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Cell_Biology_Alberts. Some transporters simply passively mediate the movement of a single solute from one side of the membrane to the other at a rate determined by their Vmax and Figure 11–6 The kinetics of simple diffusion compared with transporter-mediated diffusion. Whereas the rate of diffusion and channel-mediated transport is directly proportional to the solute concentration (within the physical limits imposed by total surface area or total channels available), the rate of transporter-mediated diffusion reaches a maximum (Vmax) when the transporter is saturated. The solute concentration when the transport rate is at half its maximal value approximates the binding constant (Km) of the transporter for the solute and is analogous to the Km of an enzyme for its substrate. The graph applies to a transporter moving a single solute; the kinetics of coupled transport of two or more solutes is more complex and exhibits cooperative behavior.


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Cell_Biology_Alberts. Km; they are called uniporters. Others function as coupled transporters, in which the transfer of one solute strictly depends on the transport of a second. Coupled transport involves either the simultaneous transfer of a second solute in the same direction, performed by symporters (also called co-transporters), or the transfer of a second solute in the opposite direction, performed by antiporters (also called exchangers) (Figure 11–8).


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Cell_Biology_Alberts. The tight coupling between the transfer of two solutes allows the coupled transporters to harvest the energy stored in the electrochemical gradient of one solute, typically an inorganic ion, to transport the other. In this way, the free energy released during the movement of an inorganic ion down an electrochemical gradient is used as the driving force to pump other solutes uphill, against their electrochemical gradient. This strategy can work in either direction; some coupled transporters function as symporters, others as antiporters. In the plasma membrane of animal cells, Na+ is the usual co-transported ion because its electrochemical gradient provides a large driving force for the active transport of a second molecule. The Na+ that enters the cell during coupled transport is subsequently pumped out by an ATP-driven Na+-K+ pump in the plasma membrane (as we discuss later), which, by maintaining the Na+ gradient, indirectly drives the coupled transport. Such ion-driven coupled


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Cell_Biology_Alberts. pumped out by an ATP-driven Na+-K+ pump in the plasma membrane (as we discuss later), which, by maintaining the Na+ gradient, indirectly drives the coupled transport. Such ion-driven coupled transporters as just described are said to mediate secondary active transport. In contrast, ATP-driven pumps are said to mediate primary active transport because in these the free energy of ATP hydrolysis is used to directly drive the transport of a solute against its concentration gradient.


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Cell_Biology_Alberts. Intestinal and kidney epithelial cells contain a variety of symporters that are driven by the Na+ gradient across the plasma membrane. Each Na+-driven symporter is specific for importing a small group of related sugars or amino acids into the cell. Because the Na+ tends to move into the cell down its electrochemical gradient, the sugar or amino acid is, in a sense, “dragged” into the cell with it. The greater the electrochemical gradient for Na+, the more solute is pumped Figure 11–7 Three ways of driving active transport. The actively transported molecule is shown in orange, and the energy source is shown in red. Redox driven active transport is discussed in Chapter 14 (see Figures 14–18 and 14–19). Figure 11–8 This schematic diagram shows transporters functioning as uniporters, symporters, and antiporters coupled transport (Movie 11.1).


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Cell_Biology_Alberts. Figure 11–9 Mechanism of glucose transport fueled by a Na+ gradient. As in the model shown in Figure 11–5, the transporter alternates between inward-open and outward-open states via an occluded intermediate state. Binding of Na+ and glucose is cooperative—that is, the binding of either solute increases the protein’s affinity for the other. Since the Na+ concentration is much higher in the extracellular space than in the cytosol, glucose is more likely to bind to the transporter in the outward-facing state. The transition to the occluded state occurs only when both Na+ and glucose are bound; their precise interactions in the solute-binding sites slightly stabilize the occluded state and thereby make this transition energetically favorable. Stochastic fluctuations caused by thermal energy drive the transporter randomly into the inward-open or outward-open conformation. If it opens outwardly, nothing is achieved, and the process starts all over. However, whenever it opens inwardly,


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Cell_Biology_Alberts. drive the transporter randomly into the inward-open or outward-open conformation. If it opens outwardly, nothing is achieved, and the process starts all over. However, whenever it opens inwardly, Na+ dissociates quickly in the low-Na+-concentration environment of the cytosol. Glucose dissociation is likewise enhanced when Na+ is lost, because of cooperativity in binding of the two solutes. The overall result is the net transport of both Na+ and glucose into the cell. Because the occluded state is not formed when only one of the solutes is bound, the transporter switches conformation only when it is fully occupied or fully empty, thereby assuring strict coupling of the transport of Na+ and glucose.


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Cell_Biology_Alberts. into the cell (Figure 11–9). Neurotransmitters (released by nerve cells to signal at synapses—as we discuss later) are taken up again by Na+ symporters after their release. These neurotransmitter transporters are important drug targets: stimulants, such as cocaine and antidepressants, inhibit them and thereby prolong signaling by the neurotransmitters, which are not cleared efficiently.


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Cell_Biology_Alberts. Despite their great variety, transporters share structural features that can explain how they function and how they evolved. Transporters are typically built from bundles of 10 or more α helices that span the membrane. Soluteand ion-binding sites are located midway through the membrane, where some helices are broken or distorted and amino acid side chains and polypeptide backbone atoms form ionand solute-binding sites. In the inward-open and outward-open conformations, these binding sites are accessible by passageways from one side of the membrane but not the other. In switching between the two conformations, the transporter protein transiently adopts an occluded conformation, in which both passageways are closed; this prevents the driving ion and the transported solute from crossing the membrane unaccompanied, which would deplete the cell’s energy store to no purpose. Because only transporters with both types of binding sites appropriately filled change their conformation, tight


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Cell_Biology_Alberts. the membrane unaccompanied, which would deplete the cell’s energy store to no purpose. Because only transporters with both types of binding sites appropriately filled change their conformation, tight coupling between ion and solute transport is assured.


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Cell_Biology_Alberts. Like enzymes, transporters can work in the reverse direction if ion and solute gradients are appropriately adjusted experimentally. This chemical symmetry is mirrored in their physical structure. Crystallographic analyses have revealed that transporters are built from inverted repeats: the packing of the transmembrane α helices in one half of the helix bundle is structurally similar to the packing in the other half, but the two halves are inverted in the membrane relative to each other. Transporters are therefore said to be pseudosymmetric, and the passageways that open and close on either side of the membrane have closely similar geometries, allowing alternating access to the ionand solute-binding sites in the center (Figure 11–10). It is thought that the two halves evolved by gene duplication of a smaller ancestor protein.


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Cell_Biology_Alberts. Some other types of important membrane transport proteins are also built from inverted repeats. Examples even include channel proteins such as the aquaporin water channel (discussed later) and the Sec61 channel through which nascent polypeptides move into the endoplasmic reticulum (discussed in Chapter 12). It is thought that these channels evolved from coupled transporters, in which the gating functions were lost, allowing them to open toward both sides of the membrane simultaneously to provide a continuous path across the membrane. In bacteria, yeasts, and plants, as well as in many membrane-enclosed organelles of animal cells, most ion-driven active transport systems depend on H+ rather than Na+ gradients, reflecting the predominance of H+ pumps in these membranes. An electrochemical H+ gradient across the bacterial plasma membrane, for example, drives the inward active transport of many sugars and amino acids. Transporters in the Plasma Membrane Regulate Cytosolic pH


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Cell_Biology_Alberts. Transporters in the Plasma Membrane Regulate Cytosolic pH Most proteins operate optimally at a particular pH. Lysosomal enzymes, for example, function best at the low pH (~5) found in lysosomes, whereas cytosolic enzymes function best at the close-to-neutral pH (~7.2) found in the cytosol. It is therefore crucial that cells control the pH of their intracellular compartments.


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Cell_Biology_Alberts. Most cells have one or more types of Na+-driven antiporters in their plasma membrane that help to maintain the cytosolic pH at about 7.2. These transporters use the energy stored in the Na+ gradient to pump out excess H+, which either leaks in or is produced in the cell by acid-forming reactions. Two mechanisms are used: either H+ is directly transported out of the cell or HCO3 is brought into the cell to neutralize H+ in the cytosol (according to the reaction HCO3– + H+ →H2O + CO2). One of the antiporters that uses the first mechanism is a Na+–H+ exchanger, which couples an influx of Na+ to an efflux of H+. Another, which uses a combination of the two mechanisms, is a Na+-driven Cl––HCO3 – exchanger that couples an influx of Na+ and HCO3 to an efflux of Cl– and H+ (so that NaHCO3 comes – in and HCl goes out). The Na+-driven Cl––HCO3 exchanger is twice as effective as the Na+–H+ exchanger: it pumps out one H+ and neutralizes another for each Na+ that enters the cell. If HCO3– is


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Cell_Biology_Alberts. – in and HCl goes out). The Na+-driven Cl––HCO3 exchanger is twice as effective as the Na+–H+ exchanger: it pumps out one H+ and neutralizes another for each Na+ that enters the cell. If HCO3– is available, as is usually the case, this antiporter is the most important transporter regulating the cytosolic pH. The pH inside the cell regulates both exchangers; when the pH in the cytosol falls, both exchangers increase their activity.


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Cell_Biology_Alberts. A Na+-independent Cl––HCO3 exchanger adjusts the cytosolic pH in the reverse direction. Like the Na+-dependent transporters, pH regulates the Na+-in– dependent Cl––HCO3 exchanger, but the exchanger’s activity increases as the – cytosol becomes too alkaline. The movement of HCO3 in this case is normally out of the cell, down its electrochemical gradient, which decreases the pH of the


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Cell_Biology_Alberts. Figure 11–10 Transporters are built from inverted repeats. (A) LeuT, a bacterial leucine/Na+ symporter related to human neurotransmitter transporters, such as the serotonin transporter, is shown. The core of the transporter is built from two bundles, each composed of five α helices (blue and yellow). The helices shown in gray differ among members of this transporter family and are thought to play regulatory roles, which are specific to a particular transporter. (B) Both core helix bundles are packed in a similar arrangement (shown as a hand, with the broken helix as the thumb), but the second bundle is inverted with respect to the first. The transporter’s structural pseudosymmetry reflects its functional symmetry: the transporter can work in either direction, depending on the direction of the ion gradient. (Adapted from K.R. Vinothkumar and R. Henderson, Q.


