468 Photosphere

oxygen than the enzyme of C3 plants, but photores- piration is still reduced because the carbon dioxide concentration is kept high by the carbon dioxide– accumulating mechanism. The cost of actively accumulating carbon dioxide in many C4 plants is about 2 ATP molecules, less ◦ than the cost of photorespiration in C3 plants at 77 F ◦ (25 C) but more than the cost of photorespiration ◦ ◦ at 68 F (20 C). However, as the carbon dioxide level in the atmosphere increases, the cost of photores- piration in C3 plants will fall, while the cost of car- bon dioxide pumping in C4 plants will remain con- stant. Hence, C3 plants benefit more than C4 plants from the increasing CO2 concentration. See PHYS- IOLOGICAL ECOLOGY (PLANT); PLANT RESPIRATION. Thomas D. Sharkey Bibliography. R. Boldt et al., D-Glycerate 3-kinase, the last unknown enzyme in the photorespira- tory cycle in Arabidopsis, belongs to a novel ki- nase family, Plant Cell, 17:2413–2420, 2005; A. J. Keys et al., Photorespiratory nitrogen cycle, Nature, 275:741–743, 1978; W. L. Ogren, Photorespiration: Pathways, regulation, and modification, Annu. Rev. Plant Physiol., 35:415–442, 1984; S. Rachmilevitch, Image of the solar photosphere made in white light near A. B. Cousins, and A. J. Bloom, Nitrate assimila- 500 nm, using the 75-cm (30-in.) Vacuum Tower Telescope tion in plant shoots depends on photorespiration, of the National Solar Observatory at Sacramento Peak, Proc. Nat. Acad. Sci. USA, 101:11506–11510, 2004; Sunspot, New Mexico. (National Solar Observatory) T. D. Sharkey, Estimating the rate of photorespiration in leaves, Physiol. Plant, 73:147–152, 1988; C. R. ing photospheric structure. Magnetic flux tubes, Somerville, An early Arabidopsis demonstration: Re- smaller than the intergranular lanes, penetrate the solving a few issues concerning photorespiration, photosphere vertically. When many of these tubes Plant Physiol., 125:20–24, 2001; L. M. Voll et al., are forced together, probably by convective motions The photorespiratory Arabidopsis shm1 mutant is deep within the Sun, they suppress the convective deficient in SHM1, Plant Physiol., 140:59–66, 2006. motions near the surface, cool the atmosphere, and form dark magnetic pores and sunspots. Gravitational, acoustic, and magnetic waves are propagated through, and at some frequencies Photosphere trapped in, the photosphere. The trapped waves, The apparent, visible surface of the Sun. The photo- most of which have periods of about 5 min, rep- sphere is a gaseous atmospheric layer a few hundred resent global oscillations of the Sun and provide in- miles deep with a diameter of 864,000 mi (1,391,000 formation about its interior when analyzed by seis- km; usually considered the diameter of the Sun) and mological techniques. See HELIOSEISMOLOGY; SUN. an average temperature of approximately 5800 K Stephen L. Keil ◦ (10,500 F). Radiation emitted from the photosphere Bibliography. R. J. Bray, R. E. Loughhead, and C. J. accounts for most of the solar energy flux at the Durrant, The Solar Granulation,2ded., 1984; R. Earth. The solar photosphere provides a natural lab- Giovanelli, Secrets of the Sun, 1984; R. W. Noyes, oratory for the study of dynamical and magnetic pro- The Sun, Our Star, 1982; H. Zirin, Astrophysics of cesses in a hot, highly ionized gas. the Sun, 1988. When studied at high resolution, the photosphere displays a rich structure. Just below it, convective motions in the Sun’s gas transport most of the solar energy flux. Convective cells penetrate into the sta- Photosynthesis ble photosphere, giving it a granular appearance (see Literally, the synthesis of chemical compounds using illus.) with bright cells (hot rising gas) surrounded light. The term photosynthesis, however, is used by dark intergranular lanes (cool descending gas). A almost exclusively to designate one particularly typical granule is approximately 600 mi (1000 km) important natural process: the use of light in the in diameter. Measurements of horizontal velocity manufacture of organic compounds (primarily cer- reveal a larger convective pattern, the supergran- tain carbohydrates) from inorganic materials by chlo- ulation, with a scale of approximately 20,000 mi rophyll- or bacteriochlorophyll-containing cells. This (30,000 km); the horizontal motion of individ- process requires a supply of energy in the form of ual granules reveals intermediate-scale (3000-mi or light, since its products contain much more chemical 5000-km) convective flows. energy than its raw materials. This is clearly shown Magnetic fields also play an important role in shap- by the liberation of energy in the