MOLECULAR CELL BIOLOGY

Transcription

1 1 Lodish Berk Kaiser Krieger scott Bretscher Ploegh Matsudaira MOLECULAR CELL BIOLOGY SEVENTH EDITION CHAPTER 11 Transmembrane Transport of Ions and Small Molecules Copyright 2013 by W. H. Freeman and Company

2 Transport mechanism 1. Passive transport 1) simple diffusion (passive diffusion) 2) facilitated diffusion 2. Active transport 1) primary active transport (ATP-powered pump) 2) secondary active transport (cotransport) gradient passive active

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4 Transport mechanism 1. Active and passive transport are biological processes that move oxygen, water and nutrients into cells and remove waste products. 1 2 Active transport requires chemical energy because it is the movement of biochemicals from areas of lower concentration to areas of higher concentration. On the other hand, passive trasport moves biochemicals from areas of high concentration to areas of low concentration; so it does not require energy. 2. Table 11-1 summarizes the four mechanisms (simple diffusion, facilitated transport, active transport, cotransport) by which small molecules and ions are transported across cellular membranes.

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6 Figure 11.2 Overview of membrane transport proteins.

7 Membrane transport proteins Thus transport of most molecules into and out of cells requires the assistance of specialized membrane proteins (Figure 11-2). 1. Pumps utilize the energy released by ATP hydrolysis to power movement of specific ions or small molecules against their electrochemical gradient. 2. Channels permit movement of specific ions (or water) down their electrochemical gradient. 3. Transporters, which fall into three groups, facilitate movement of specific small molecules or ions. Uniporters transport a single type of molecule down its concentration gradient. Cotransport proteins (symporters, and antiporters) catalyze the movement of one molecule against its concentration gradient (black circles), driven by movement of one or more ions down an electrochemical gradient (red circles). Differences in the mechanisms of transport by these three major classes of proteins account for their varying rates of solute movement.

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9 Simple diffusion Figure 11.1 Relative permeability of a pure phospholipid bilayer to various molecules and ions.

10 Simple diffusion With its dense hydrophobic core, a phospholipid bilayer is largely impermeable to water-soluble molecules and ions. Only gases, such as O 2 and CO 2, and small, uncharged polar molecules, such as urea and ethanol, can readily move by simple diffusion across an artificial membrane composed of pure phospholipid or of phospholipid and cholesterol. Such molecules also can diffuse across cellular membranes without the aid of transport proteins (Figure 11-1).

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12 Facilitated diffusion 1. glucose 2. amino acids 3. ions: ion channel 4. water Experimental Figure 11.4 Cellular uptake of glucose mediated by GLUT proteins exhibits simple enzyme kinetics. Figure 11.5 Model of uniport transport by GLUT1.

13 Ion channel: facilitated diffusion

14 Facilitated diffusion 1. The protein-mediated transport of a single type of molecule, such as glucose or other small hydrophilic molecules, down a concentration gradient across a cellular membrane is known as uniport (Experimental Figure 11-4, Figure 11-5). 1 2 The rate of substrate movement by uniporters is far higher than simple diffusion through a pure phospholipid bilayer. A measure of the affinity of a tranporter for its substrate is Km, which is the concentration of substrate at which transport is half maximal. Uniport proteins, such as the glucose transporters (GLUTs), are thought to shuttle between two conformational states, one in which the substrate-binding site faces outward and one in which the binding site faces inward. 2. Channels transport water, specific ions, or hydrophilic small molecules across membranes down their concentration or electric potential gradients. Because this process requires transport proteins but not energy, it is sometimes referred to as passive transport or facilitated diffusion, but it is more properly called facilitated transport. Voltage-gated ion channels open and close in response to membrane potential. Ligand-gated ion channels are a group of transmembrane ion channel proteins which open to allow ions such as Na +, K +, Ca 2+, or Cl to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter.

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16 Figure 11.8 Structure of the water-channel protein aquaporin.

17 Figure 11.6 Osmotic pressure.

18 Aquaporin Aquaporins are water-channel proteins that specifically increase the permeability of biomembranes for water. Aquaporin 2 in the plasma membrane of certain kidney cells is essential for resorption of water from the forming urine; its absence leads to a form of diabetes. In its functional form, aquaporin is a tetramer of identical 28-kDa subunits (a). Each subunit contains six membrane-spanning α helices that form a central pore through which water moves (b, c). Water tends to move across a semipermeable membrane from a solution of low solute concentration to one of high concentration, a process termed osmosis, or osmotic flow. In other words, since solutions with a high concentration of dissolved solute have a lower concentration of water, water will spontaneously move from a solution of high water concentration to one of lower. In effect, osmosis is equivalent to diffusion of water. Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions. In this context, the membrane may be a layer of cells or a plasma membrane that is permeable to water but not to the solutes. The osmotic pressure is directly proportional to the difference in the concentration of the total number of solute molecules on each side of the membrane.

