20.2 Photochemical Reaction Centers in Chapter 20 Photosynthesis and Carbohydrate Synthesis in Plants

20.2 Photochemical Reaction Centers

Studies on a variety of bacteria that carry out photosynthesis have been helpful in understanding the mechanisms of photosynthesis in cyanobacteria, algae and vascular plants. Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of photosystems. Both systems send electrons through a cytochrome complex that pumps protons, producing the electrochemical gradient that drives ATP synthesis.

Photosynthetic Bacteria Have Two Types of Reaction Center

The type II photosystem in purple bacteria consists of three basic modules (Fig. 20-11a): a single P870 reaction center; a cytochrome $b c_{1}$ $b c 1$ electron-transfer complex similar to Complex III of the mitochondrial electron-transfer chain; and an ATP synthase, also similar to that of mitochondria. Illumination lifts an electron in the reaction center to its excited state (P870*), from which it passes through pheophytin (chlorophyll a lacking its central ${Mg}^{2 +}$ $Mg Superscript 2 plus$ ) and a quinone to the cytochrome $b c_{1}$ $b c 1$ complex. After passing through the $b c_{1}$ $b c 1$ complex, electrons flow through cytochrome $c_{2}$ $c 2$ back to the reaction center, restoring its preillumination state and completing one cycle. This light-driven cyclic electron transfer provides the energy for proton pumping by the cytochrome $b c_{1}$ $b c 1$ complex. Powered by the resulting proton gradient, ATP synthase produces ATP, exactly as in mitochondria.

The type I photosystem in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and includes additional enzymatic reactions (Fig. 20-11b). Excitation by light causes an electron to move from the excited reaction center to the cytochrome $b c_{1}$ $b c 1$ complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. However, in contrast to the cyclic electron transfer path in purple bacteria, some electrons follow a linear electron transfer path from the reaction center to the soluble iron-sulfur protein ferredoxin (see Fig. 19-5), which then passes electrons via ferredoxin: ${NAD}^{+}$ $bold upper N upper A upper D Superscript plus$ reductase to ${NAD}^{+}$ $NAD Superscript plus$ , producing NADH. The electrons taken from the reaction center to reduce ${NAD}^{+}$ $NAD Superscript plus$ are replaced by the oxidation of $H_{2} S$ $upper H Subscript 2 Baseline upper S$ to elemental S in the reaction that defines the green sulfur bacteria. This oxidation of $H_{2} S$ $upper H Subscript 2 Baseline upper S$ by bacteria is chemically analogous to the oxidation of $H_{2} O$ $upper H Subscript 2 Baseline upper O$ by oxygenic plants. Note that the path of electrons in the purple bacteria is cyclic; the path in the green sulfur bacteria can be either cyclic or linear, leading to ${NAD}^{+}$ $NAD Superscript plus$ and producing NADH.

A two-part figure shows the functional modules of the photosynthetic machinery in purple bacteria in part a and in green sulfur bacteria in part b. — FIGURE 20-11 Functional Modules of Photosynthetic Machinery in Purple Bacteria and Green Sulfur Bacteria. The position on the vertical scale of each electron carrier reflects its standard reduction potential. (a) In purple bacteria, light energy excites an electron in the reaction center P870. The electron passes through pheophytin (Pheo), a quinone (Q), and the cytochrome $b c_{1}$ $b c 1$ complex, then through cytochrome $c_{2}$ $c 2$ and thus back to the reaction center. Electron transfer through the cytochrome $b c_{1}$ $b c 1$ complex causes proton pumping, creating an electrochemical potential that powers ATP synthesis. (b) Green sulfur bacteria have two routes for electrons driven by excitation of P840. A cyclic electron transfer route that goes through a quinone to the cytochrome $b c_{1}$ $b c 1$ complex and back to the reaction center via cytochrome $c_{553}$ $c 553$ causes proton pumping. A linear electron transfer route that goes from the reaction center through the iron-sulfur protein ferredoxin (Fd) reduces ${NAD}^{+}$ $NAD Superscript plus$ to NADH in a reaction catalyzed by ferredoxin: ${NAD}^{+}$ $NAD Superscript plus$ reductase.

FIGURE 20-11 Functional Modules of Photosynthetic Machinery in Purple Bacteria and Green Sulfur Bacteria. The position on the vertical scale of each electron carrier reflects its standard reduction potential. (a) In purple bacteria, light energy excites an electron in the reaction center P870. The electron passes through pheophytin (Pheo), a quinone (Q), and the cytochrome $b c_{1}$ $b c 1$ complex, then through cytochrome $c_{2}$ $c 2$ and thus back to the reaction center. Electron transfer through the cytochrome $b c_{1}$ $b c 1$ complex causes proton pumping, creating an electrochemical potential that powers ATP synthesis. (b) Green sulfur bacteria have two routes for electrons driven by excitation of P840. A cyclic electron transfer route that goes through a quinone to the cytochrome $b c_{1}$ $b c 1$ complex and back to the reaction center via cytochrome $c_{553}$ $c 553$ causes proton pumping. A linear electron transfer route that goes from the reaction center through the iron-sulfur protein ferredoxin (Fd) reduces ${NAD}^{+}$ $NAD Superscript plus$ to NADH in a reaction catalyzed by ferredoxin: ${NAD}^{+}$ $NAD Superscript plus$ reductase.

A vertical scale along the left side of the figure is labeled E prime degree symbol (volts) and ranges from 0.5 to negative 1.0, labeled in increments of 0.5. Part a is labeled purple bacteria (pheophytin-quinone type). A wavy red arrow points from light to a purple box labeled P 870 at 0.5 on the scale. A blue arrow that widens as it goes up points from this purple box to a purple box labeled P 870 asterisk with short lines radiating from all sides positioned at just below negative 1.0 on the scale. A blue arrow labeled e minus points from this box to P h e o at just below negative 0.5, from which a blue arrow points to Q at negative 0.2, from which a blue arrow points to a blue box labeled C y t italicized b end italics c subscript 1 end subscript complex at 0. From this box, a red arrow widens as it approaches proton gradient to the lower right and a blue arrow points down to a gray circle labeled C y t italicized c end italics subscript 2 end subscript at 0.4, from which an arrow points back to the purple box labeled P 870. Part b is labeled green sulfur bacteria (F e – S type). A wavy red arrow points from light to a light red box labeled P 840 at 0 on the vertical scale. A blue arrow that widens as it goes up points to a light red box labeled P 840 asterisk with short lines radiating from all sides located at negative 1.0. A short vertical arrow labeled e minus points from this box to F d, from which an arrow points down to a blue box labeled F d: N A D plus reductase at negative 0.8. An arrow curves from N A D plus to the blue box labeled F d: N A D plus reductase to an orange oval labeled N A D H. A second blue arrow labeled e minus points from the light red box labeled P 840 asterisk to Q at negative 0.2, from which a short arrow points to a blue box labeled C y t italicized b end italics c subscript 1 end subscript complex at negative 0.1. From this box, a red arrow widens as it approaches proton gradient to the lower right and a short blue arrow curves down to a gray circle labeled C y t italicized c end italics subscript 553 end subscript at 0.1, from which an arrow points back to the light red box labeled P 840.

In Vascular Plants, Two Reaction Centers Act in Tandem

The photosynthetic apparatus of cyanobacteria, algae, and vascular plants is more complex than the one-center bacterial systems, and it most likely evolved through the combination of two simpler bacterial photosystems. The Z scheme diagram in Figure 20-12 outlines the path of electron flow between the two photosystems and the energy relationships in the light-dependent reactions. (The Z scheme takes its name from the zigzag pattern of the pathways in the diagram.) The thylakoid membranes of chloroplasts have two different kinds of photosystems, each with its own type of photochemical reaction center and set of antenna molecules. The two systems have distinct and complementary functions. Photosystem II (PSII) is a pheophytin-quinone type of system (like the single photosystem of purple bacteria) containing roughly equal amounts of chlorophylls a and b. Excitation of the P680 special pair in its reaction center drives electrons through the cytochrome $b_{6} f$ $b 6 f$ complex discussed below, with concomitant pumping of protons across the thylakoid membrane and ATP synthesis. Photosystem I (PSI) is structurally and functionally related to the photosynthetic machinery of green sulfur bacteria. It has a P700 reaction center and a high ratio of chlorophyll a to chlorophyll b. The excited P700 passes electrons through a linear chain of carriers to ferredoxin, then to ${NADP}^{+}$ $NADP Superscript plus$ , producing NADPH. An alternative pathway for electrons is cyclic: instead of following the linear path that leads to ${NADP}^{+}$ $NADP Superscript plus$ reduction, electrons pass to plastoquinone (PQ) through a membrane-embedded protein complex, cytochrome $b_{6} f$ $bold-italic b bold 6 bold-italic f$ (again, with the movement of protons into the chloroplast lumen). The thylakoid membranes of a single spinach chloroplast have many hundreds of each kind of photosystem.