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Cell_Biology_Alberts. K.R. Vinothkumar and R. Henderson, Q. Rev. Biophys. 43:65–158, 2010. With permission from Cambridge University Press. PDB code: 3F3E.) cytosol. A Na+-independent Cl––HCO3 exchanger in the membrane of red blood cells (called band 3 protein—see Figure 10–38) facilitates the quick discharge of CO2 (as HCO3–) as the cells pass through capillaries in the lung. The intracellular pH is not entirely regulated by transporters in the plasma membrane: ATP-driven H+ pumps are used to control the pH of many intracellular compartments. As discussed in Chapter 13, H+ pumps maintain the low pH in lysosomes, as well as in endosomes and secretory vesicles. These H+ pumps use the energy of ATP hydrolysis to pump H+ into these organelles from the cytosol. An Asymmetric Distribution of Transporters in Epithelial Cells Underlies the Transcellular Transport of Solutes


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Cell_Biology_Alberts. An Asymmetric Distribution of Transporters in Epithelial Cells Underlies the Transcellular Transport of Solutes In epithelial cells, such as those that absorb nutrients from the gut, transporters are distributed nonuniformly in the plasma membrane and thereby contribute to the transcellular transport of absorbed solutes. By the actions of the transporters in these cells, solutes are moved across the epithelial cell layer into the extracellular fluid from where they pass into the blood. As shown in Figure 11–11, Na+-linked symporters located in the apical (absorptive) domain of the plasma membrane actively transport nutrients into the cell, building up substantial concentration gradients for these solutes across the plasma membrane. Uniporters in the basal and lateral (basolateral) domains allow the nutrients to leave the cell passively down these concentration gradients.


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Cell_Biology_Alberts. In many of these epithelial cells, the plasma membrane area is greatly increased by the formation of thousands of microvilli, which extend as thin, fingerlike projections from the apical surface of each cell. Such microvilli can increase the total absorptive area of a cell as much as 25-fold, thereby enhancing its transport capabilities. As we have seen, ion gradients have a crucial role in driving many essential transport processes in cells. Ion pumps that use the energy of ATP hydrolysis establish and maintain these gradients, as we discuss next. transporter mediating passive transport of glucose


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Cell_Biology_Alberts. Figure 11–11 Transcellular transport. The transcellular transport of glucose across an intestinal epithelial cell depends on the nonuniform distribution of transporters in the cell’s plasma membrane. The process shown here results in the transport of glucose from the intestinal lumen to the extracellular fluid (from where it passes into the blood). Glucose is pumped into the cell through the apical domain of the membrane by a Na+-powered glucose symporter. Glucose passes out of the cell (down its concentration gradient) by passive movement through a glucose uniporter in the basal and lateral membrane domains. The Na+ gradient driving the glucose symport is maintained by the Na+-K+ pump in the basal and lateral plasma membrane domains, which keeps the internal concentration of Na+ low (Movie 11.2). Adjacent cells are connected by impermeable tight junctions, which have a dual function in the transport process illustrated: they prevent solutes from crossing the epithelium between cells,


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Cell_Biology_Alberts. 11.2). Adjacent cells are connected by impermeable tight junctions, which have a dual function in the transport process illustrated: they prevent solutes from crossing the epithelium between cells, allowing a concentration gradient of glucose to be maintained across the cell sheet (see Figure 19–18). They also serve as diffusion barriers (fences) within the plasma membrane, which help confine the various transporters to their respective membrane domains (see Figure 10–34).


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Cell_Biology_Alberts. There Are Three Classes of ATP-Driven Pumps ATP-driven pumps are often called transport ATPases because they hydrolyze ATP to ADP and phosphate and use the energy released to pump ions or other solutes across a membrane. There are three principal classes of ATP-driven pumps (Figure 11–12), and representatives of each are found in all prokaryotic and eukaryotic cells. 1. P-type pumps are structurally and functionally related multipass trans-membrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. This class includes many of the ion pumps that are responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes. 2. ABC transporters (ATP-Binding Cassette transporters) differ structurally from P-type ATPases and primarily pump small molecules across cell membranes. 3.


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Cell_Biology_Alberts. 2. ABC transporters (ATP-Binding Cassette transporters) differ structurally from P-type ATPases and primarily pump small molecules across cell membranes. 3. V-type pumps are turbine-like protein machines, constructed from multiple different subunits. The V-type proton pump transfers H+ into organelles such as lysosomes, synaptic vesicles, and plant or yeast vacuoles (V = vacuolar), to acidify the interior of these organelles (see Figure 13–37).


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Cell_Biology_Alberts. Structurally related to the V-type pumps is a distinct family of F-type ATPases, more commonly called ATP synthases because they normally work in reverse: instead of using ATP hydrolysis to drive H+ transport, they use the H+ gradient across the membrane to drive the synthesis of ATP from ADP and phosphate (see Figure 14–30). ATP synthases are found in the plasma membrane of bacteria, the inner membrane of mitochondria, and the thylakoid membrane of chloroplasts. The H+ gradient is generated either during the electron-transport steps of oxidative phosphorylation (in aerobic bacteria and mitochondria), during photosynthesis (in chloroplasts), or by the light-driven H+ pump (bacteriorhodopsin) in Halobacterium. We discuss some of these proteins in detail in Chapter 14. For the remainder of this section, we focus on P-type pumps and ABC transporters. A P-type ATPase Pumps Ca2+ into the Sarcoplasmic Reticulum in Muscle Cells


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Cell_Biology_Alberts. For the remainder of this section, we focus on P-type pumps and ABC transporters. A P-type ATPase Pumps Ca2+ into the Sarcoplasmic Reticulum in Muscle Cells Eukaryotic cells maintain very low concentrations of free Ca2+ in their cytosol (~10–7 M) in the face of a very much higher extracellular Ca2+ concentration (~10–3 M). Therefore, even a small influx of Ca2+ significantly increases the concentration of free Ca2+ in the cytosol, and the flow of Ca2+ down its steep concentration gradient in response to extracellular signals is one means of transmitting these signals rapidly across the plasma membrane (discussed in Chapter 15). It is thus


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Cell_Biology_Alberts. Figure 11–12 Three types of aTP-driven pumps. Like any enzyme, all ATP-driven pumps can work in either direction, depending on the electrochemical gradients of their solutes and the ATP/ADP ratio. When the ATP/ADP ratio is high, they hydrolyze ATP; when the ATP/ADP ratio is low, they can synthesize ATP. The F-type ATPase in mitochondria normally works in this “reverse” mode to make most of the cell’s ATP. important that the cell maintains a steep Ca2+ gradient across its plasma membrane. Ca2+ transporters that actively pump Ca2+ out of the cell help maintain the gradient. One of these is a P-type Ca2+ ATPase; the other is an antiporter (called a Na+–Ca2+ exchanger) that is driven by the Na+ electrochemical gradient (discussed in Chapter 15).


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Cell_Biology_Alberts. The Ca2+ pump, or Ca2+ ATPase, in the sarcoplasmic reticulum (SR) membrane of skeletal muscle cells is a well-understood P-type transport ATPase. The SR is a specialized type of endoplasmic reticulum that forms a network of tubular sacs in the muscle cell cytoplasm, and it serves as an intracellular store of Ca2+. When an action potential depolarizes the muscle cell plasma membrane, Ca2+ is released into the cytosol from the SR through Ca2+-release channels, stimulating the muscle to contract (discussed in Chapters 15 and 16). The Ca2+ pump, which accounts for about 90% of the membrane protein of the SR, moves Ca2+ from the cytosol back into the SR. The endoplasmic reticulum of nonmuscle cells contains a similar Ca2+ pump, but in smaller quantities.


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Cell_Biology_Alberts. Enzymatic studies and analyses of the three-dimensional structures of transport intermediates of the SR Ca2+ pump and related pumps have revealed the molecular mechanism of P-type transport ATPases in great detail. They all have similar structures, containing 10 transmembrane α helices connected to three cytosolic domains (Figure 11–13). In the Ca2+ pump, amino acid side chains protruding from the transmembrane helices form two centrally positioned binding sites for Ca2+. As shown in Figure 11–14, in the pump’s ATP-bound nonphosphorylated state, these binding sites are accessible only from the cytosolic side of the SR membrane. Ca2+ binding triggers a series of conformational changes that close the passageway to the cytosol and activate a phosphotransfer reaction in which the terminal phosphate of the ATP is transferred to an aspartate that is highly conserved among all P-type ATPases. The ADP then dissociates and is replaced with a fresh ATP, causing another conformational change


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Cell_Biology_Alberts. phosphate of the ATP is transferred to an aspartate that is highly conserved among all P-type ATPases. The ADP then dissociates and is replaced with a fresh ATP, causing another conformational change that opens a passageway to the SR lumen through which the two Ca2+ ions exit. They are replaced by two H+ ions and a water molecule that stabilize the empty Ca2+-binding sites and close the passageway to the SR lumen. Hydrolysis of the labile phosphoryl-aspartate bond returns the pump to the initial conformation, and the cycle starts again. The transient self-phosphorylation of the pump during its cycle is an essential characteristic of all P-type pumps.


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Cell_Biology_Alberts. The Plasma Membrane Na+-K+ Pump Establishes Na+ and K+ Gradients Across the Plasma Membrane The concentration of K+ is typically 10–30 times higher inside cells than outside, whereas the reverse is true of Na+ (see Table 11–1, p. 598). A Na+-K+ pump, or Na+K+ ATPase, found in the plasma membrane of virtually all animal cells maintains


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Cell_Biology_Alberts. Figure 11–13 The structure of the sarcoplasmic reticulum Ca2+ pump. The ribbon model (left), derived from x-ray crystallographic analyses, shows the pump in its phosphorylated, ATP-bound state. The three globular cytosolic domains of the pump—the nucleotide-binding domain (dark green), the activator domain (blue), and the phosphorylation domain (red), also shown schematically on the right—change conformation dramatically during the pumping cycle. These changes in turn alter the arrangement of the transmembrane helices, which allows the Ca2+ to be released from its binding cavity into the SR lumen (Movie 11.3). (PDB code: 3B9B.) these concentration differences. Like the Ca2+ pump, the Na+-K+ pump belongs to the family of P-type ATPases and operates as an ATP-driven antiporter, actively pumping Na+ out of the cell against its steep electrochemical gradient and pumping K+ in (Figure 11–15).