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20 2 Active transport 1) primary active transport (ATP-powered pump) 2) secondary active transport (cotransport)

21 Figure 11.9 The four classes of ATP-powered transport proteins.

22 The four classes of ATP-powered transport proteins All ATP-powered pumps are transmembrane proteins with one or more binding sites for ATP located on the cytosolic face of the membrane. Although these proteins commonly are called ATPases, they normally do not hydrolyze ATP into ADP and Pi unless ions or other molecules are simultaneously transported. Because of this tight coupling between ATP hydrolysis and transport, the energy stored in the phosphoanhydride bond is not dissipated but rather used to move ions or other molecules uphill against an electrochemical gradient. The general structures of the four classes of ATP-powered pumps are depicted in Figure 11-9, with specific examples in each class listed below. Note that the members of three classes (P, F, and V) transport ions only, whereas members of the ABC superfamily primarily transport small molecules.

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24 P-class pump

25 P-class pump All P-class ion pumps possess two identical catalytic α subunits that contain an ATP-binding site. Most also have two smaller β subunits that usually have regulatory functions. During the transport process, at least one of the α subunits is phosphorylated (hence the name P class), and the transported ions are thought to move through the phosphorylated subunit. 1. This class includes the Na + /K + ATPase in the plasma membrane, which maintains the low cytosolic Na + and high cytosolic K + concentrations typical of animal cells. 2. Certain Ca 2+ ATPases pump Ca 2+ ions out of the cytosol into the external medium; others pump Ca 2+ from the cytosol into the endoplasmic reticulum or into the specialized ER called the sarcoplasmic reticulum, which is found in muscle cells. 3. Another member of the P class, found in acid-secreting cells of the mammalian stomach, transports protons (H + ions) out of and K + ions into the cell. 4. The H + pump that generates and maintains the membrane electric potential in plant, fungal, and bacterial cells also belongs to this class.

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27 Na +, K +, Ca 2+ gradient outside inside Na + Cl - Ca 2+ K + ER & SR Ca 2+

28 Na +, K +, Ca 2+ gradient The specific ionic composition of the cytosol usually differs greatly from that of the surrounding extracellular fluid. Also, the cytosolic concentration of K + is much higher than that of Na +. Some Ca 2+ in the cytosol is bound to the negatively charged groups in ATP and other molecules, but it is the concentration of free, unbound Ca 2+ that is critical to its functions in signaling pathways and muscle contraction (Table 11-2).

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30 Na + /K + ATPase

31 Figure Operational model of the plasma membrane Na + /K + ATPase.

32 The Nobel Prize in Chemistry 1997: elucidation of the enzymatic mechanism underlying the synthesis of adenosine triphosphate (ATP) The Nobel Prize in Chemistry 1997: discovery of an ion-transporting enzyme, Na+, K+ -ATPase

33 Na + /K + ATPase 1. An important P-class ion pump present in the plasma membrane of all animal cells is the Na+/K+ ATPase. This ion pump is a tetramer of subunit composition α2β2, and shares structural homology with the Ca2+ pump (Figure 11-13) During the catalytic cycle of the Na+/K+ ATPase it moves three Na+ ions out of and two K+ ions into the cell per ATP molecule hydrolyzed. In its E1 conformation, the Na+/K+ ATPase has three high-affinity Na+-binding sites and two lowaffinity K+-binding sites accessible to the cytosolic surface of the protein. Conversely, the affinity of the cytosolic K+-binding sites is low enough that K+ ions, transported inward through the protein, dissociate from E1 into the cytosol despite the high intracellular K+ concentration. During the E1 E2 transition, the three bound Na+ ions become accessible to the exoplasmic face, and stimultaneously the affinity of the three Na+-binding sites drops. The three Na+ ions now bound to low-affinity Na+ sites dissociate one at a time into the extracellular medium despite the high extracellular Na+ concentration. Transition to the E2 conformation also generates two high-affinity K+ sites accessible to the exoplasmic face. Similarly, during the subsequent E2 E1 transition, the two bound K+ ions are transported inward and then released into the cytosol. 2. Ion pumping by the Na+/K+ ATPase involves phosphorylation, dephosphorylation, and conformational changes similar to those in the muscle Ca2+ ATPase. In this case, hydrolysis of the E2 P intermediate powers the E2 E1 conformational change and concomitant transport of two K+ ions inward (Figure 11-13).

