20.3 Evolution of a Universal Mechanism for ATP Synthesis in Chapter 20 Photosynthesis and Carbohydrate Synthesis in Plants

20.3 Evolution of a Universal Mechanism for ATP Synthesis

The combined activities of the two plant photosystems move electrons from water to ${NADP}^{+}$ $NADP Superscript plus$ , conserving some of the energy of absorbed light as NADPH (Fig. 20-12). Simultaneously, protons are pumped across the thylakoid membrane and energy is conserved as an electrochemical potential. We turn now to the process by which this proton gradient drives the synthesis of ATP, the other energy-conserving product of the light-dependent reactions.

A Proton Gradient Couples Electron Flow and Phosphorylation

Although the energy source and electron carriers in photophosphorylation in chloroplasts differ from those of oxidative phosphorylation in mitochondria, they use essentially the same mechanism to capture the energy of the proton gradient. Electron-transferring molecules in the chain of carriers connecting PSII and PSI are oriented asymmetrically in the thylakoid membrane, so photoinduced electron flow results in the net movement of protons across the membrane, from the stromal side to the thylakoid lumen (Fig. 20-21).

A figure shows how proton and electron circuits interact during photophosphorylation. — FIGURE 20-21 Proton and electron circuits during photophosphorylation. In the linear electron pathway (blue arrows), electrons move from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ through PSII, through the intermediate chain of carriers of the cytochrome $b_{6} f$ $b 6 f$ complex, through PSI, and finally to ${NADP}^{+}$ $NADP Superscript plus$ . In the cyclic pathway, electrons move from PSI back to plastoquinone and cytochrome $b_{6} f$ $b 6 f$ . Protons (red arrows) are pumped into the thylakoid lumen by the flow of electrons through cytochrome $b_{6} f$ $b 6 f$ , and they reenter the stroma through proton channels formed by ${CF}_{o}$ $CF Subscript o$ of ATP synthase. The ${CF}_{1}$ $CF Subscript 1$ subunit catalyzes synthesis of ATP.

FIGURE 20-21 Proton and electron circuits during photophosphorylation. In the linear electron pathway (blue arrows), electrons move from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ through PSII, through the intermediate chain of carriers of the cytochrome $b_{6} f$ $b 6 f$ complex, through PSI, and finally to ${NADP}^{+}$ $NADP Superscript plus$ . In the cyclic pathway, electrons move from PSI back to plastoquinone and cytochrome $b_{6} f$ $b 6 f$ . Protons (red arrows) are pumped into the thylakoid lumen by the flow of electrons through cytochrome $b_{6} f$ $b 6 f$ , and they reenter the stroma through proton channels formed by ${CF}_{o}$ $CF Subscript o$ of ATP synthase. The ${CF}_{1}$ $CF Subscript 1$ subunit catalyzes synthesis of ATP.