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Cell_Biology_Alberts. We mentioned earlier that the Na+ gradient produced by the Na+-K+ pump drives the transport of most nutrients into animal cells and also has a crucial role in regulating cytosolic pH. A typical animal cell devotes almost one-third of its energy to fueling this pump, and the pump consumes even more energy in nerve cells and in cells that are dedicated to transport processes, such as those forming kidney tubules. Since the Na+-K+ pump drives three positively charged ions out of the cell for every two it pumps in, it is electrogenic: it drives a net electric current across the membrane, tending to create an electrical potential, with the cell’s inside being negative relative to the outside. This electrogenic effect of the pump, however, seldom directly contributes more than 10% to the membrane potential. The remaining 90%, as we discuss later, depends only indirectly on the Na+-K+ pump. Figure 11–14 The pumping cycle of the sarcoplasmic reticulum Ca2+ pump.


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Cell_Biology_Alberts. Figure 11–14 The pumping cycle of the sarcoplasmic reticulum Ca2+ pump. Ion pumping proceeds by a series of stepwise conformational changes in which movements of the pump’s three cytosolic domains [the nucleotide-binding domain (N), the phosphorylation domain (P), and the activator domain (A)] are mechanically coupled to movements of the transmembrane α helices. Helix movement opens and closes passageways through which Ca2+ enters from the cytosol and binds to the two centrally located Ca2+ binding sites. The two Ca2+ then exit into the SR lumen and are replaced by two H+, which are transported in the opposite direction. The Ca2+-dependent phosphorylation and H+-dependent dephosphorylation of aspartic acid are universally conserved steps in the reaction cycle of all P-type pumps: they cause the conformational transitions to occur in an orderly manner, enabling the proteins to do useful work. (Adapted from C. Toyoshima et al., Nature 432:361–368, 2004 and


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Cell_Biology_Alberts. J.V. Møller et al., Q. Rev. Biophys. 43:501– 566, 2010.) Figure 11–15 The function of the Na+-K+ pump. This P-type ATPase actively pumps Na+ out of and K+ into a cell against their electrochemical gradients. It is structurally closely related to the Ca2+ ATPase but differs in its selectivity for ions: for every molecule of ATP hydrolyzed by the pump, three Na+ are pumped out and two K+ are pumped in. As in the Ca2+ pump, an aspartate is phosphorylated and dephosphorylated during the pumping cycle (Movie 11.4). ABC Transporters Constitute the Largest Family of Membrane Transport Proteins


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Cell_Biology_Alberts. The last type of transport ATPase that we discuss is the family of the ABC transporters, so named because each member contains two highly conserved ATPase domains, or ATP-Binding “Cassettes,” on the cytosolic side of the membrane. ATP binding brings together the two ATPase domains, and ATP hydrolysis leads to their dissociation (Figure 11–16). These movements of the cytosolic domains are transmitted to the transmembrane segments, driving cycles of conformational changes that alternately expose solute-binding sites on one side of the membrane and then on the other, as we have seen for other transporters. In this way, ABC transporters harvest the energy released upon ATP binding and hydrolysis to drive transport of solutes across the bilayer. The transport is directional toward inside or toward outside, depending on the particular conformational change in the solute binding site that is linked to ATP hydrolysis (see Figure 11–16).


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Cell_Biology_Alberts. ABC transporters constitute the largest family of membrane transport proteins and are of great clinical importance. The first of these proteins to be characterized was found in bacteria. We have already mentioned that the plasma membranes of all bacteria contain transporters that use the H+ gradient across the membrane to actively transport a variety of nutrients into the cell. In addition, bacteria use ABC transporters to import certain small molecules. In bacteria such as E. coli that have double membranes (Figure 11–17), the ABC transporters are located in the inner membrane, and an auxiliary mechanism operates to capture the nutrients and deliver them to the transporters (Figure 11–18).


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Cell_Biology_Alberts. In E. coli, 78 genes (an amazing 5% of the bacterium’s genes) encode ABC transporters, and animal genomes encode an even larger number. Although each transporter is thought to be specific for a particular molecule or class of molecules, the variety of substrates transported by this superfamily is great and includes inorganic ions, amino acids, monoand polysaccharides, peptides, lipids, drugs, and, in some cases, even proteins that can be larger than the transporter itself.


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Cell_Biology_Alberts. Figure 11–16 Small-molecule transport by typical abC transporters. ABC transporters consist of multiple domains. Typically, two hydrophobic domains, each built of six membrane-spanning α helices, together form the translocation pathway and provide substrate specificity. Two ATPase domains protrude into the cytosol. In some cases, the two halves of the transporter are formed by a single polypeptide, whereas in other cases they are formed by two or more separate polypeptides that assemble into a similar structure. Without ATP bound, the transporter exposes a substrate-binding site on one side of the membrane. ATP binding induces a conformational change that exposes the substrate-binding site on the opposite side; ATP hydrolysis followed by ADP dissociation returns the transporter to its original conformation. Most individual ABC transporters are unidirectional. (A) Both importing and exporting ABC transporters are found in bacteria; an ABC importer is shown in this cartoon. The crystal


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Cell_Biology_Alberts. conformation. Most individual ABC transporters are unidirectional. (A) Both importing and exporting ABC transporters are found in bacteria; an ABC importer is shown in this cartoon. The crystal structure of a bacterial ABC transporter is shown in Figure 3–76.


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Cell_Biology_Alberts. (B) In eukaryotes, most ABC transporters export substances—either from the cytosol to the extracellular space or from the cytosol to a membrane-bound intracellular compartment such as the endoplasmic reticulum—or from the mitochondrial matrix to the cytosol.


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Cell_Biology_Alberts. The first eukaryotic ABC transporters identified were discovered because of their ability to pump hydrophobic drugs out of the cytosol. One of these transporters is the multidrug resistance (MDR) protein, also called P-glycoprotein. It is present at elevated levels in many human cancer cells and makes the cells simultaneously resistant to a variety of chemically unrelated cytotoxic drugs that are widely used in cancer chemotherapy. Treatment with any one of these drugs can result in the selective survival and overgrowth of those cancer cells that express an especially large amount of the MDR transporter. These cells pump drugs out of the cell very efficiently and are therefore relatively resistant to the drugs’ toxic effects (Movie 11.5). Selection for cancer cells with resistance to one drug can thereby lead to resistance to a wide variety of anticancer drugs. Some studies indicate that up to 40% of human cancers develop multidrug resistance, making it a major hurdle in the battle


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Cell_Biology_Alberts. drug can thereby lead to resistance to a wide variety of anticancer drugs. Some studies indicate that up to 40% of human cancers develop multidrug resistance, making it a major hurdle in the battle against cancer.


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Cell_Biology_Alberts. A related and equally sinister phenomenon occurs in the protist Plasmodium falciparum, which causes malaria. More than 200 million people are infected worldwide with this parasite, which remains a major cause of human death, killing almost a million people every year. The development of resistance to the antimalarial drug chloroquine has hampered the control of malaria. The resistant P. falciparum have amplified a gene encoding an ABC transporter that pumps out the chloroquine.


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Cell_Biology_Alberts. Figure 11–17 a small section of the double membrane of an E. coli bacterium. The inner membrane is the cell’s plasma membrane. Between the inner and outer membranes is a highly porous, rigid peptidoglycan layer, composed of protein and polysaccharide that constitute the bacterial cell wall. It is attached to lipoprotein molecules in the outer membrane and fills the periplasmic space (only a little of the peptidoglycan layer is shown). This space also contains a variety of soluble protein molecules. The dashed threads (shown in green) at the top represent the polysaccharide chains of the special lipopolysaccharide molecules that form the external monolayer of the outer membrane; for clarity, only a few of these chains are shown. Bacteria with double membranes are called Gram-negative because they do not retain the dark blue dye used in Gram staining. Bacteria with single membranes (but thicker peptidoglycan cell walls), such as staphylococci and streptococci, retain the blue dye and


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Cell_Biology_Alberts. they do not retain the dark blue dye used in Gram staining. Bacteria with single membranes (but thicker peptidoglycan cell walls), such as staphylococci and streptococci, retain the blue dye and are therefore called Gram-positive; their single membrane is analogous to the inner (plasma) membrane of Gram-negative bacteria.


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Cell_Biology_Alberts. Figure 11–18 The auxiliary transport system associated with transport aTPases in bacteria with double membranes. The solute diffuses through channel proteins (porins) in the outer membrane and binds to a periplasmic substrate-binding protein that delivers it to the ABC transporter, which pumps it across the plasma membrane. The peptidoglycan is omitted for simplicity; its porous structure allows the substrate-binding proteins and water-soluble solutes to move through it by diffusion.


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Cell_Biology_Alberts. In most vertebrate cells, an ABC transporter in the endoplasmic reticulum (ER) membrane (named transporter associated with antigen processing, or TAP transporter) actively pumps a wide variety of peptides from the cytosol into the ER lumen. These peptides are produced by protein degradation in proteasomes (discussed in Chapter 6). They are carried from the ER to the cell surface, where they are displayed for scrutiny by cytotoxic T lymphocytes, which kill the cell if the peptides are derived from a virus or other microorganism lurking in the cytosol of an infected cell (discussed in Chapter 24).


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Cell_Biology_Alberts. Yet another member of the ABC transporter family is the cystic fibrosis trans-membrane conductance regulator protein (CFTR), which was discovered through studies of the common genetic disease cystic fibrosis. This disease is caused by a mutation in the gene encoding CFTR, a Cl– transport protein in the plasma membrane of epithelial cells. CFTR regulates ion concentrations in the extracellular fluid, especially in the lung. One in 27 Caucasians carries a gene encoding a mutant form of this protein; in 1 in 2900, both copies of the gene are mutated, causing the disease. In contrast to other ABC transporters, ATP binding and hydrolysis in the CFTR protein do not drive the transport process. Instead, they control the opening and closing of a continuous channel, which provides a passive conduit for Cl– to move down its electrochemical gradient. Thus, some ABC proteins can function as transporters and others as gated channels.