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35 3 Physilogical roles of Na + /K + gradient 1. Osmolarity & cell volume maintenance 2. Resting membrane potential (K+) 3. Energy for secondary active transport : electrochemical gradient of Na+

36 The Na+/K+-ATPase helps maintain resting potential, effect transport, and regulate cellular volume. 1. Resting potential : In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). The sodiumpotassium pump moves 3 sodium ions out and moves 2 potassium ions in, thus, in total, removing one positive charge carrier from the intracellular space. The action of the sodium-potassium pump is not the only mechanism responsible for the generation of the resting membrane potential. 2. Transport : Export of sodium from the cell provides the driving force for several secondary active transporters membrane transport proteins, which import glucose, amino acids, and other nutrients into the cell by use of the sodium gradient. Another important task of the Na+-K+ pump is to provide a Na+ gradient that is used by certain carrier processes. 3. Controlling cell volume : Failure of the Na+-K+ pumps can result in swelling of the cell. A cell's osmolarity is the sum of the concentrations of the various ion species and many proteins and other organic compounds inside the cell. When this is higher than the osmolarity outside of the cell, water flows into the cell through osmosis. This can cause the cell to swell up and lyse. The Na+-K+ pump helps to maintain the right concentrations of ions.

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38 Resting membrane potential Experimental Figure The electric potential across the plasma membrane of living cells can be measured. Experimental Figure Generation of a transmembrane electric potential (voltage) depends on the selective movement of ions across a semipermeable membrane.

39 Resting membrane potential 1. The plasma membranes of animal cells contain many open K+ channels but few open Na+, Cl-, or Ca2+ channels. As a result, the major ionic movement across the plasma membrane is that of K+ from the inside outward, powered by the K+ concentration gradient, leaving an excess of negative charge on the inside and creating an excess of positive charge on the outside, similar to the experimental system shown in Figure 11-18c. Thus the outward flow of K+ ions through these channels, called resting K+ channels, is the major determinant of the inside-negative membrane potential (Experimental Figure 11-18). 2. The potential across the plasma membrane of large cells can be measured with a microelectrode inserted inside the cell and a reference electrode placed in the extracellular fluid. The two are connected to a potentiometer capable of measuring small potential differences. In virtually all cells the inside (cytosolic face) of the cell membrane is negative relative to the outside; typical membrane potentials range between 30 and 70 mv. The potential across the surface membrane of most animal cells generally does not vary with time. In contrast, neurons and muscle cells the principal types of electrically active cells undergo controlled changes in their membrane potential that we discuss later (Experimental Figure 11-19).

40 Resting K + channels 1. Like all other K + channels, bacterial K + channels are built of four identical subunits symmetrically arranged around a central pore. Each subunit contains two membrane-spanning α helices (S5 and S6) and a short P (pore domain) segment that partly penetrates the membrane bilayer. Four extended loops that are part of the P segments form the actual ion-selectivity filter in the narrow part of the pore near the exoplasmic surface above the vestibule (Figure 11-20). 2. The ability of the ion-selectivity filter in K + channels to select K + over Na + is due mainly to backbone carbonyl oxygens on glycine residues located in a Gly-Tyr-Gly sequence that is found in an analogous position in the P segment in every known K + channel. As a K + ion enters the narrow selectivity filter, it loses its water of hydration but becomes bound to eight backbone carbonyl oxygens, two from the extended loop in each P segment lining the channel. As a result, a relatively low activation energy is required for passage of K + ions through the channel. Because a dehydrated Na + ion is too small to bind to all eight carbonyl oxygens that line the selectivity filter, the activation energy for passage of Na + ions is relatively high. This difference in activation energies favors passage of K + ions over Na + by a factor of thousand. Calcium ions are too large to pass through a K + channel with or without their bound water (Figure 11-21).

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42 Energy for secondary active transport Figure Transmembrane forces acting on Na + ions.

43 Energy for secondary active transport The electrochemical gradient across a semipermeable membrane determines the direction of ion movement through channel proteins. The two forces constituting the electrochemical gradient, the membrane electric potential and the ion concentration gradient, may act in the same or opposite directions. As with all ions, the movement of Na+ ions across the plasma membrane is governed by the sum of two separate forces the ion concentration gradient and the membrane electric potential. At the internal and external Na+ concentrations typical of mammalian cells, these forces usually act in the same direction, making the inward movement of Na+ ions energetically favorable (Figure 11-25).