A yellow horizontal membrane is above a green lumen (P side) above a second yellow membrane labeled thylakoid membrane. The region below the thylakoid membrane is labeled stroma (N side). P S Roman numeral 2 in the top membrane is a roughly horizontal purple structure with two large rounded protrusions below and two smaller rounded protrusions above. C y t italicized b end italics subscript 6 end subscript italicized f end italics complex is a blue structure with two vertical rectangles joined at the top and joined at the bottom, where they flare to the left and right beneath the membrane. P S I is a pink vertical rectangle with a protrusion to the upper left above the membrane and a small sphere labeled F d attached to the upper right side. A T P synthase has a rectangular portion labeled C F subscript 0 end subscript embedded in the thylakoid membrane with a vertical rectangular piece to its right, a stalk extending down to a rounded portion labeled C F subscript 1 end subscript, and a strand that loops from the bottom of the rounded portion back around along the right side to end adjacent to the vertical rectangular piece in the membrane. A wavy yellowish-orange arrow points from light at the upper left to P S Roman numeral 2 in the top membrane. Red highlighted H 2 nonhighlighted O in the lumen has a blue arrow pointing to M n 4 C a O 5 in P S Roman numeral 2 and a red arrow pointing right to one-half O 2 plus red highlighted 2 H plus. A blue arrow points from M n 4 C a O g to P Q red highlighted H 2 in the upper membrane, from which a blue arrow curves up to P Q, from which an arrow that curves back down to P Q red highlighted H 2 is joined by an arrow pointing down from red highlighted 2 H plus above. Two additional arrows point from P Q red highlighted H 2. A blue arrow points into the left side of the C y t italicized b end italics subscript 6 end subscript italicized f end italics complex, then a blue arrow points from the right side of the complex to a light red circle labeled plastocyanin below in the lumen, then a blue arrow points up to P S Roman numeral 1 above. A wavy yellowish-orange arrow points from light to P S Roman numeral 1. A second blue arrow points from P S Roman numeral 1 to F d at the upper right, which is met by a curved arrow from N A D P plus plus H plus to N A D P H. The other arrow from P Q red highlighted H 2 is a red arrow down to red highlighted 4 H plus. Red arrows point from this red highlighted 4 H plus and the red highlighted 2 H plus previously produced by splitting water through the vertical rectangular piece of A T P synthase, then right across the right-hand strand of A T P synthase to end at H plus in the stroma. A curved arrow from A D P plus P I to an orange oval labeled A T P crosses the left side of the rounded portion of A T P synthase.

The Approximate Stoichiometry of Photophosphorylation Has Been Established

As electrons move from water to ${NADP}^{+}$ $NADP Superscript plus$ in chloroplasts, about 12 protons move from the stroma into the thylakoid lumen per 4 electrons passed (that is, per $O_{2}$ $upper O Subscript 2$ formed). Of these protons, 4 are moved by the oxygen-evolving center, and up to 8 are moved by the cytochrome $b_{6} f$ $b 6 f$ complex. The measurable result is a 1,000-fold difference in $H^{+}$ $upper H Superscript plus$ concentration across the thylakoid membrane $(Δ pH = 3)$ $left-parenthesis normal upper Delta p upper H equals 3 right-parenthesis$ . Recall that the energy stored in a proton gradient (the electrochemical potential) has two components: a proton concentration difference $(Δ pH)$ $left-parenthesis normal upper Delta pH right-parenthesis$ and an electrical potential $(Δ ψ)$ $left-parenthesis normal upper Delta psi right-parenthesis$ due to charge separation. In chloroplasts, $Δ pH$ $normal upper Delta pH$ is the dominant component; counterion movement apparently dissipates most of the electrical potential. In illuminated chloroplasts, the energy stored in the proton gradient per mole of protons is

Δ G = 2.3 R T Δ pH + Z F Δ ψ = - 17 kJ/mol

$normal upper Delta upper G equals 2.3 upper R upper T normal upper Delta p upper H plus upper Z upper F normal upper Delta psi equals negative 17 kJ slash mol$

so the movement of 12 mol of protons across the thylakoid membrane represents conservation of about 200 kJ of energy — enough energy to drive the synthesis of several moles of ATP $(Δ G^{'} ° = 30.5 kJ/mol)$ $left-parenthesis normal upper Delta upper G Superscript prime Baseline degree equals 30.5 kJ slash mol right-parenthesis$ . Experimental measurements yield values of about 3 ATP per $O_{2}$ $upper O Subscript 2$ produced.

At least 8 photons must be absorbed to drive 4 electrons from $2 H_{2} O$ $2 upper H Subscript 2 Baseline upper O$ to 2 NADPH (one photon per electron at each reaction center). The energy in 8 photons of visible light is more than enough for the synthesis of three molecules of ATP.