A figure shows the integration of photosystems Roman numeral 1 and 2 in chloroplasts. — FIGURE 20-12 Integration of photosystems I and II in chloroplasts. This “Z scheme” shows the pathway of linear electron transfer from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ (lower left) to ${NADP}^{+}$ $NADP Superscript plus$ (far right). The position on the vertical scale of each electron carrier reflects its standard reduction potential. To raise the energy of electrons derived from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ to the energy level required to reduce ${NADP}^{+}$ $NADP Superscript plus$ to NADPH, each electron must be “lifted” twice (heavy arrows) by photons absorbed in PSII and PSI. One photon is required per electron in each photosystem. After excitation, the high-energy electrons flow “downhill” through the carrier chains as shown. Protons move across the thylakoid membrane during the water-splitting reaction and during electron transfer through the cytochrome $b_{6} f$ $b 6 f$ complex, producing the proton gradient that is essential to ATP formation. An alternative path of electrons is cyclic electron transfer, in which electrons move from ferredoxin back to the plastoquinone and cytochrome $b_{6} f$ $b 6 f$ complex, instead of reducing ${NADP}^{+}$ $NADP Superscript plus$ to NADPH. The cyclic pathway produces more ATP and less NADPH than the linear pathway.

A vertical scale along the left side of the figure is labeled E prime degree symbol (volts) and ranges from 1.0 to above negative 1.0, labeled in increments of 1.0. Key: P Q subscript A end subscript: Plastiquinone. P Q subscript B end subscript: Second quinone. A subscript 0: Electron acceptor chlorophyll. A subscript 1 end subscript: Phylloquinone. Photosystem Roman numeral 2 is shown in a light blue rectangle on the left that begins with a wavy arrow labeled light pointing to a purple box labeled P 680 at 1.0 on the vertical scale. H 2 O has a curved arrow that meets a blue box labeled O 2 – evolving center, then points to one-half O 2. At the place where this curved arrow meets the blue box, a blue arrow points to e minus, from which another blue arrow points to the purple box labeled P 680. A blue arrow widens as it points upward to a purple box labeled P 680 asterisk with radiating lines extending from all sides located at negative 0.9. A short arrow labeled e minus points from this box to Pheo at negative 0.75, from which an arrow points to P Q subscript A end subscript at negative 0.6, from which a blue arrow points to P Q subscript B end subscript at negative 3.8, from which a blue arrow points out of the blue rectangle representing photosystem Roman numeral 2 to a blue box labeled C y t italicized b end italics subscript 6 end subscript italicized f end italics complex at negative 0.1. A red arrow widens as it points down to text reading, proton gradient. From the box labeled C y t italicized b end italics subscript 6 end subscript italicized f end italics, a blue arrow points to plastocyanin at 0.2, from which a blue arrow extends into a light red rectangle representing photo system Roman numeral 1 to point to a light red box labeled P 700 at 0.3. A wavy arrow labeled light points to this box, from which a blue arrow points upward and widens until it reaches a similar light blue box labeled P 700 asterisk with lines radiating from all sides located at negative 1.45. A short blue arrow labeled e minus points down to A subscript 0 end subscript at negative 1.2, from which a short blue arrow points to A subscript 1 end subscript at negative 1.0, from which a short arrow points down to F e – S at negative 0.9, from which a blue arrow points down to F d at negative 0.75. A long blue arrow labeled cyclic points from F d diagonally down to P Q subscript B end subscript at the lower left in photosystem Roman numeral 2. A short blue arrow points from F d out of the light red rectangle representing photosystem Roman numeral 1 to F d: N A D P plus reductase at negative 0.5, from which a blue arrow points down to N A D P plus at negative 0.35, from which an blue arrow points down to an orange oval labeled N A D P H at 0. The steps from F d: N A D P plus reductase to N A D P H are enclosed in a bracket labeled linear. All data are approximate.

These two photosystems in plants act in tandem to catalyze the light-driven movement of electrons from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ to ${NADP}^{+}$ $NADP Superscript plus$ . The electron carriers include large, integral protein complexes (PSI, PSII, and the proton-pumping complex cytochrome $b_{6} f$ $b 6 f$ ); quinones that are lipid-soluble and move through the membrane between the protein complexes; and two soluble proteins, plastocyanin (analogous to cytochrome c in mitochondria) and ferredoxin.

To replace the electrons that move from PSII through PSI to ${NADP}^{+}, H_{2} O$ $NADP Superscript plus Baseline comma upper H Subscript 2 Baseline upper O$ is oxidized, producing $O_{2}$ $upper O Subscript 2$ (Fig. 20-12, bottom left). All $O_{2}$ $upper O Subscript 2$ -evolving photosynthetic cells — those of plants, algae, and cyanobacteria — contain both PSI and PSII. The Z scheme thus describes the complete route by which electrons flow from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ to ${NADP}^{+}$ $NADP Superscript plus$ , according to the equation

2 H_{2} O + 2 {NADP}^{+} + 8 photons \to O_{2} + 2 NADPH + 2 H^{+}

$2 normal upper H 2 normal upper O plus 2 upper N upper A upper D upper P Superscript plus Baseline plus 8 p h o t o n s right-arrow normal upper O 2 plus 2 upper N upper A upper D upper P upper H plus 2 normal upper H Superscript plus$

For every two photons absorbed (one by each photosystem), one electron is transferred from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ to ${NADP}^{+}$ $NADP Superscript plus$ . To form one molecule of $O_{2}$ $upper O Subscript 2$ , which requires transfer of four electrons from two $H_{2} O$ $upper H Subscript 2 Baseline upper O$ to two ${NADP}^{+}$ $NADP Superscript plus$ , a total of eight photons must be absorbed, four by each photosystem.

Having seen the overall process, we’ll now look at how the structure of the photosystems informs our understanding of the electrochemistry.

Photosystem II

PSII is dimeric (Fig. 20-13). Each monomer is a huge complex of 19 proteins, including the accessory proteins CP47 and CP43, and the core complex of P680 reaction-center proteins D1 and D2; 2 chlorophyll-binding proteins; and associated chromophores, including carotenoids, a nonheme iron, and the critically important inorganic cofactor, ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ . Of the proteins in PSII, 16 have transmembrane segments, but 3 are peripheral proteins on the lumenal side that stabilize the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor. Surrounding PSII are additional chlorophyll-binding proteins and light-harvesting complexes. When a photon is absorbed by any of these antenna molecules, the resulting exciton moves very rapidly from one to another of the antenna chlorophylls until it reaches the reaction center and excites P680, the special pair of chlorophyll a molecules ${(Chl a)}_{2}$ $left-parenthesis Chl a right-parenthesis Subscript 2$ , to initiate the photochemistry.

A figure shows the structure of photosystem Roman numeral 2 of the cyanobacterium italicized Thermosynechococcus vulcanus end italics. — FIGURE 20-13 Structure of photosystem II of the cyanobacterium *Thermosynechococcus vulcanus*. The enormous complex, visualized by x-ray crystallography, is a dimer; each monomer has its own reaction center. Chlorophyll-binding proteins CP43 and CP47 form the core antenna, directly associated with the PSII reaction-center proteins D1 and D2. Each PSII monomer contains 35 chlorophylls, 2 pheophytins, 11 β-carotenes, 2 plastoquinones, and 1 each of b-type cytochrome, c-type cytochrome, and nonheme iron. Water is oxidized to form $O_{2}$ $upper O Subscript 2$ at the oxygen-evolving center $({Mn}_{4} {CaO}_{5})$ $left-parenthesis Mn Subscript 4 Baseline CaO Subscript 5 Baseline right-parenthesis$ . [Data from PDB ID 3WU2, Y. Umena et al., *Nature* 473:55, 2011.]

FIGURE 20-13 Structure of photosystem II of the cyanobacterium *Thermosynechococcus vulcanus*. The enormous complex, visualized by x-ray crystallography, is a dimer; each monomer has its own reaction center. Chlorophyll-binding proteins CP43 and CP47 form the core antenna, directly associated with the PSII reaction-center proteins D1 and D2. Each PSII monomer contains 35 chlorophylls, 2 pheophytins, 11 β-carotenes, 2 plastoquinones, and 1 each of b-type cytochrome, c-type cytochrome, and nonheme iron. Water is oxidized to form $O_{2}$ $upper O Subscript 2$ at the oxygen-evolving center $({Mn}_{4} {CaO}_{5})$ $left-parenthesis Mn Subscript 4 Baseline CaO Subscript 5 Baseline right-parenthesis$ . [Data from PDB ID 3WU2, Y. Umena et al., *Nature* 473:55, 2011.]

A horizontal piece of membrane is shown with the region above labeled lumen (P side) and the region below labeled stroma (N side). A complex structure embedded in the membrane has two halves against a largely white surrounding pieces in the background. The left-hand piece has a roughly vertical purple right-hand part labeled C P 47 that is wider at the bottom. To its left, there is a yellow region labeled D 2 that runs along its right side to meet a small dark purple piece labeled D 1 at the upper left and a small light pink piece labeled C P 43 between C 1 and the top protrusion of the main vertical purple piece. A complex wireframe model runs horizontally in an almost rectangular shape across the center of the lower piece of C P 47 and D 2, where they are embedded in the membrane. A yellow vertical wireframe model runs up along the right side and a purple structure runs in a curved shape across the lower left. The lower part is mostly light green and the upper part is mostly dark green. There is a red sphere at the bottom center. The dark green parts come together in a rectangular shape with a yellow ring at the upper left and a blue ring at the upper right just below a rounded structure in C P 4 7 just above where it emerges from the membrane that consist of alternating purple and red spheres with one green sphere below labeled M n 4 C a O 5. The right-hand piece is similar except that the right-hand piece is pink instead of purple and labeled C P 43, a purple piece of C P 47 is visible above the upper left of C P 43, a large vertical rectangular piece of dark purple D 1 runs along the left side with a small piece of yellow above, the yellow wireframe model is on the left of the green rectangular wireframe piece, and the purple curved wireframe model is on the right of the green rectangular wireframe piece. A small yellow piece of D 2 is visible at the bottom center. A bracket enclosing all of the green wireframe pieces is labeled, chlorophylls.

