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Cell_Biology_Alberts. the respiratory Chain Forms a Supercomplex in the Crista Membrane By using cryoelectron microscopy to examine proteins that have been very gently isolated, it can be shown that the three protein complexes that form the respiratory chain assemble into an even larger supercomplex in the crista membrane. As illustrated in Figure 14–26, this structure is thought to help the mobile electron carriers ubiquinone (in the crista membrane) and cytochrome c (in the crista space) transfer electrons with high efficiency. The formation of the supercomplex depends on the presence of the mitochondrial lipid cardiolipin (see Figure 14–11), which presumably works like a hydrophobic glue that holds the components together.


Cell_Biology_Alberts

Cell_Biology_Alberts. In addition to the three proton pumps in the supercomplex just discussed, one of the enzymes in the citric acid cycle, succinate dehydrogenase, is embedded in the mitochondrial crista membrane. In the course of oxidizing succinate to fumarate in the matrix, this enzyme complex captures electrons in the form of a tightly bound FADH2 molecule (see Panel 2–9, pp. 106–107) and passes them to


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 14–25The reaction of O2 with electrons in cytochrome c oxidase. Electrons from cytochrome c pass through the complex via bound copper ions (blue spheres)and hemes (red) to an O2 molecule bound between heme a3 and a copper ion. Iron ions are shown as red spheres. the iron atom in heme aserves as an electron queuing point where electrons are held so that they can be released to an O2 molecule (not shown) that is held at the bimetallic center active site, which is formed by the central iron of the other heme (heme a3) and a closely apposed copper atom. the four protons needed to reduce O2 to water are removed from the matrix. For each O2 molecule that undergoes the reaction 4e – + 4h+ + O2→ 2h2O, another four protons are pumped out of the matrix by mechanisms that are driven by allosteric changes in protein conformation (see Figure 14–28).


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Cell_Biology_Alberts. iron–sulfur cytochrome c Figure 14–26 The respiratory-chain mitochondria. the three proton-pumping CRISTA complexes of the mitochondrial respiratory chain of mammalian mitochondria assemble into large supercomplexes in the crista membrane. Supercomplexes can be isolated by mild detergent treatment of mitochondria, and their structure has been deciphered by single-particle cryoelectron microscopy. the bovine heart supercomplex has a total mass of 1.7 megadaltons. Shown is a schematic of such a complex that consists of NaDh dehydrogenase, cytochrome c reductase, and cytochrome c oxidase, as indicated. the facing quinol-binding sites of NaDh dehydrogenase and cytochrome c reductase, plus the short distance between the cytochrome c-binding sites in cytochrome c reductase and cytochrome c oxidase, facilitate fast, efficient electron transfer. Cofactors active in electron transport are marked as a yellow dot (flavin mononucleotide), red and yellow dots (iron–sulfur clusters), Q (quinone),


Cell_Biology_Alberts

Cell_Biology_Alberts. facilitate fast, efficient electron transfer. Cofactors active in electron transport are marked as a yellow dot (flavin mononucleotide), red and yellow dots (iron–sulfur clusters), Q (quinone), red squares (hemes), and a blue dot (copper atom). Only cofactors participating in the linear flow of electrons from NaDh to water are shown. Blue arrows indicate the path of the electrons through the supercomplex. (adapted from t. athoff et al., EMBO J. 30:4652–4664, 2011.) a molecule of ubiquinone. The reduced ubiquinol then passes its two electrons to the respiratory chain via cytochrome c reductase (see Figure 14–18). Succinate dehydrogenase is not a proton pump, and it does not contribute directly to the electrochemical potential utilized for ATP production in mitochondria. Thus, it is not considered to be an integral part of the respiratory chain.


Cell_Biology_Alberts

Cell_Biology_Alberts. The protons in water are highly mobile: by rapidly dissociating from one water molecule and associating with its neighbor, they can rapidly flit through a hydrogen-bonded network of water molecules (see Figure 2–5). But how can a pro ton move through the hydrophobic interior of a protein embedded in the lipid bilayer? Proton-translocating proteins contain so-called proton wires, which are rows of polar or ionic side chains, or water molecules spaced at short distances, so that the protons can jump from one to the next (Figure 14–27). Along such predefined pathways, protons move up to 40 times faster than through bulk water. The three-dimensional structure of cytochrome c oxidase indicates two different proton-uptake pathways. This confirmed earlier mutagenesis studies, which had shown that replacing the side chains of particular aspartate or arginine residues, whose side chains can bind and release protons, made the cytochrome c oxidase less efficient as a proton pump.


Cell_Biology_Alberts

Cell_Biology_Alberts. But how can electron transport cause allosteric changes in protein conformations that pump protons? From the most basic point of view, if electron transport drives sequential allosteric changes in protein conformation that alter the redox state of the components, these conformational changes can be connected to protein wires that allow the protein to pump H+ across the crista membrane. This type of H+ pumping requires at least three distinct conformations for the pump protein, as schematically illustrated in Figure 14–28. Figure 14–27 Proton movement through water and proteins. (a) protons move rapidly through water, hopping from one h2O molecule to the next by the continuous formation and dissociation of hydronium ions, h3O+ (see Chapter 2). In this diagram, proton jumps are indicated by red arrows.


Cell_Biology_Alberts

Cell_Biology_Alberts. wires.” these are predefined proton paths consisting of suitably spaced amino acid side chains that accept and release protons easily (asp, Glu) or carry a waterlike hydroxyl group (Ser, thr), along with water molecules trapped in the protein interior.


Cell_Biology_Alberts

Cell_Biology_Alberts. The respiratory chain embedded in the inner mitochondrial membrane contains three respiratory enzyme complexes, through which electrons pass on their way from NADH to O2. In these complexes, electrons are transferred along a series of protein-bound electron carriers, including hemes and iron–sulfur clusters. The energy released as the electrons move to lower and lower energy levels is used to pump protons by different mechanisms in the three respiratory enzyme complexes, each coupling lateral electron transport to vectorial proton transport across the membrane. Electrons are shuttled between enzyme complexes by the mobile electron carriers ubiquinone and cytochrome c to complete the electron-transport chain. The path of electron flow is NADH → NADH dehydrogenase complex → ubiquinone → cytochrome c reductase → cytochrome c → cytochrome c oxidase complex → molecular oxygen (O2).


Cell_Biology_Alberts

Cell_Biology_Alberts. As we have just discussed, the three proton pumps of the respiratory chain each contribute to the formation of an electrochemical proton gradient across the inner mitochondrial membrane. This gradient drives ATP synthesis by ATP synthase, a large membrane-bound protein complex that performs the extraordinary feat of converting the energy contained in this electrochemical gradient into biologically useful, chemical-bond energy in the form of ATP (see Figure 14–10). Protons flow down their electrochemical gradient through the membrane part of this proton turbine, thereby driving the synthesis of ATP from ADP and Pi in the extramembranous part of the complex. As discussed in Chapter 2, the formation of ATP from ADP and inorganic phosphate is highly unfavorable energetically. As we shall see, ATP synthase can produce ATP only because of allosteric shape changes in this protein complex that directly couple ATP synthesis to the energetically favorable flow of protons across its membrane.


Cell_Biology_Alberts

Cell_Biology_Alberts. the Large Negative Value of ∆G for atp hydrolysis Makes atp Useful to the Cell An average person turns over roughly 50 kg of ATP per day. In athletes running a marathon, this figure can go up to several hundred kilograms. The ATP produced in mitochondria is derived from the energy available in the intermediates NADH, FADH2, and GTP. These three energy-rich compounds are produced both by the oxidation of glucose (Table 14–1A), and by the oxidation of fats (Table 14–1B; see also Figure 2–56). Glycolysis alone can produce only two molecules of ATP for every molecule of glucose that is metabolized, and this is the total energy yield for the fermentation processes that occur in the absence of O2 (discussed in Chapter 2). In oxidative Figure 14–28 A general model for H+ pumping coupled to electron transport.


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 14–28 A general model for H+ pumping coupled to electron transport. this mechanism for h+ pumping by a transmembrane protein is thought to be used by NaDh dehydrogenase and cytochrome c oxidase, and by many other proton pumps. the protein is driven through a cycle of three conformations. In one of these conformations, the protein has a high affinity for h+, causing it to pick up an h+ on the inside of the membrane. In another conformation, the protein has a low affinity for h+, causing it to release an h+ on the outside of the membrane. as indicated, the transitions from one conformation to another occur only in one direction, because they are being driven by being allosterically coupled to the energetically favorable process of electron transport (discussed in Chapter 11).


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Cell_Biology_Alberts. phosphorylation, each pair of electrons donated by the NADH produced in mitochondria can provide energy for the formation of about 2.5 molecules of ATP. Oxidative phosphorylation also produces 1.5 ATP molecules per electron pair from the FADH2 produced by succinate dehydrogenase in the mitochondrial matrix, and from the NADH molecules produced by glycolysis in the cytosol. From the product yields of glycolysis and the citric acid cycle, we can calculate that the complete oxidation of one molecule of glucose—starting with glycolysis and ending with oxidative phosphorylation—gives a net yield of about 30 molecules of ATP. Nearly all this ATP is produced by the mitochondrial ATP synthase.