ATP synthesis is not the only energy-conserving reaction of photosynthesis in plants; the NADPH formed in the final electron transfer is also energetically rich. The overall equation for this linear photophosphorylation is

2 H_{2} O + 8 photons + 2 {NADP}^{+} + \sim 3 {ADP}^{+} + \sim 3 P_{i} \to O_{2} + \sim 3 ATP + 2 NADPH

$2 normal upper H 2 normal upper O plus 8 p h o t o n s plus 2 upper N upper A upper D upper P Superscript plus Baseline plus tilde 3 upper A upper D upper P Superscript plus Baseline plus tilde 3 normal upper P Subscript normal i Baseline right-arrow normal upper O 2 plus tilde 3 ATP plus 2 NADPH$

(20-6)

The ATP Synthase Structure and Mechanism Are Nearly Universal

The enzyme responsible for ATP synthesis in chloroplasts is a large complex with two functional components, ${CF}_{o}$ $CF Subscript o$ and ${CF}_{1}$ $CF Subscript 1$ (C denoting its location in chloroplasts). ${CF}_{o}$ $CF Subscript o$ is a transmembrane proton pore composed of several integral membrane proteins and is homologous to mitochondrial $F_{o}$ $upper F Subscript o$ . ${CF}_{1}$ $CF Subscript 1$ is a peripheral membrane protein complex very similar in subunit composition, structure, and function to mitochondrial $F_{1}$ $upper F Subscript 1$ .

Electron microscopy of sectioned chloroplasts shows ATP synthase complexes as projections on the outside (stromal, or n) surface of thylakoid membranes; these complexes correspond to the ATP synthase complexes that project on the inside (matrix, or n) surface of the inner mitochondrial membrane. Thus, the relationship between the orientation of the ATP synthase and the direction of proton pumping is the same in chloroplasts and mitochondria. In both cases, the $F_{1}$ $upper F Subscript 1$ portion of ATP synthase is located on the more alkaline (n) side of the membrane through which protons flow down their concentration gradient; the direction of proton flow relative to $F_{1}$ $upper F Subscript 1$ is the same in both cases: p to n (Fig. 20-22).

A figure shows the orientation of A T P synthase in a mitochondrion, a chloroplast, and a bacterium to show that it is fixed relative to the proton gradient. — FIGURE 20-22 Orientation of ATP synthase is fixed relative to the proton gradient. Superficially, the direction of proton pumping in chloroplasts may seem to be opposite to that in mitochondria and bacteria. In mitochondria and bacteria, protons are pumped *out of* the organelle or cell, and $F_{1}$ $upper F Subscript 1$ is on the *inside* of the membrane; in chloroplasts, protons are pumped *into* the thylakoid lumen, and ${CF}_{1}$ $CF Subscript 1$ is on the *outside* of the thylakoid membrane. However, exactly the same mechanism of energy conversion (from proton gradient to ATP) occurs in all three cases. ATP is synthesized in the matrix of mitochondria, the stroma of chloroplasts, and the cytosol of bacteria.

FIGURE 20-22 Orientation of ATP synthase is fixed relative to the proton gradient. Superficially, the direction of proton pumping in chloroplasts may seem to be opposite to that in mitochondria and bacteria. In mitochondria and bacteria, protons are pumped *out of* the organelle or cell, and $F_{1}$ $upper F Subscript 1$ is on the *inside* of the membrane; in chloroplasts, protons are pumped *into* the thylakoid lumen, and ${CF}_{1}$ $CF Subscript 1$ is on the *outside* of the thylakoid membrane. However, exactly the same mechanism of energy conversion (from proton gradient to ATP) occurs in all three cases. ATP is synthesized in the matrix of mitochondria, the stroma of chloroplasts, and the cytosol of bacteria.