Excitation of P680 in PSII (Fig. 20-14) produces $P 680 *$ $upper P 680 asterisk$ , an excellent electron donor that, within picoseconds, transfers an electron to pheophytin, giving it a negative charge $(^{•} {Pheo}^{-})$ $left-parenthesis Superscript bullet Baseline zero width space Pheo Superscript minus Baseline right-parenthesis$ . With the loss of its electron, $P 680 *$ $upper P 680 asterisk$ is transformed into a radical cation, $P 680^{+}$ $upper P 680 Superscript plus$ . $^{•} {Pheo}^{-}$ $Superscript bullet Baseline zero width space Pheo Superscript minus$ very rapidly passes its extra electron to a protein-bound plastoquinone, $P Q_{A}$ $bold upper P bold upper Q Subscript bold upper A$ , which in turn passes its electron to another, more loosely bound plastoquinone, ${PQ}_{B}$ $PQ Subscript upper B$ . When ${PQ}_{B}$ $PQ Subscript upper B$ has acquired two electrons in two such transfers from $P Q_{A}$ $upper P upper Q Subscript upper A Baseline$ and two protons from the solvent water, it is in its fully reduced quinol form, ${PQ}_{B} H_{2}$ $PQ Subscript upper B Baseline upper H Subscript 2$ . The overall reaction initiated by light in PSII is

4 P 680 + 4 H^{+} + 2 {PQ}_{B} + 4 photons \to 4 P 680^{+} + 2 {PQ}_{B} H_{2}

$4 upper P 680 plus 4 upper H Superscript plus Baseline plus 2 PQ Subscript upper B Baseline plus 4 p h o t o n s right-arrow 4 upper P 680 Superscript plus Baseline plus 2 PQ Subscript upper B Baseline upper H Subscript 2 Baseline$

(20-1)

Eventually, the electrons in ${PQ}_{B} H_{2}$ $PQ Subscript upper B Baseline upper H Subscript 2$ pass through the cytochrome $b_{6} f$ $b 6 f$ complex (see Fig. 20-12). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below.

A figure shows electron transfer through photosystem Roman numeral 2 of the cyanobacterium italicized Synechcoccus elongatus nonitalicized. — FIGURE 20-14 Electron transfer through photosystem II of the cyanobacterium *Synechococcus elongatus*. The monomeric form of the core complex shown here has two major transmembrane proteins, D1 and D2, each with its set of electron carriers. Although the two subunits are nearly symmetrical, electron transfer occurs through only one of the two branches of electron carriers: that on the right (in D1). The arrows show the path of electron transfer from the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ ion cofactor of the oxygen-evolving complex to plastoquinone ${PQ}_{B}$ $PQ Subscript upper B$ . The photochemical events occur in the sequence indicated by the step numbers. The role of the Tyr residues and the detailed structure of the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor are discussed below (see Fig. 20-20b).

FIGURE 20-14 Electron transfer through photosystem II of the cyanobacterium *Synechococcus elongatus*. The monomeric form of the core complex shown here has two major transmembrane proteins, D1 and D2, each with its set of electron carriers. Although the two subunits are nearly symmetrical, electron transfer occurs through only one of the two branches of electron carriers: that on the right (in D1). The arrows show the path of electron transfer from the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ ion cofactor of the oxygen-evolving complex to plastoquinone ${PQ}_{B}$ $PQ Subscript upper B$ . The photochemical events occur in the sequence indicated by the step numbers. The role of the Tyr residues and the detailed structure of the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor are discussed below (see Fig. 20-20b).

A horizontal piece of phospholipid membrane is shown with the region above labeled lumen (P side) and the region below labeled stroma (N side). Two vertical rectangles are visible embedded in the membrane. Yellow D 2 is on the left and purple D 1 is on the right. At the bottom right of D 2, a yellow, rounded lobe extends across D 1. A purple lobe of D 1 is visible at the lower right of D 2. At the upper right of D 1, a cuboidal structure with projections below and to the right is labeled M n C a O 5. An arrow curves through this structure to show that 2 H 2 O enter and 4 H plus plus O 2 are produced. Two green chlorophyll molecules are present in the center, one in D 2 and one in D 1. Each has another chlorophyll to each side at a different angle. T y r subscript D end subscript in the upper center of E 2 is shown as a six-membered ring with bonds to the upper right and lower left vertices. An arrow labeled 5 points down from M n 4 C a O 5 to T y r subscript Z end subscript, shown as a six-membered ring with bonds to the upper left and lower right vertices. A blue arrow labeled 5 points from T y r subscript Z end subscript to the right-hand chlorophyll in D 1, joined to the left-hand chlorophyll in D 2. These are labeled special pair of chlorophylls and P 680. A blue arrow labeled 1 points from the right-hand chlorophyll of this pair to the chlorophyll to its right, from which a blue arrow labeled 2 points to a blue multi-ring structure labeled Pheo. A blue arrow labeled 3 points from Pheo into the lower right lobe of D 2 below , where P Q subscript A end subscript is visible as a ring with bonds from the left and right side vertices and a chain bonded to the upper left vertex. An arrow labeled 6 points into the lobe of D 1 at the lower right of D 2, past a red circle labeled F e where D 1 and D 2 meet, to P Q subscript B end subscript in this lobe of D 1. At the left and right sides of the overall structure, single chlorophyll molecules are visible.

Photosystem I

PSI and its antenna molecules are part of a supramolecular complex composed of at least 16 proteins, including 4 chlorophyll-binding proteins arranged around the periphery of the reaction center (Fig. 20-15). The complex also includes 35 carotenoids of several types, three 4Fe-4S clusters, and two phylloquinones.

A five-part figure shows the structure of photosystem Roman numeral 1 in the cyanobacterium italicized Synechoccous elongatus nonitalicized by showing P S I in the plane of the thylakoid membrane in part a, perpendicular to the plane of the thylakoid membrane in part b, one of the three large units in P S I in part c, the ligand alone in part d, and the reaction center in part e. — FIGURE 20-15 Structure of photosystem I in the cyanobacterium *Synechococcus elongatus.* PSI is a symmetric trimer, viewed here (a) in the plane of the thylakoid membrane and (b) from the stroma (n side of the membrane). (c) One of the three core complexes in PSI, displayed as the protein without its ligands and (d) the ligands alone. Note the four peripheral light-harvesting complexes (LHC) and the many chlorophyll molecules surrounding the reaction center. (e) Close-up view of the reaction center without the surrounding chlorophylls, showing the chlorophyll special pair, phylloquinones, and Fe-S centers. [Data from PDB ID 1JBO, P. Jordan et al., *Nature* 411:909, 2001; PDB ID 4RKU, Y. Mazor et al.]

Part a shows a horizontal piece of membrane with the region above labeled lumen (P side) and the region below labeled stroma (N side). The central region of the membrane is filled with green wireframe structures mixed with some yellow wireframe structures. Light yellow cylinders are dispersed throughout the membrane, with most running vertically or almost vertically and a few short ones running almost horizontally. Above the membrane, there are dark green cylinders and two dark blue cylinders, then gray cylinders, then horizontal dark blue cylinders above a region with red cylinders within the membrane, then turquoise cylinders, then dark blue cylinders that extend through the membrane. Beneath the central part of this larger structure, there are many light purple cylinders with some interspersed circles made up of orange and yellow spheres. A horizontal line is encircled by an arrow labeled 90 degrees that runs across the front and around the back. Part b shows a roughly circular structure with three lobes: one at the upper left, one at the upper right, and one below. Each lobe contains many green wireframe models. There are red cylinders in the center and between the lobes. Each lobe has a more open region near the outside, then an outer layer. These outer layers contain gray cylinders. Purple cylinders are towards the inside. Turquoise and blue cylinders are toward the outside of the inner part of each lobe. On the outer part of the outer lobe, four regions are circled hat each contain some green structures and several gray cylinders. An arrow points left to a structure labeled core structure that has an oval center made up of many light blue, darker blue, and turquoise helices. There are pink helices in the center and dark blue helices at the bottom center. There are red helices at the top, left side, and right side. Four ovals below form a semicircle below the main structure. Each oval contains gray cylinders and is labeled L H C. From structure b, a second arrow points right to structure d. This is labeled core complex and shows a roughly oval ring of green wireframe structures mixed with a few brown wireframe structures. A line of green across the center with some yellow contains two clusters of yellow and orange spheres labeled reaction center. Four ovals across the bottom form a semicircle. Each is labeled L H C and contains many green wireframe pieces. The left-hand three also contain brown pieces. An arrow points down to part e, labeled reaction center. A yellow vertical wireframe is next to a green vertical wireframe. The yellow wireframe extends above and below a cluster of orange and yellow spheres forming a vertical bar in the center. The yellow wireframe is labeled chlorophyll special pair. The clusters of orange and yellow spheres are labeled F e – S centers. The green wireframe models are labeled phylloquinones.