Cell_Biology_Alberts

Cell_Biology_Alberts. In Chapter 2, we introduced the concept of free energy (G). The free-energy change for a reaction, ∆G, determines whether that reaction will occur in a cell. We showed on pp. 60–63 that the ∆G for a given reaction can be written as the sum of two parts: the first, called the standard free-energy change, ∆G°, depends only on the intrinsic characters of the reacting molecules; the second depends only on their concentrations. For the simple reaction A →B, [A] where [A] and [B] denote the concentrations of A and B, and ln is the natural logarithm. ∆G° is the standard reference value, which can be seen to be equal to the value of ∆G when the molar concentrations of A and B are equal (since ln 1 = 0). In Chapter 2, we discussed how the large, favorable free-energy change (large negative ∆G) for ATP hydrolysis is used, through coupled reactions, to drive many other chemical reactions in the cell that would otherwise not occur (see pp. 65–66). The ATP hydrolysis reaction produces two


Cell_Biology_Alberts

Cell_Biology_Alberts. ∆G) for ATP hydrolysis is used, through coupled reactions, to drive many other chemical reactions in the cell that would otherwise not occur (see pp. 65–66). The ATP hydrolysis reaction produces two products, ADP and Pi; it is therefore of the type A → B + C, where, as demonstrated in Figure 14–29, [B][C]


Cell_Biology_Alberts

Cell_Biology_Alberts. When ATP is hydrolyzed to ADP and Pi under the conditions that normally exist in a cell, the free-energy change is roughly –46 to –54 kJ/mole (–11 to –13 kcal/mole). This extremely favorable ∆G depends on maintaining a high concentration of ATP compared with the concentrations of ADP and Pi. When ATP, ADP, and Pi are all present at the same concentration of 1 mole/liter (so-called standard conditions), the ∆G for ATP hydrolysis drops to the standard free-energy change (∆G°), which is only –30.5 kJ/mole (–7.3 kcal/mole). At much lower concentrations of ATP relative to ADP and Pi, ∆G becomes zero. At this point, the rate at AT EQUILIBRIUM: synthesis conc. of × conc. of hydrolysis conc. of + synthesis rate constant ×conc. of phosphate ×conc. of ADP synthesis synthesis rate = 2 4 ATPADPPi conc. of conc. of hydrolysis ADP phosphate rate constant thus, = = equilibrium constant K concentration synthesis of ATP rate constant or abbreviated,


Cell_Biology_Alberts

Cell_Biology_Alberts. ADP phosphate rate constant thus, = = equilibrium constant K concentration synthesis of ATP rate constant or abbreviated, At equilibrium the reaction has no net effect on the disorder of For the reaction the universe, so °G = 0. Therefore, at equilibrium, [ATP]the following equation applies: But the concentrations of reactants at equilibrium must satisfy the equilibrium equation: Therefore, at equilibrium, where °G and °Go are in Joules per mole, R is the gas constant (8.3 J/mole K), T is the absolute temperature (K), and all the concentrations are in moles per liter. We thus see that whereas °Go indicates the equilibrium point for a


Cell_Biology_Alberts

Cell_Biology_Alberts. When the concentrations of all reactants are at 1 M, °G = °Go reaction, °G reveals how far the reaction is from equilibrium. °G is (since RT ln 1 = 0). °Go is thus a constant defned as the a measure of the “driving force” for any chemical reaction, just as the standard free-energy change for the reaction. proton-motive force is the driving force for the translocation of protons. which ADP and Pi will join to form ATP will be equal to the rate at which ATP hydrolyzes to form ADP and Pi. In other words, when ∆G = 0, the reaction is at equilibrium (see Figure 14–29).


Cell_Biology_Alberts

Cell_Biology_Alberts. which ADP and Pi will join to form ATP will be equal to the rate at which ATP hydrolyzes to form ADP and Pi. In other words, when ∆G = 0, the reaction is at equilibrium (see Figure 14–29). It is ∆G, not ∆G°, that indicates how far a reaction is from equilibrium and determines whether it can drive other reactions. Because the efficient conversion of ADP to ATP in mitochondria maintains such a high concentration of ATP relative to ADP and Pi, the ATP hydrolysis reaction in cells is kept very far from equilibrium and ∆G is correspondingly very negative. Without this large disequilibrium, ATP hydrolysis could not be used to drive the reactions of the cell. At low ATP concentrations, many biosynthetic reactions would run backward and the cell would die. the atp Synthase Is a Nanomachine that produces atp by rotary Catalysis


Cell_Biology_Alberts

Cell_Biology_Alberts. the atp Synthase Is a Nanomachine that produces atp by rotary Catalysis The ATP synthase is a finely tuned nanomachine composed of 23 or more separate protein subunits, with a total mass of about 600,000 daltons. The ATP synthase can work both in the forward direction, producing ATP from ADP and phosphate in response to an electrochemical gradient, or in reverse, generating an electrochemical gradient by ATP hydrolysis. To distinguish it from other enzymes that hydrolyze ATP, it is also called an F1Fo ATP synthase or F-type ATPase.


Cell_Biology_Alberts

Cell_Biology_Alberts. Resembling a turbine, ATP synthase is composed of both a rotor and a stator (Figure 14–30). To prevent the catalytic head from rotating, a stalk at the periphery of the complex (the stator stalk) connects the head to stator subunits embedded in the membrane. A second stalk in the center of the assembly (the rotor stalk) is connected to the rotor ring in the membrane that turns as protons flow through it, driven by the electrochemical gradient across the membrane. As a result, proton


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 14–29 The basic relationship between free-energy changes and equilibrium in the ATP hydrolysis reaction. the rate constants in boxes 1 and 2 are determined from experiments in which product accumulation is measured as a function of time (conc., concentration). the equilibrium constant shown here, K, is in units of moles per liter. (See panel 2–7, pp. 102–103, for a discussion of free energy and see Figure 3–44 for a discussion of the equilibrium constant.) flow makes the rotor stalk rotate inside the stationary head, where the catalytic sites that assemble ATP from ADP and Pi are located. Three α and three β subunits of similar structure alternate to form the head. Each of the three β subunits has a catalytic nucleotide-binding site at the α/βinterface. These catalytic sites are all in different conformations, depending on their interaction with the rotor stalk. This stalk acts like a camshaft, the device that opens and closes the valves in a combustion engine. As it rotates


Cell_Biology_Alberts

Cell_Biology_Alberts. all in different conformations, depending on their interaction with the rotor stalk. This stalk acts like a camshaft, the device that opens and closes the valves in a combustion engine. As it rotates within the head, the stalk changes the conformations of the β subunits sequentially. One of the possible conformations of the catalytic sites has high affinity for ADP and Pi, and as the rotor stalk pushes the binding site into a different conformation, these two substrates are driven to form ATP. In this way, the mechanical force exerted by the central rotor stalk is directly converted into the chemical energy of the ATP phosphate bond.


Cell_Biology_Alberts

Cell_Biology_Alberts. Serving as a proton-driven turbine, the ATP synthase is driven by H+ flow into the matrix to spin at about 8000 revolutions per minute, generating three molecules of ATP per turn. In this way, each ATP synthase can produce roughly 400 molecules of ATP per second. proton-driven turbines are of ancient Origin


Cell_Biology_Alberts

Cell_Biology_Alberts. The membrane-embedded rotors of ATP synthases consist of a ring of identical c subunits (Figure 14–31). Each c subunit is a hairpin of two membrane-spanning α helices that contain a proton-binding site defined by a glutamate or aspartate in the middle of the lipid bilayer. The a subunit, which is part of the stator (see Figure 14–30), makes two narrow channels at the interface between the rotor and stator, each spanning half of the membrane and converging on the proton-binding site at the middle of the rotor subunit. Protons flow through the two half-channels down their electrochemical gradient from the crista space back into the matrix. A negatively charged side chain in the binding site accepts a proton arriving from the crista space through the first half-channel, as it rotates past the a subunit. The bound proton then rides round in the ring for a full cycle, whereupon it is thought to be displaced by a positively charged arginine in the a subunit, and escapes


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 14–30 ATP synthase. the three-dimensional structure of the F1Fo atp synthase, determined by x-ray crystallography. also known as an F-type atpase, it consists of an Fo part (from “oligomycin-sensitive factor”) in the membrane and the large, catalytic F1 head in the matrix. Under mild dissociation conditions, this complex separates into its F1 and Fo components, which can be isolated and studied individually. (a) Diagram of the enzyme complex showing how its globular head portion (green) is kept stationary as proton-flow across the membrane drives a rotor (blue) that turns inside it. (B) In bovine heart mitochondria, the Fo rotor ring in the membrane (light blue) has eight c subunits. It is attached to the γ subunit of the central stalk (dark blue) by the ε subunit (purple). the catalytic F1 head consists of a ring of three α and three β subunits (light and dark green), and it directly converts mechanical energy into chemical-bond energy in atp, as described in the text. the


Cell_Biology_Alberts

Cell_Biology_Alberts. catalytic F1 head consists of a ring of three α and three β subunits (light and dark green), and it directly converts mechanical energy into chemical-bond energy in atp, as described in the text. the elongated peripheral stalk of the stator (orange) is connected to the F1 head by the small δ subunit (red) at one end, and to the a subunit in the membrane (pink oval) at the other. together with the c subunits of the ring rotating past it, the a subunit creates a path for protons through the membrane. (C) the symbol for atp synthase used throughout this book.


Cell_Biology_Alberts

Cell_Biology_Alberts. the closely related atp synthases of mitochondria, chloroplasts, and bacteria synthesize atp by harnessing the proton-motive force across a membrane. this powers the rotation of the rotor against the stator in a counterclockwise direction, as seen from the F1 head. the same enzyme complex can also pump protons against their electrochemical gradient by hydrolyzing atp, which then drives the clockwise rotation of the rotor. the direction of operation depends on the net free-energy change (∆G) for the coupled processes of h+ translocation across the membrane and the synthesis of atp from aDp and pi (Movie 14.5 and Movie 14.6).


Cell_Biology_Alberts

Cell_Biology_Alberts. Measurement of the torque that the atp synthase can produce by atp hydrolysis reveals that the atp synthase is 60 times more powerful than a diesel engine of equal dimensions. (B, courtesy of K. Davies. pDB codes: 2WpD, 2CLy, 2WSS, 2BO5.) through the second half-channel into the matrix. Thus proton flow causes the rotor ring to spin against the stator like a proton-driven turbine. The mitochondrial ATP synthase is of ancient origin: essentially the same enzyme occurs in plant chloroplasts and in the plasma membrane of bacteria or archaea. The main difference between them is the number of c subunits in the rotor ring. In mammalian mitochondria, the ring has 8 subunits. In yeast mitochondria, the number is 10; in bacteria and archaea, it ranges from 11 to 13; in plant chloroplasts, there are 14; and the rings of some cyanobacteria contain 15 c subunits.


Cell_Biology_Alberts

Cell_Biology_Alberts. The c subunits in the rotor ring can be thought of as cogs in the gears of a bicycle. A high gear, with a small number of cogs, is advantageous when the supply of protons is limited, as in mitochondria, but a low gear, with a large number of cogs in the wheel, is preferable when the proton gradient is high. This is the case in chloroplasts and cyanobacteria, where protons produced through the action of sunlight are plentiful. Because each rotation produces three molecules of ATP in the head, the synthesis of one ATP requires around three protons in mitochondria but up to five in photosynthetic organisms. It is the number of c subunits in the ring that defines how many protons need to pass through this marvelous device to make each molecule of ATP, and thereby how high a ratio of ATP to ADP can be maintained by the ATP synthase.