Three squares are shown. The left-hand square is labeled mitochondrion and has an illustration of a mitochondrion on top. A square shows that a small piece of the inner membrane is magnified below. Beneath, a membrane is shown with the matrix (N side) above and the intermembrane space (P side) below. The parts of the respiratory chain are shown in the membrane as follows: a pink horizontal piece that curves up to a rounded piece above the membrane; a tan vertical piece that joins a rounded piece above the membrane; an elongated rounded piece that narrows as it reaches the top of the membrane, then protrudes horizontally on each side, then becomes wider with an invagination at the top; a green roughly rectangular piece with a small protrusion at the bottom and a depression at the top; and an A T P synthase at the right with a rectangular piece in the membrane and a rounded piece extending up into the matrix. A red arrow points from the matrix down across the membrane between the tan and light blue pieces, representing the second and third components of the respiratory chain, to red highlighted H plus below. A second red arrow points up from red highlighted H plus through the vertical rectangular piece of A T P synthase and right across a strand on its right side to end above the membrane in the matrix. A curved arrow meets the rounded part of A T P synthase and shows A T P production. The middle square is labeled chloroplast and has an illustration of a chloroplast on top. A square shows that a small piece of a thylakoid is magnified below. Beneath, a membrane is shown with the thylakoid lumen (P side) above and the stroma (N side) below. From left to right, the structures embedded in the membrane are as follows: a pink horizontal piece that has a rounded protrusion at its lower right that extends into the stroma; a blue piece with two horizontal halves that join below and that join above before flaring to the left and right above the membrane; a purple horizontal piece with two large rounded protrusions above the membrane and two smaller protrusions below the membrane; and an A T P synthase at the right with a rectangular piece in the membrane and a rounded piece extending down into the stroma. A red arrow points up from the stroma across the left side of the purple horizontal piece, which is the third piece from the left, and across the right-hand flare of the blue second piece from the left, before reaching red highlighted H plus above. A red arrow points from red highlighted H plus in the thylakoid lumen above the membrane through the vertical rectangular right side of A T P synthase and across a strand to its right to end in the stroma. A curved arrow across the lower left of A T P synthase shows that A T P is produced. The third square is labeled bacterium (italicized E. coli end italics). The membrane contains purple ovals along its inner side all around the oval bacterium. A square shows that a small piece of the bottom membrane is magnified below. Beneath, a membrane is shown with the cytosol (N side) above and the intermembrane space (P side) below. A purple horizontal oval is embedded in the membrane to the left and an A T P synthase is on the right with a rectangular piece in the membrane and a rounded piece extending up into the cytosol. A red arrow points from the cytosol through the purple oval to red highlighted H plus in the intermembrane space. A second arrow points up from red highlighted H plus through the vertical rectangular piece of A T P synthase and right across a strand on its right side to end above the membrane in the matrix. A curved arrow meets the rounded part of A T P synthase and shows A T P production.

The mechanism of chloroplast ATP synthase is essentially identical to that of its mitochondrial analog; ADP and $P_{i}$ $upper P Subscript i$ readily condense to form ATP on the enzyme surface, and the release of this enzyme-bound ATP requires a proton-motive force. Rotational catalysis sequentially engages each of the three β subunits of the ATP synthase in ATP synthesis, ATP release, and $ADP + P_{i}$ $upper A upper D upper P plus normal upper P Subscript normal i$ binding (see Figs. 19-26 and 19-27).

The appearance of oxygenic photosynthesis on Earth about 2.5 billion years ago was a crucial event in the evolution of the biosphere. Before that, Earth’s atmosphere was composed of methane, ${CO}_{2}$ $CO Subscript 2$ , and $N_{2}$ $upper N Subscript 2$ . The planet was essentially devoid of molecular oxygen and lacked the ozone layer that protects organisms from solar UV radiation. Oxygenic photosynthesis made available a nearly limitless supply of reducing agent $(H_{2} O)$ $left-parenthesis upper H Subscript 2 Baseline upper O right-parenthesis$ to drive the production of organic compounds by reductive biosynthetic reactions. And mechanisms evolved that allowed organisms to use $O_{2}$ $upper O Subscript 2$ as a terminal electron acceptor in highly energetic electron transfers from organic substrates, employing the energy of oxidation to support metabolism. The complex photosynthetic apparatus of a modern vascular plant is the culmination of a series of evolutionary events, the most recent of which was the acquisition by eukaryotic cells of a cyanobacterial endosymbiont.

