial cells in-creases the osmolality in that region compared with the in-testinal lumen and drives the osmotic absorption of water.Absorption of glucose increases the absorption of NaCland water and helps to compensate for excessive diarrheallosses of salt and water.

THE RESTING MEMBRANE POTENTIAL

The different passive and active transport systems are coor-dinated in a living cell to maintain intracellular ions andother solutes at concentrations compatible with life. Con-sequently, the cell does not equilibrate with the extracellu-lar fluid, but rather exists in a steady state with the extra-cellular solution. For example, intracellular Na�

concentration (10 mmol/L in a muscle cell) is much lowerthan extracellular Na� concentration (140 mmol/L), soNa� enters the cell by passive transport through nongatedNa� channels. The rate of Na� entry is matched, however,by the rate of active transport of Na� out of the cell via thesodium-potassium pump (Fig. 2.16). The net result is thatintracellular Na� is maintained constant and at a low level,even though Na� continually enters and leaves the cell.The reverse is true for K�, which is maintained at a highconcentration inside the cell relative to the outside. Thepassive exit of K� through nongated K� channels ismatched by active entry via the pump (see Fig. 2.16). Main-tenance of this steady state with ion concentrations insidethe cell different from those outside the cell is the basis forthe difference in electrical potential across the plasmamembrane or the resting membrane potential.


Ion Movement Is Driven by the

Electrochemical Potential

If there are no differences in temperature or hydrostaticpressure between the two sides of a plasma membrane, twoforces drive the movement of ions and other solutes acrossthe membrane. One force results from the difference in theconcentration of a substance between the inside and theoutside of the cell and the tendency of every substance tomove from areas of high concentration to areas of low con-centration. The other force results from the difference inelectrical potential between the two sides of the membrane,and it applies only to ions and other electrically chargedsolutes. When a difference in electrical potential exists,positive ions tend to move toward the negative side, whilenegative ions tend to move toward the positive side.

The sum of these two driving forces is called the gradi-ent (or difference) of electrochemical potential across themembrane for a specific solute. It measures the tendency ofthat solute to cross the membrane. The expression of thisforce is given by:

�� RT ln �CC

o

i� � zF (Ei � Eo) (5)

where � represents the electrochemical potential (�� isthe difference in electrochemical potential between twosides of the membrane); Ci and Co are the concentrationsof the solute inside and outside the cell, respectively; Ei isthe electrical potential inside the cell measured with re-spect to the electrical potential outside the cell (Eo); R is theuniversal gas constant (2 cal/mol�K); T is the absolute tem-

K+

2K+

3 Na+

Passive exitvia nongated

channel

Passive entryvia nongated

channel

Active transport byNa+/K+-ATPase

ADP

ATPK+

Na+

Na+

3 Na+

The concept of a steady state. Na� enters acell through nongated Na� channels, moving

passively down the electrochemical gradient. The rate of Na� en-try is matched by the rate of active transport of Na� out of thecell via the Na�/K�-ATPase. The intracellular concentration ofNa� remains low and constant. Similarly, the rate of passive K�

exit through nongated K� channels is matched by the rate of ac-tive transport of K� into the cell via the pump. The intracellularK� concentration remains high and constant. During each cycleof the ATPase, two K� are exchanged for three Na� and onemolecule of ATP is hydrolyzed to ADP. Large type and smalltype indicate high and low ion concentrations, respectively.

FIGURE 2.16


perature (K); z is the valence of the ion; and F is the Fara-day constant (23 cal/mV�mol). By inserting these units inequation 5 and simplifying, the electrochemical potentialwill be expressed in cal/mol, which are units of energy. Ifthe solute is not an ion and has no electrical charge, then z� 0 and the last term of the equation becomes zero. In thiscase, the electrochemical potential is defined only by thedifferent concentrations of the uncharged solute, called thechemical potential. The driving force for solute transportbecomes solely the difference in chemical potential.

Net Ion Movement Is Zero

at the Equilibrium Potential

Net movement of an ion into or out of a cell continues as longas the driving force exists. Net movement stops and equilib-rium is reached only when the driving force of electrochemi-cal potential across the membrane becomes zero. The condi-tion of equilibrium for any permeable ion will be �� 0.Substituting this condition into equation 5, we obtain:

0 � RT ln �CC

o

i� � zF (Ei � Eo)

Ei � Eo � � �RzTF� ln �

CC

o

i� (6)

Ei � Eo � �RzTF� ln �

CC

o

i�

Equation 6, known as the Nernst equation, gives thevalue of the electrical potential difference (Ei � Eo) neces-sary for a specific ion to be at equilibrium. This value isknown as the Nernst equilibrium potential for that partic-ular ion and it is expressed in millivolts (mV), units of volt-age. At the equilibrium potential, the tendency of an ion tomove in one direction because of the difference in concen-trations is exactly balanced by the tendency to move in theopposite direction because of the difference in electricalpotential. At this point, the ion will be in equilibrium andthere will be no net movement. By converting to log10 andassuming a physiological temperature of 37C and a valueof �1 for z (for Na� or K�), the Nernst equation can be ex-pressed as:

Ei � Eo � 61 log10 �CC

o

i� (7)

Since Na� and K� (and other ions) are present at differ-ent concentrations inside and outside a cell, it follows fromequation 7 that the equilibrium potential will be differentfor each ion.