reverse process, Photosynthesis 469 namely the combustion of organic material with reaction possible. A considerable part of the light oxygen, which takes place during respiration. See energy used for this process is stored as chemical CHLOROPHYLL; PLANT RESPIRATION; RESPIRATION. energy. Among chlorophyll-containing plant and algal Temporal phases of photosynthesis. Photosynthe- cells, and in cyanobacteria (formerly known as blue- sis is a complex multistage process that consists of green algae), photosynthesis involves the oxidation nearly a hundred physical processes and chemical re- of water (H2O) to produce oxygen molecules, which actions. To make this complex process more under- are then released into the environment. This is standable, it is useful to divide it into four temporal called oxygenic photosynthesis. In contrast, bacte- stages. Each phase is based roughly on the time scale rial photosynthesis does not involve O2 evolution in which it occurs. These phases are (1) photon ab- (production). In this case, other electron donors, sorption and energy transfer processes in antennas such as H2S, are used instead of H2O. This process (or antenna chlorophylls, molecules which collect is called anoxygenic photosynthesis. Both types of light quanta); (2) primary electron transfer in pho- photosynthesis are discussed below. tochemical reaction centers; (3) electron transport The light energy absorbed by the pigments of and adenosine triphosphate (ATP) formation; and photosynthesizing cells, especially by chlorophyll (4) carbon fixation and export of stable products. or bacteriochlorophyll pigments, is efficiently con- Phase 1 takes place in the femtosecond [1 fem- verted into stored chemical energy. Together, the tosecond (fs) = 10−15 s] to picosecond [1 picoseond two aspects of photosynthesis—the conversion of (ps) = 10−12 s] time scale. Phase 2 takes place in the inorganic matter into organic matter, and the con- picosecond to nanosecond time scale [1 nanosecond version of light energy into chemical energy—make (ns) = 10−9 s]. Phase 3 takes place on the microsec- it the fundamental process of life on Earth: it is the ond [1 microsecond (µs) = 10−6 s] to millisecond ultimate source of all living matter and of almost all of [1 millisecond (ms) = 10−3 s) time scale. Phase 4 the energy of life. All types of photosynthesis require takes place in the millisecond to second time scale. light; therefore, photosynthetic organisms are gener- We will consider each of these phases of photo- ally restricted to the photic zone, that is, the narrow synthesis in more detail below. The time scales are region of the Earth near the surface that receives shown schematically in Fig. 1. sunlight. The only known exceptions are the anoxy- Sites of photosynthesis. The photosynthetic pro- genic photosynthetic bacteria that live near deep-sea cess takes place within certain structural elements hydrothermal vents and utilize the very weak light found in plant cells. All algae, as well as all given out by the hot vents. higher plants, contain pigment-bearing subcellular The total productivity of photosynthesis on Earth organelles called chloroplasts. In the leaves of the has been estimated in various ways. The most ac- higher land plants, these are usually flat ellipsoids curate modern method uses satellite remote-sensing about 5 micrometers in diameter and 2.3 µmin data to estimate both land-based and oceanic pho- thickness. Ten to 100 of them may be present in tosynthesis. These estimates take into consideration the average parenchyma cell of a leaf. Under the the fact that the vegetation in these areas make use electron microscope, all chloroplasts show a lay- of only 1% of the entire solar spectrum on average, or ered structure with alternating lighter and darker 2% if only visible light is considered. The net primary layers of roughly 0.01 µminthickness. These layers productivity of photosynthesis on Earth has been de- are membranes that contain proteins. These mem- termined to be about 115 petagrams of carbon per branes are called thylakoid membranes (thylakoid year [1 petagram (Pg) equals 1015 grams or 109 met- ric tons]. The contributions of land- and ocean-based primary production are roughly equal, although the accumulation of biomass on land is much higher than photon absorption in the ocean. This is due to the fact that organisms 1 rapidly recycle the fixed carbon in the oceans by and energy transfer consuming it. Oxygenic photosynthesis. The net overall chemi- primary 2 cal reaction of oxygenic photosynthesis (by plants, photochemistry algae and cyanobacteria) is shown in Eq. (1), where