Cell_Biology_Alberts

Cell_Biology_Alberts. In principle, ATP synthase can also run in reverse as an ATP-powered proton pump that converts the energy of ATP back into a proton gradient across the membrane. In many bacteria, the rotor of the ATP synthase in the plasma membrane changes direction routinely, from ATP synthesis mode in aerobic respiration, to ATP hydrolysis mode in anaerobic metabolism. In this latter case, ATP hydrolysis serves to maintain the proton gradient across the plasma membrane, which is used to power many other essential cell functions including nutrient transport and the rotation of bacterial flagella. The V-type ATPases that acidify certain cellular organelles are architecturally similar to the F-type ATP synthases, but they normally function in reverse (see Figure 13–37). Mitochondrial Cristae help to Make atp Synthesis Efficient (A)


Cell_Biology_Alberts

Cell_Biology_Alberts. In the electron microscope, the mitochondrial ATP synthase complexes can be seen to project like lollipops on the matrix side of cristae membranes. Recent studies by cryoelectron microscopy and tomography have shown that this large complex is not distributed randomly in the membrane, but forms long rows of dimers along the cristae ridges (Figure 14–32). The dimer rows induce or stabilize these regions of high membrane curvature, which are otherwise energetically unfavorable. Indeed, the formation of ATP synthase dimers and their assembly into rows are required for cristae formation and have far-reaching consequences for cellular fitness. By contrast with bacterial or chloroplast ATP synthases, which do not form dimers, the mitochondrial complex contains additional subunits, located mostly near the membrane end of the stator stalk. Several of these subunits are found to be dimer-specific. If these subunits are mutated in yeast, the ATP synthase in the membrane remains monomeric, the


Cell_Biology_Alberts

Cell_Biology_Alberts. near the membrane end of the stator stalk. Several of these subunits are found to be dimer-specific. If these subunits are mutated in yeast, the ATP synthase in the membrane remains monomeric, the mitochondria have no cristae, cellular respiration drops by half, and the cells grow more slowly.


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Cell_Biology_Alberts. Figure 14–31 Fo ATP synthase rotor rings. (a) atomic force microscopy image of atp synthase rotors from the cyanobacterium Synechococcus elongatus in a lipid bilayer. Whereas 8 c subunits form the rotor in Figure 14–30, there are 13 c subunits in this ring. (B) the x-ray structure of the Fo ring of the atp synthase from Spirulina platensis, another cyanobacterium, shows that this rotor has 15 c subunits. In all atp synthases, the c subunits are hairpins of two membrane-spanning α helices (one subunit is highlighted in gray). the helices are highly hydrophobic, except for two glutamine and glutamate side chains (yellow) that create proton-binding sites in the membrane. (a, courtesy of thomas Meier and Denys pogoryelov; B, pDB code: 2WIE.)


Cell_Biology_Alberts

Cell_Biology_Alberts. Electron tomography suggests that the proton pumps of the respiratory chain are located in the membrane regions at either side of the dimer rows. Protons pumped into the crista space by these respiratory-chain complexes are thought to diffuse very rapidly along the membrane surface, with the ATP synthase rows creating a proton “sink” at the cristae tips (Figure 14–33). In vitro studies suggest that the ATP synthase needs a proton gradient of about 2 pH units to produce ATP at the rate required by the cell, irrespective of the membrane potential. The H+ gradient across the inner mitochondrial membrane is only 0.5 to 0.6 pH units. The cristae thus seem to work as proton traps that enable the ATP synthase to make efficient use of the protons pumped out of the mitochondrial matrix. As we shall see in the next section, this elaborate arrangement of membrane protein complexes is absent in chloroplasts, where the H+ gradient is much higher.


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Cell_Biology_Alberts. Special transport proteins Exchange atp and aDp through the Inner Membrane Like all biological membranes, the inner mitochondrial membrane contains numerous specific transport proteins that allow particular substances to pass through. One of the most abundant of these is the ADP/ATP carrier protein (Figure 14–34). This carrier shuttles the ATP produced in the matrix through the inner membrane to the intermembrane space, from where it diffuses through the outer mitochondrial membrane to the cytosol. In exchange, ADP passes from the cytosol into the matrix for recycling into ATP. ATP4– has one more negative charge than ADP3–, and the exchange of ATP and ADP is driven by the electrochemical gradient across the inner membrane, so that the more negatively charged ATP is pushed out of the matrix, and the less negatively charged ADP is pulled in. The ADP/ATP carrier is but one member of a mitochondrial carrier family: the inner CYTOSOL pH 7.4 inner membraneMATRIX pH 7.9 ATP synthase


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Cell_Biology_Alberts. CYTOSOL pH 7.4 inner membraneMATRIX pH 7.9 ATP synthase Figure 14–32 Dimers of mitochondrial ATP synthase in cristae membranes. a three-dimensional map of a small mitochondrion obtained by electron microscope tomography shows that atp synthases form long paired rows along cristae ridges. the outer membrane is gray, the inner membrane and cristae membranes have been colored light blue. Each head of an atp synthase is indicated by a yellow sphere. (B) a three-dimensional map of a mitochondrial atp synthase dimer in the crista membrane obtained by subtomogram averaging, with fitted x-ray structures (Movie 14.7). (a, from K. Davies et al., Proc. Natl Acad. Sci. USA 108:14121–14126, 2011. With permission from the National academy of Sciences; B, from K. Davies et al., Proc. Natl Acad. Sci. USA 109:13602–13607, 2012. With permission from the National academy of Sciences.)


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Cell_Biology_Alberts. Figure 14–33 ATP synthase dimers at cristae ridges and ATP production. at the crista ridges, the atp synthases (yellow) form a sink for protons (red). the proton pumps of the electron-transport chain (green) are located in the membrane regions on either side of the crista. as illustrated, protons tend to diffuse along the membrane from their source to the proton sink created by the atp synthase. this allows efficient atp production despite the small h+ gradient between the cytosol and matrix. Red arrows show the direction of the proton flow.


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 14–34 The ADP/ATP carrier protein. (a) the aDp/atp carrier protein is a small membrane protein that carries the atp produced on the matrix side of the inner membrane to the intermembrane space, and the aDp that is needed for atp synthesis into the matrix. (B) In the aDp/atp carrier, six transmembrane α helices define a cavity that binds either aDp or atp. In this x-ray structure, the substrate is replaced by a tightly bound inhibitor instead (colored). When aDp binds from outside the inner membrane, it triggers a conformational change and is released into the matrix. In exchange, a molecule of atp quickly binds to the matrix side of the carrier and is transported to the intermembrane space. From there the atp diffuses through the outer mitochondrial membrane to the cytoplasm, where it powers the energy-requiring processes in the cell. (B, pDB code: 1OKC.) mitochondrial membrane contains about 20 related carrier proteins exchanging various other metabolites, including the


Cell_Biology_Alberts

Cell_Biology_Alberts. where it powers the energy-requiring processes in the cell. (B, pDB code: 1OKC.) mitochondrial membrane contains about 20 related carrier proteins exchanging various other metabolites, including the phosphate that is required along with ADP for ATP synthesis.


Cell_Biology_Alberts

Cell_Biology_Alberts. In some specialized fat cells, mitochondrial respiration is uncoupled from ATP synthesis by the uncoupling protein, another member of the mitochondrial carrier family. In these cells, known as brown fat cells, most of the energy of oxidation is dissipated as heat rather than being converted into ATP. In the inner membranes of the large mitochondria in these cells, the uncoupling protein allows protons to move down their electrochemical gradient without passing through ATP synthase. This process is switched on when heat generation is required, causing the cells to oxidize their fat stores at a rapid rate and produce heat rather than ATP. Tissues containing brown fat serve as “heating pads,” helping to revive hibernating animals and to protect newborn human babies from the cold.


Cell_Biology_Alberts

Cell_Biology_Alberts. Bacteria use enormously diverse energy sources. Some, like animal cells, are aerobic; they synthesize ATP from sugars they oxidize to CO2 and H2O by glycolysis, the citric acid cycle, and a respiratory chain in their plasma membrane that is similar to the one in the inner mitochondrial membrane. Others are strict anaerobes, deriving their energy either from glycolysis alone (by fermentation, see Figure 2–47) or from an electron-transport chain that employs a molecule other than oxygen as the final electron acceptor. The alternative electron acceptor can be a nitrogen compound (nitrate or nitrite), a sulfur compound (sulfate or sulfite), or a carbon compound (fumarate or carbonate), for example. A series of electron carriers in the plasma membrane that are comparable to those in mitochondrial respiratory chains transfers the electrons to these acceptors.


Cell_Biology_Alberts

Cell_Biology_Alberts. Despite this diversity, the plasma membrane of the vast majority of bacteria contains an ATP synthase that is very similar to the one in mitochondria. In bacteria that use an electron-transport chain to harvest energy, the electron-transport chain pumps H+ out of the cell and thereby establishes a proton-motive force across the plasma membrane that drives the ATP synthase to make ATP. In other bacteria, the ATP synthase works in reverse, using the ATP produced by glycolysis to pump H+ and establish a proton gradient across the plasma membrane.


Cell_Biology_Alberts

Cell_Biology_Alberts. Bacteria, including the strict anaerobes, maintain a proton gradient across their plasma membrane that is harnessed to drive many other processes. It can be used to drive a flagellar motor, for example (Figure 14–35). This gradient is harnessed to pump Na+ out of the bacterium via a Na+–H+ antiporter that takes the place of the Na+-K+ pump of eukaryotic cells. The gradient is also used for the active inward transport of nutrients, such as most amino acids and many sugars: each nutrient is dragged into the cell along with one or more protons through a specific symporter (Figure 14–36; see also Chapter 11). In animal cells, by contrast, most inward transport across the plasma membrane is driven by the Na+ gradient (high Na+ outside, low Na+ inside) that is established by the Na+-K+ pump (see Figure 11–15).


Cell_Biology_Alberts

Cell_Biology_Alberts. Some unusual bacteria have adapted to live in a very alkaline environment and yet must maintain their cytoplasm at a physiological pH. For these cells, any attempt to generate an electrochemical H+ gradient would be opposed by a large H+ concentration gradient in the wrong direction (H+ higher inside than outside). Presumably for this reason, some of these bacteria substitute Na+ for H+ in all of their chemiosmotic mechanisms. The respiratory chain pumps Na+ out of the cell, the transport systems and flagellar motor are driven by an inward flux of Na+, and a Na+-driven ATP synthase synthesizes ATP. The existence of such bacteria demonstrates a critical point: the principle of chemiosmosis is more fundamental than the proton-motive force on which it is normally based. As we discuss next, an ATP synthase coupled to chemiosmotic processes is also a central feature of plants, where it plays critical roles in both mitochondria and chloroplasts.


Cell_Biology_Alberts

Cell_Biology_Alberts. As we discuss next, an ATP synthase coupled to chemiosmotic processes is also a central feature of plants, where it plays critical roles in both mitochondria and chloroplasts. Figure 14–36 The importance of H+-driven transport in bacteria. a proton-motive force generated across the plasma membrane pumps nutrients into the cell and expels Na+. (a) In an aerobic bacterium, a respiratory chain fed by the oxidation of substrates produces an electrochemical proton gradient across the plasma membrane. this gradient is then harnessed to make atp, as well as to transport nutrients (proline, succinate, lactose, and lysine) into the cell and to pump Na+ out of the cell.