The photochemical events that follow the excitation of PSI at the reaction-center P700 (Fig. 20-16) are formally similar to those occurring in PSII. The excited reaction-center $P 700 *$ $upper P 700 asterisk$ loses an electron to an acceptor, designated $A_{0}$ $upper A Subscript 0$ (a chlorophyll a molecule, functionally homologous to the pheophytin of PSII), creating $A_{0}^{-}$ $upper A Subscript 0 Superscript minus$ and $P 700^{+}$ $upper P 700 Superscript plus$ . Again, excitation results in charge separation at the photochemical reaction center. $P 700^{+}$ $upper P 700 Superscript plus$ is a strong oxidizing agent, which quickly acquires an electron from plastocyanin, a soluble Cu-containing electron-transfer protein. $A_{0}^{-}$ $upper A Subscript 0 Superscript minus$ is an exceptionally strong reducing agent that passes its electron through a chain of carriers that leads to ${NADP}^{+}$ $NADP Superscript plus$ (Fig. 20-12, right side). Phylloquinone $(Q_{K})$ $left-parenthesis bold upper Q Subscript bold upper K Baseline right-parenthesis$ accepts the electron and passes it to an iron-sulfur protein through three Fe-S centers in PSI. From here, the electron moves to ferredoxin (Fd). Recall that ferredoxin contains a 2Fe-2S center (see Fig. 19-5) that undergoes one-electron oxidation and reduction reactions. The fourth electron carrier in the chain is the flavoprotein ferredoxin: ${NADP}^{+}$ $bold upper N upper A upper D upper P Superscript plus$ reductase, which transfers electrons from reduced ferredoxin $({Fd}_{red} ) to {NADP}^{+}$ $left-parenthesis Fd Subscript red Baseline zero width space right-parenthesis to NADP Superscript plus$ :

2 {Fd}_{red} + 2 H^{+} + {NADP}^{+} \to 2 {Fd}_{ox} + NADPH + H^{+}

$2 Fd Subscript red Baseline plus 2 upper H Superscript plus Baseline plus NADP Superscript plus Baseline right-arrow 2 Fd Subscript ox Baseline plus NADPH plus upper H Superscript plus$

A figure shows the path of electrons through P S I. — FIGURE 20-16 The path of electrons through PSI. The path of electrons (blue arrows) through PSI, viewed in the plane of the membrane. When the reaction-center P700, the special pair of chlorophylls, is excited by a photon or an exciton, its reduction potential is dramatically reduced, making it a good electron donor. P700 then passes an electron through a nearby chlorophyll (referred to as $A_{0}$ $upper A Subscript 0$ ) to phylloquinone $(Q_{K})$ $left-parenthesis upper Q Subscript upper K Baseline right-parenthesis$ . Reduced $Q_{K}$ $upper Q Subscript upper K$ is reoxidized as it passes two electrons, one at a time, to an Fe-S center $(F_{X})$ $left-parenthesis upper F Subscript upper X Baseline right-parenthesis$ near the n side of the membrane. From $F_{X}$ $upper F Subscript upper X$ , electrons move through two more Fe-S centers $(F_{A} and F_{B})$ $left-parenthesis upper F Subscript upper A Baseline and upper F Subscript upper B Baseline right-parenthesis$ to ferredoxin in the stroma. Ferredoxin then donates electrons to ${NADP}^{+}$ $NADP Superscript plus$ (not shown), reducing it to NADPH, one of the forms in which the energy of photons is trapped in chloroplasts.

FIGURE 20-16 The path of electrons through PSI. The path of electrons (blue arrows) through PSI, viewed in the plane of the membrane. When the reaction-center P700, the special pair of chlorophylls, is excited by a photon or an exciton, its reduction potential is dramatically reduced, making it a good electron donor. P700 then passes an electron through a nearby chlorophyll (referred to as $A_{0}$ $upper A Subscript 0$ ) to phylloquinone $(Q_{K})$ $left-parenthesis upper Q Subscript upper K Baseline right-parenthesis$ . Reduced $Q_{K}$ $upper Q Subscript upper K$ is reoxidized as it passes two electrons, one at a time, to an Fe-S center $(F_{X})$ $left-parenthesis upper F Subscript upper X Baseline right-parenthesis$ near the n side of the membrane. From $F_{X}$ $upper F Subscript upper X$ , electrons move through two more Fe-S centers $(F_{A} and F_{B})$ $left-parenthesis upper F Subscript upper A Baseline and upper F Subscript upper B Baseline right-parenthesis$ to ferredoxin in the stroma. Ferredoxin then donates electrons to ${NADP}^{+}$ $NADP Superscript plus$ (not shown), reducing it to NADPH, one of the forms in which the energy of photons is trapped in chloroplasts.

A horizontal piece of phospholipid membrane is shown with the region above labeled lumen (P side) and the region below labeled stroma (N side). Two vertical rectangles are visible embedded in the membrane. Two vertical rectangular pieces are lined up in the center with light blue B on the left and light green A on the right. Each has a protrusion at its center and a blue structure labeled plastocyanin that is somewhat oval but flat on the bottom fits between these protrusions. A green oval labeled P 700 is at the top center where pieces A and B meet. A blue arrow labeled e minus points from plastocyanin to P 700, from which arrows point to the left and right. Both of these arrows point to C h l (A subscript 0 end subscript), from which a blue arrow labeled e minus points to Q subscript K end subscript. Blue arrows point from Q subscript K end subscript on each side to a pair of spheres labeled F subscript X end subscript where parts A and B meet. One sphere is red and one is orange. An arrow points down to the right to a similar pair of spheres, now arranged diagonally with the orange sphere at the upper right, labeled F subscript A end subscript and located in a horizontal pink rectangle labeled C located below the membrane with a rectangular protrusion to the lower right curved where it fits against a blue oval labeled ferredoxin. A blue arrow labeled e minus points from F subscript A end subscript to F subscript B end subscript next to a similar pair of spheres that is now horizontal, then a final blue arrow points to a similar pair of spheres, now diagonal, in ferredoxin below.

The Cytochrome $b_{6} f$ $bold-italic b bold 6 bold-italic f$ Complex Links Photosystems II and I, Conserving the Energy of Electron Transfer

Electrons temporarily held in plastoquinol as a result of the excitation of P680 in PSII are carried to P700 of PSI via the cytochrome $b_{6} f$ $b 6 f$ complex and the soluble protein plastocyanin (see Fig. 20-12, center). With a structure and role analogous with that of Complex III in mitochondria, the cytochrome $b_{6} f$ $b 6 f$ complex (Fig. 20-17) contains a b-type cytochrome with two heme groups (designated $b_{H}$ $b Subscript upper H$ and $b_{L}$ $b Subscript upper L$ ), a Rieske iron-sulfur protein $(M_{r} 20,000)$ $left-parenthesis upper M Subscript r Baseline 20,000 right-parenthesis$ , and cytochrome f (named for the Latin frons, “leaf”). Electrons flow through the cytochrome $b_{6} f$ $b 6 f$ complex from ${PQ}_{B} H_{2}$ $PQ Subscript upper B Baseline upper H Subscript 2$ to cytochrome f, then to plastocyanin, and finally to $P 700^{+}$ $upper P 700 Superscript plus$ , thereby reducing it.

A two-part figure shows electron and proton flow through the cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex with part a showing the positions of four hemes and beta-carotene and part b showing the oxidation of plastiquinone. — FIGURE 20-17 Electron and proton flow through the cytochrome $b_{6} f$ $bold-italic b bold 6 bold-italic f$ complex. (a) In addition to the hemes of cytochrome b (heme $b_{H}$ $b Subscript upper H$ and $b_{L}$ $b Subscript upper L$ ; also called heme $b_{N}$ $b Subscript upper N$ and $b_{P}$ $b Subscript upper P$ , respectively, because of their proximity to the n and p sides of the bilayer) and cytochrome f (heme f), there is a fourth heme (heme x) near heme $b_{H}$ $b Subscript upper H$ ; also present is a β-carotene of unknown function. Two sites bind plastoquinone: the ${PQH}_{2}$ $PQH Subscript 2$ site near the p side of the bilayer, and the PQ site near the n side. The Fe-S center of the Rieske protein lies just outside the bilayer on the p side, and the heme f site is on a protein domain that extends well into the thylakoid lumen. The electron path is shown for just one of the monomers, but both sets of carriers in the dimer carry electrons to plastocyanin. (b) Plastoquinol $({PQH}_{2})$ $left-parenthesis PQH Subscript 2 Baseline right-parenthesis$ , formed in PSII, is oxidized by the cytochrome $b_{6} f$ $b 6 f$ complex in a series of steps like those of the Q cycle in Complex III of mitochondria (see Fig. 19-11). One electron from ${PQH}_{2}$ $PQH Subscript 2$ passes to the Fe-S center of the Rieske protein, the other to heme $b_{L}$ $b Subscript upper L$ of cytochrome $b_{6}$ $b 6$ . The net effect is passage of electrons from ${PQH}_{2}$ $PQH Subscript 2$ to the soluble protein plastocyanin, which carries them to PSI. [Data from PDB ID 1VF5, G. Kurisu et al., *Science* 302:1009, 2003; PDB ID 2Q5B, Y. S. Bukhman-DeRuyter et al.]