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Cell_Biology_Alberts. Transporters bind specific solutes and transfer them across the lipid bilayer by undergoing conformational changes that alternately expose the solute-binding site on one side of the membrane and then on the other. Some transporters move a single solute “downhill,” whereas others can act as pumps to move a solute “uphill” against its electrochemical gradient, using energy provided by ATP hydrolysis, by a downhill flow of another solute (such as Na+ or H+), or by light to drive the requisite series of conformational changes in an orderly manner. Transporters belong to a small number of protein families. Each family evolved from a common ancestral protein, and its members all operate by a similar mechanism. The family of P-type transport ATPases, which includes Ca2+ and Na+-K+ pumps, is an important example; each of these ATPases sequentially phosphorylates and dephosphorylates itself during the pumping cycle. The superfamily of ABC transporters is the largest family of membrane


Cell_Biology_Alberts

Cell_Biology_Alberts. is an important example; each of these ATPases sequentially phosphorylates and dephosphorylates itself during the pumping cycle. The superfamily of ABC transporters is the largest family of membrane transport proteins and is especially important clinically. It includes proteins that are responsible for cystic fibrosis, for drug resistance in both cancer cells and malaria-causing parasites, and for pumping pathogen-derived peptides into the ER for cytotoxic lymphocytes to reorganize on the surface of infected cells.


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Cell_Biology_Alberts. Unlike transporters, channels form pores across membranes. One class of channel proteins found in virtually all animals forms gap junctions between adjacent cells; each plasma membrane contributes equally to the formation of the channel, which connects the cytoplasm of the two cells. These channels are discussed in Chapter 19 and will not be considered further here. Both gap junctions and porins, the channels in the outer membranes of bacteria, mitochondria, and chloroplasts (discussed in Chapter 10), have relatively large and permissive pores, and it would be disastrous if they directly connected the inside of a cell to an extracellular space. Indeed, many bacterial toxins do exactly that to kill other cells (discussed in Chapter 24).


Cell_Biology_Alberts

Cell_Biology_Alberts. In contrast, most channels in the plasma membrane of animal and plant cells that connect the cytosol to the cell exterior necessarily have narrow, highly selective pores that can open and close rapidly. Because these proteins are concerned specifically with inorganic ion transport, they are referred to as ion channels. For transport efficiency, ion channels have an advantage over transporters, in that they can pass up to 100 million ions through one open channel each second—a rate 105 times greater than the fastest rate of transport mediated by any known transporter. As discussed earlier, however, channels cannot be coupled to an energy source to perform active transport, so the transport they mediate is always passive (downhill). Thus, the function of ion channels is to allow specific inorganic ions—primarily Na+, K+, Ca2+, or Cl–—to diffuse rapidly down their electrochemical gradients across the lipid bilayer. In this section, we will see that the ability to control ion fluxes


Cell_Biology_Alberts

Cell_Biology_Alberts. inorganic ions—primarily Na+, K+, Ca2+, or Cl–—to diffuse rapidly down their electrochemical gradients across the lipid bilayer. In this section, we will see that the ability to control ion fluxes through these channels is essential for many cell functions. Nerve cells (neurons), in particular, have made a specialty of using ion channels, and we will consider how they use many different ion channels to receive, conduct, and transmit signals. Before we discuss ion channels, however, we briefly consider the aquaporin water channels that we mentioned earlier.


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Cell_Biology_Alberts. Aquaporins Are Permeable to Water But Impermeable to Ions


Cell_Biology_Alberts

Cell_Biology_Alberts. Because cells are mostly water (typically ~70% by weight), water movement across cell membranes is fundamentally important for life. Cells also contain a high concentration of solutes, including numerous negatively charged organic molecules that are confined inside the cell (the so-called fixed anions) and their accompanying cations that are required for charge balance. This creates an osmotic gradient, which mostly is balanced by an opposite osmotic gradient due to a high concentration of inorganic ions—chiefly Na+ and Cl–—in the extracellular fluid. The small remaining osmotic force tends to “pull” water into the cell, causing it to swell until the forces are balanced. Because all biological membranes are moderately permeable to water (see Figure 11–2), cell volume equilibrates in minutes or less in response to an osmotic gradient. For most animal cells, however, osmosis has only a minor role in regulating cell volume. This is because most of the cytoplasm is in a gel-like state and


Cell_Biology_Alberts

Cell_Biology_Alberts. or less in response to an osmotic gradient. For most animal cells, however, osmosis has only a minor role in regulating cell volume. This is because most of the cytoplasm is in a gel-like state and resists large changes in its volume in response to changes in osmolarity.


Cell_Biology_Alberts

Cell_Biology_Alberts. In addition to the direct diffusion of water across the lipid bilayer, some prokaryotic and eukaryotic cells have water channels, or aquaporins, embedded in their plasma membrane to allow water to move more rapidly. Aquaporins are particularly abundant in animal cells that must transport water at high rates, such as the epithelial cells of the kidney or exocrine cells that must transport or secrete large volumes of fluids, respectively (Figure 11–19).


Cell_Biology_Alberts

Cell_Biology_Alberts. Aquaporins must solve a problem that is opposite to that facing ion channels. To avoid disrupting ion gradients across membranes, they have to allow the rapid passage of water molecules while completely blocking the passage of ions. The three-dimensional structure of an aquaporin reveals how it achieves this remarkable selectivity. The channels have a narrow pore that allows water molecules to traverse the membrane in single file, following the path of carbonyl oxygens that line one side of the pore (Figure 11–20A and B). Hydrophobic amino acids line the other side of the pore. The pore is too narrow for any hydrated ion to enter, and the energy cost of dehydrating an ion would be enormous because the hydrophobic wall of the pore cannot interact with a dehydrated ion to compensate for the loss of water. This design readily explains why the aquaporins cannot conduct K+,


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–19 The role of aquaporins in fluid secretion. Cells lining the ducts of exocrine glands (as found, for example, in the pancreas and liver, and in mammary, sweat, and salivary glands) secrete large volumes of body fluids. These cells are organized into epithelial sheets in which their apical plasma membrane faces the lumen of the duct. Ion pumps and channels situated in the basolateral and apical plasma membrane move ions (mostly Na+ and Cl–) into the ductal lumen, creating an osmotic gradient between the surrounding tissue and the duct. Water molecules rapidly follow the osmotic gradient through aquaporins that are present in high concentrations in both the apical and basolateral membranes.


Cell_Biology_Alberts

Cell_Biology_Alberts. Na+, Ca2+, or Cl– ions. These channels are also impermeable to H+, which is mainly present in cells as H3O+. These hydronium ions diffuse through water extremely rapidly, using a molecular relay mechanism that requires the making and breaking of hydrogen bonds between adjacent water molecules (Figure 11–20C). Aquaporins contain two strategically placed asparagines, which bind to the oxygen atom of the central water molecule in the line of water molecules traversing the pore, imposing a bipolarity on the entire column of water molecules (Figure 11–20C and D). This makes it impossible for the “making and breaking” sequence of hydrogen bonds (shown in Figure 11–20C) to get past the central asparagine-bonded water molecule. Because both valences of this central oxygen are unavailable for hydrogen-bonding, the central water molecule cannot participate in an H+ relay, and the pore is therefore impermeable to H+. We now turn to ion channels, the subject of the rest of the chapter.


Cell_Biology_Alberts

Cell_Biology_Alberts. We now turn to ion channels, the subject of the rest of the chapter. Two important properties distinguish ion channels from aqueous pores. First, they show ion selectivity, permitting some inorganic ions to pass, but not others. This suggests that their pores must be narrow enough in places to force permeating ions into intimate contact with the walls of the channel so that only ions of appropriate size and charge can pass. The permeating ions have to shed most or all of their associated water molecules to pass, often in single file, through the narrowest part of the channel, which is called the selectivity filter; this limits their rate of passage (Figure 11–21). Thus, as the ion concentration increases, the flux of the ion through a channel increases proportionally but then levels off (saturates) at a maximum rate.


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Cell_Biology_Alberts. Figure 11–20 The structure of aquaporins. (A) A ribbon diagram of an aquaporin monomer. In the membrane, aquaporins form tetramers, with each monomer containing an aqueous pore in its center (not shown). Each individual aquaporin channel passes about 109 water molecules per second. (B) A longitudinal cross section through one aquaporin monomer, in the plane of the central pore. One face of the pore is lined with hydrophilic amino acids, which provide transient hydrogen bonds to water molecules; these bonds help line up the transiting water molecules in a single row and orient them as they traverse the pore. (C and D) A model explaining why aquaporins are impermeable to H+.


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Cell_Biology_Alberts. (C) In water, H+ diffuses extremely rapidly by being relayed from one water molecule to the next. (D) Carbonyl groups (C=O) lining the hydrophilic face of the pore align water molecules, and two strategically placed asparagines in the center help tether a central water molecule such that both valences on its oxygen are occupied. This arrangement bipolarizes the entire line of water molecules, with each water molecule acting as a hydrogen-bond acceptor from its inner neighbor (Movie 11.6). (A and B, adapted from R.M. Stroud et al., Curr. Opin. Struct. Biol. 13:424–431, 2003. With permission from Elsevier.)


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–21 a typical ion channel, which fluctuates between closed and open conformations. The ion channel shown here in cross section forms a pore across the lipid bilayer only in the “open” conformational state. The pore narrows to atomic dimensions in one region (the selectivity filter), where the ion selectivity of the channel is largely determined. Another region of the channel forms the gate. Figure 11–22 The gating of ion channels. This schematic drawing shows several kinds of stimuli that open ion channels. Mechanically gated channels often have cytoplasmic extensions (not shown) that link the channel to the cytoskeleton.


Cell_Biology_Alberts

Cell_Biology_Alberts. The second important distinction between ion channels and aqueous pores is that ion channels are not continuously open. Instead, they are gated, which allows them to open briefly and then close again. Moreover, with prolonged (chemical or electrical) stimulation, most ion channels go into a closed “desensitized,” or “inactivated,” state, in which they are refractory to further opening until the stimulus has been removed, as we discuss later. In most cases, the gate opens in response to a specific stimulus. As shown in Figure 11–22, the main types of stimuli that are known to cause ion channels to open are a change in the voltage across the membrane (voltage-gated channels), a mechanical stress (mechanically gated channels), or the binding of a ligand (ligand-gated channels). The ligand can be either an extracellular mediator—specifically, a neurotransmitter (transmitter-gated channels)—or an intracellular mediator such as an ion (ion-gated channels) or a nucleotide (nucleotide-gated


Cell_Biology_Alberts

Cell_Biology_Alberts. be either an extracellular mediator—specifically, a neurotransmitter (transmitter-gated channels)—or an intracellular mediator such as an ion (ion-gated channels) or a nucleotide (nucleotide-gated channels). In addition, protein phosphorylation and dephosphorylation regulates the activity of many ion channels; this type of channel regulation is discussed, together with nucleotide-gated ion channels, in Chapter 15.