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Cell_Biology_Alberts. (B) When the same bacterium grows under anaerobic conditions, it derives its atp from glycolysis. as indicated, the atp synthase in the plasma membrane then hydrolyzes some of this atp to establish an electrochemical proton gradient that drives the same transport processes that depend on respiratory chain proton-pumping in (a). Figure 14–35 The rotation of the bacterial flagellum driven by H+ flow. the flagellum is attached to a series of protein rings (pink), which are embedded in the outer and inner membranes and rotate with the flagellum. the rotation is driven by a flow of protons through an outer ring of proteins (the stator) by mechanisms that may resemble those used by the atp synthase. however, the flow of protons in the flagellar motor is always toward the cytosol, both during clockwise and counterclockwise rotation, whereas in atp synthase this flow reverses with the direction of rotation (Movie 14.8).


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Cell_Biology_Alberts. The large amount of free energy released when H+ flows back into the matrix from the cristae provides the basis for ATP production on the matrix side of mitochondrial cristae membranes by a remarkable protein machine—the ATP synthase. The ATP synthase functions like a miniature turbine, and it is a reversible device that can couple proton flow to either ATP synthesis or ATP hydrolysis. The transmembrane electrochemical gradient that drives ATP production in mitochondria also drives the active transport of selected metabolites across the inner mitochondrial membrane, including an efficient ADP/ATP exchange between the mitochondrion and the cytosol that keeps the cell’s ATP pool highly charged. The resulting high cellular concentration of ATP makes the free-energy change for ATP hydrolysis extremely favorable, allowing this hydrolysis reaction to drive a very large number of energy-requiring processes throughout the cell. The universal presence of ATP synthase in bacteria, mitochondria,


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Cell_Biology_Alberts. favorable, allowing this hydrolysis reaction to drive a very large number of energy-requiring processes throughout the cell. The universal presence of ATP synthase in bacteria, mitochondria, and chloroplasts testifies to the central importance of chemiosmotic mechanisms in cells.


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Cell_Biology_Alberts. All animals and most microorganisms rely on the continual uptake of large amounts of organic compounds from their environment. These compounds provide both the carbon-rich building blocks for biosynthesis and the metabolic energy for life. It is likely that the first organisms on the primitive Earth had access to an abundance of organic compounds produced by geochemical processes, but it is clear that these were used up billions of years ago. Since that time, virtually all of the organic materials required by living cells have been produced by photosynthetic organisms, including plants and photosynthetic bacteria. The core machinery that drives all photosynthesis appears to have evolved more than 3 billion years ago in the ancestors of present-day bacteria; today it provides the only major solar energy storage mechanism on Earth.


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Cell_Biology_Alberts. The most advanced photosynthetic bacteria are the cyanobacteria, which have minimal nutrient requirements. They use electrons from water and the energy of sunlight to convert atmospheric CO2 into organic compounds—a process called carbon fixation. In the course of the overall reaction nH2O + nCO2 → (light) (CH2O)n + nO2, they also liberate into the atmosphere the molecular oxygen that then powers oxidative phosphorylation. In this way, it is thought that the evolution of cyanobacteria from more primitive photosynthetic bacteria eventually made possible the development of the many different aerobic life-forms that populate the Earth today.


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Cell_Biology_Alberts. Plants (including algae) developed much later than cyanobacteria, and their photosynthesis occurs in a specialized intracellular organelle—the chloroplast (Figure 14–37). Chloroplasts use chemiosmotic mechanisms to carry out their energy interconversions in much the same way that mitochondria do. Although much larger than mitochondria, they are organized on the same principles. They have a highly permeable outer membrane; a much less permeable inner membrane, in which membrane transport proteins are embedded; and a narrow intermembrane space in between. Together, these two membranes form the chloroplast envelope (Figure 14–37D). The inner chloroplast membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix. The stroma contains many metabolic enzymes and, as for the mitochondrial matrix, it is the place where ATP is made by the head of an ATP synthase. Like the mitochondrion, the chloroplast has its own genome and genetic system. The stroma


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Cell_Biology_Alberts. and, as for the mitochondrial matrix, it is the place where ATP is made by the head of an ATP synthase. Like the mitochondrion, the chloroplast has its own genome and genetic system. The stroma therefore also contains a special set of ribosomes, RNAs, and the chloroplast DNA.


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Cell_Biology_Alberts. An important difference between the organization of mitochondria and chloroplasts is highlighted in Figure 14–38. The inner membrane of the chloroplast is not folded into cristae and does not contain electron-transport chains. Instead, the electron-transport chains, photosynthetic light-capturing systems, and ATP synthase are all contained in the thylakoid membrane, a separate, distinct mem-brane that forms a set of flattened, disc-like sacs, the thylakoids. The thylakoid membrane is highly folded into numerous local stacks of flattened vesicles called grana, interconnected by nonstacked thylakoids. The lumen of each thylakoid is connected with the lumen of other thylakoids, thereby defining a third internal compartment called the thylakoid space. This space represents a separate com-partment in each chloroplast that is not connected to either the intermembrane space or the stroma. Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon We can group the reactions that occur


Cell_Biology_Alberts

Cell_Biology_Alberts. in each chloroplast that is not connected to either the intermembrane space or the stroma. Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon We can group the reactions that occur during photosynthesis in chloroplasts into two broad categories: 1. The photosynthetic electron-transfer reactions (also called the “light reactions”) occur in two large protein complexes, called reaction centers, embedded in the thylakoid membrane. A photon (a quantum of light) knocks an electron out of the green pigment molecule chlorophyll in the first reaction center, creating a positively charged chlorophyll ion. This electron then moves along an electron-transport chain and through a sec-ond reaction center in much the same way that an electron moves along the respiratory chain in mitochondria. During this electron-transport process, H+ is pumped across the thylakoid membrane, and the resulting LEAF upper epidermis lower epidermis nucleus cytosol vacuole air space cell wall


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Cell_Biology_Alberts. in mitochondria. During this electron-transport process, H+ is pumped across the thylakoid membrane, and the resulting LEAF upper epidermis lower epidermis nucleus cytosol vacuole air space cell wall mitochondrion (B) (C)(A) 10 mm chloroplast chloroplast (D) 5 µm 0.5 µm


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Cell_Biology_Alberts. Figure 14–37 Chloroplasts in the cell. Schematic cross section through the leaf of a green plant. (B) Light microscopy of a plant leaf cell—here, a mesophyll cell from Zinnia elegans—shows chloroplasts as bright green bodies, measuring several micrometers across, in the transparent cell interior. (C) the electron micrograph of a thin, stained section through a wheat leaf cell shows a thin rim of cytoplasm— containing chloroplasts, the nucleus, and mitochondria—surrounding a large, water-filled vacuole. (D) at higher magnification, electron microscopy reveals the chloroplast envelope membrane and the thylakoid membrane within the chloroplast that is highly folded into grana stacks (Movie 14.9). (B, courtesy of John Innes Foundation; C and D, courtesy of K. plaskitt.)


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Cell_Biology_Alberts. K. plaskitt.) Figure 14–38 A mitochondrion and chloroplast compared. Chloroplasts are generally larger than mitochondria. In addition to an outer and inner envelope membrane, they contain the thylakoid membrane with its internal thylakoid space. the chloroplast thylakoid membrane, which is the site of solar energy conversion in plants and algae, corresponds to the mitochondrial cristae, which are the sites of energy conversion by cellular respiration. Unlike the crista membrane, which is continuous with the inner mitochondrial membrane at cristae junctions, the thylakoid membrane is not connected to the inner chloroplast membrane at any point.


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Cell_Biology_Alberts. electrochemical proton gradient drives the synthesis of ATP in the stroma. As the final step in this series of reactions, electrons are loaded (together with H+) onto NADP+, converting it to the energy-rich NADPH molecule. Because the positively charged chlorophyll in the first reaction center quickly regains its electrons from water (H2O), O2 gas is produced as a by-product. All of these reactions are confined to the chloroplast.


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Cell_Biology_Alberts. 2. The carbon-fixation reactions do not require sunlight. Here the ATP and NADPH generated by the light reactions serve as the source of energy and reducing power, respectively, to drive the conversion of CO2 to carbohydrate. These carbon-fixation reactions begin in the chloroplast stroma, where they generate the three-carbon sugar glyceraldehyde 3-phosphate. This simple sugar is exported to the cytosol, where it is used to produce sucrose and many other organic metabolites in the leaves of the plant. The sucrose is then exported to meet the metabolic needs of the nonphotosynthetic plant tissues, serving as a source of both carbon skeletons and energy for growth.


Cell_Biology_Alberts

Cell_Biology_Alberts. Thus, the formation of ATP, NADPH, and O2 (which requires light energy directly) and the conversion of CO2 to carbohydrate (which requires light energy only indirectly) are separate processes (Figure 14–39). However, they are linked by elaborate feedback mechanisms that allow a plant to manufacture sugars only when it is appropriate to do so. Several of the chloroplast enzymes required for carbon fixation, for example, are inactive in the dark and reactivated by light-stimulated electron-transport processes. Carbon Fixation Uses atp and NaDph to Convert CO2 into Sugars


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Cell_Biology_Alberts. Carbon Fixation Uses atp and NaDph to Convert CO2 into Sugars We have seen earlier in this chapter how animal cells produce ATP by using the large amount of free energy released when carbohydrates are oxidized to CO2 and H2O. The reverse reaction, in which plants make carbohydrate from CO2 and H2O, takes place in the chloroplast stroma. The large amounts of ATP and NADPH produced by the photosynthetic electron-transfer reactions are required to drive this energetically unfavorable reaction. Figure 14–39 A summary of the energy-converting metabolism in chloroplasts.


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Cell_Biology_Alberts. Chloroplasts require only water and carbon dioxide as inputs for their light-driven photosynthesis reactions, and they produce the nutrients for most other organisms on the planet. Each oxidation of two water molecules by a photochemical reaction center in the thylakoid membrane produces one molecule of oxygen, which is released into the atmosphere. at the same time, protons are concentrated in the thylakoid space. these protons create a large electrochemical gradient across the thylakoid membrane, which is utilized by the chloroplast atp synthase to produce atp from aDp and phosphate. the electrons withdrawn from water are transferred to a second type of photochemical reaction center to produce NaDph from NaDp+. as indicated, the NaDph and atp are fed into the carbon-fixation cycle to reduce carbon dioxide, thereby producing the precursors for sugars, amino acids, and fatty acids. the CO2 that is taken up from the atmosphere here is the source of the carbon atoms for most organic


Cell_Biology_Alberts

Cell_Biology_Alberts. reduce carbon dioxide, thereby producing the precursors for sugars, amino acids, and fatty acids. the CO2 that is taken up from the atmosphere here is the source of the carbon atoms for most organic molecules on Earth.