The chloroplasts of modern organisms share several properties with mitochondria and originated by the same mechanism that gave rise to mitochondria: endosymbiosis. Like mitochondria, chloroplasts contain their own DNA and protein-synthesizing machinery. Some of the polypeptides of chloroplast proteins are encoded by chloroplast genes and synthesized in the chloroplast; others are encoded by nuclear genes, synthesized outside the chloroplast, and imported (Chapter 27). When plant cells grow and divide, chloroplasts give rise to new chloroplasts by division, during which their DNA is replicated and divided between daughter chloroplasts. The machinery and mechanisms for light capture, electron flow, and ATP synthesis in modern cyanobacteria are similar in many respects to those in plant chloroplasts. These observations led to the now widely accepted hypothesis that the evolutionary progenitors of modern plant cells were primitive eukaryotes that engulfed photosynthetic cyanobacteria and established stable endosymbiotic relationships with them (see Fig. 1-37).

At least half of the photosynthetic activity on Earth now occurs in microorganisms — algae, other photosynthetic eukaryotes, and photosynthetic bacteria. Cyanobacteria have PSII and PSI in tandem, and the PSII has an associated oxygen-evolving activity resembling that of plants. However, the other groups of photosynthetic bacteria have single reaction centers and do not split $H_{2} O$ $upper H Subscript 2 Baseline upper O$ or produce $O_{2}$ $upper O Subscript 2$ . Many are obligate anaerobes and cannot tolerate $O_{2}$ $upper O Subscript 2$ ; they must use some compound other than $H_{2} O$ $upper H Subscript 2 Baseline upper O$ as an electron donor. Some photosynthetic bacteria use inorganic compounds as electron (and hydrogen) donors. For example, green sulfur bacteria use hydrogen sulfide:

2 H_{2} S + {CO}_{2} \overset{light}{\to} ({CH}_{2} O) + H_{2} O + 2 S

$2 normal upper H 2 normal upper S plus upper C upper O 2 right-arrow Overscript l i g h t Endscripts left-parenthesis upper C upper H 2 normal upper O right-parenthesis plus normal upper H 2 normal upper O plus 2 normal upper S$

These bacteria, instead of producing molecular $O_{2}$ $upper O Subscript 2$ , form elemental sulfur as the oxidation product of $H_{2} S$ $upper H Subscript 2 Baseline upper S$ . (They further oxidize the S to ${SO}_{4}^{2 -}$ $SO Subscript 4 Superscript 2 minus$ .) Other photosynthetic bacteria use organic compounds such as lactate as electron donors:

2 Lactate + {CO}_{2} \overset{light}{\to} ({CH}_{2} O) + H_{2} O + 2 pyruvate

$2 upper L a c t a t e plus upper C upper O 2 right-arrow Overscript light Endscripts left-parenthesis upper C upper H 2 normal upper O right-parenthesis plus normal upper H 2 normal upper O plus 2 p y r u v a t e$

The fundamental similarity of photosynthesis in plants and bacteria, despite the differences in the electron donors they employ, becomes more obvious when the equation of photosynthesis is written in the more general form

2 H_{2} D + {CO}_{2} \overset{light}{\to} ({CH}_{2} O) + H_{2} O + 2 D

$2 normal upper H 2 normal upper D plus upper C upper O 2 right-arrow Overscript l i g h t Endscripts left-parenthesis upper C upper H 2 normal upper O right-parenthesis plus normal upper H 2 normal upper O plus 2 normal upper D$

in which $H_{2} D$ $upper H Subscript 2 Baseline upper D$ is an electron (and hydrogen) donor and D is its oxidized form. $H_{2} D$ $upper H Subscript 2 Baseline upper D$ may be water, hydrogen sulfide, lactate, or some other organic compound, depending on the species. Most likely, the bacteria that first developed photosynthetic ability used $H_{2} S$ $upper H Subscript 2 Baseline upper S$ as their electron source.

Modern cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (Fig. 20-23). Three protein components function in both processes, giving evidence that the processes have a common evolutionary origin (Fig. 20-24). First, the proton-pumping cytochrome $b_{6} f$ $b 6 f$ complex carries electrons from plastoquinone to cytochrome $c_{6}$ $c 6$ in photosynthesis, and also carries electrons from ubiquinone to cytochrome $c_{6}$ $c 6$ in oxidative phosphorylation — the role played by cytochrome $b c_{1}$ $b c 1$ in mitochondria. Second, cytochrome $c_{6}$ $c 6$ , homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria; it can also carry electrons from the cytochrome $b_{6} f$ $b 6 f$ complex to PSI — a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome $b_{6} f$ $b 6 f$ complex and the mitochondrial cytochrome $b c_{1}$ $b c 1$ complex, and between cyanobacterial cytochrome $c_{6}$ $c 6$ and plant plastocyanin. The third conserved component is the ATP synthase, which functions in oxidative phosphorylation and photophosphorylation in cyanobacteria, and in the mitochondria and chloroplasts of photosynthetic eukaryotes. The structure and remarkable mechanism of this enzyme have been strongly conserved throughout evolution.