Cell_Biology_Alberts

Cell_Biology_Alberts. More than 100 types of ion channels have been identified thus far, and new ones are still being discovered, each characterized by the ions it conducts, the mechanism by which it is gated, and its abundance and localization in the cell and in specific cells. Ion channels are responsible for the electrical excitability of muscle cells, and they mediate most forms of electrical signaling in the nervous system. A single neuron typically contains 10 or more kinds of ion channels, located in different domains of its plasma membrane. But ion channels are not restricted to electrically excitable cells. They are present in all animal cells and are found in plant cells and microorganisms: they propagate the leaf-closing response of the mimosa plant, for example (Movie 11.7), and allow the single-celled Paramecium to reverse direction after a collision.


Cell_Biology_Alberts

Cell_Biology_Alberts. Ion channels that are permeable mainly to K+ are found in the plasma membrane of almost all cells. An important subset of K+ channels opens even in an unstimulated or “resting” cell, and hence these are called K+ leak channels. Although this term applies to many different K+ channels, depending on the cell type, they serve a common purpose: by making the plasma membrane much more permeable to K+ than to other ions, they have a crucial role in maintaining the membrane potential across all plasma membranes, as we discuss next. The Membrane Potential in Animal Cells Depends Mainly on K+ Leak Channels and the K+ Gradient Across the Plasma Membrane


Cell_Biology_Alberts

Cell_Biology_Alberts. The Membrane Potential in Animal Cells Depends Mainly on K+ Leak Channels and the K+ Gradient Across the Plasma Membrane A membrane potential arises when there is a difference in the electrical charge on the two sides of a membrane, due to a slight excess of positive ions over negative ones on one side and a slight deficit on the other. Such charge differences can result both from active electrogenic pumping (see p. 608) and from passive ion diffusion. As we discuss in Chapter 14, electrogenic H+ pumps in the mitochondrial inner membrane generate most of the membrane potential across this membrane. Electrogenic pumps also generate most of the electrical potential across the plasma membrane in plants and fungi. In typical animal cells, however, passive ion movements make the largest contribution to the electrical potential across the plasma membrane.


Cell_Biology_Alberts

Cell_Biology_Alberts. As explained earlier, due to the action of the Na+-K+ pump, there is little Na+ inside the cell, and other intracellular inorganic cations have to be plentiful enough to balance the charge carried by the cell’s fixed anions—the negatively charged organic molecules that are confined inside the cell. This balancing role is performed largely by K+, which is actively pumped into the cell by the Na+K+ pump and can also move freely in or out through the K+ leak channels in the plasma membrane. Because of the presence of these channels, K+ comes almost to equilibrium, where an electrical force exerted by an excess of negative charges attracting K+ into the cell balances the tendency of K+ to leak out down its concentration gradient. The membrane potential (of the plasma membrane) is the manifestation of this electrical force, and we can calculate its equilibrium value from the steepness of the K+ concentration gradient. The following argument may help to make this clear.


Cell_Biology_Alberts

Cell_Biology_Alberts. Suppose that initially there is no voltage gradient across the plasma membrane (the membrane potential is zero) but the concentration of K+ is high inside the cell and low outside. K+ will tend to leave the cell through the K+ leak channels, driven by its concentration gradient. As K+ begins to move out, each ion leaves behind an unbalanced negative charge, thereby creating an electrical field, or membrane potential, which will tend to oppose the further efflux of K+. The net efflux of K+ halts when the membrane potential reaches a value at which this electrical driving force on K+ exactly balances the effect of its concentration gradient—that is, when the electrochemical gradient for K+ is zero. Although Cl– ions also equilibrate across the membrane, the membrane potential keeps most of these ions out of the cell because their charge is negative.


Cell_Biology_Alberts

Cell_Biology_Alberts. The equilibrium condition, in which there is no net flow of ions across the plasma membrane, defines the resting membrane potential for this idealized cell. A simple but very important formula, the Nernst equation, quantifies the equilibrium condition and, as explained in Panel 11–1, makes it possible to calculate the theoretical resting membrane potential if we know the ratio of internal and external ion concentrations. As the plasma membrane of a real cell is not exclusively permeable to K+ and Cl–, however, the actual resting membrane potential is usually not exactly equal to that predicted by the Nernst equation for K+ or Cl–. The Resting Potential Decays Only Slowly When the Na+-K+ Pump Is Stopped


Cell_Biology_Alberts

Cell_Biology_Alberts. The Resting Potential Decays Only Slowly When the Na+-K+ Pump Is Stopped Movement of only a minute number of inorganic ions across the plasma membrane through ion channels suffices to set up the membrane potential. Thus, we can think of the membrane potential as arising from movements of charge that leave ion concentrations practically unaffected and result in only a very slight discrepancy in the number of positive and negative ions on the two sides of the membrane (Figure 11–23). Moreover, these movements of charge are generally rapid, taking only a few milliseconds or less. Consider the change in the membrane potential in a real cell after the sudden inactivation of the Na+-K+ pump. A slight drop in the membrane potential occurs immediately. This is because the pump is electrogenic and, when active, makes a


Cell_Biology_Alberts

Cell_Biology_Alberts. The fow of any inorganic ion through a membrane channel is driven by the electrochemical gradient for that ion. This gradient represents the combination of two infuences: the voltage gradient and the concentration gradient of the ion across the membrane. When these two infuences just balance each other, the electrochemical gradient for the ion is zero, and there is no net fow of the ion through the channel. The voltage gradient (membrane potential) at which this equilibrium is reached is called the equilibrium potential for the ion. It can be calculated from an equation that will be derived below, called the Nernst equation. The Nernst equation is V = RT In Co zF Ci V = the equilibrium potential in volts (internal Co and Ci = outside and inside concentrations of the ion, respectively R = the gas constant (8.3 J mol–1 K–1) T = the absolute temperature (K) F = Faraday’s constant (9.6 × 104 J V–1 mol–1) z = the valence (charge) of the ion In = logarithm to the base e


Cell_Biology_Alberts

Cell_Biology_Alberts. R = the gas constant (8.3 J mol–1 K–1) T = the absolute temperature (K) F = Faraday’s constant (9.6 × 104 J V–1 mol–1) z = the valence (charge) of the ion In = logarithm to the base e The Nernst equation is derived as follows: A molecule in solution (a solute) tends to move from a region of high concentration to a region of low concentration simply due to the random movement of molecules, which results in their equilibrium. Consequently, movement down a concentration gradient is accompanied by a favorable free-energy change (°G < 0), whereas movement up a concentration gradient is accompanied by an unfavorable free-energy change (°G > 0). (Free energy is introduced in Chapter 2 and discussed in the context of redox reactions in Panel 14–1, p. 765.) The free-energy change per mole of solute moved across the plasma membrane (°Gconc) is equal to –RT In Co / Ci.


Cell_Biology_Alberts

Cell_Biology_Alberts. The free-energy change per mole of solute moved across the plasma membrane (°Gconc) is equal to –RT In Co / Ci. If the solute is an ion, moving it into a cell across a membrane whose inside is at a voltage V relative to the outside will cause an additional free-energy change (per mole of solute moved) of °Gvolt = zFV. At the point where the concentration and voltage gradients just balance, and the ion distribution is at equilibrium across the membrane. Thus, and, therefore, or, using the constant that converts natural logarithms to base 10, V = 2.3 log10 For a univalent cation, 2.3 = 58 mV at 20oC and 61.5 mV at 37oC. Thus, for such an ion at 37ºC, V = + 61.5 mV for Co/ Ci = 10, whereas V = 0 for Co / Ci= 1. The K+ equilibrium potential (VK), for example, is 61.5 log10([K+]o / [K+]i) millivolts (–89 mV for a typical cell, where [K+]o = 5 mM and [K+]i = 140 mM). At VK, there is no net fow of K+ across the membrane.


Cell_Biology_Alberts

Cell_Biology_Alberts. At VK, there is no net fow of K+ across the membrane. Similarly, when the membrane potential has a value of 61.5 log10([Na+]o /[Na+]i), the Na+ equilibrium potential (VNa), there is no net fow of Na+. For any particular membrane potential, VM, the net force tending to drive a particular type of ion out of the cell, is proportional to the difference between VM and the equilibrium potential for the ion: hence, for K+ it is VM – VK and for Na+ it is VM – VNa.


Cell_Biology_Alberts

Cell_Biology_Alberts. When there is a voltage gradient across the membrane, the ions responsible for it—the positive ions on one side and the negative ions on the other—are concentrated in thin layers on either side of the membrane because of the attraction between positive and negative electric charges. The number of ions that go to form the layer of charge adjacent to the membrane is minute compared with the total number inside the cell. For example, the movement of 6000 Na+ ions across 1 µm2 of membrane will carry suffcient charge to shift the membrane potential by about 100 mV. Because there are about 3 × 107 Na+ ions in a typical cell (1 µm3 of bulk cytoplasm), such a movement of charge will generally have a negligible effect on the ion concentration gradients across the membrane.


Cell_Biology_Alberts

Cell_Biology_Alberts. exact balance of charges on each side of the a few of the positive ions (red) cross the membrane; membrane potential = 0 membrane from right to left, leaving their small direct contribution to the membrane potential by pumping out three Na+ for every two K+ that it pumps in (see Figure 11–15). However, switching off the pump does not abolish the major component of the resting potential, which is generated by the K+ equilibrium mechanism just described. This component of the membrane potential persists as long as the Na+ concentration inside the cell stays low and the K+ ion concentration high—typically for many minutes. But the plasma membrane is somewhat permeable to all small ions, including Na+. Therefore, without the Na+-K+ pump, the ion gradients set up by the pump will eventually run down, and the membrane potential established by diffusion through the K+ leak channels will fall as well. As Na+ enters, the cell eventually comes to a new resting state where Na+, K+, and Cl– are


Cell_Biology_Alberts

Cell_Biology_Alberts. down, and the membrane potential established by diffusion through the K+ leak channels will fall as well. As Na+ enters, the cell eventually comes to a new resting state where Na+, K+, and Cl– are all at equilibrium across the membrane. The membrane potential in this state is much less than it was in the normal cell with an active Na+-K+ pump.