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Cell_Biology_Alberts. In a plant cell, a variety of metabolites produced in the chloroplast are exported to the cytoplasm for biosyntheses. Some of the sugar produced is stored in the form of starch granules in the chloroplast, but the rest is transported throughout the plant as sucrose or converted to starch in special storage tissues. these storage tissues serve as a major food source for animals. carbon ribulose 1,5-bisphosphate intermediate 2 molecules of dioxide 3-phosphoglycerate


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 14–40 illustrates the central reaction of carbon fixation, in which an atom of inorganic carbon is converted to organic carbon: CO2 from the atmosphere combines with the five-carbon compound ribulose 1,5-bisphosphate plus water to yield two molecules of the three-carbon compound 3-phosphoglycerate. This carboxylation reaction is catalyzed in the chloroplast stroma by a large enzyme called ribulose bisphosphate carboxylase, or Rubisco for short. Because the reaction is so slow (each Rubisco molecule turns over only about 3 molecules of substrate per second, compared to 1000 molecules per second for a typical enzyme), an unusually large number of enzyme molecules are needed. Rubisco often constitutes more than 50% of the chloroplast protein mass, and it is thought to be the most abundant protein on Earth. In a global context, Rubisco also keeps the amount of the greenhouse gas CO2 in the atmosphere at a low level.


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Cell_Biology_Alberts. Although the production of carbohydrates from CO2 and H2O is energetically unfavorable, the fixation of CO2 catalyzed by Rubisco is an energetically favorable reaction. Carbon fixation is energetically favorable because a continuous supply of the energy-rich ribulose 1,5-bisphosphate is fed into the process. This compound is consumed by the addition of CO2, and it must be replenished. The energy and reducing power needed to regenerate ribulose 1,5-bisphosphate come from the ATP and NADPH produced by the photosynthetic light reactions.


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Cell_Biology_Alberts. The elaborate series of reactions in which CO2 combines with ribulose 1,5-bisphosphate to produce a simple sugar—a portion of which is used to regenerate ribulose 1,5-bisphosphate—forms a cycle, called the carbon-fixation cycle, or the Calvin cycle (Figure 14–41). This cycle was one of the first metabolic pathways to be worked out by applying radioisotopes as tracers in biochemistry. As indicated, each turn of the cycle converts six molecules of 3-phosphoglycerate to three molecules of ribulose 1,5-bisphosphate plus one molecule of glyceraldehyde 3-phosphate. Glyceraldehyde 3-phosphate, the three-carbon sugar produced by the cycle, then provides the starting material for the synthesis of many other sugars and all of the other organic molecules that form the plant. Sugars Generated by Carbon Fixation Can Be Stored as Starch or Consumed to produce atp


Cell_Biology_Alberts

Cell_Biology_Alberts. Sugars Generated by Carbon Fixation Can Be Stored as Starch or Consumed to produce atp The glyceraldehyde 3-phosphate generated by carbon fixation in the chloroplast stroma can be used in a number of ways, depending on the needs of the plant. During periods of excess photosynthetic activity, much of it is retained in the chloroplast stroma and converted to starch. Like glycogen in animal cells, starch is a large polymer of glucose that serves as a carbohydrate reserve, and it is stored as large granules in the chloroplast stroma. Starch forms an important part of the diet of all animals that eat plants. Other glyceraldehyde 3-phosphate molecules are converted to fat in the stroma. This material, which accumulates as fat droplets, likewise serves as an energy reserve. At night, this stored starch and fat can be broken down to sugars and fatty acids, which are exported to the cytosol to help support the metabolic needs of the plant. Some of the exported sugar enters the


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Cell_Biology_Alberts. Figure 14–40 The initial reaction in carbon fixation. this carboxylation reaction allows one molecule each of carbon dioxide and water to be incorporated into organic carbon molecules. It is catalyzed in the chloroplast stroma by the abundant enzyme ribulose bisphosphate carboxylase, or rubisco. as indicated, the product is two molecules of 3-phosphoglycerate.


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Cell_Biology_Alberts. 6 6 6 6 6 6 × 3-phosphoglycerate 6 × 1,3-bisphosphoglycerate 6 × glyceraldehyde 3-phosphate 1 MOLECULE OF GLYCERALDEHYDE 3-PHOSPHATE LEAVES THE CYCLE 5 × glyceraldehyde 3-phosphate 3 ×ribulose 1,5-bisphosphate 3C 5C 3C 3C3C Rubisco ATPADPATPADPNADPHNADP+Pi Pi CARBON-FIXATION (CALVIN) CYCLE NET RESULT: For every 3 molecules of CO2 that enter the cycle, 1 molecule of glyceraldehyde 3-phosphate is produced and 9 molecules of ATP + 6 molecules of NADPH are consumed REGENERATIONCARBOXYLATIONREDUCTION sugars, fats, amino acids glycolytic pathway (see Figure 2–46), where it is converted to pyruvate. Both that pyruvate and the fatty acids can enter the plant cell mitochondria and be fed into the citric acid cycle, ultimately leading to the production of large amounts of ATP by oxidative phosphorylation (Figure 14–42). Plants use this ATP in the same way that animal cells and other nonphotosynthetic organisms do to power a variety of metabolic reactions.


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Cell_Biology_Alberts. The glyceraldehyde 3-phosphate exported from chloroplasts into the cytosol can also be converted into many other metabolites, including the disaccharide sucrose. Sucrose is the major form in which sugar is transported between the cells of a plant: just as glucose is transported in the blood of animals, so sucrose is exported from the leaves to provide carbohydrate to the rest of the plant. the thylakoid Membranes of Chloroplasts Contain the protein Complexes required for photosynthesis and atp Generation


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Cell_Biology_Alberts. We next need to explain how the large amounts of ATP and NADPH required for carbon fixation are generated in the chloroplast. Chloroplasts are much larger and less dynamic than mitochondria, but they make use of chemiosmotic energy conversion in much the same way. As we saw in Figure 14–38, chloroplasts and mitochondria are organized on the same principles, although the chloroplast contains a separate thylakoid membrane system in which its chemiosmotic mechanisms occur. The thylakoid membranes contain two large membrane protein complexes, called photosystems, which endow plants and other photosynthetic organisms with the ability to capture and convert solar energy for their own use. Two other protein complexes in the thylakoid membrane that work together with the photosystems in photophosphorylation—the generation of ATP with sunlight— have mitochondrial equivalents. These are the heme-containing cytochrome


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Cell_Biology_Alberts. Figure 14–41 The carbon-fixation cycle. this central metabolic pathway allows organic molecules to be produced from CO2 and h2O. In the first stage of the cycle (carboxylation), CO2 is added to ribulose 1,5-bisphosphate, as shown in Figure 14–40. In the second stage (reduction), atp and NaDph are consumed to produce glyceraldehyde 3-phosphate molecules. In the final stage (regeneration), some of the glyceraldehyde 3-phosphate produced is used to regenerate ribulose 1,5-bisphosphate. Other glyceraldehyde 3-phosphate molecules are either converted to starch and fat in the chloroplast stroma, or transported out of the chloroplast into the cytosol. the number of carbon atoms in each type of molecule is indicated in yellow. there are many intermediates between glyceraldehyde 3-phosphate and ribulose 5-phosphate, but they have been omitted here for clarity. the entry of water into the cycle is also not shown (but see Figure 14–40).


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Cell_Biology_Alberts. Figure 14–42 How chloroplasts and mitochondria collaborate to supply cells with both metabolites and ATP. (a)the inner chloroplast membrane is impermeable to the atp and NaDph that are produced in the stroma during the light reactions of photosynthesis. these molecules are therefore funneled into the carbon-fixation cycle, where they are used to make sugars. the resulting sugars and their metabolites are either stored within the chloroplast—in the form of starch or fat—or exported to the rest of the plant cell. there, they can enter the energy-generating pathway that ends in atp synthesis linked to oxidative phosphorylation inside the mitochondrion. Unlike the chloroplast, mitochondrial membranes contain a specific transporter that makes them permeable to atp (see Figure 14–34). Note that the O2 released to the atmosphere by photosynthesis in chloroplasts is used for oxidative phosphorylation in mitochondria; similarly, the CO2 released by the citric acid cycle in mitochondria is used


Cell_Biology_Alberts

Cell_Biology_Alberts. the O2 released to the atmosphere by photosynthesis in chloroplasts is used for oxidative phosphorylation in mitochondria; similarly, the CO2 released by the citric acid cycle in mitochondria is used for carbon fixation in chloroplasts. (B) In a leaf, mitochondria (red) tend to cluster close to the chloroplasts (green), as seen in this light micrograph. (B, courtesy of Olivier Grandjean.) b6-f complex, which both functionally and structurally resembles cytochrome CH2 c reductase in the respiratory chain; and the chloroplast ATP synthase, which closely resembles the mitochondrial ATP synthase and works in the same way.


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Cell_Biology_Alberts. The photosystems in the thylakoid membrane are multiprotein assemblies of a complexity comparable to that of the protein complexes in the mitochondrial electron-transport chain. They contain large numbers of specifically bound chlorophyll molecules, in addition to cofactors that will be familiar from our discus-CH2 sion of mitochondria (heme, iron–sulfur clusters, and quinones). Chlorophyll, the green pigment of photosynthetic organisms, has a long hydrophobic tail that makes it behave like a lipid, plus a porphyrin ring that has a central Mg atom and an extensive system of delocalized electrons in conjugated double bonds (Figure 14–43). When a chlorophyll molecule absorbs a quantum of sunlight (a photon), the energy of the photon causes one of these electrons to move from a low-energy molecular orbital to another orbital of higher energy.


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Cell_Biology_Alberts. The excited electron in a chlorophyll molecule tends to return quickly to its ground state, which can occur in one of three ways: 1. By converting the extra energy into heat (molecular motion) or to some combination of heat and light of a longer wavelength (fluorescence); this is what usually happens when light is absorbed by an isolated chlorophyll molecule in solution. 2. By transferring the energy—but not the electron—directly to a neighboring chlorophyll molecule by a process called resonance energy transfer. 3. By transferring the excited electron with its negative charge to another nearby molecule, an electron acceptor, after which the positively charged chlorophyll returns to its original state by taking up an electron from some other molecule, an electron donor.