FIGURE 20-17 Electron and proton flow through the cytochrome $b_{6} f$ $bold-italic b bold 6 bold-italic f$ complex. (a) In addition to the hemes of cytochrome b (heme $b_{H}$ $b Subscript upper H$ and $b_{L}$ $b Subscript upper L$ ; also called heme $b_{N}$ $b Subscript upper N$ and $b_{P}$ $b Subscript upper P$ , respectively, because of their proximity to the n and p sides of the bilayer) and cytochrome f (heme f), there is a fourth heme (heme x) near heme $b_{H}$ $b Subscript upper H$ ; also present is a β-carotene of unknown function. Two sites bind plastoquinone: the ${PQH}_{2}$ $PQH Subscript 2$ site near the p side of the bilayer, and the PQ site near the n side. The Fe-S center of the Rieske protein lies just outside the bilayer on the p side, and the heme f site is on a protein domain that extends well into the thylakoid lumen. The electron path is shown for just one of the monomers, but both sets of carriers in the dimer carry electrons to plastocyanin. (b) Plastoquinol $({PQH}_{2})$ $left-parenthesis PQH Subscript 2 Baseline right-parenthesis$ , formed in PSII, is oxidized by the cytochrome $b_{6} f$ $b 6 f$ complex in a series of steps like those of the Q cycle in Complex III of mitochondria (see Fig. 19-11). One electron from ${PQH}_{2}$ $PQH Subscript 2$ passes to the Fe-S center of the Rieske protein, the other to heme $b_{L}$ $b Subscript upper L$ of cytochrome $b_{6}$ $b 6$ . The net effect is passage of electrons from ${PQH}_{2}$ $PQH Subscript 2$ to the soluble protein plastocyanin, which carries them to PSI. [Data from PDB ID 1VF5, G. Kurisu et al., *Science* 302:1009, 2003; PDB ID 2Q5B, Y. S. Bukhman-DeRuyter et al.]

Part a shows a horizontal piece of phospholipid membrane with the region above labeled lumen (P side) and the region below labeled stroma (N side). Several complexes are within the membrane in a roughly spherical configuration. A red square complex of spheres angled toward the viewer and labeled heme italicized b end italics subscript H end subscript is at the lower right in front of a pink similar shape oriented almost perpendicularly and labeled heme italicized x end italics. Behind the pink shape, a rounded portion of an orange shape made of spheres becomes a chain labeled beta-carotene that bends right and then left. The middle of this orange chain is labeled e minus. A similar complex with red, pink, and orange structures is on the right but oriented with the orange part on the left, the pink part in front, and the red part in the back. To the upper left, a similar structure has the orange component in the front with the chain extending left, a blue relatively square shape of spheres behind labeled heme italicized b end italics subscript L end subscript, and red spheres visible behind. A similar structure is to the right but has the blue portion in front, red visible at the top, and the chain of orange extending to the right. F e – S centers consisting of two orange spheres and two yellow spheres are to the upper left and right above the structures in the membrane. Above these, there is a red roughly square shape of spheres labeled heme italicized f end italics to the upper left and a similar red shape to the upper right. In the stroma, red highlighted H plus has an arrow pointing to P Q in the membrane between the bottom two red, pink, and orange structures. A blue arrow labeled e minus points from the light blue part of the upper left complex to the red heme italicized b end italics subscript H end subscript in the lower left complex, from which a blue arrow points to P Q. P Q red highlighted H 2 to the left of the upper left orange, light blue, and red complex has a blue arrow that points through the orange chain extending to the left, then splits. A blue arrow points right to the light blue part, a red arrow points up to red highlighted 2 H plus above the membrane, and a second blue arrow labeled e minus points up to the F e – S center to the upper left above the membrane. Another blue arrow labeled e minus points from the F e – S center to the red structure labeled heme italicized f end italics, from which a longer blue arrow labeled e minus points to a dark reddish structure labeled plastocyanin that has a helix on the right, a small coil on the left, strands in the middle and above, and a wireframe model on the left to upper left side adjacent to a yellow sphere. Part b shows a horizontal piece of membrane that separates into upper and lower pieces on the left and is labeled thylakoid. An arrow points down from this box to show a structure below. It has a horizontal piece of phospholipid membrane with the region above labeled lumen (P side) and the region below labeled stroma (N side). A large vertical purple oval labeled subunit Roman numeral 4 extends through the membrane and a smaller green vertical oval overlapping its upper to middle right side is labeled C y t italicized b end italics subscript 6. A large dark pink oval labeled Rieske iron-sulfur protein is angled from above the green oval in the membrane down across the very top of purple subunit Roman numeral 4 to its left, but still above the membrane. This dark pink oval contains a sphere of two orange and two yellow spheres at its bottom left. A darker purple oval structure labeled C y t italicized f end italics extends from behind the Rieske iron-sulfur protein up to the left. A brown sphere labeled plastocyanin is in the opening formed where the purple and pink ovals meet. Red highlighted 2 H plus beneath the membrane has an arrow pointing to a clockwise circle within the membrane from a yellow box labeled P Q on the left to a yellow box labeled P Q red highlighted H 2 on the right. A second arrow curves back from P Q red highlighted H 2 to P Q and this entire circle of reactions is labeled the Q cycle. A blue arrow from the curved arrow from P Q red highlighted H 2 to P Q splits into two arrows labeled e minus. The bottom arrow points to a red diamond labeled italicized b end italics subscript L end subscript inside the green oval, from which a blue arrow points down to a red diamond labeled italicized b end italics subscript H end subscript, from which a blue arrow points to the curved arrow from P Q to P Q red highlighted H 2. The second blue arrow from the Q cycle points to the spherical structure of two orange and two yellow spheres in the pink oval above the membrane, from which a blue arrow points to a red parallelogram in the dark purple oval labeled C y t italicized f end italics and a red arrow from this blue arrow points to red highlighted 4 H plus above the complex embedded in the membrane. Another blue arrow points from the red parallelogram in the dark purple oval labeled C y t italicized f end italics to C u in the brown sphere labeled plastocyanin.

Like Complex III of mitochondria, cytochrome $b_{6} f$ $b 6 f$ conveys electrons from a reduced quinone — a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, ${PQ}_{B}$ $PQ Subscript upper B$ in chloroplasts; P for plastoquinone) — to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts) (Fig 20-17a). As in mitochondria, the function of this complex involves a Q cycle (Fig. 20-17b; see Fig. 19-11) in which electrons pass, one at a time, from ${PQ}_{B} H_{2}$ $PQ Subscript upper B Baseline upper H Subscript 2$ to cytochrome $b_{6}$ $b 6$ . This cycle results in the pumping of protons across the membrane, from the stromal compartment to the thylakoid lumen. Up to four protons enter the lumen for each pair of electrons that passes through the cytochrome $b_{6} f$ $b 6 f$ complex. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration — a powerful driving force for ATP synthesis.

Cyclic Electron Transfer Allows Variation in the Ratio of ATP/NADPH Synthesized

Cyclic electron flow between PSI and cytochrome $b_{6} f$ $b 6 f$ increases the production of ATP relative to NADPH. The linear path of electrons from water, through PSII, cytochrome $b_{6} f$ $b 6 f$ , and PSI to ${NADP}^{+}$ $NADP Superscript plus$ produces both a proton gradient, which is used to drive ATP synthesis, and NADPH, which is used in reductive biosynthetic processes (see Fig. 20-12). Some fraction of electrons passing from $P 700 *$ $upper P 700 asterisk$ to ferredoxin do not continue to ${NADP}^{+}$ $NADP Superscript plus$ , but cycle back through plastoquinone and the cytochrome $b_{6} f$ $b 6 f$ complex to plastocyanin. Plastocyanin then donates electrons to P700. In this way, electrons are repeatedly recycled through the cytochrome $b_{6} f$ $b 6 f$ complex and the reaction center of PSI, each electron propelled around the cycle by the energy of one photon. Cyclic electron flow is not accompanied by net formation of NADPH or evolution of $O_{2}$ $upper O Subscript 2$ . However, it is accompanied by proton pumping by the cytochrome $b_{6} f$ $b 6 f$ complex and by phosphorylation of ADP to ATP, referred to as cyclic photophosphorylation. The overall equation for cyclic electron flow and photophosphorylation is simply

ADP + P_{i} \overset{light}{\to} ATP + H_{2} O

$upper A upper D upper P plus normal upper P Subscript normal i Baseline right-arrow Overscript light Endscripts upper A upper T upper P plus normal upper H 2 upper O$

By regulating the partitioning of electrons between ${NADP}^{+}$ $NADP Superscript plus$ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the ${CO}_{2}$ $CO Subscript 2$ -assimilation reactions and other biosynthetic processes. As we shall see in Section 20.4, the ${CO}_{2}$ $CO Subscript 2$ -assimilation reactions require ATP and NADPH in the ratio 3:2. This regulation of electron-transfer pathways is part of a short-term adaptation to changes in light color (wavelength) and quantity (intensity).