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45 Figure Operational model of the Ca 2+ ATPase in the SR membrane of skeletal muscle cells.

46 Ca 2+ ATPase 1. In skeletal muscle cells, Ca2+ ions are concentrated and stored in the sarcoplasmic reticulum (SR), a specialized type of endoplasmic reticulum (ER). The release via ion channels of stored Ca2+ ions from the SR lumen into the cytosol causes muscle contraction. A P-class Ca2+ ATPase located in the SR membrane of skeletal muscle pumps Ca2+ from the cytosol back into the lumen of the SR, thereby inducing muscle relaxation (Figure 11-10). 1 2 The current model for the mechanism of action of the Ca2+ ATPase in the SR membrane involves multiple conformational states. For simplicity, we group these into E1 states, in which the two binding sites for Ca2+, located in the center of the membrane-spanning domain, face the cytosol, and E2 states, in which these binding sites face the exoplasmic face of the membrane, pointing into the lumen of the SR. Coupling of ATP hydrolysis with ion pumping involves several conformational changes in the protein that must occur in a defined order. When the protein is in the E1 conformation, two Ca2+ ions bind to two high-affinity binding sites accessible from the cytosolic side; even though the cytosolic Ca2+ concentration is low, calcium ions still fill these sites. All P-class ion pumps, regardless of which ion they transport, are phosphorylated on a highly concserved aspartate residue during the transport process. 2. This suggestion is borne out by the similarities of the three-dimensional structures of the membranespanning segments of the Na+/K+ ATPase with that of the Ca2+ pump; the molecular structures of the three cytoplasmic domains are also very similar. Thus, the operational model in Figure is generally applicable to all of the P-class ATP-powered ion pumps.

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48 H + /K + ATPase in stomach parietal cell Figure Acidification of the stomach lumen by parietal cells in the gastric lining.

49 H + /K + ATPase in stomach parietal cell The overall process by which parietal cells acidify the stomach lumen is illustrated in Figure In a reaction catalyzed by carbonic anhydrase the excess cytosolic OH - combines with CO 2 that diffuses in from the blood, forming HCO 3-. Catalyzed by the basolateral anion antiporter, this bicarbonate ion is exported across the basolateral membrane (and ultimately into the blood) in exchange for a Cl - ion. The Cl - ions then exit through Cl - channels in the apical membrane, entering the stomach lumen. To preserve electroneutrality, each Cl - ion that moves into the stomach lumen across the apical membrane is accompanied by a K + ion that moves outward through a separate K + channel. In this way, the excess K + ions pumped inward by the H + /K + ATPase are returned to the stomach lumen, thus maintaining the normal intracellular K + concentration. The net result is secretion of equal amounts of H + and Cl - ions (i.e., HCl) into the stomach lumen, while the ph of the cytosol remains neutral and the excess OH - ions, as HCO3 -, are transported into the blood.

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51 V-class H + pump lysosome & endosome 4 Figure Concentration of ions and sucrose by the plant vacuole. Figure Effect of V-class H + pumps on H + concentration gradients and electric potential gradients Figure Dissolution of bone by polarized osteoclast cells requires a V-class proton pump and the ClC-7 chloride channel protein.