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Cell_Biology_Alberts. Figure 14–43 The structure of chlorophyll. a magnesium atom is held in a porphyrin ring, which is related to the porphyrin ring that binds iron in heme (see Figure 14–15). Electrons are delocalized over the bonds shaded in blue. The latter two mechanisms occur when chlorophylls are attached to proteins in a chlorophyll–protein complex. The protein coordinates the central Mg atom in the chlorophyll porphyrin, most often through a histidine side chain located in the hydrophobic interior of a membrane, causing each of the chlorophylls in a protein complex to be held at exactly defined distances and orientations. The flow of excitation energy or electrons then depends on both the precise spatial arrangement and the local protein environment of the protein-bound chlorophylls.


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Cell_Biology_Alberts. When excited by a photon, most protein-bound chlorophylls simply transmit the absorbed energy to another nearby chlorophyll by the process of resonance energy transfer. However, in a few specially positioned chlorophylls, the energy difference between the ground state and the excited state is just right for the photon to trigger a light-induced chemical reaction. The special state of such chlorophyll molecules derives from their close interaction with a second chlorophyll molecule in the same chlorophyll–protein complex. Together, these two chlorophylls form a special pair.


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Cell_Biology_Alberts. The photosynthetic electron transfer process starts when a photon of suitable energy ionizes a chlorophyll molecule in such a special pair, dissociating it into an electron and a positively charged chlorophyll ion. The energized electron is passed rapidly to a quinone in the same protein complex, preventing its unproductive reassociation with the chlorophyll ion. This light-induced transfer of an electron from a chlorophyll to a mobile electron carrier is the central charge-separation step in photosynthesis, in which a chlorophyll becomes positively charged and an electron carrier becomes negatively charged (Figure 14–44). The chlorophyll ion is a very strong oxidant that is able to withdraw an electron from a low-energy substrate; in the first step of oxygenic photosynthesis, this low-energy substrate is water.


Cell_Biology_Alberts

Cell_Biology_Alberts. Upon transfer to a mobile carrier in the electron-transport chain, the electron is stabilized as part of a strong electron donor and made available for subsequent reactions. These subsequent reactions require more time to complete, and they result in light-generated energy-rich compounds. a photosystem Consists of an antenna Complex and a reaction Center There are two distinct types of chlorophyll–protein complexes in the photosynthetic membrane. One type, called a photochemical reaction center, contains the special pair of chlorophylls just described. The other type engages exclusively in light absorption and resonance energy transfer and is called an antenna complex. Together, the two types of complex make up a photosystem (Figure 14–45).


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Cell_Biology_Alberts. The role of the antenna complex in the photosystem is to collect the energy of a sufficient number of photons for photosynthesis. Without it, the process would be slow and inefficient, as each reaction-center chlorophyll would absorb only about one light quantum per second, even in broad daylight, whereas hundreds per second are needed for effective photosynthesis. When light excites a chlorophyll molecule in the antenna complex, the energy passes rapidly from one protein-bound chlorophyll to another by resonance energy transfer until it reaches the special pair in the reaction center. The antenna complex is also known as a


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Cell_Biology_Alberts. Figure 14–44 A general scheme for the charge-separation step in a photosynthetic reaction center. In a reaction center, light energy is harnessed to generate electrons that are held at a high energy level by mobile electron carriers in a membrane. Light energy is thereby converted to chemical energy. the process starts when a photon absorbed by the special pair of chlorophylls in the reaction center knocks an electron out of one of the chlorophylls. the electron is taken up by a mobile electron carrier (orange) bound at the opposite membrane surface. a set of intermediary carriers embedded in the reaction center provide the path from the special pair to this carrier (not shown). the physical distance between the positively charged chlorophyll ion and the negatively charged electron carrier stabilizes the charge-separated state for a short time, during which the chlorophyll ion, a strong oxidant, withdraws an electron from a suitable compound (for example, from water, an event we will


Cell_Biology_Alberts

Cell_Biology_Alberts. stabilizes the charge-separated state for a short time, during which the chlorophyll ion, a strong oxidant, withdraws an electron from a suitable compound (for example, from water, an event we will discuss in detail shortly). the electron carrier then diffuses away from the reaction center as a strong electron donor that will transfer its electron to an electron-transport chain.


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Cell_Biology_Alberts. light-harvesting complex, or LHC. In addition to many chlorophyll molecules, an LHC contains orange carotenoid pigments. The carotenoids collect light of a different wavelength from that absorbed by chlorophylls, helping to make the antenna complex more efficient. They also have an important protective role in preventing the formation of harmful oxygen radicals in the photosynthetic membrane. the thylakoid Membrane Contains two Different photosystems Working in Series The excitation energy collected by the antenna complex is delivered to the special pair in the photochemical reaction center. The reaction center is a transmembrane chlorophyll–protein complex that lies at the heart of photosynthesis. It harbors the special pair of chlorophyll molecules, which acts as an irreversible trap for excitation energy (see Figure 14–45).


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Cell_Biology_Alberts. Chloroplasts contain two functionally different although structurally related photosystems, each of which feeds electrons generated by the action of sunlight into an electron-transfer chain. In the chloroplast thylakoid membrane, photo-system I is confined to the unstacked stroma thylakoids, while the stacked grana thylakoids contain photosystem II. The two photosystems were named in order of their discovery, not of their actions in the photosynthetic pathway, and electrons are first activated in photosystem II before being transferred to photosystem I (Figure 14–46). The path of the electron through the two photosystems can be described as a Z-like trajectory and is known as the Z scheme. In the Z scheme, the reaction center of photosystem II first withdraws an electron from water. The electron passes via an electron-transport chain (composed of the electron carrier plastoquinone, the cytochrome b6-f complex, and the protein plastocyanin) to photosystem I, which propels the electron


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Cell_Biology_Alberts. passes via an electron-transport chain (composed of the electron carrier plastoquinone, the cytochrome b6-f complex, and the protein plastocyanin) to photosystem I, which propels the electron across the membrane in a second light-driven charge-separation reaction that leads to NADPH production.


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Cell_Biology_Alberts. Figure 14–45 A photosystem. Each photosystem consists of a reaction center plus a number of light-harvesting antenna complexes. the solar energy for photosynthesis is collected by the antenna complexes, which account for most of the chlorophyll in a plant cell. the energy hops randomly by resonance energy transfer (red arrows) from one chlorophyll molecule to another, until it reaches the reaction center complex, where it ionizes a chlorophyll in the special pair. the chlorophyll special pair holds its electrons at a lower energy than the chlorophyll in the antenna complexes, causing the energy transferred to it from the antenna complex to become trapped there. Note that it is only energy that moves from one chlorophyll molecule to another in the antenna complex, not electrons (Movie 14.10).


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Cell_Biology_Alberts. Figure 14–46 The Z scheme for photosynthesis. the thylakoids of plants and cyanobacteria contain two different photosystems, known as photosystem I and photosystem II, which work in series. Each of the photosystem I and II reaction centers receives excitation energy from its own set of tightly associated antenna complexes, known as LhC-I and LhC-II, by resonance energy transfer. Note that, for historical reasons, the two photosystems were named opposite to the order in which they act, with photosystem II passing its electrons to photosystem I.


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Cell_Biology_Alberts. The Z scheme is necessary to bridge the very large energy gap between water and NADPH (Figure 14–47). A single quantum of visible light does not contain enough energy both to withdraw electrons from water, which holds on to its electrons very tightly (redox potential +820 mV) and therefore is a very poor electron donor, and to force them on to NADP+, which is a very poor electron acceptor (redox potential –320 mV). The Z scheme first evolved in cyanobacteria to enable them to use water as a universally available electron source. Other, simpler photosynthetic bacteria have only one photosystem. As we shall see, they cannot use water as an electron source and must rely on other, more energy-rich substrates instead, from which electrons are more readily withdrawn. The ability to extract electrons from water (and thereby to produce molecular oxygen) was acquired by plants when their ancestors took up the endosymbiotic cyanobacteria that later evolved into chloroplasts (see Figure 1–31).


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Cell_Biology_Alberts. photosystem II Uses a Manganese Cluster to Withdraw Electrons From Water In biology, only photosystem II is able to withdraw electrons from water and to generate molecular oxygen as a waste product. This remarkable specialization of photosystem II is conferred by the unique properties of one of the two chlorophyll molecules of its special pair and by a manganese cluster linked to the protein. These chlorophyll molecules and the manganese cluster form the catalytic core of the photosystem II reaction center, whose mechanism is outlined in Figure 14–48.


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Cell_Biology_Alberts. Water is an inexhaustible source of electrons, but it is also extremely stable; therefore a large amount of energy is required to make it part with its electrons. The only compound in living organisms that is able to achieve this feat after its ionization by light, is the chlorophyll special pair called P680 (P680/P680+ redox potential = +1270 mV). The reaction 2H2O + 4 photons →4H+ + 4e – + O2 is catalyzed by its adjacent manganese cluster. The intermediates remain firmly attached to the manganese cluster until two water molecules have been fully oxidized to O2, thus


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Cell_Biology_Alberts. Figure 14–47 Changes in redox potential during photosynthesis. the redox potential for each molecule is indicated by its position along the vertical axis. photosystem II passes electrons derived from water to photosystem I, which in turn passes them to NaDp+ through ferredoxin-NaDp+ reductase. the net electron flow through the two photosystems is from water to NaDp+, and it produces NaDph as well as an electrochemical proton gradient. this proton gradient is used by the atp synthase to produce atp. Details in this figure will be explained in the subsequent text.


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Cell_Biology_Alberts. ensuring that no dangerous oxygen radicals are released as the reaction proceeds. The protons released by the two water molecules are discharged to the thylakoid space, contributing to the proton gradient across the thylakoid membrane (pH lower in the thylakoid space than in the stroma). The unique protein environment that endows life with this all-important ability to oxidize water has remained essentially unchanged throughout billions of years of evolution (Figure 14–49). All of the oxygen in the Earth’s atmosphere has been generated in this way. Although the exact details of the water-oxidation reaction in photosystem II are still not fully understood, scientists are trying to construct an artificial system that mimics the process. If successful, this might provide a virtually endless supply of clean energy, helping to solve the world’s energy crisis. the Cytochrome b6-f Complex Connects photosystem II to photosystem I


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Cell_Biology_Alberts. the Cytochrome b6-f Complex Connects photosystem II to photosystem I Following the path shown previously in Figure 14–48, the electrons extracted from water by photosystem II are transferred to plastoquinol, a strong electron donor similar to ubiquinol in mitochondria. This quinol, which can diffuse rapidly in the lipid bilayer of the thylakoid membrane, transfers its electrons to the cytochrome b6-f complex, whose structure is homologous to the cytochrome c reductase in mitochondria. The cytochrome b6-f complex pumps H+ into the thylakoid space using the same Q cycle that is utilized in mitochondria (see Figure 14–21), thereby adding to the proton gradient across the thylakoid membrane. The cytochrome b6-f complex forms the connecting link between photosystems II and I in the chloroplast electron-transport chain. It passes its electrons


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Cell_Biology_Alberts. Figure 14–48 The conversion of light energy to chemical energy in the photosystem II complex. (a) Schematic diagram of the photosystem II reaction center, whose special pair of chlorophyll molecules is designated as p680 based on the wavelength of its absorbance maximum (680 nm). (B) Cofactors and pigments at the core of the reaction center. Shown are the manganese (Mn) cluster, the tyrosine side chain that links it to the p680 special pair, four chlorophylls (green), two pheophytins (light blue), two plastoquinones (pink), and an iron atom (red). the path of electrons is shown by blue arrows. In the manganese cluster, four manganese atoms (light blue), one calcium atom (purple), and five oxygen atoms (red) work together to catalyze the oxidation of water. the water-splitting reaction occurs in four successive steps, each requiring the energy of one photon. Each photon turns a p680 reaction-center chlorophyll into a positively charged chlorophyll ion. through an ionized tyrosine side


Cell_Biology_Alberts

Cell_Biology_Alberts. in four successive steps, each requiring the energy of one photon. Each photon turns a p680 reaction-center chlorophyll into a positively charged chlorophyll ion. through an ionized tyrosine side chain (yellow), this chlorophyll ion pulls an electron away from a water molecule bound at the manganese cluster. In this way, a total of four electrons are withdrawn from two water molecules to generate molecular oxygen, which is released into the atmosphere.