State Transitions Change the Distribution of LHCII between the Two Photosystems

Photosynthetic organisms are exposed to light of highly variable intensity and wavelength in the course of a day or a season, and, although they can alter their growth patterns somewhat, they cannot uproot themselves and move to optimize their light exposure. Instead, cellular mechanisms have evolved that allow plants to accommodate changing light conditions. The energy needed to excite PSI (P700) is less (light of longer wavelength, lower energy) than the energy needed to excite PSII (P680). If PSI and PSII were physically contiguous, excitons originating in the antenna system of PSII would migrate to the reaction center of PSI, leaving PSII chronically underexcited and thus interfering with the operation of the two-center system. This imbalance in the supply of excitons is prevented by physically separating the two photosystems in the thylakoid membrane (Fig. 20-18). PSII is located almost exclusively in the tightly appressed membrane stacks of granal thylakoids; its associated light-harvesting complex (LHCII) mediates the tight association of adjacent membranes in the grana. PSI and the ATP synthase complex are located almost exclusively in the nonappressed membranes of the stromal thylakoids, where they have access to the contents of the stroma, including ADP and ${NADP}^{+}$ $NADP Superscript plus$ . The cytochrome $b_{6} f$ $b 6 f$ complex is present primarily in the granal thylakoids.

A two-part figure shows the localization of P S Roman numeral 1 and P S Roman numeral 2 in thylakoid membranes with part a showing the structures of complexes and soluble proteins of the photosynthetic apparatus of a vascular plant or alga and part b showing light-harvesting complexes and A T P synthase in the granal and stromal thylakoids. — FIGURE 20-18 Localization of PSI and PSII in thylakoid membranes. (a) Structures of the complexes and soluble proteins of the photosynthetic apparatus of a vascular plant or alga, drawn to the same scale. The bovine ATP synthase is shown. (b) Light-harvesting complex LHCII and ATP synthase are located both in appressed regions of the thylakoid membrane (granal thylakoids, in which several membranes are in contact) and in nonappressed regions (stromal thylakoids), and have ready access to ADP and ${NADP}^{+}$ $NADP Superscript plus$ in the stroma. PSII is present almost exclusively in the appressed granal regions, and PSI almost exclusively in nonappressed stromal regions. LHCII is the “adhesive” that holds appressed thylakoid membranes together (see Fig. 20-19). [(a) Data from PSII: PDB ID 3WU2, Y. Umena et al., *Nature* 473:55, 2011; cyt $b_{6} f$ $b 6 f$ complex: PDB ID 2E74, E. Yamashita et al., *J. Mol. Biol.* 370:39, 2007; plastocyanin: PDB ID 1AG6, Y. Xue et al., *Protein Sci.* 7:2099, 1998; PSI: PDB ID 4RKU, Y. Mazor et al.; ferredoxin: PDB ID 1A70, C. Binda et al., *Acta Crystallogr. D Biol. Crystallogr.* 54:1353, 1998; ferredoxin:NADP reductase: PDB ID 1QG0, Z. Deng et al., *Nat. Struct. Biol.* 6:847, 1999; ATP synthase: PDB ID 5ARA, A. Zhou et al., *eLife* 4:e10180, 2015.]

FIGURE 20-18 Localization of PSI and PSII in thylakoid membranes. (a) Structures of the complexes and soluble proteins of the photosynthetic apparatus of a vascular plant or alga, drawn to the same scale. The bovine ATP synthase is shown. (b) Light-harvesting complex LHCII and ATP synthase are located both in appressed regions of the thylakoid membrane (granal thylakoids, in which several membranes are in contact) and in nonappressed regions (stromal thylakoids), and have ready access to ADP and ${NADP}^{+}$ $NADP Superscript plus$ in the stroma. PSII is present almost exclusively in the appressed granal regions, and PSI almost exclusively in nonappressed stromal regions. LHCII is the “adhesive” that holds appressed thylakoid membranes together (see Fig. 20-19). [(a) Data from PSII: PDB ID 3WU2, Y. Umena et al., *Nature* 473:55, 2011; cyt $b_{6} f$ $b 6 f$ complex: PDB ID 2E74, E. Yamashita et al., *J. Mol. Biol.* 370:39, 2007; plastocyanin: PDB ID 1AG6, Y. Xue et al., *Protein Sci.* 7:2099, 1998; PSI: PDB ID 4RKU, Y. Mazor et al.; ferredoxin: PDB ID 1A70, C. Binda et al., *Acta Crystallogr. D Biol. Crystallogr.* 54:1353, 1998; ferredoxin:NADP reductase: PDB ID 1QG0, Z. Deng et al., *Nat. Struct. Biol.* 6:847, 1999; ATP synthase: PDB ID 5ARA, A. Zhou et al., *eLife* 4:e10180, 2015.]

Part a shows a horizontal piece of phospholipid membrane with the region above labeled lumen (P side) and the region below labeled stroma (N side). Photosystem Roman numeral 2 is shown on the left as two roughly rectangular complexes of many approximately vertical helices that cross the membrane. The left-hand portion has white, then pink, then blue, then white helices before the open region between the two halves. A complex green wireframe structure is throughout. Many strands are beneath he membrane and there are some helices above the membrane. Some of the pink helices extend up above the membrane, but many white helices are present in various orientations to the left, across the top above the pink helices, and above the center region between the two halves. The right half is similar except that it contains purple helices instead of pink helices. The white helices above each half come close together and meet above and below the open region between them. To the right, the cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex has two roughly rectangular pieces of vertical or almost vertical purple helices joined together by a horizontal purple helix across the bottom and with many complex purple helices and strands extending up above the membrane. Plastocyanin is shown as a small roughly circular structure above the membrane with a series of strands connected by threads with a yellow sphere at the bottom. Photosystem Roman numeral 1 is a roughly rectangular vertical structure with many almost vertical helices and a dense network of green wireframe structures. It has red helices at the left side, then some blue helices, then red helices, then a w hite helix, then more white helices from the middle to the right side. Some dark blue helices are visible above and below the membrane to the right of the center. A region of light purple strands extends beneath the center and contains two roughly spherical structures consisting of orange and yellow spheres. To the right, A T P synthase has a roughly rectangular portion within the membrane with brown vertical helices to the left and a larger portion of orange and yellow helices to the right. The orange helices each have a purple sphere near the middle. Just below the membrane, there is a region of blue helices, then a region of green helices. Next, there is a wide region that is spherical overall and has black helices on the right, purple helices in the center, and gray helices to the left with green helices from below visible in the center. There is a small sphere of orange helices above that join to long yellow helices and a few shorter yellow helices that run along the left side back into the membrane. Ferredoxin is shown as silver strands with a sphere at the right side consisting of two orange and two yellow spheres. Ferredoxin: N A D P plus oxidoreductase is a roughly spherical structure with brown helices on the left, brown almost vertical strands in the center, more helices to the right, and diagonal strands at the upper right. Part b shows a structure on the right with membrane above and below and a green lumen in between. To the left, the lumen widens and the structure separates into two horizontal bars. A third horizontal bar is visible beneath them. The region above the structure is labeled stroma. The membranes contain the following components. Photosystem Roman numeral 1: Red rectangles with rounded protrusions extending into the stroma. A T P synthase: An orange structure that has a square portion in the membrane next to a vertical rectangular portion with a stalk that extends into the stroma to a rounded portion from which a strand runs back down along the side to end adjacent to the vertical rectangle. Cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex: A blue structure with two roughly rectangular halves that are joined one end, open in the center, and then joined again at the other end with two sides that flare to the left and right within the lumen. Light harvesting complex Roman numeral 2: A green structure consisting of three roughly rectangular components that each have protrusions into the stroma. Photosystem Roman numeral 2: A purple horizontal piece within the membrane that has two rounded protrusions into the lumen. The single piece on the right with no other bars above or below is labeled nonappressed membranes (stromal thylakoids). This region contains alternating photosystem Roman numeral 1 and A T P synthase structures. Along the top membrane of the top left-hand bar, there is a photosystem Roman numeral 1 on the right, then a light harvesting complex Roman numeral 2, then a cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex. Where the membrane bends down to form the left end of the bar, there is an A T P synthase with the rounded portion extending out into the stroma. The bottom membrane of this top bar and the top membrane of the bar below are labeled appressed membranes (granal thylakoids). The bottom membrane of the top bar contains a cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex above a similar complex in the membrane below, then a light harvesting complex Roman numeral 2, then an adjacent photosystem Roman numeral 2, then an adjacent cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex. The top membrane of the bar beneath has a cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex, then an adjacent photosystem Roman numeral 2, then an adjacent cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex. Next, there is the cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex beneath a similar complex in the membrane above. The remaining part of the membrane before it curves up to join the membrane above does not have embedded structures. There is a cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex on the left end of the second bar. The bottom membrane of the second bar has a cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex adjacent to photosystem Roman numeral 2, adjacent to another cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex., then a space, then a cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex at its right end before a photosystem Roman numeral 1 just before the start of the nonappressed membranes to the right. The upper membrane of the bottom bar has cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex, then a space beneath the structures in the membrane above, then cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex, photosystem Roman numeral 2, and cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex beneath a piece of membrane above with no embedded structures. The right end of the bottom bar has an A T P synthase. The bottom membrane of the bottom bar has photosystem Roman numeral 1 at the right, cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex, light-harvesting complex Roman numeral 2, cytochrome italicized b end italics subscript 6 end subscript italicized f end italics complex, light-harvesting compound Roman numeral 2, then photosystem Roman numeral 1.