52 Figure Effect of V-class H + pumps on H + concentration gradients and electric potential gradients across cellular

53 V-Class H ATPases Pump Protons Across Lysosomal and Vacuolar Membranes By themselves, ATP-powered proton pumps cannot acidify the lumen of an organelle (or the extracellular space) because these pumps are electrogenic; that is, a net movement of electric charge occurs during transport. Pumping of just a few protons causes a buildup of positively charged H+ ions on the exoplasmic (inside) face of the organelle membrane. For each H+ pumped across, a negative ion (e.g., OH- or Cl-) will be left behind on the cytosolic face, causing a buildup of negatively charged ions there. These oppositely charged ions attract each other on opposite faces of the membrane, generating a charge separation, or electric potential, across the membrane. The lysosome membrane thus functions as a capacitor in an electric circuit, storing opposing charges (anions and cations) on opposite sides of a barrier impermeable to movement of charged particles. As more and more protons are pumped and build up excess positive charge on the exoplasmic face, the energy required to move additional protons against this rising electric potential gradient increases dramatically and prevents pumping of additional protons long beforea significant transmembrane H+ concentration gradient is established (Figure 11-14a). In fact, this is the way that P-class H+ pumps generate a cytosol-negative potential across plant and yeast plasma membranes. In order for an organelle lumen or an extracellular space (e.g., the lumen of the stomach) to become acidic, movement of protons must be accompanied either by (1) movement of an equal number of anions (e.g., Cl-) in the same direction or by (2) movement of equal numbers of a different cation in the opposite direction. The first process occurs in lysosomes and plant vacuoles, whose membranes contain V-class H+ ATPases and anion channels through which accompanying Clions move (Figure 11-14b). The second process occurs in the lining of the stomach, which contains a P-class H+/K+ ATPase that is not electrogenic and pumps one H+ outward and one K+ inward. Operation of this pump is discussed later in the chapter. The ATP-powered proton pumps in lysosomal and vacuolar membranes have been solubilized, purified, and incorporated into liposomes. As shown in Figure 11-9, these V-class proton pumps contain two discrete domains: a cytosolic hydrophilic domain (V1) and a transmembrane domain (V0) with multiple subunits forming each domain. Binding and hydrolysis of ATP by the B subunits in V1 provide the energy for pumping of H+ ions through the proton-conducting channel formed by the c and a subunits in V0. Unlike P-class ion pumps, V-class proton pumps are not phosphorylated and dephosphorylated during proton transport. The structurally similar F-class proton pumps, which we describe in Chapter 12, normally operate in the reverse direction to generate ATP rather than pump protons; their structure and mechanism of action is understood in great detail.

54 Figure Concentration of ions and sucrose by the plant vacuole.

55 Figure Dissolution of bone by polarized osteoclast cells requires a V-class proton pump and the ClC-7 chloride channel protein.

56 V-class H + pump All V-class ATPase transport only H+ ions. These proton pumps, present in the membranes of lysosomes, endosomes, and plant vacuoles, function to acidify the lumen of these organelles. 1. If an intracellular organelle contains only V-class pumps, proton pumping generates an electric potential across the membrane (cytosol-facing side negative and luminal-side positive) but no significant change in the intraluminal ph. If the organelle membrane also contains Cl- channels, anions passively follow the pumped protons, resulting in an accumulation of H+ and Cl- ions in the lumen (low luminal ph) but no electric potential across the membrane (Figure 11-14). 2. The vacuolar membrane contains two types of proton pumps: a V-class H+ ATPase and a pyrophosphate-hydrolyzing proton pump that differs from all other ion transport proteins and probably is unique to plants. These pumps generate a low luminal ph as well as an inside-positive electric potential across the vacuolar membrane owing to the inward pumping of H+ ions. The inside-positive potential powers the movement of Cl- and NO3- from the cytosol through separate channel proteins. Proton antiporters powered by the H+ gradient, accumulate Na+, Ca2+, and sucrose inside the vacuole (Figure 11-29). 3. The osteoclast plasma membrane is divided into two domains separated by the tight seal between a ring of membrane and the bone surface. The membrane domain facing the bone contains V-class proton pumps and ClC-7 Cl- channels. The opposing membrane domain contains anion antiporters that exchange HCO3- and Cl- ions. The combined operation of these three transport proteins and carbonic anhydrase acidifies the enclosed space and allows bone resorption while maintaining the neutral ph of the cytosol (Figure 11-32).

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58 F-class pump

59 F-class pump 1. F-class pumps are found in bacterial plasma membranes and in mitochondria and chloroplasts. In contrast to V pumps, they generally function to power the synthesis of ATP from ADP and Pi by movement of protons from the exoplasmic to the cytosolic face of the membrane down the proton electrochemical gradient. Because of their importance in ATP synthesis in chloroplasts and mitochondria, F-class proton pumps, commonly called ATP synthases, are treated separately in Chapter. 2. In chemiosmosis, a proton-motive force generated by proton pumping across a membrane is used to power ATP synthesis. The mechanism and membrane orientation of the process are similar in bacteria, mitochondria, and chloroplasts. 1 2 During electron transport, protons are always pumped from the cytosolic face to the exoplasmic face, creating a proton concentration gradient (exoplasmic face > cytosolic face) and an electric potential (negative cytosolic face and positive exoplasmic face) across the membrane. During the synthesis of ATP, protons flow in the reverse direction (down their electrochemical gradient) through ATP synthase (F0F1 complex), which protrudes in a knob at the cytosolic face in all cases.