Cell_Biology_Alberts

Cell_Biology_Alberts. The resting potential of an animal cell varies between –20 mV and –120 mV, depending on the organism and cell type. Although the K+ gradient always has a major influence on this potential, the gradients of other ions (and the disequilibrating effects of ion pumps) also have a significant effect: the more permeable the membrane for a given ion, the more strongly the membrane potential tends to be driven toward the equilibrium value for that ion. Consequently, changes in a membrane’s permeability to ions can cause significant changes in the membrane potential. This is one of the key principles relating the electrical excitability of cells to the activities of ion channels. To understand how ion channels select their ions and how they open and close, we need to know their atomic structure. The first ion channel to be crystallized and studied by x-ray diffraction was a bacterial K+ channel. The details of its structure revolutionized our understanding of ion channels.


Cell_Biology_Alberts

Cell_Biology_Alberts. The Three-Dimensional Structure of a Bacterial K+ Channel Shows How an Ion Channel Can Work Scientists were puzzled by the remarkable ability of ion channels to combine exquisite ion selectivity with a high conductance. K+ leak channels, for example, conduct K+ 10,000-fold faster than Na+, yet the two ions are both featureless spheres and have similar diameters (0.133 nm and 0.095 nm, respectively). A single amino acid substitution in the pore of an animal cell K+ channel can result in a loss of ion selectivity and cell death. We cannot explain the normal K+ selectivity by pore size, because Na+ is smaller than K+. Moreover, the high conductance rate is incompatible with the channel’s having selective, high-affinity K+-binding sites, as the binding of K+ ions to such sites would greatly slow their passage.


Cell_Biology_Alberts

Cell_Biology_Alberts. The puzzle was solved when the structure of a bacterial K+ channel was determined by x-ray crystallography. The channel is made from four identical trans-membrane subunits, which together form a central pore through the membrane. Each subunit contributes two transmembrane α helices, which are tilted outward in the membrane and together form a cone, with its wide end facing the outside of


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–23 The ionic basis of a membrane potential. A small flow of inorganic ions through an ion channel carries sufficient charge to cause a large change in the membrane potential. The ions that give rise to the membrane potential lie in a thin (< 1 nm) surface layer close to the membrane, held there by their electrical attraction to their oppositely charged counterparts (counterions) on the other side of the membrane. For a typical cell, 1 microcoulomb of charge (6 × 1012 monovalent ions) per square centimeter of membrane, transferred from one side of the membrane to the other, changes the membrane potential by roughly 1 V. This means, for example, that in a spherical cell of diameter 10 μm, the number of K+ ions that have to flow out to alter the membrane potential by 100 mV is only about 1/100,000 of the total number of K+ ions in the cytosol. This amount is so minute that the intracellular K+ concentration remains virtually unchanged.


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Cell_Biology_Alberts. 618 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes


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Cell_Biology_Alberts. Figure 11–24 The structure of a bacterial K+ channel. (A) The transmembrane α helices from only two of the four identical subunits are shown. From the cytosolic side, the pore (schematically shaded in blue) opens up into a vestibule in the middle of the membrane. The pore vestibule facilitates transport by allowing the K+ ions to remain hydrated even though they are more than halfway across the membrane. The narrow selectivity filter of the pore links the vestibule to the outside of the cell. Carbonyl oxygens line the walls of the selectivity filter and form transient binding sites for dehydrated K+ ions. Two K+ ions occupy different sites in the selectivity filter, while a third K+ ion is located in the center of the vestibule, where it is stabilized by electrical interactions with the more negatively charged ends of the pore helices. The ends of the four short “pore helices” (only two of which are shown) point precisely toward the center of the vestibule, thereby guiding K+ ions


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Cell_Biology_Alberts. more negatively charged ends of the pore helices. The ends of the four short “pore helices” (only two of which are shown) point precisely toward the center of the vestibule, thereby guiding K+ ions into the selectivity filter (Movie 11.8). (B) Peptide bonds have an electric dipole, with more negative charge accumulated at the oxygen of the C=O bond and at the nitrogen of the N–H bond. In an α helix, hydrogen bonds (red) align the dipoles. As a consequence, every α helix has an electric dipole along its axis, resulting from summation of the dipoles of the individual peptide bonds, with a more negatively charged C-terminal end (δ–) and a more positively charged N-terminal end (δ+). (A, adapted from D.A. Doyle et al., Science 280:69–77, 1998.) the cell where K+ ions exit from the channel (Figure 11–24). The polypeptide chain that connects the two transmembrane helices forms a short αhelix (the pore helix) and a crucial loop that protrudes into the wide section of the cone to form the


Cell_Biology_Alberts

Cell_Biology_Alberts. (Figure 11–24). The polypeptide chain that connects the two transmembrane helices forms a short αhelix (the pore helix) and a crucial loop that protrudes into the wide section of the cone to form the selectivity filter. The selectivity loops from the four subunits form a short, rigid, narrow pore, which is lined by the carbonyl oxygen atoms of their polypeptide backbones. Because the selectivity loops of all known K+ channels have similar amino acid sequences, it is likely that they form a closely similar structure.


Cell_Biology_Alberts

Cell_Biology_Alberts. The structure of the selectivity filter explains the ion selectivity of the channel. A K+ ion must lose almost all of its bound water molecules to enter the filter, where it interacts instead with the carbonyl oxygens lining the filter; the oxygens are rigidly spaced at the exact distance to accommodate a K+ ion. A Na+ ion, in contrast, cannot enter the filter because the carbonyl oxygens are too far away from the smaller Na+ ion to compensate for the energy expense associated with the loss of water molecules required for entry (Figure 11–25).


Cell_Biology_Alberts

Cell_Biology_Alberts. Structural studies of K+ channels and other ion channels have also indicated some general principles of how these channels open and close. The gating involves movement of the helices in the membrane so that they either obstruct or open the path for ion movement. Depending on the particular type of channel, helices tilt, rotate, or bend during gating. The structure of a closed K+ channel shows that by tilting the inner helices, the pore constricts like a diaphragm at its cytosolic end (Figure 11–26). Bulky hydrophobic amino acid side chains block the small opening that remains, preventing the entry of ions. Many other ion channels operate on similar principles: the channel’s gating helices are allosterically coupled to domains that form the ion-conducting pathway; and a conformational change in the gate—in response, say, to ligand binding or altered membrane potential—brings about conformational change in the conducting pathway, either opening it or blocking it off.


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Cell_Biology_Alberts. All organisms, from single-cell bacteria to multicellular animals and plants, must sense and respond to mechanical forces in their external environment (such as sound, touch, pressure, shear forces, and gravity) and in their internal environment (such as osmotic pressure and membrane bending). Numerous proteins are known to be capable of responding to such mechanical forces, and a large subset of those proteins has been identified as possible mechanosensitive channels, but very few of the candidate proteins have been shown directly to be mechanically activated ion channels. One reason for this dearth in our knowledge is that most such channels are extremely rare. Auditory hair cells in the human cochlea, for example, contain extraordinarily sensitive mechanically gated ion channels, but each of the approximately 15,000 individual hair cells is thought to have a total of only 50–100 of them (Movie 11.9). Additional difficulties arise because the gating mechanisms of many


Cell_Biology_Alberts

Cell_Biology_Alberts. but each of the approximately 15,000 individual hair cells is thought to have a total of only 50–100 of them (Movie 11.9). Additional difficulties arise because the gating mechanisms of many mechanosensitive channel types require the channels to be embedded in complex architectures that require attachment to the extracellular matrix or to the cytoskeleton and are difficult to reconstitute in the test tube. The study of mechanosensitive receptors is a field of active investigation.


Cell_Biology_Alberts

Cell_Biology_Alberts. A well-studied class of mechanosensitive channels is found in the bacterial plasma membrane. These channels open in response to mechanical stretching of the lipid bilayer in which they are embedded. When a bacterium experiences a low-ionic-strength external environment (hypotonic conditions), such as


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Cell_Biology_Alberts. Figure 11–25 K+ specificity of the selectivity filter in a K+ channel. The drawings show K+ and Na+ ions (A) in the vestibule and (B) in the selectivity filter of the pore, viewed in cross section. In the vestibule, the ions are hydrated. In the selectivity filter, they have lost their water, and the carbonyl oxygens are placed to accommodate a dehydrated K+ ion. The dehydration of the K+ ion requires energy, which is precisely balanced by the energy regained by the interaction of the ion with all of the carbonyl oxygens that serve as surrogate water molecules. Because the Na+ ion is too small to interact with the oxygens, it can enter the selectivity filter only at a great energetic expense. The filter therefore selects K+ ions with high specificity. (A, adapted from Y. Zhou et al., Nature 414:43–48, 2001. With permission from Macmillan Publishers Ltd.)


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Cell_Biology_Alberts. Figure 11–26 a model for the gating of a bacterial K+ channel. The channel is viewed in cross section. To adopt the closed conformation, the four inner transmembrane helices that line the pore on the cytosolic side of the selectivity filter (see Figure 11–24) rearrange to close the cytosolic entrance to the channel. (Adapted from E. Perozo et al., Science 285:73–78, 1999.) (A) CLOSED (B) OPEN Figure 11–27 The structure of mechanosensitive channels. The crystal structures of MscS in its (A) closed and (B) open conformation are shown. The side views (lower panels) show the entire protein, including the large intracellular domain. The face views (upper panels) show the transmembrane domains only. The open structure occupies more area in the lipid bilayer and is energetically favored when a membrane is stretched. This may explain why the MscS channel opens as pressure builds up inside the cell. (PDB codes: 2OAU, 2VV5.) rainwater, the cell swells as water seeps in due to an increase in the


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Cell_Biology_Alberts. is stretched. This may explain why the MscS channel opens as pressure builds up inside the cell. (PDB codes: 2OAU, 2VV5.) rainwater, the cell swells as water seeps in due to an increase in the osmotic pressure. If the pressure rises to dangerous levels, the cell opens mechanosensitive channels that allow small molecules to leak out. Bacteria that are experimentally placed in fresh water can rapidly lose more than 95% of their small molecules in this manner, including amino acids, sugars, and potassium ions. However, they keep their macromolecules safely inside and thus can recover quickly after environmental conditions return to normal.