+ + −chlorophyll−−−−−→ { }+ phase H2O CO2 light energy CH2O O2 (1) secondary electron enzymes 3 transfer and ATP synthesis

{CH2O} stands for a carbohydrate (sugar). The pho- tochemical reaction in photosynthesis belongs to the carbon fixation and type known as oxidation-reduction, with CO act- 4 2 export of products ing as the oxidant (electron acceptor) and water as the reductant (electron donor). The unique charac- teristic of this particular oxidation-reduction is that −15 −12 −9 −6 −3 100 it is energetically unfavorable; that is, it converts 10 10 10 10 10 chemically stable materials into chemically unstable time, s products. Light energy is used to make this “uphill” Fig. 1. Four temporal phases of photosynthesis. 470 Photosynthesis

stroma lamellae intermembrane space by experiments of Robert Emerson. Emerson dis- (site of PSI) covered that the “maximum quantum yield of pho- outer tosynthesis” (number of O2 molecules evolved per thylakoid envelope absorbed quantum), while constant at the shorter wavelengths of light (red, orange, yellow, green), de- grana lamellae clines in the far-red above 680 nanometers (the “red (stack of drop”). Later, Emerson and coworkers showed that thylakoids and site of PSII) this low yield could be enhanced if both chlorophyll a and b are simultaneously excited (only chlorophyll a absorbs above 680 nm). This effect, now known as thylakoid the Emerson enhancement effect, suggests that two stroma stroma different pigments must be excited to perform effi- thylakoid lamella cient photosynthesis, thus indicating that two light inner lumen granum (a) envelope (stack of thylakoids) reactions are involved in photosynthesis. Further, it suggests that one of these pigments is sensitized by light absorption in chlorophyll a and one by absorp- outer and inner stroma tion in another pigment (for example, chlorophyll b). membranes Experiments by others enlarged Emerson’s observa- grana stroma tion by suggesting that plants contained two pigment lamellae systems. One (called photosystem I, or PSI; sensitiz- ing reaction I) is primarily composed of chlorophyll a; the other (called photosystem II, or PSII; sensitiz- ing reaction II) is also composed of chlorophyll a, but includes most of chlorophyll b or other auxiliary pigments. These other auxiliary pigments include red and blue pigments, called phycobilins, in red algae and cyanobacteria, for example, and the brown pigment fucoxanthol in brown algae and diatoms. Efficient photosynthesis requires the absorption of (b) an equal number of quanta in PSI and in PSII. This Fig. 2. A typical chloroplast. (a) Schematic diagram of a higher plant chloroplast. ensures that within both systems excitation energy (b) Electron micrograph of a chloroplast. (After L. Taiz and E. Zeiger, 2006) is absorbed by the antenna system and partitioned to each photosystem, where the energy drives the chemical reactions. Robert Hill proposed that one of these reactions stands for a membrane sac). The proteins bind all involves the transfer of an electron from an interme- of the chlorophyll. The membranes are the sites of diate to cytochrome b6 (shown later to be a plasto- the first three phases of photosynthesis. In algae, the quinone molecule) during the conversion of water to number and shape of chloroplasts vary much more. oxygen). The other reaction, he proposed, involves The green unicellular alga Chlorella contains only the transfer of an electron from cytochrome f to an one bell-shaped chloroplast. A typical chloroplast is intermediate in the conversion of CO2 to carbo- shown in Fig. 2. See CELL PLASTIDS. hydrate. The intermediate transfer of an electron The photochemical apparatus is less complex in from reduced plastoquinone (plastoquinol) to cy- cyanobacteria. These cells are prokaryotes and there- tochrome f can occur without energy input because fore lack a nucleus and organelles such as chloro- the former is a stronger reductant than the latter. plasts and mitochondria. The early phases of photo- There is a great deal of experimental evidence for synthesis take place on thylakoid membranes, which the existence of two pigment systems and for the extend throughout the interior of the cell. key role of plastoquinone and cytochrome f in this Quantum energy storage process. In photosynthe- sequence. Louis N. M. Duysens and Jan Amesz con- sis, the energy of light quanta is converted into chem- ducted a crucial early experiment showing the ex- ical energy. As seen in Eq. (1), the conversion of istence of two photosystems. In this, they showed 1 mole of CO2 and 1 mole of H2Oproduces 1 mole that light 1, absorbed by chlorophyll a,oxidizes cy- of a carbohydrate group and 1 mole of oxygen. This tochrome f, and that light 2 (absorbed in other pho- reaction results in the storage of about 469 kilojoules tosynthetic pigments, including chlorophyll a)re- of total energy per mole or, under natural conditions, duces this oxidized cytochrome f. The results sug- about 502 kJ of potential chemical energy (free en- gest that the two light reactions are arranged in ergy) per mole. Pigments absorb light in the form of series. quanta (photons). See ABSORPTION OF ELECTROMAG- Z scheme and the proteins that carry it out. The two NETIC RADIATION.; PHOTON. photochemical events that were discovered at that Discovery of two photosystems. A specific mecha- time are now known to take place on large protein nism in which two photochemical events cooperate complexes embedded in the thylakoid membrane. to carry out oxygenic photosynthesis was suggested Figure 3 shows the four large protein complexes Photosynthesis 471