The association of LHCII with PSI and PSII depends on light intensity and wavelength, which can change in the short term and lead to state transitions in the chloroplast. In state 1, LHCII, PSII, and PSI are poised to maximize the capture of light energy. A critical Thr residue in LHCII is unphosphorylated, and LHCII associates with PSII. Under conditions of intense or blue light, which favor absorption by PSII, that photosystem reduces plastoquinone to plastoquinol $({PQH}_{2})$ $left-parenthesis PQH Subscript 2 Baseline right-parenthesis$ faster than PSI can oxidize it. The resulting accumulation of ${PQH}_{2}$ $PQH Subscript 2$ activates a protein kinase that triggers the transition to state 2 by phosphorylating a Thr residue on LHCII (Fig. 20-19). Phosphorylation weakens the interaction of LHCII with the appressed membrane and with PSII; some LHCII dissociates and moves to the stromal thylakoids. Here it captures photons (excitons) for PSI, speeding the oxidation of ${PQH}_{2}$ $PQH Subscript 2$ and reversing the imbalance between electron flow in PSI and PSII. In less intense light (in the shade, with more red light), PSI oxidizes ${PQH}_{2}$ $PQH Subscript 2$ faster than PSII can make it, and the resulting increase in [PQ] triggers dephosphorylation of LHCII, reversing the effect of phosphorylation. The state transition in LHCII localization and the transition from cyclic to linear electron transfer are coordinately regulated: the path of electrons is primarily linear in state 1 and primarily cyclic in state 2.

A figure illustrates the role of state transitions in electron transfer between photosystem Roman numeral 1 and photosystem Roman numeral 2. — FIGURE 20-19 Electron transfer in PSI and PSII is balanced through state transitions. In granal thylakoids, a hydrophobic domain of LHCII in one membrane inserts into the neighboring membrane and closely appresses the two (state 1). Accumulation of plastoquinol (not shown) stimulates a protein kinase that phosphorylates a Thr residue in the hydrophobic domain of LHCII, which reduces its affinity for the neighboring membrane and converts appressed granal thylakoids to nonappressed stromal thylakoids (state 2). A specific protein phosphatase reverses this regulatory phosphorylation when the $[PQ] / [{PQH}_{2}]$ $left-bracket PQ right-bracket slash left-bracket PQH Subscript 2 Baseline right-bracket$ ratio increases.

FIGURE 20-19 Electron transfer in PSI and PSII is balanced through state transitions. In granal thylakoids, a hydrophobic domain of LHCII in one membrane inserts into the neighboring membrane and closely appresses the two (state 1). Accumulation of plastoquinol (not shown) stimulates a protein kinase that phosphorylates a Thr residue in the hydrophobic domain of LHCII, which reduces its affinity for the neighboring membrane and converts appressed granal thylakoids to nonappressed stromal thylakoids (state 2). A specific protein phosphatase reverses this regulatory phosphorylation when the $[PQ] / [{PQH}_{2}]$ $left-bracket PQ right-bracket slash left-bracket PQH Subscript 2 Baseline right-bracket$ ratio increases.

The left half of the illustration is labeled appressed (state 1). Two horizontal pieces of thylakoid membrane are shown, one above the other. The top membrane contains three roughly rectangular vertical green structures that each have a protrusion extending down to the membrane below. The center structure overlaps the others and is labeled L H C Roman numeral 2. Each protrusion is attached to a diagonal chain of T h r bonded to O H extending through the membrane below. A right-pointing arrow joined by a curved arrow showing the addition of A T P and loss of A D P is labeled protein kinase. A left-pointing arrow with a curved arrow to show the loss of P subscript I end subscript is labeled protein phosphatase. The right-half of the illustration, to the right of these arrows, is labeled nonappressed (state 2). The two horizontal membranes are farther apart. The protrusions are gone from the green vertical rectangles. Each green rectangle has a diagonal bond to T h r bonded to P in a red circle extending into the space above the bottom membrane.

When light is so intense that the combined activity of PSII and PSI cannot synthesize ATP and NADPH fast enough to keep up with the supply of photons, carotenoids in LHCII absorb the excitons and very rapidly quench the excited chlorophyll before it can create damaging reactive oxygen species (ROS). The trigger for switching from an efficient light-harvesting state to an energy-dissipating state is the lowering of pH in the lumenal space, but the detailed mechanism for this transition is not yet known.

Water Is Split at the Oxygen-Evolving Center

The ultimate source of the electrons passed to NADPH in plant (oxygenic) photosynthesis is water. Having given up an electron to pheophytin, $P 680^{+}$ $upper P 680 Superscript plus$ (of PSII) must acquire an electron to return to its ground state in preparation for capture of another photon. In principle, the required electron might come from any number of organic or inorganic compounds. Photosynthetic bacteria use a variety of electron donors for this purpose — acetate, succinate, malate, or sulfide — depending on what is available in a particular ecological niche. About 2.5 billion years ago, evolution of primitive photosynthetic bacteria (progenitors of the modern cyanobacteria) produced a photosystem capable of taking electrons from a donor that is always available: water. Two water molecules are oxidized, yielding four electrons, four protons, and molecular oxygen:

2 H_{2} O \to 4 H^{+} + 4 e^{-} + O_{2}

$2 normal upper H 2 normal upper O right-arrow 4 normal upper H Superscript plus Baseline plus 4 e Superscript minus Baseline plus normal upper O 2$

A single photon of visible light does not have enough energy to break the bonds in water; four photons are required in this photolytic cleavage reaction.

The four electrons abstracted from water do not pass directly to $P 680^{+}$ $upper P 680 Superscript plus$ , which can accept only one electron at a time. Instead, a remarkable molecular device, the oxygen-evolving center, passes four electrons one at a time to $upper P 680 Superscript plus$ $upper P 680 Superscript plus$ (Fig. 20-20a). The immediate electron donor to $P 680^{+}$ $upper P 680 Superscript plus$ is a Tyr residue (sometimes designated Z or ${Tyr}_{Z}$ $Tyr Subscript upper Z$ ) in subunit D1 of the PSII reaction center. The Tyr residue loses both a proton and an electron, generating the electrically neutral Tyr free radical, $^{•} Tyr$ $Superscript bullet Baseline zero width space Tyr$ :

4 P 680^{+} + 4 Tyr \to 4 P 680 + 4^{•} Tyr

$4 upper P 680 Superscript plus Baseline plus 4 upper T y r right-arrow 4 upper P 680 plus 4 Superscript bullet Baseline Tyr$

(20-2)

The Tyr radical regains its missing electron and proton by oxidizing a cofactor of four manganese ions and one calcium ion in the oxygen-evolving center. With each single-electron transfer, the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor becomes more oxidized; four single-electron transfers, each corresponding to the absorption of one photon, produce a charge of $4 +$ $4 plus$ on the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor (Fig. 20-20a):

4^{•} Tyr + {[{Mn}_{4} {CaO}_{5}]}^{0} \to 4 Tyr + {[{Mn}_{4} {CaO}_{5}]}^{4 +}

$4 Superscript bullet Baseline upper T y r plus left-bracket Mn Subscript 4 Baseline CaO Subscript 5 Baseline right-bracket Superscript 0 Baseline right-arrow 4 upper T y r plus left-bracket Mn Subscript 4 Baseline CaO Subscript 5 Baseline right-bracket Superscript 4 plus$

(20-3)

In this state, the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor can take four electrons from a pair of water molecules, releasing four $H^{+}$ $upper H Superscript plus$ and $O_{2}$ $upper O Subscript 2$ :

{[{Mn}_{4} {CaO}_{5}]}^{4 +} + 2 H_{2} O \to {[{Mn}_{4} {CaO}_{5}]}^{0} + 4 H^{+} + O_{2}

$left-bracket Mn Subscript 4 Baseline CaO Subscript 5 Baseline right-bracket Superscript 4 plus Baseline plus 2 upper H Subscript 2 Baseline normal upper O right-arrow left-bracket Mn Subscript 4 Baseline CaO Subscript 5 Baseline right-bracket Superscript 0 Baseline plus 4 upper H Superscript plus Baseline plus upper O Subscript 2$

(20-4)

Because the four protons produced in this reaction are released into the thylakoid lumen, the oxygen-evolving center acts as a proton pump, driven by electron transfer.