The Resting Membrane Potential Is Determined

by the Passive Movement of Several Ions

The resting membrane potential is the electrical potentialdifference across the plasma membrane of a normal livingcell in its unstimulated state. It can be measured directly bythe insertion of a microelectrode into the cell with a refer-ence electrode in the extracellular fluid. The resting mem-brane potential is determined by those ions that can crossthe membrane and are prevented from attaining equilib-rium by active transport systems. Potassium, sodium, and

chloride ions can cross the membranes of every living cell,and each of these ions contributes to the resting membranepotential. By contrast, the permeability of the membrane ofmost cells to divalent ions is so low that it can be ignoredin this context.

The Goldman equation gives the value of the mem-brane potential (in mV) when all the permeable ions are ac-counted for:

Ei � Eo � �RFT� ln (8)

where PK, PNa, and PCl represent the permeability of themembrane to potassium, sodium, and chloride ions, re-spectively; and brackets indicate the concentration of theion inside (i) and outside (o) the cell. If a certain cell isnot permeable to one of these ions, the contribution ofthe impermeable ion to the membrane potential will bezero. If a specific cell is permeable to an ion other thanthe three considered in equation 8, that ion’s contribu-tion to the membrane potential must be included in theequation.

It can be seen from equation 8 that the contribution ofany ion to the membrane potential is determined by themembrane’s permeability to that particular ion. The higherthe permeability of the membrane to one ion relative to theothers, the more that ion will contribute to the membranepotential. The plasma membranes of most living cells aremuch more permeable to potassium ions than to any otherion. Making the assumption that PNa and PCl are zero rela-tive to PK, equation 8 can be simplified to:

Ei � Eo � �RFT� ln �

PP

K

K

[[KK

�

�

]]o

i�

Ei � Eo � �RFT� ln �

[[KK

�

�

]]o

i�

(9)

which is the Nernst equation for the equilibrium potentialfor K� (see equation 6). This illustrates two importantpoints:• In most cells, the resting membrane potential is close to

the equilibrium potential for K�.• The resting membrane potential of most cells is domi-

nated by K� because the plasma membrane is more per-meable to this ion compared to the others.As a typical example, the K� concentrations outside and

inside a muscle cell are 3.5 mmol/L and 155 mmol/L, re-spectively. Substituting these values in equation 7 gives anequilibrium potential for K� of �100 mV, negative insidethe cell relative to the outside. The resting membrane po-tential in a muscle cell is �90 mV (negative inside). Thisvalue is close to, although not the same as, the equilibriumpotential for K�.

The reason the resting membrane potential in the mus-cle cell is less negative than the equilibrium potential forK� is as follows. Under physiological conditions, there ispassive entry of Na� ions. This entry of positively chargedions has a small but significant effect on the negative po-tential inside the cell. Assuming intracellular Na� to be 10mmol/L and extracellular Na� to be 140 mmol/L, theNernst equation gives a value of �70 mV for the Na� equi-librium potential (positive inside the cell). This is far from

PK[K�]o � PNa[Na�]o � PCl[Cl�]i��PK[K�]i � PNa[Na�]i � PCl[Cl�]o

the resting membrane potential of �90 mV. Na� makesonly a small contribution to the resting membrane poten-tial because membrane permeability to Na� is very lowcompared to that of K�.

The contribution of Cl� ions need not be consideredbecause the resting membrane potential in the muscle cellis the same as the equilibrium potential for Cl�. Therefore,there is no net movement of chloride ions.

In most cells, as shown above using a muscle cell as anexample, the equilibrium potentials of K� and Na� are dif-ferent from the resting membrane potential, which indi-cates that neither K� ions nor Na� ions are at equilibrium.

Consequently, these ions continue to cross the plasmamembrane via specific nongated channels, and these pas-sive ion movements are directly responsible for the restingmembrane potential.

The Na�/K�-ATPase is important indirectly for main-taining the resting membrane potential because it sets upthe gradients of K� and Na� that drive passive K� exit andNa� entry. During each cycle of the pump, two K� ions aremoved into the cell in exchange for three Na�, which aremoved out (see Fig. 2.16). Because of the unequal exchangemechanism, the pump’s activity contributes slightly to thenegative potential inside the cell.


DIRECTIONS: Each of the numbereditems or incomplete statements in thissection is followed by answers or bycompletions of the statement. Select theONE lettered answer or completion that isBEST in each case.