NADPH

+ ATP ADP Pi stroma cyclic electron transfer ferredoxin ferredoxin- H+ NADP reductase light + thylakoid light H membrane

PQH2

F-ATPase

cytochrome b6f 2H2O + + 2O2 4 H complex Photosystem I Photosystem II H+ n H+ plastocyanin

thylakoid lumen

Fig. 3. Structural diagram of the four protein complexes present in the thylakoid membrane, as characterized by x-ray crystallography. Photosystem II (PSII), the cytochrome b6f complex (cyt b6f), photosystem I (PSI), and the ATP synthase are shown in the figure. Electron transfer steps are shown as solid arrows while proton flow is indicated by dashed arrows.

found in the thylakoid membrane, and demonstrates a particular way in the membrane. This orientation the interactions between photosystems I and II; the is essential to the proper functioning of photosyn- cytochrome b6 f complex functions between the thesis. photosystems, and the ATP synthase which makes Figure 4 shows the Z scheme, summarizing the ATP. Arrows show the pathway of electrons and pro- wayinwhich the two photosystems cooperate to tons (H+), the latter is essential for the formation of carry out electron transfer reactions involved in pho- ATP. Note that the membrane has an inherent asym- tosynthesis. It is an energetic diagram, in that the metry in that the protein complexes are oriented in energy of the component, which is measured as the

− − 1.2 P700* 1 ps A 40–200 ps Cyt b f complex 0 −0.8 6 P680* ~1 ps A1 15–200 ps Fx 20–200 ns − Pheo + 0.4 H s F /F ~2 ms 200 ps − Photosystem I A B HCO Cyt bL 3 1 ms F 2NADPH , eV Q Q d A B Cyt bH ,7 0.0 PQ hv m 100–600 µs

E Photosystem II FNR 1–20 ms FeS <1 ms Cyt f 0.4 200 µs − PC + HCO hv 2NADP 3 P700 1 ms 100–800 µs 4Mn 0.8 2H2O 200 ns + − Yz + Ca Cl P680 H s 4H+ + O 1.2 2 donor slide of PSII acceptor slide of PSII donor slide of PSI acceptor slide of PSI

Fig. 4. Z scheme of photosynthesis, representating electron flow and chemical reactions in the two light steps in photosynthesis. Yz (a tyrosine residue, also referred to as Z) is the electron donor to the oxidized chlorophyll a P680 of PSII, and Mn (manganese-containing protein) is the charge accumulator that leads to O2 evolution. Pheo (pheophytin) and Chl (a A0 chlorophyll a)are the primary electron acceptors of PSII and PSI, respectively. QA and FX (an iron-sulfur center) are the stable electron acceptors of PSII and PSI, respectively. QB is another bound plastoquinone, while PQ is plastoquinone, Cyt is cytochrome, and PC is plastocyanin. A1 is an electron acceptor of PSI, FB and FA are iron-sulfur centers, Fd is ferredoxin, and + FNR is ferredoxin NADP reductase. Em is the reduction-oxidation (redox) potential. 472 Photosynthesis

+ stroma (low H+) H

+ + + + NADP H ADP Pi ATP NADPH

+ light H light FNR ATP synthase Fd low

PSII PQ Cytochrome PSI b6f − − e − e PQH2 e P680 P700

plastoquinone PC high electrochemical H+ potential gradient + H2O + n H plastocyanin 1 + /2 O2 2 H

oxidation of water lumen (high H+)

Fig. 5. A diagram of the four major protein complexes of the thylakoid membrane.