Two micrographs show structures in a cyanobacterium at different levels of magnification. The top micrograph shows an oval bacterium with many concentric membranes around the outside. The oval region inside has gray mottling and rough black spheres. The oval is about 1300 n m in length. The second micrograph shows a close-up of a similar bacterium showing an outer membrane with many concentric layers of membrane beneath above a small mottled region below. The image is about 500 n m in length. — FIGURE 20-23 The photosynthetic membranes of a cyanobacterium. In these thin sections of a cyanobacterium, viewed with a transmission electron microscope, the multiple layers of the internal membranes are seen to fill half the total volume of the cell. The extensive membrane system serves the same role as the thylakoid membranes of vascular plants, providing a large surface area containing all of the photosynthetic machinery. $(Bar = 100 nm .)$ $left-parenthesis upper B a r equals 100 nm period right-parenthesis$ [S. R. Miller et al. Discovery of a free-living chlorophyll d-producing cyanobacterium with a hybrid proteobacterial/cyanobacterial small-subunit rRNA gene. *Proc. Natl. Acad. Sci. USA* 102:850, 2005, Fig. 2. © 2005 National Academy of Sciences.]

FIGURE 20-23 The photosynthetic membranes of a cyanobacterium. In these thin sections of a cyanobacterium, viewed with a transmission electron microscope, the multiple layers of the internal membranes are seen to fill half the total volume of the cell. The extensive membrane system serves the same role as the thylakoid membranes of vascular plants, providing a large surface area containing all of the photosynthetic machinery. $(Bar = 100 nm .)$ $left-parenthesis upper B a r equals 100 nm period right-parenthesis$ [S. R. Miller et al. Discovery of a free-living chlorophyll d-producing cyanobacterium with a hybrid proteobacterial/cyanobacterial small-subunit rRNA gene. *Proc. Natl. Acad. Sci. USA* 102:850, 2005, Fig. 2. © 2005 National Academy of Sciences.]

A two-part figure illustrates evolutionary origins of cytochrome italicized b end italics subscript 6 end subscript italicized f end italics and cytochrome italicized c end italics subscript 6 end subscript by comparing photophosphorylation in part a with oxidative phosphorylation in part b. — FIGURE 20-24 Dual roles of cytochrome $b_{6} f$ $bold-italic b bold 6 bold-italic f$ and cytochrome $c_{6}$ $c 6$ in cyanobacteria reflect evolutionary origins. Cyanobacteria use cytochrome $b_{6} f$ $b 6 f$ , cytochrome $c_{6}$ $c 6$ , and plastoquinone for both oxidative phosphorylation and photophosphorylation. (a) In photophosphorylation, electrons flow (blue arrows) from water to ${NADP}^{+}$ $NADP Superscript plus$ . (b) In oxidative phosphorylation, electrons flow from NADH to $O_{2}$ $upper O Subscript 2$ . Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle.

FIGURE 20-24 Dual roles of cytochrome $b_{6} f$ $bold-italic b bold 6 bold-italic f$ and cytochrome $c_{6}$ $c 6$ in cyanobacteria reflect evolutionary origins. Cyanobacteria use cytochrome $b_{6} f$ $b 6 f$ , cytochrome $c_{6}$ $c 6$ , and plastoquinone for both oxidative phosphorylation and photophosphorylation. (a) In photophosphorylation, electrons flow (blue arrows) from water to ${NADP}^{+}$ $NADP Superscript plus$ . (b) In oxidative phosphorylation, electrons flow from NADH to $O_{2}$ $upper O Subscript 2$ . Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle.