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Cell_Biology_Alberts. Each electron that is energized by light passes from the special pair along an electron-transfer chain inside the complex, along the indicated path to the permanently bound plastoquinone Qa and then to plastoquinone QB as electron acceptors. Once QB has picked up two electrons (plus two protons; see Figure 14–17), it dissociates from its binding site in the complex and enters the lipid bilayer as a mobile electron carrier, being immediately replaced by a new, nonreduced molecule of plastoquinone. Note that the chlorophylls and pheophytins form two symmetrical branches of a potential electron-transport chain. Only one branch is active, thus ensuring that the plastoquinones become fully reduced in minimum time.


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Cell_Biology_Alberts. Figure 14–49 The structure of the complete photosystem II complex. this photosystem contains at least 16 protein subunits, along with 36 chlorophylls, two pheophytins, two hemes, and a number of protective carotenoids (colored). Most of these pigments and cofactors are deeply buried, tightly complexed to protein (gray). the path of electrons is indicated by the blue arrows, and is explained in Figure 14–48B. the photosystem II complex presented here is the cyanobacterial complex, which is simpler and more stable than the plant complex, which works in the same way. (pDB code: 3arC.) one at a time to the mobile electron carrier plastocyanin (a small copper-containing protein that takes the place of the cytochrome c in mitochondria), which will transfer them to photosystem I (Figure 14–50). As we discuss next, photosystem I then harnesses a second photon of light to further energize the electrons that it receives.


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Cell_Biology_Alberts. photosystem I Carries Out the Second Charge-Separation Step in the Z Scheme Photosystem I receives electrons from plastocyanin in the thylakoid space and transfers them, via a second charge-separation reaction, to the small protein ferredoxin on the opposite membrane surface (Figure 14–51). Then, in a final step, ferredoxin feeds its electrons to a membrane-associated enzyme complex, the ferredoxin-NADP+ reductase, which uses the electrons to produce NADPH from NADP+ (see Figure 14–50).


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Cell_Biology_Alberts. The redox potential of the NADP+/NADPH pair (–320 mV) is already very low, and reduction of NADP+ therefore requires a compound with an even lower redox potential. This turns out to be a chlorophyll molecule near the stromal membrane surface of photosystem I that has a redox potential of –1000 mV (chlorophyll A0), making it the strongest known electron donor in biology. The reduced NADPH is released into the chloroplast stroma, where it is used for biosynthesis of glyceraldehyde 3-phosphate, amino acid precursors, and fatty acids, much of it to be exported to the cytoplasm. Figure 14–50 Electron flow through the cytochrome b6-f complex to NADPH.


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Cell_Biology_Alberts. Figure 14–50 Electron flow through the cytochrome b6-f complex to NADPH. the cytochrome b6-f complex is the functional equivalent of cytochrome c reductase (the cytochrome b-c1 complex) in mitochondria (see Figure 14–22). Like its mitochondrial homolog, the b6-f complex receives its electrons from a quinone and engages in a complicated Q cycle that pumps two protons across the membrane (details not shown). It hands its electrons, one at a time, to plastocyanin (pC). plastocyanin diffuses along the membrane surface to photosystem I and transfers the electrons via ferredoxin (Fd) to the ferredoxin-NaDp+ reductase (FNr), where they are utilized to produce NaDph. p700 is a special pair of chlorophylls that absorbs light of wavelength 700 nm.


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Cell_Biology_Alberts. Figure 14–51 Structure and function of photosystem I. at the heart of the photosystem I complex assembly is the electron-transfer chain shown. at one end is a special pair of chlorophylls called p700 (because it absorbs light of 700 nm wavelength), receiving electrons from plastocyanin (pC). at the other end are the a0 chlorophylls, which hand the electrons on to ferredoxin via two plastoquinones (pQ; purple) and three iron–sulfur clusters. Even though the roles of photosystems I and II in photosynthesis are very different, their central electron-transfer chains are structurally similar, indicating a common evolutionary origin (see Figure 14–53). Note that in photosystem I both branches of the electron-transfer chain are active, unlike in photosystem II (see Figure 14–48). (pDB code: 3LW5.)


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Cell_Biology_Alberts. Figure 14–52 Summary of electron and proton movements during photosynthesis in the thylakoid membrane. Electrons are withdrawn, through the action of light energy, from a water molecule that is held by the manganese cluster in photosystem II. the electrons pass on to plastoquinone, which delivers them to the cytochrome b6-f complex that resembles the cytochrome c reductase of mitochondria and the b-c complex of bacteria. they are then carried to photosystem I by the soluble electron carrier plastocyanin, the functional equivalent of cytochrome c in mitochondria. From photosystem I they are transferred to ferredoxin-NaDp+ reductase (FNr) by the soluble carrier ferredoxin (Fd; a small protein containing an iron–sulfur center). protons are pumped into the thylakoid space by the cytochrome b6-f complex, in the same way that protons are pumped into mitochondrial cristae by cytochrome c reductase (see Figure 14–21). In addition, the h+ released into the thylakoid space by water oxidation,


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Cell_Biology_Alberts. complex, in the same way that protons are pumped into mitochondrial cristae by cytochrome c reductase (see Figure 14–21). In addition, the h+ released into the thylakoid space by water oxidation, and the h+ consumed during NaDph formation in the stroma, contribute to the generation of the electrochemical h+ gradient across the thylakoid membrane. as illustrated, this gradient drives atp synthesis by an atp synthase that sits in the same membrane (see Figure 14–47).


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Cell_Biology_Alberts. the Chloroplast atp Synthase Uses the proton Gradient Generated by the photosynthetic Light reactions to produce atp The sequence of events that results in light-driven production of ATP and NADPH in chloroplasts and cyanobacteria is summarized in Figure 14–52. Starting with the withdrawal of electrons from water, the light-driven charge-separation steps in photosystems II and I enable the energetically unfavorable (uphill) flow of electrons from water to NADPH (see Figure 14–47). Three small mobile electron carriers—plastoquinone, plastocyanin, and ferredoxin—participate in this process. Together with the electron-driven proton pump of the cytochrome b6-f complex, the photosystems generate a large proton gradient across the thylakoid membrane. The ATP synthase molecules embedded in the thylakoid membranes then harness this proton gradient to produce large amounts of ATP in the chloroplast stroma, mimicking the synthesis of ATP in the mitochondrial matrix.


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Cell_Biology_Alberts. The linear Z scheme for photosynthesis thus far discussed can switch to a circular mode of electron flow through photosystem I and the b6-f complex. Here, the reduced ferredoxin diffuses back to the b6-f complex to reduce plastoquinone, instead of passing its electrons to the ferredoxin-NADP+ reductase enzyme complex. This, in effect, turns photosystem I into a light-driven proton pump, thereby increasing the proton gradient and thus the amount of ATP made by the ATP synthase. An elaborate set of regulatory mechanisms control this switch, which enables the chloroplast to generate either more NADPH (linear mode) or more ATP (circular mode), depending on the metabolic needs of the cell.


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Cell_Biology_Alberts. Evidence for the prokaryotic origins of mitochondria and chloroplasts abounds in their genetic systems, as we will see in the next section. But strong and direct evidence for the evolutionary origins of chloroplasts can also be found in the molecular structures of photosynthetic reaction centers revealed in recent years by crystallography. The positions of the chlorophylls in the special pair and the two branches of the electron-transfer chain are basically the same in photosystem I, photosystem II, and the photochemical reaction centers of photosynthetic bacteria (Movie 14.11). As a result, one can conclude that they all have evolved from a common ancestor. Evidently, the molecular architecture of the photosynthetic reaction center originated only once and has remained essentially unchanged during evolution. By contrast, the less critical antenna systems have evolved in several different ways and are correspondingly diverse in present-day photosynthetic organisms (Figure 14–53).


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Cell_Biology_Alberts. the proton-Motive Force for atp production in Mitochondria and Chloroplasts Is Essentially the Same The proton gradient across the thylakoid membrane depends both on the proton-pumping activity of the cytochrome b6-f complex and on the photosynthetic activity of the two photosystems, which in turn depends on light intensity. In chloroplasts exposed to light, H+ is pumped out of the stroma (pH around 8, similar to the mitochondrial matrix) into the thylakoid space (pH 5–6), creating a gradient of 2–3 pH units across the thylakoid membrane, representing a proton-motive force of about 180 mV. This is very similar to the proton-motive force in respiring mitochondria. However, a membrane potential across the inner mitochondrial membrane makes the largest contribution to the proton-motive force that drives the mitochondrial ATP synthase to make ATP, whereas a H+ gradient predominates for chloroplasts.


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Cell_Biology_Alberts. In contrast to mitochondrial ATP synthase, which forms long rows of dimers along the cristae ridges, the chloroplast ATP synthase is monomeric and located in flat membrane regions (Figure 14–54). Evidently, the H+ gradient across the thylakoid membrane is high enough for ATP synthesis without the need for the elaborate arrangement of ATP synthase seen in mitochondria. The first living cells on Earth may have consumed geochemically produced organic molecules and generated their ATP by fermentation. Because oxygen was not yet present in the atmosphere, such anaerobic fermentation reactions would have dumped organic acids—such as lactic or formic acids, for example—into the Figure 14–53 Evolution of photosynthetic reaction centers. pigments involved in light-harvesting are colored green; those involved in the central photochemical events are colored red.