+ midpoint redox potential Em,isshown on the y axis when P700 is oxidized to P700 in PSI. P700 has been and the progress of the reaction is shown on the x shown to be a pair of Chl a and Chl a molecules. In axis. The two vertical arrows represent energy input the above, the P stands for pigment and the numeri- to the system due to photon absorption. cal designation gives the location, in nanometers, of The structures of all four of the large protein com- the maximum decrease in the absorption in the red plexes are now known due to x-ray crystallography or the infrared region of the spectrum. studies. The combination of the structural pictures Detailed reactions of PSI and II. The PSII reaction in Fig. 3 and Fig. 5 and the energetic picture in is the one most closely associated with O2 evolu- Fig. 4 provides an understanding of the process that tion. The final result of this set of reactions is the neither diagram individually can portray. oxidation of water to O2 and the reduction of a plas- Chemical role of chlorophyll. How does the chloro- toquinone (an oxidation-reduction catalyst). Current phyll a molecule, after excitation by the absorbed evidence suggests that light absorbed by the major quantum of energy, utilize it for an energy-storing part of the accessory pigments is ultimately trans- photochemical process, such as the transfer of an ferred to a special chlorophyll a molecule in the electron from a reluctant donor, H2O, to a reluctant PSII reaction center, which is in a favorable posi- acceptor (perhaps NADP+)? The chemical proper- tion to act as an energy trap. There is a charge sep- ties of chlorophyll are very different in the excited aration within 3 picoseconds; this special chloro- state from the unexcited or ground state. The excited phyll molecule is oxidized, and a specific pheo- chlorophyll acts as a very strong reducing agent, so phytin (Pheo) molecule is reduced (Chl+ Pheo−). that it easily loses an electron to a nearby acceptor. The positive charge is transferred again within a This is the primary photochemical process in pho- few picoseconds to a pair of special Chl’s nearby tosynthesis. Support for this concept is provided by (P680). The P680 recovers by accepting an electron observations of reversible photochemical oxidation from a tyrosine amino acid residue that is repre- and of reversible photochemical reduction of chloro- sented by YZ (referred to as Z below), within 20– phyll in solution, and the comparison of these data 400 nanoseconds, as shown in reactions (2). with those in more intact systems. Studies of changes ∗ in the absorption spectrum of photosynthetic cells Chl + hII → Chl (2a) in light show that a small fraction of a special form ∗ + → + + − of chlorophyll a (including P700), absorbing max- Chl Pheo Chl Pheo (2b) imally at 700 and 430 nm, is in an oxidized state + + Chl P680 → P680 + Chl (2c) during illumination. This is at the heart of the re- action center of PS I. The reaction center Chl’s of P680+ + Z → P680 + Z+ (2d) PS II (including P680) have been suggested also to undergo oxidation-reduction. Furthermore, a chloro- The components of reactions (2), that is, the phyll a molecule appears to be chemically reduced special chlorophyll, P680, pheophytin, and Z, and Photosynthesis 473