1. Which one of the following is acommon property of all phospholipidmolecules?(A) Hydrophilic(B) Steroid structure(C) Water-soluble(D) Amphipathic(E) Hydrophobic

2. Select the true statement aboutmembrane phospholipids.(A) A phospholipid containscholesterol(B) Phospholipids move rapidly in theplane of the bilayer(C) Specific phospholipids are alwayspresent in equal proportions in the twohalves of the bilayer(D) Phospholipids form ion channelsthrough the membrane(E) Na�-glucose symport is mediatedby phospholipids

3. Several segments of the polypeptidechain of integral membrane proteinsusually span the lipid bilayer. Thesesegments frequently(A) Adopt an �-helical configuration(B) Contain many hydrophilic aminoacids(C) Form covalent bonds withcholesterol(D) Contain unusually strong peptidebonds(E) Form covalent bonds withphospholipids

4. The electrical potential differencenecessary for a single ion to be atequilibrium across a membrane is bestdescribed by the(A) Goldman equation

(B) van’t Hoff equation(C) Fick’s law(D) Nernst equation(E) Permeability coefficient

5. The ion present in highestconcentration inside most cells is(A) Sodium(B) Potassium(C) Calcium(D) Chloride(E) Phosphate

6. Solute movement by active transportcan be distinguished from solutetransport by equilibrating carrier-mediated transport because activetransport(A) Is saturable at high soluteconcentration(B) Is inhibited by other moleculeswith structures similar to that of thesolute(C) Moves the solute against itselectrochemical gradient(D) Allows movement of polarmolecules(E) Is mediated by specific membraneproteins

7. A sodium channel that opens inresponse to an increase in intracellularcyclic GMP is an example of(A) A ligand-gated ion channel(B) An ion pump(C) Sodium-coupled solute transport(D) A peripheral membrane protein(E) Receptor-mediated endocytosis

8. During regulatory volume decrease,many cells will increase(A) Their volume(B) Influx of Na�

(C) Efflux of K�

(D) Synthesis of sorbitol(E) Influx of water

9. At equilibrium the concentrations ofCl� inside and outside a cell are 8mmol/L and 120 mmol/L, respectively.

The equilibrium potential for Cl� at37C is calculated to be(A) �4.07 mV(B) –4.07 mV(C) �71.7 mV(D) –71.7 mV(E) �91.5 mV(F) –91.5 mV

10.What is the osmotic pressure (in atm)of an aqueous solution of 100 mmol/LCaCl2 at 27 C? (Assume the osmoticcoefficient is 0.86 and the gas constantis 0.0821 L�atm/mol�K).(A) 738 atm(B) 635 atm(C) 211 atm(D) 7.38 atm(E) 6.35 atm(F) 2.11 atm

SUGGESTED READING

Barrett MP, Walmsley AR, Gould GW.Structure and function of facilitativesugar transporters. Curr Opin Cell Biol1999;11:496–502.

DeWeer P. A century of thinking aboutcell membranes. Annu Rev Physiol2000;62:919–926.

Barrett KE, Keely SJ. Chloride secretionby the intestinal epithelium: Molecularbasis and regulatory aspects. Annu RevPhysiol 2000;62:535–572.

Giebisch G. Physiological roles of renalpotassium channels. Semin Nephrol1999;19:458–471.

Hebert SC. Molecular mechanisms. SeminNephrol 1999;19:504–523.

Hwang TC, Sheppard DN. Molecularpharmacology of the CFTR Cl� chan-nel. Trends Pharmacol Sci1999;20:448–453.

Kanai Y. Family of neutral and acidicamino acid transporters: Molecular bi-ology, physiology and medical implica-tions. Curr Opin Cell Biol1997;9:565–572.

R E V I E W Q U E S T I O N S

(continued)


Ma T, Verkman AS. Aquaporin waterchannels in gastrointestinal physiology.J Physiol (London) 1999;517:317–326.

Nielsen S, Kwon TH, Christensen BM, etal. Physiology and pathophysiology ofrenal aquaporins. J Am Soc Nephrol1999;10:647–663.

O’Neill WC. Physiological significance ofvolume-regulatory transporters. Am JPhysiol 1999;276:C995–C1011.

Pilewski JM, Frizzell RA. Role of CFTR inairway disease. Physiol Rev1999;79(Suppl):S215–S255.

Rojas CV. Ion channels and human ge-netic diseases. News Physiol Sci1996;11:36–42.

Reuss L. One hundred years of inquiry: themechanism of glucose absorption in theintestine. Annu Rev Physiol2000;62:939–946.

Saier MH. Families of proteins formingtransmembrane channels. J Membr Biol2000;175:165–180.

Wright EM. Glucose galactose malabsorp-tion. Am J Physiol1998;275:G879–G882.

Yeaman C, Grindstaff KK, Nelson WJ.New perspectives on mechanisms in-volved in generating epithelial cell po-larity. Physiol Rev 1999;79:73–98.

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