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Cell_Biology_Alberts. Mechanical gating has been demonstrated using biophysical techniques in which force is exerted on pure lipid bilayers containing the bacterial mechanosensitive channels; for example, by applying suction with a micropipette. Such measurements demonstrate that the cell has several different channels that open at different levels of pressure. The mechanosensitive channel of small conductance, called the MscS channel, opens at low and moderate pressures (Figure 11–27). It is composed of seven identical subunits, which in the open state form a pore about 1.3 nm in diameter—just big enough to pass ions and small molecules. Large cytoplasmic domains limit the size of molecules that can reach the pore. The mechanosensitive channel of large conductance, called the MscL channel, opens to over 3 nm in diameter when the pressure gets so high that the cell might burst. The Function of a Neuron Depends on Its Elongated Structure


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Cell_Biology_Alberts. The Function of a Neuron Depends on Its Elongated Structure The cells that make most sophisticated use of channels are neurons. Before discussing how they do so, we digress briefly to describe how a typical neuron is organized. The fundamental task of a neuron, or nerve cell, is to receive, conduct, and transmit signals. To perform these functions, neurons are often extremely elongated. In humans, for example, a single neuron extending from the spinal cord to a muscle in the foot may be as long as 1 meter. Every neuron consists of a cell body (containing the nucleus) with a number of thin processes radiating outward from it. Usually one long axon conducts signals away from the cell body toward distant


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Cell_Biology_Alberts. Figure 11–28 a typical vertebrate neuron. The arrows indicate the direction in which signals are conveyed. The single axon conducts signals away from the cell body, while the multiple dendrites (and the cell body) receive signals from the axons of other neurons. The axon terminals end on the dendrites or cell body of other neurons or on other cell types, such as muscle or gland cells. targets, and several shorter, branching dendrites extend from the cell body like antennae, providing an enlarged surface area to receive signals from the axons of other neurons (Figure 11–28), although the cell body itself also receives such signals. A typical axon divides at its far end into many branches, passing on its message to many target cells simultaneously. Likewise, the extent of branching of the dendrites can be very great—in some cases sufficient to receive as many as 100,000 inputs on a single neuron.


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Cell_Biology_Alberts. Despite the varied significance of the signals carried by different classes of neurons, the form of the signal is always the same, consisting of changes in the electrical potential across the neuron’s plasma membrane. The signal spreads because an electrical disturbance produced in one part of the membrane spreads to other parts, although the disturbance becomes weaker with increasing distance from its source, unless the neuron expends energy to amplify it as it travels. Over short distances, this attenuation is unimportant; in fact, many small neurons conduct their signals passively, without amplification. For long-distance communication, however, such passive spread is inadequate. Thus, larger neurons employ an active signaling mechanism, which is one of their most striking features. An electrical stimulus that exceeds a certain threshold strength triggers an explosion of electrical activity that propagates rapidly along the neuron’s plasma membrane and is sustained by automatic


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Cell_Biology_Alberts. An electrical stimulus that exceeds a certain threshold strength triggers an explosion of electrical activity that propagates rapidly along the neuron’s plasma membrane and is sustained by automatic amplification all along the way. This traveling wave of electrical excitation, known as an action potential, or nerve impulse, can carry a message without attenuation from one end of a neuron to the other at speeds of 100 meters per second or more. Action potentials are the direct consequence of the properties of voltage-gated cation channels, as we now discuss.


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Cell_Biology_Alberts. The plasma membrane of all electrically excitable cells—not only neurons, but also muscle, endocrine, and egg cells—contains voltage-gated cation channels, which are responsible for generating the action potentials. An action potential is triggered by a depolarization of the plasma membrane—that is, by a shift in the membrane potential to a less negative value inside. (We shall see later how the action of a neurotransmitter causes depolarization.) In nerve and skeletal muscle cells, a stimulus that causes sufficient depolarization promptly opens the voltage-gated Na+ channels, allowing a small amount of Na+ to enter the cell down its electrochemical gradient. The influx of positive charge depolarizes the membrane further, thereby opening more Na+ channels, which admit more Na+ ions, causing still further depolarization. This self-amplification process (an example of positive feedback, discussed in Chapters 8 and 15) continues until, within a fraction of a millisecond, the electrical


Cell_Biology_Alberts

Cell_Biology_Alberts. still further depolarization. This self-amplification process (an example of positive feedback, discussed in Chapters 8 and 15) continues until, within a fraction of a millisecond, the electrical potential in the local region of membrane has shifted from its resting value of about –70 mV (in squid giant axon; about –40 mV in human) to almost as far as the Na+ equilibrium potential of about +50 mV (see


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Cell_Biology_Alberts. Panel 11–1, p. 616). At this point, when the net electrochemical driving force for the flow of Na+ is almost zero, the cell would come to a new resting state, with all of its Na+ channels permanently open, if the open conformation of the channel were stable. Two mechanisms act in concert to save the cell from such a permanent electrical spasm: the Na+ channels automatically inactivate and voltage-gated K+ channels open to restore the membrane potential to its initial negative value. The Na+ channel is built from a single polypeptide chain that contains four structurally very similar domains. It is thought that these domains evolved by gene duplication followed by fusion into a single large gene (Figure 11–29A). In bacteria, in fact, the Na+ channel is a tetramer of four identical polypeptide chains, supporting this evolutionary idea.


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Cell_Biology_Alberts. Each domain contributes to the central channel, which is very similar to the K+ channel. Each domain also contains a voltage sensor that is characterized by an unusual transmembrane helix, S4, that contains many positively charged amino acids. As the membrane depolarizes, the S4 helices experience an electrostatic pulling force that attracts them to the now negatively charged extracellular side of the plasma membrane. The resulting conformational change opens the channel. The structure of a bacterial voltage-gated Na+ channel provides insights how the structural elements are arranged in the membrane (Figure 11–29B and C).


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Cell_Biology_Alberts. The Na+ channels also have an automatic inactivating mechanism, which causes the channels to reclose rapidly even though the membrane is still depolarized (see Figure 11–30). The Na+ channels remain in this inactivated state, unable to reopen, until after the membrane potential has returned to its initial negative value. The time necessary for a sufficient number of Na+ channels to recover from inactivation to support a new action potential, termed the refractory period, limits


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Cell_Biology_Alberts. Figure 11–29 Structural models of voltage-gated Na+ channels. (A) The channel in animal cells is built from a single polypeptide chain that contains four homologous domains. Each domain contains two transmembrane α helices (green) that surround the central ion-conducting pore. They are separated by sequences (blue) that form the selectivity filter. Four α additional helices (gray and red) in each domain constitute the voltage sensor. The S4 helices (red) are unique in that they contain an abundance of positively charged arginines. An inactivation gate that is part of a flexible loop connecting the third and fourth domains acts as a plug that obstructs the pore in the channel’s inactivated state, as shown in Figure 11–30. (B) Side and top views of a homologous bacterial channel protein showing its arrangement within the membrane. (C) A cross section of the pore domain of the channel shown in (B) shows lateral portals, through which the central cavity is accessible from the hydrophobic


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Cell_Biology_Alberts. its arrangement within the membrane. (C) A cross section of the pore domain of the channel shown in (B) shows lateral portals, through which the central cavity is accessible from the hydrophobic core of the lipid bilayer. In the crystals, lipid acyl chains were found to intrude into the pore. These lateral portals are large enough to allow entry of small, hydrophobic, pore-blocking drugs that are commonly used as anesthetics and block ion conductance. (PDB code: 3RVZ.)


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Cell_Biology_Alberts. Figure 11–30 Na+ channels and an action potential. (A) An action potential is triggered by a brief pulse of current, which (B) partially depolarizes the membrane, as shown in the plot of membrane potential versus time. The green curve shows how the membrane potential would have simply relaxed back to the resting value after the initial depolarizing stimulus if there had been no voltage-gated Na+ channels in the membrane. The red curve shows the course of the action potential that is caused by the opening and subsequent inactivation of voltage-gated Na+ channels. The states of the Na+ channels are indicated in (B). The membrane cannot fire a second action potential until the Na+ channels have returned from the inactivated to the closed conformation; until then, the membrane is refractory to stimulation. (C) The three states of the Na+ channel. When the membrane is at rest (highly polarized), the closed conformation of the channel has the lowest free energy and is therefore most stable;


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Cell_Biology_Alberts. stimulation. (C) The three states of the Na+ channel. When the membrane is at rest (highly polarized), the closed conformation of the channel has the lowest free energy and is therefore most stable; when the membrane is depolarized, the energy of the open conformation is lower, so the channel has a high probability of opening. But the free energy of the inactivated conformation is lower still; therefore, after a randomly variable period spent in the open state, the channel becomes inactivated. Thus, the open conformation corresponds to a metastable state that can exist only transiently when the membrane depolarizes (Movie 11.10).


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Cell_Biology_Alberts. the repetitive firing rate of a neuron. The cycle from initial stimulus to the return to the original resting state takes a few milliseconds or less. The Na+ channel can therefore exist in three distinct states—closed, open, and inactivated—which contribute to the rise and fall of the action potential (Figure 11–30). This description of an action potential applies only to a small patch of plasma membrane. The self-amplifying depolarization of the patch, however, is sufficient to depolarize neighboring regions of membrane, which then go through the same cycle. In this way, the action potential sweeps like a wave from the initial site of depolarization over the entire plasma membrane, as shown in Figure 11–31. The Use of Channelrhodopsins Has Revolutionized the Study of Neural Circuits


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Cell_Biology_Alberts. The Use of Channelrhodopsins Has Revolutionized the Study of Neural Circuits Channelrhodopsins are photosensitive ion channels that open in response to light. They evolved as sensory receptors in photosynthetic green algae to allow the algae to swim toward light. The structure of channelrhodopsin closely resembles that of bacteriorhodopsin (see Figure 10–31). It contains a covalently bound retinal group that absorbs light and undergoes an isomerization reaction, which triggers a conformational change in the protein, opening an ion channel in the plasma membrane. In contrast to bacteriorhodopsin, which is a light-driven proton pump, channelrhodopsin is a light-driven cation channel.