We saw in Equation 20-1 that the overall reaction initiated by light in PSII is

4 P 680 + 4 H^{+} + 2 {PQ}_{B} + 4 photons \to 4 P 680^{+} + 2 {PQ}_{B} H_{2}

$4 upper P 680 plus 4 upper H Superscript plus Baseline plus 2 upper P upper Q Subscript upper B Baseline plus 4 p h o t o n s right-arrow 4 upper P 680 Superscript plus Baseline plus 2 PQ Subscript upper B Baseline upper H Subscript 2 Baseline$

The sum of Equations 20-1 through 20-4 is

2 H_{2} O + 2 {PQ}_{B} + 4 photons \to O_{2} + 2 {PQ}_{B} H_{2}

$2 normal upper H 2 normal upper O plus 2 upper P upper Q Subscript normal upper B Baseline plus 4 p h o t o n s right-arrow normal upper O 2 plus 2 PQ Subscript upper B Baseline upper H Subscript 2 Baseline$

(20-5)

A two-part figure shows the water-splitting activity of the oxygen-evolving center by showing the process that produces a four-electron oxidizing agent in the oxygen-evolving center in part a and the center of the oxygen-evolving center in part b. — FIGURE 20-20 Water-splitting activity of the oxygen-evolving center. (a) The process that produces a four-electron oxidizing agent — a multinuclear center with four Mn ions, one Ca ion, and five oxygen atoms — in the oxygen-evolving center of PSII. The sequential absorption of four photons (excitons), each absorption causing the loss of one electron from the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor, produces an oxidizing agent that can remove four electrons from two molecules of water, producing $O_{2}$ $upper O Subscript 2$ . The electrons lost from the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor pass one at a time to an oxidized Tyr residue in a PSII protein, then to $P 680^{+}$ $upper P 680 Superscript plus$ . (b) The chair-shaped metallic center of the oxygen-evolving center. ${Tyr}^{161}$ $Tyr Superscript 161$ , known to participate in the oxidation of water, is seen hydrogen-bonded to a network of water molecules, including several directly in contact with the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor. This is the site of one of the most important reactions in the biosphere. [(b) Data from PDB ID 3WU2, Y. Umena et al., *Nature* 473:55, 2011.]

Part a begins with 2 H 2 O to the left of a dashed vertical line with an arrow pointing right to a cuboidal structure within brackets next to the word lumen. There is a superscript 0 on the outside of the brackets and the structure inside the brackets is labeled M n 4 C a O 5. The cube has a bright purple sphere at the left upper rear vertex bonded to two gray spheres, a red sphere at the right upper rear vertex bonded to a purple sphere above further bonded to two gray spheres and to a red sphere below further bonded to a purple sphere at the right front upper vertex of the cube, which is further bonded to a red sphere at the left front upper vertex. The bottom of the cube has spheres of the following colors: red at the lower left rear vertex, purple at the lower right rear vertex, red at the lower right front vertex, and purple at the lower left front vertex. A blue arrow labeled e minus points up to an orange six-membered ring with bonds from its upper left and lower right vertices labeled T y r subscript Z end subscript. A blue arrow labeled e minus points up to a pair of two central chlorophyll molecules, shown as molecules with a relatively square center with a dot in the center with four pentagons around the outside and joined by a shared pentagon in the middle center. Each of these chlorophylls is joined with a second chlorophyll that extends diagonally up away from the center. Text below reads, P 680 plus. A red arrow pointing between the two central chlorophylls reads, first exciton. An arrow points right from M n 4 C a O 5 below with a curved red arrow pointing down to red highlighted H plus below before crossing a vertical dashed line to indicate another cube in brackets with superscript 1 plus at the upper right. A blue arrow labeled e minus points up to T y r subscript Z end subscript, from which a blue arrow labeled e minus points up to a similar complex of chlorophylls with a red arrow labeled second exciton pointing down between them. An arrow points right from M n 4 C a O 5 below with a curved red arrow pointing down to red highlighted H plus below before crossing a vertical dashed line to indicate another cube in brackets with superscript 2 plus at the upper right. A blue arrow labeled e minus points up to T y r subscript Z end subscript, from which a blue arrow labeled e minus points up to a similar complex of chlorophylls with a red arrow labeled third exciton pointing down between them. An arrow points right from M n 4 C a O 5 below with a curved red arrow pointing down to red highlighted H plus below before crossing a vertical dashed line to indicate another cube in brackets with superscript 3 plus at the upper right. A blue arrow labeled e minus points up to T y r subscript Z end subscript, from which a blue arrow labeled e minus points up to a similar complex of chlorophylls with a red arrow labeled fourth exciton pointing down between them. An arrow points right from M n 4 C a O 5 below with a curved red arrow pointing down to red highlighted H plus below before crossing a vertical dashed line to indicate another cube in brackets with superscript 4 plus at the upper right. T y r subscript Z end subscript is shown above beneath a similar complex of chlorophylls, from which an arrow points back to the left across all of the steps to end to the left of the first set of chlorophylls. An arrow points right from the final M n 4 C a O 5 across a final vertical dashed line to O 2. Part b shows the structure of a cuboidal molecule in which purple spheres represent M n, red spheres represent O, and a bright purple sphere represents C a. The cube has a purple sphere at the left upper rear vertex bonded to two gray spheres, a red sphere at the right upper rear vertex bonded to a purple sphere above further bonded to two gray spheres and to a red sphere below further bonded to a purple sphere at the right front upper vertex of the cube, which is further bonded to a red sphere at the left front upper vertex. The bottom of the cube has spheres of the following colors: red at the lower left rear vertex, purple at the lower right rear vertex, red at the lower right front vertex, and purple at the lower left front vertex. Small blue spheres are labeled waters. Lines made up of horizontal bars join each light blue sphere to nearby light blue spheres and to a red tip at the end of a wireframe of T y r superscript 161, shown as a ring with a bottom vertex that bonds to a piece that ends in red and a top vertex that bonds to a chain. A line of horizontal bars joins this red tip to a blue vertex of H I s superscript 190 end superscript, shown as a ring with a blue vertex at the right side, a single white vertex, then a blue vertex at the lower left followed by a left side vertex bonded to a chain.

The oxygen-evolving cofactor takes the shape of a chair (Fig. 20-20b). The seat and legs of the chair are made up of three Mn ions, one Ca ion, and four O atoms; the fourth Mn and another O form the back of the chair. Four water molecules are also seen in the crystal structure, two associated with one of the Mn ions, the other two with the Ca ion. It may be one (or more) of these water molecules that undergoes oxidation to produce $O_{2}$ $upper O Subscript 2$ . This metal cofactor is associated with several peripheral membrane proteins on the lumenal side of the thylakoid membrane that are believed to stabilize the cofactor. The Tyr residue designated Z, through which electrons move between water and the PSII reaction center, is connected with a network of hydrogen-bonded water molecules that includes the four associated with the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor. The detailed mechanism of water oxidation by the ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor is not known but is under intense investigation. The reaction is central to life on Earth and may involve novel bioinorganic chemistry. Determination of the structure of the polymetallic center has inspired several reasonable and testable hypotheses. Stay tuned.

SUMMARY 20.2 Photochemical Reaction Centers

Bacteria have a single photochemical reaction center. Purple bacteria have a type II photosystem where electrons from an excited special pair of chlorophyll molecules $(P 870 *)$ $left-parenthesis upper P 870 asterisk right-parenthesis$ flow through pheophytin, quinones, and a proton-pumping cytochrome complex, back to the special pair of chlorophylls. Green sulfur bacteria have a type I photosystem that can send electrons through a similar cyclic path or through a linear path that reduces ${NAD}^{+}$ $NAD Superscript plus$ to NADH.
In cyanobacteria, algae, and plants, two different reaction centers are arranged in tandem. In the reaction center of PSII, when the special pair of chlorophylls (P680) is excited by light, it passes electrons to plastoquinone, and the electrons lost from P680 are replaced by electrons from $H_{2} O$ $upper H Subscript 2 Baseline upper O$ . PSI passes electrons from the excited special pair $(P 700 *)$ $left-parenthesis upper P 700 asterisk right-parenthesis$ in its reaction center through a series of carriers to ferredoxin, which then reduces ${NADP}^{+}$ $NADP Superscript plus$ to NADPH.
Electron flow from either photosystem through the cytochrome $b_{6} f$ $b 6 f$ complex drives protons across the thylakoid membrane, creating a proton-motive force that provides the energy for ATP synthesis by an ATP synthase.
Linear electron transfer through the photosystems produces NADPH and ATP. Cyclic electron transfer produces only ATP and allows variability in the proportions of NADPH and ATP formed.
The distribution of PSI and PSII between the granal and stromal thylakoids can change and is indirectly controlled by light intensity, optimizing the distribution of excitons between PSI and PSII for efficient energy capture.
The oxygen-evolving center, which contains a ${Mn}_{4} {CaO}_{5}$ $Mn Subscript 4 Baseline CaO Subscript 5$ cofactor, uses energy from light to split water, producing $O_{2}$ $upper O Subscript 2$ . For each $O_{2}$ $upper O Subscript 2$ formed at the oxygen-evolving center, four protons are pumped into the thylakoid lumen, contributing to the proton motive force.