The figure is divided into two vertical halves that share components in the center. Part a, the left half, is labeled photophosphorylation. Part b, the right half, is labeled oxidative phosphorylation. The two halves share a white circle labeled plastoquinone, a blue square labeled C y t italicized b end italics subscript 6 end subscript italicized f end italics, and a gray square labeled C y t italicized c end italics subscript 6 end subscript. Part a begins with a wavy yellowish-orange arrow pointing from light to a purple circle labeled P S Roman numeral 2. A curved arrow from H 2 O to one-half O 2 meets P S Roman numeral 2 at its inflection point, with e minus shown below. The first half of the curved arrow, from H 2 O until it meets P S Roman numeral 2, is blue and then the arrow becomes gray. At the inflection point, a series of blue arrows travels from e minus to a white sphere labeled plastoquinone that extends into the right half of the illustration. A blue arrow points down from plastoquinone to a blue square labeled C y t italicized b end italics subscript 6 end subscript italicized f end italics complex 3, from which a blue arrow curves through a gray circle labeled C y s italicized c end italics subscript 6 end subscript into an orange circle labeled P S Roman numeral 1. A wavy yellowish-orange arrow points from light to P S Roman numeral 1. Beneath the orange circle labeled P S Roma numeral 1, a gray arrow curves from 2 F d subscript ox end subscript, meets the blue arrow continuing down, and continues to curve to reach 2 F d subscript red end subscript on the right. A curved arrow from 2 F d subscript red to 2 F d subscript ox end subscript is blue at first, then meets a curved arrow from N A D P plus to N A D P H plus H plus. The part of the arrow from 2 F d subscript red is blue and the part to 2 F d subscript ox end subscript is gray. The arrow from N A D P plus is gray, then becomes red as it meets the blue arrow from 2 F d subscript red end subscript before continuing to N A D P H plus H plus. Part b begins with a curved arrow from N A D H plus H plus that is blue until it meets an orange square labeled N A D H dehydrogenase complex Roman numeral 1 at a point labeled e minus and becomes gray as it continues up to N A D plus. The blue arrow moves through the orange square to plastoquinone, then through C y t italicized b end italics subscript 6 end subscript italicized f end italics complex 3, then down through C y t italicized c end italics subscript 6 end subscript into a green square labeled C y t italicized a end italics plus italicized a end italics subscript 6 end subscript complex Roman numeral 4. A blue arrow continues down to meet a gray curved arrow from one-half O 2, which becomes blue as it continues to H 2 O.

SUMMARY 20.3 Evolution of a Universal Mechanism for ATP Synthesis

In plants, both the water-splitting reaction and electron flow through the cytochrome $b_{6} f$ $b 6 f$ complex are accompanied by proton pumping across the thylakoid membrane. The proton-motive force thus created drives ATP synthesis by a ${CF}_{o} {CF}_{1}$ $CF Subscript o Baseline CF Subscript 1$ complex similar to the mitochondrial $F_{o} F_{1}$ $upper F Subscript o Baseline upper F Subscript 1$ complex in both structure and catalytic mechanism.
Direct measurements show that eight photons drive the production of one $O_{2}$ $upper O Subscript 2$ from oxidation of two $H_{2} O$ $upper H Subscript 2 Baseline upper O$ , making three ATP molecules.
About 2.5 billion years ago, cyanobacteria appeared on Earth. They had acquired two photosystems — one of the type now found in purple bacteria, the other of the type found in green sulfur bacteria — that operated in tandem, and a water-splitting activity that released oxygen into the atmosphere.
Many photosynthetic microorganisms obtain electrons for photosynthesis not from water but from donors such as $H_{2} S$ $upper H Subscript 2 Baseline upper S$ , forming an oxidized product such as elemental sulfur (not oxygen).
Chloroplasts, like mitochondria, evolved from bacteria living as endosymbionts in early eukaryotic cells. The ATP synthases of bacteria, cyanobacteria, mitochondria, and chloroplasts share a common evolutionary precursor and a common enzymatic mechanism.