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61 ABC superfamily

62 ABC superfamily The final class of ATP-powered pumps contains more members and is more diverse than the other classes. Referred to as the ABC (ATP-binding cassette) superfamily, this class includes several hundred different transport proteins found in organisms ranging from bacteria to humans. Each ABC protein is specific for a single substrate or group of related substrates, which may be ions, sugars, amino acids, phospholipids, peptides, polysaccharides, or even proteins. All ABC transport proteins share a structural organization consisting of four core domains: two transmembrane (T) domains, forming the passageway through which transported molecules cross the membrane, and two cytosolic ATP-binding (A) domains. In some ABC proteins, mostly in bacteria, the core domains are present in four separate polypeptides; in others, the core domains are fused into one or two multidomain polypeptides (Table 11-3).

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64 5 Active transport 1) primary active transport (ATP-powered pump) 2) secondary active transport (cotransport)

65 Secondary active transport: cotransport Energy 1) Na + motive force 2) H + motive force Transported molecules 1) Glucose 2) Amino acids 3) Ca 2+ Figure Transmembrane forces acting on Na + ions.

66 Figure Transcellular transport of glucose from the intestinal lumen into the blood. [Glucose] low high low

67 Cotransport 1. Cotransporters use the energy released by movement of an ion (usually H+ or Na+) down its electrochemical gradient to power the import or export of a small molecule or different ion against its concentration gradient. The cells lining the small intestine and kidney tubules express symport proteins that couple the energetically favorable entry of Na+ to the import of glucose and amino acids against their concentration gradients (Figure 11-25). 2. The Na+/K+ ATPase in the basolateral surface membrane generates Na+ and K+ concentration gradients (step1). The outward movement of K+ ions through nongated K+ channels generates an inside-negative membrane potential. Both the Na+ concentration gradient and the membrane potential are used to drive the uptake of glucose from the intestinal lumen by the two-na+/one-glucose symporter located in the apical surface membrane (step2). Glucose leaves the cell via facilitated diffusion catalyzed by GLUT2, a glucose uniporter located in the basolateral membrane (step3) (Figure 11-30).

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69 Figure Operational model for the two-na + /one-glucose symporter.

70 Two-Na + /one-glucose symporter Thus by coupling the transport of two Na+ ions to the transport of one glucose, the two-na+/oneglucose symporter permits cell to accumulate a very high concentration of glucose relative to the external concentration. This means that glucose present even at very low concentrations in the lumen of the intestine or in the kidney tubules can be efficiently transported into the lining cells and not lost from the body. Simultaneous binding of Na+ and glucose to the conformation with outward-facing binding sites (step 1) causes a conformational change in the protein such that the bound substrates are transiently occluded, unable to dissociate into either medium (step 2). In step 3 the protein assumes a third conformation with inward-facing sites. Dissociation of the bound Na+ and glucose into the cytosol (step 4) allows the protein to revert to its original outward-facing conformation (step 5), ready to transport additional substrate (Figure 11-26).

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72 ph regulation

73 ph and respiration Figure Carbon dioxide transport in blood requires a Cl /HCO 3 antiporter. 1. Erythroblast 2. Reticulocyte: 핵, 세포내소기관없음, 단백질합성기구존재 3. Erythrocyte: 단백질합성기구없음

74 ph regulation 1. Two cotransporters that are activated at low ph help maintain the cytosolic ph in animal cells very close to 7.4 despite metabolic production of carbonic and latic acids. 1 One, a Na+/H+ antiporter, exports excess protons. 2 The other, a Na+HCO3-/Cl- cotransporter, importers HCO3-, which dissociates in the cytosol to yield ph-raising OH- ions. 3 A Cl-/HCO3- antiporter that is activated at high ph functions to export HCO3- when the cytosolic ph rises above normal and causes a decrease in ph. AE1, a Cl-/HCO3- antiporter in the erythrocyte membrane, increases the ability of blood to transport CO2 from tissues to the lungs. 2. In systemic capillaries, carbon dioxide gas diffuses across the erythrocyte plasma membrane and is converted into soluble HCO3- by the enzyme carbonic anhydrase; at the same time, oxygen leaves the cell and hemoglobin binds a proton. The anion antiporter AE1 catalyzes the reversible exchange of Cland HCO3- ions across the membrane. The overall reaction causes HCO3- to be released from the cell, which is essential for maximal CO2 transport from the tissues to the lungs, and for maintaining ph neutrality in the blood cell. In the lungs, where carbon dioxide is excreted, the overall reaction is reversed (Figure 11-28).