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Cell_Biology_Alberts. Figure 14–53 Evolution of photosynthetic reaction centers. pigments involved in light-harvesting are colored green; those involved in the central photochemical events are colored red. the primitive photochemical reaction center of purple bacteria contains two related protein subunits, L and M, that bind the pigments involved in the central process of photosynthesis, including a special pair of chlorophyll molecules. Electrons are fed into the excited chlorophylls by a cytochrome. Lh1 is a bacterial antenna complex.


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Cell_Biology_Alberts. photosystem II contains the D1 and D2 proteins, which are homologous to the L and M subunits in (a). the excited p680 chlorophyll in the special pair withdraws electrons from water held by the manganese cluster. LhC-II is the light-harvesting complex that feeds energy into the core antenna proteins. (C) photosystem I contains the psa a and psa B proteins, each of which is equivalent to a fusion of the D1 or D2 protein to a core antenna protein of photosystem II. the loosely bound plastocyanin (pC) feeds electrons into the excited chlorophyll pair. as indicated, in photosystem I, electrons are passed from a bound quinone (Q) through a series of three iron–sulfur centers (red circles). (Modified from K. rhee, E. Morris, J.


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Cell_Biology_Alberts. J. Barber and W. Kühlbrandt, Nature 396:283–286, 1998; and W. Kühlbrandt, Nature 411:896–899, 2001. With permission from Macmillan publishers Ltd.) matrix pH 7.9 intermembrane space pH 7.4 thylakoid space pH 5.5 intermembrane space pH 7.4 environment (see Figure 2–47). Perhaps such acids lowered the pH of the environment, favoring the survival of cells that evolved transmembrane proteins that could pump H+ out of the cytosol, thereby preventing the cell from becoming too acidic (stage 1 in Figure 14–55). One of these pumps may have used the energy available from ATP hydrolysis to eject H+ from the cell; such a proton pump could have been the ancestor of present-day ATP synthases.


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Cell_Biology_Alberts. As the Earth’s supply of geochemically produced nutrients began to dwindle, organisms that could find a way to pump H+ without consuming ATP would have been at an advantage: they could save the small amounts of ATP they derived from the fermentation of increasingly scarce foodstuffs to fuel other important activities. This need to conserve resources might have led to the evolution of electron-transport proteins that allowed cells to use the movement of electrons between molecules of different redox potentials as a source of energy for pumping H+ across the plasma membrane (stage 2 in Figure 14–55). Some of these cells might have used the nonfermentable organic acids that neighboring cells had excreted as waste to provide the electrons needed to feed this electron-transport system. Some present-day bacteria grow on formic acid, for example, using the small amount of redox energy derived from the transfer of electrons from formic acid to fumarate to pump H+.


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Cell_Biology_Alberts. Figure 14–55 How ATP synthesis by chemiosmosis might have evolved in stages. the first stage could have involved the evolution of an atpase that pumped protons out of the cell using the energy of atp hydrolysis. Stage 2 could have involved the evolution of a different proton pump, driven by an electron-transport chain. Stage 3 would then have linked these two systems together to generate a primitive atp synthase that used the protons pumped by the electron-transport chain to synthesize atp. an early bacterium with this final system would have had a selective advantage over bacteria with neither of the systems or only one.


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Cell_Biology_Alberts. Figure 14–54 A comparison of H+ concentrations and the arrangement of ATP synthase in mitochondria and chloroplasts. In both organelles, the ph in the intermembrane space is 7.4, as in the cytoplasm. the ph of the mitochondrial matrix and the ph of the chloroplast stroma are both about 8 (light gray). the ph in the thylakoid space is around 5.5, depending on photosynthetic activity. this results in a high proton-motive force across the thylakoid membrane, consisting largely of the h+ gradient (a high permeability of this membrane to Mg2+ and Cl– ions allows the flow of these ions to dissipate most of the membrane potential).


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Cell_Biology_Alberts. In contrast to chloroplasts, the h+ gradient across the inner mitochondrial membrane is insufficient for atp production, and mitochondria need a membrane potential to bring the proton-motive force to the same level as in chloroplasts. the arrangement of the mitochondrial atp synthase in rows of dimers along the cristae ridges (see Figure 14–32) next to the respiratory-chain proton pumps may help the flow of protons along the membrane surface toward the atp synthase, as the availability of protons is limiting for atp production. In the chloroplast, the atp synthase is distributed randomly in thylakoid membranes. ATP-driven proton pump working in reverse to make ATP


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Cell_Biology_Alberts. ATP-driven proton pump working in reverse to make ATP Eventually, some bacteria would have developed H+-pumping electron-transport systems that were so efficient that they could harvest more redox energy than they needed to maintain their internal pH. Such cells would probably have generated large electrochemical proton gradients, which they could then use to produce ATP. Protons could leak back into the cell through the ATP-driven H+ pumps, essentially running them in reverse so that they synthesized ATP (stage 3 in Figure 14–55). Because such cells would require much less of the dwindling supply of fermentable nutrients, they would have proliferated at the expense of their neighbors. By providing an Inexhaustible Source of reducing power, photosynthetic Bacteria Overcame a Major Evolutionary Obstacle


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Cell_Biology_Alberts. By providing an Inexhaustible Source of reducing power, photosynthetic Bacteria Overcame a Major Evolutionary Obstacle The gradual depletion of nutrients from the environment on the early Earth meant that organisms had to find some alternative source of carbon to make the sugars that serve as the precursors for so many other cell components. Although the CO2 in the atmosphere provides an abundant potential carbon source, to convert it into an organic molecule such as a carbohydrate requires reducing the fixed CO2 with a strong electron donor, such as NADPH, which can generate (CH2O) units from CO2 (see Figure 14–41). Early in cellular evolution, strong reducing agents (electron donors) are thought to have been plentiful. But once an ancestor of ATP synthase began to generate most of the ATP, it would have become imperative for cells to evolve a new way of generating strong reducing agents.


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Cell_Biology_Alberts. A major evolutionary breakthrough in energy metabolism came with the development of photochemical reaction centers that could use the energy of sunlight to produce molecules such as NADPH. It is thought that this occurred early in the process of cellular evolution in the ancestors of the green sulfur bacteria. Present-day green sulfur bacteria use light energy to transfer hydrogen atoms (as an electron plus a proton) from H2S to NADPH, thereby producing the strong reducing power required for carbon fixation. Because the redox potential of H2S is much lower than that of H2O (–230 mV for H2S compared with +820 mV for H2O), one quantum of light absorbed by the single photosystem in these bacteria is sufficient to generate NADPH via a relatively simple photosynthetic electron-transport chain. the photosynthetic Electron-transport Chains of Cyanobacteria produced atmospheric Oxygen and permitted New Life-Forms


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Cell_Biology_Alberts. The next evolutionary step, which is thought to have occurred with the development of the cyanobacteria perhaps 3 billion years ago, was the evolution of organisms capable of using water as the electron source for CO2 reduction. This entailed the evolution of a water-splitting enzyme and also required the addition of a second photosystem, acting in series with the first, to bridge the large gap in redox potential between H2O and NADPH. The biological consequences of this evolutionary step were far-reaching. For the first time, there would have been organisms that could survive on water, CO2, and sunlight (plus a few trace elements). These cells would have been able to spread and evolve in ways denied to the earlier photosynthetic bacteria, which needed H2S or organic acids as a source of electrons. Consequently, large amounts of biologically synthesized, reduced organic materials accumulated and oxygen entered the atmosphere for the first time.


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Cell_Biology_Alberts. Oxygen is highly toxic because the oxidation of biological molecules alters their structure and properties indiscriminately and irreversibly. Most anaerobic bacteria, for example, are rapidly killed when exposed to air. Thus, organisms on the primitive Earth would have had to evolve protective mechanisms against the rising O2 levels in the environment. Late evolutionary arrivals, such as ourselves, have numerous detoxifying mechanisms that protect our cells from the ill effects of oxygen. Even so, an accumulation of oxidative damage to our macromolecules has been postulated to contribute to human aging, as we discuss in the next section.


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Cell_Biology_Alberts. Figure 14–56 Major events during the evolution of living organisms on Earth. With the evolution of the membrane-based process of photosynthesis, organisms were able to make their own organic molecules from CO2 gas. the delay of more than 109 years between the appearance of bacteria that split water and released O2 during photosynthesis and the accumulation of high levels of O2 in the atmosphere is thought to be due to the initial reaction of the oxygen with the abundant ferrous iron (Fe2+) that was dissolved in the early oceans. Only when the ferrous iron was used up would oxygen have started to accumulate in the atmosphere. In response to the rising oxygen levels, nonphotosynthetic oxygen-consuming organisms evolved, and the concentration of oxygen in the atmosphere equilibrated at its present-day level.


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Cell_Biology_Alberts. The increase in atmospheric O2 was very slow at first and would have allowed a gradual evolution of protective devices. For example, the early seas contained large amounts of iron in its reduced, ferrous state (Fe2+), and nearly all the O2 produced by early photosynthetic bacteria would have been used up in oxidizing Fe2+ to ferric Fe3+. This conversion caused the precipitation of huge amounts of stable oxides, and the extensive banded iron formations in sedimentary rocks, beginning about 2.7 billion years ago, help to date the spread of the cyanobacteria. By about 2 billion years ago, the supply of Fe2+ was exhausted, and the deposition of further iron precipitates ceased. Geological evidence reveals how O2 levels in the atmosphere have changed over billions of years, approximating current levels only about 0.5 billion years ago (Figure 14–56).


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Cell_Biology_Alberts. The availability of O2 enabled the rise of bacteria that developed an aerobic metabolism to make their ATP. These organisms could harness the large amount of energy released by breaking down carbohydrates and other reduced organic molecules all the way to CO2 and H2O, as explained when we discussed mitochondria. Components of preexisting electron-transport complexes were modified to produce a cytochrome oxidase, so that the electrons obtained from organic or inorganic substrates could be transported to O2 as the terminal electron acceptor. Some present-day purple photosynthetic bacteria can switch between photosynthesis and respiration depending on the availability of light and O2, with only relatively minor reorganizations of their electron-transport chains.


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Cell_Biology_Alberts. In Figure 14–57, we relate these postulated evolutionary pathways to different types of bacteria. By necessity, evolution is always conservative, taking parts of the old and building on them to create something new. Thus, parts of the electron-transport chains that were derived to service anaerobic bacteria 3–4 billion years ago survive, in altered form, in the mitochondria and chloroplasts of today’s higher eukaryotes. A good example is the overall similarity in structure and function between the cytochrome c reductase that pumps H+ in the central segment of the mitochondrial respiratory chain and the analogous cytochrome b-f complex in the electron-transport chains of both bacteria and chloroplasts, revealing their common evolutionary origin (Figure 14–58).