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Cell_Biology_Alberts. Using genetic engineering techniques, channelrhodopsin can be expressed in virtually any cell type in vertebrates and invertebrates. Researchers first introduced the gene into cultured neurons and showed that flashing light could now activate the channelrhodopsin and induce the neurons to fire action potentials. Because the frequency of the light flashes determined the frequency of the action potentials, one can control the frequency of neuronal firing with millisecond precision. Next, neurobiologists used the approach to activate specific neurons in the brain of experimental animals. Using a tiny fiber optic cable implanted near the axon at time = 0 (triggering of action potential)


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Cell_Biology_Alberts. Figure 11–31 The propagation of an action potential along an axon. (A) The voltages that would be recorded from a set of intracellular electrodes placed at intervals along the axon. (B) The changes in the Na+ channels and the current flows (curved red arrows) that give rise to a traveling action potential. The region of the axon with a depolarized membrane is shaded in blue. Note that once an action potential has started to progress, it has to continue in the same direction, traveling only away from the site of depolarization, because Na+-channel inactivation prevents the depolarization from spreading backward.


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Cell_Biology_Alberts. relevant brain region, they could flash light to specifically activate the channelrhodopsin-containing neurons to fire action potentials. One group of researchers expressed channelrhodopsin in a subset of mouse neurons thought to be involved in aggression: when these cells were activated by light, the mouse immediately attacked anything in its environment—including other mice or even an inflated rubber glove (Figure 11–32); when the light was switched off, the neurons fell silent and the mouse’s behavior returned to normal. Since these pioneering studies, researchers have engineered additional light-responsive ion channels and transporters, including some that can rapidly Figure 11–32 Optogenetic control of aggression neurons in a living mouse.


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Cell_Biology_Alberts. A gene encoding channelrhodopsin was introduced into a subpopulation of neurons in the hypothalamus of a mouse. When the neurons were exposed to flashing blue light using a tiny, implanted fiber optic cable, the channelrhodopsin channels opened, depolarizing and activating the cells. When the light was switched on, the mouse immediately became aggressive and attacked the inflated rubber glove; when the light was switched off, its behavior immediately returned to normal (Movie 11.11). (From D. Lin et al., Nature 470:221–226, 2011. With permission from Macmillan Publishers Ltd.) inactivate specific neurons. It is therefore now possible to transiently activate or inhibit specific neurons in the brains of awake animals with remarkable spatial and temporal precision. In this way, the rapidly expanding new field of optogenetics is revolutionizing neurobiology, allowing neuroscientists to analyze the neurons and circuits underlying even the most complex behaviors in experimental animals,


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Cell_Biology_Alberts. expanding new field of optogenetics is revolutionizing neurobiology, allowing neuroscientists to analyze the neurons and circuits underlying even the most complex behaviors in experimental animals, including nonhuman primates.


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Cell_Biology_Alberts. Myelination Increases the Speed and Efficiency of Action Potential Propagation in Nerve Cells The axons of many vertebrate neurons are insulated by a myelin sheath, which greatly increases the rate at which an axon can conduct an action potential. The importance of myelination is dramatically demonstrated by the demyelinating disease multiple sclerosis, in which the immune system destroys myelin sheaths in some regions of the central nervous system; in the affected regions, nerve impulse propagation greatly slows or even fails, often with devastating neurological consequences.


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Cell_Biology_Alberts. Myelin is formed by specialized non-neuronal supporting cells called glial cells. Schwann cells are the glial cells that myelinate axons in peripheral nerves, and oligodendrocytes do so in the central nervous system. These myelinating glial cells wrap layer upon layer of their own plasma membrane in a tight spiral around the axon (Figure 11–33A and B), thereby insulating the axonal membrane so that little current can leak across it. The myelin sheath is interrupted at regularly spaced nodes of Ranvier, where almost all the Na+ channels in the axon are concentrated (Figure 11–33C). This arrangement allows an action potential to propagate along a myelinated axon by jumping from node to node, a process called saltatory conduction. This type of conduction has two main advantages: action potentials travel very much faster, and metabolic energy is conserved because the active excitation is confined to the small regions of axonal plasma membrane at nodes of Ranvier.


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Figure 11–33 Myelination.

Cell_Biology_Alberts. Figure 11–33 Myelination.


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Cell_Biology_Alberts. (A) A myelinated axon from a peripheral nerve. Each Schwann cell wraps its plasma membrane concentrically around the axon to form a segment of myelin sheath about 1 mm long. For clarity, the membrane layers of the myelin are shown less compacted than they are in reality (see part B). (B) An electron micrograph of a nerve in the leg of a young rat. Two Schwann cells can be seen: one near the bottom is just beginning to myelinate its axon; the one above it has formed an almost mature myelin sheath. (C) Fluorescence micrograph and diagram of individual myelinated axons teased apart in a rat optic nerve, showing the confinement of the voltage-gated Na+ channels (green) in the axonal membrane at the node of Ranvier. A protein called Caspr (red) marks the junctions where the myelinating glial cell plasma membrane tightly abuts the axon on either side of the node. Voltage-gated K+ channels (blue) localize to regions in the axon plasma membrane well away from the node. (B, from Cedric S.


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Cell_Biology_Alberts. cell plasma membrane tightly abuts the axon on either side of the node. Voltage-gated K+ channels (blue) localize to regions in the axon plasma membrane well away from the node. (B, from Cedric S. Raine, in Myelin [P. Morell, ed.]. New York: Plenum, 1976; C, from M.N. Rasband and P. Shrager, J. Physiol. 525:63–73, 2000. With permission from Blackwell Publishing.)


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Cell_Biology_Alberts. Neuron and skeletal muscle cell plasma membranes contain many thousands of voltage-gated Na+ channels, and the current crossing the membrane is the sum of the currents flowing through all of these. An intracellular microelectrode can record this aggregate current, as shown in Figure 11–31A. Remarkably, however, it is also possible to record current flowing through individual channels. Patch-clamp recording, developed in the 1970s and 1980s, revolutionized the study of ion channels and made it possible to examine transport through a single channel in a small patch of membrane covering the mouth of a micropipette (Figure 11–34). With this simple but powerful technique, one can study the detailed properties of ion channels in all sorts of cell types. This work led to the discovery that even cells that are not electrically excitable usually have a variety of ion channels in their plasma membrane. Many of these cells, such as yeasts, are too small to be investigated by the traditional


Cell_Biology_Alberts

Cell_Biology_Alberts. cells that are not electrically excitable usually have a variety of ion channels in their plasma membrane. Many of these cells, such as yeasts, are too small to be investigated by the traditional electrophysiologist’s method of impalement with an intracellular microelectrode.


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Cell_Biology_Alberts. Patch-clamp recording indicates that individual ion channels open in an allor-nothing fashion. For example, a voltage-gated Na+ channel opens and closes at random, but when open, the channel always has the same large conductance, allowing more than 1000 ions to pass per millisecond (Figure 11–35). Therefore, the aggregate current crossing the membrane of an entire cell does not indicate the degree to which a typical individual channel is open but rather the total number of channels in its membrane that are open at any one time.


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Cell_Biology_Alberts. Some simple physical principles allow us to refine our understanding of voltage-gating from the perspective of a single Na+ channel. The interior of the resting neuron or muscle cell is at an electrical potential about 40–100 mV more negative than the external medium. Although this potential difference seems small, it exists across a plasma membrane only about 5 nm thick, so that the resulting voltage gradient is about 100,000 V/cm. Charged proteins in the membrane such as Na+ channels are thus subjected to a very large electrical field that can profoundly affect their conformation. Each conformation can “flip” to another conformation if given a sufficient jolt by the random thermal movements of the surroundings, and it is the relative stability of the closed, open, and inactivated conformations against flipping that is altered by changes in the membrane potential (see Figure 11–30C).


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Cell_Biology_Alberts. Na+ channels are not the only kind of voltage-gated cation channel that can generate an action potential. The action potentials in some muscle, egg, and endocrine cells, for example, depend on voltage-gated Ca2+ channels rather than on Na+ channels. pull micropipette away from cellglass to detach themicropipette patch of


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Cell_Biology_Alberts. Figure 11–34 The technique of patch-clamp recording. Because of the extremely tight seal between the micropipette and the membrane, current can enter or leave the micropipette only by passing through the ion channels in the patch of membrane covering its tip. The term clamp is used because an electronic device is employed to maintain, or “clamp,” the membrane potential at a set value while recording the ionic current through individual channels. The current through these channels can be recorded with the patch still attached to the rest of the cell, as in (A), or detached, as in (B). The advantage of the detached patch is that it is easy to alter the composition of the solution on either side of the membrane to test the effect of various solutes on channel behavior. A detached patch can also be produced with the opposite orientation, so that the cytoplasmic surface of the membrane faces the inside of the pipette.


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Cell_Biology_Alberts. There is a surprising amount of structural and functional diversity within each of the different classes of voltage-gated cation channels, generated both by multiple genes and by the alternative splicing of RNA transcripts produced from the same gene. Nonetheless, the amino acid sequences of the known voltage-gated Na+, K+, and Ca2+ channels show striking similarities, demonstrating that they all belong to a large superfamily of evolutionarily and structurally related proteins and share many of the design principles. Whereas the single-celled yeast S. cerevisiae contains a single gene that codes for a voltage-gated K+ channel, the genome of the worm C. elegans contains 68 genes that encode different but related K+ channels. This complexity indicates that even a simple nervous system made up of only 302 neurons uses a large number of different ion channels to compute its responses.


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Cell_Biology_Alberts. Humans who inherit mutant genes encoding ion channels can suffer from a variety of nerve, muscle, brain, or heart diseases, depending in which cells the channel encoded by the mutant gene normally functions. Mutations in genes that encode voltage-gated Na+ channels in skeletal muscle cells, for example, can cause myotonia, a condition in which there is a delay in muscle relaxation after voluntary contraction, causing painful muscle spasms. In some cases, this occurs because the abnormal channels fail to inactivate normally; as a result, Na+ entry persists after an action potential finishes and repeatedly reinitiates membrane depolarization and muscle contraction. Similarly, mutations that affect Na+ or K+ channels in the brain can cause epilepsy, in which excessive synchronized firing of large groups of neurons causes epileptic seizures (convulsions, or fits).


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Cell_Biology_Alberts. The particular combination of ion channels conducting Na+, K+, and Ca2+ that are expressed in a neuron largely determines how the cell fires repetitive sequences of action potentials. Some nerve cells can repeat action potentials up to 300 times per second; other neurons fire short bursts of action potentials separated by periods of silence; while others rarely fire more than one action potential at a time. There is a remarkable diversity of neurons in the brain.