+ the next intermediates, QA and QB (bound plasto- ferredoxin-NADP-reductase, eventually to NADP , quinones), are located on a protein complex consist- reducing it (Figs. 4 and 5). ing of two polypeptides, D-1 and D-2 (approximately Mechanism of O2 evolution. The mechanism of O2 32,000 molecular weight each). The oxidation prod- evolution is the least known part of photosynthe- uct, the strong oxidant Z+,isutilized to oxidize water sis. All oxygen liberated in photosynthesis originates and liberate O2 and protons (Figs. 4 and 5). in water. Based on the measurements, by Pierre Jo- The PS I reaction is the one most closely associ- liot and Bessel Kok, of the amount of O2 evolved ated with the reduction of NADP+. Light absorbed in single brief (10-µs) saturating light flashes, it has by most of the chlorophyll a molecules is transferred been shown that four oxidizing equivalents must ac- to a special chlorophyll a molecule P700, where the cumulate on the O2-evolving complex before it can primary charge separation occurs: P700 is oxidized, oxidize H2OtoO2. There is a period 4 oscillation in and another special Chl a (A0)isreduced in about a the pattern of O2/flash as a function of flash number picosecond, as shown in reactions (3). (Fig. 6a, left). The system is an oxygen “clock” in which the state (S) of the O2-evolving complex un- ∗ P700 + hI → P700 (3a) dergoes sequential reactions, and the subscripts on S represent the oxidizing equivalents accumulated on ∗ + − P700 + Chl → P700 + Chl (3b) the complex; the system starts in the dark in S1 state (Fig. 6b,right). After the primary photochemical processes, sev- Manganese (Mn) is required for O2 evolution. eral additional steps take place. In PSII, the elec- Plants grown in a manganese-deficient medium lose trons flow on the electron donor side from H2O their capacity to evolve O2. The charge accumula- to oxidized tyrosine Z+ via a manganese (Mn) com- tor of the reaction mechanism is a manganese com- plex. On the electron acceptor side, electrons flow plex; manganese is known to exist in several va- from Pheo− to two special plastoquinones called lence states. In addition to manganese, chloride and QA and QB.Abound bicarbonate ion is essential calcium ions also function at the O2-evolving site, for the electron and proton transfer from photosys- though the mechanism of action is not known. The tem II to the plastoquinone pool. Additional reac- O2-evolving complex includes at least four “intrin- tions and cofactors connect the two photosystems, sic” proteins having molecular weights of 34,000, including a pool of PQ, the cytochrome b6 f com- 32,000, 9000, and 4500 (D- 1, D-2, and two subunits plex which contains cytochrome b6, cytochrome f of cytochrome b-559). In addition, three “extrinsic” and the Rieske iron-sulfur protein, and the soluble polypeptides of molecular weights 33,000, 24,000, copper protein plastocyanin. Plastocyanin finally re- and 18,000 are also involved. In cyanobacteria, the reduces P700+ in PSI. On the electron acceptor side 24,000 and the 18,000 molecular weight proteins are of PSI, the electron that is lost from P700 flows replaced by a cytochrome c and another low molec- − − from Chl a (A0), through an intermediate A1 (phyl- ular weight protein. Further research is needed to loquinone), the iron-sulfur centers (FX,FA,FB), and understand the detailed chemical mechanism of O2 soluble ferredoxin (Fd) and a flavoprotein called evolution.

3+ 3+ 80 (Mn , Mn ) − − e to QA e to QA S1 4 L 1 (Mn2+, Mn3+) 60 H+ + S0 S2 L L (Mn3+, Mn4+) 2H+ 40 2 O 2 H+

− + + e to QA 2H2O

elative oxygen yield per flash S r 20 4 S3 + + + L (Mn4 , Mn4 ) L+ 3+ 4+ 3 (Mn , Mn )

0 510152025 e− to Q (a) flash number (b) A

Fig. 6. Oxygen clock. (a) Pattern of oxygen evolution from dark-adapted chloroplasts when illuminated by a series of intense brief flashes. (b) S-state mechanism to describe oxygen evolution pattern. The oxygen-evolving complex can exist in five distinct states, S0–S4. Light activates PSII, which in turn advances the system through the states one step for each photochemical turnover of the PSII reaction center. When state S4 is reached, oxygen is produced and the clock is reset to S0. 474 Photosynthesis

Photophosphorylation. Inner cytoplasmic mem- to the reaction centers. The collection of antenna branes from photosynthetic bacteria, cyanobacterial molecules along with their associated photosystems cells, and chloroplasts from green plants and algae— are often referred to as photosynthetic units. when illuminated in the presence of adenosine Robert Emerson and William Arnold showed how diphosphate (ADP) and inorganic phosphate (Pi)— the light reaction in photosynthesis can be sepa- use light energy to synthesize adenosine triphos- rated from the dark reaction by the use of brief, phate (ATP). About 42 kJ of converted light en- intense light flashes, separated by intervals of dark- ergy in this reaction is stored in each mole of the ness of variable duration. They found that the yield high-energy phosphate, ATP. This photophospho- of a single flash was maximum when the interval rylation is coupled with energy-releasing steps in between the consecutive light flashes was at least ◦ ◦ photosynthesis, such as the electron flow from PSII 0.04 s at 1 C (33 F). This, then, is the minimum time to PSI. When phosphorylation is associated with required for the efficient utilization of the products + noncyclic electron flow from H2OtoNADP ,itis from the light reactions. Emerson and Arnold further called noncyclic photophosphorylation. See ADENO- observed that, under optimal conditions, the maxi- SINE DIPHOSPHATE (ADP); ADENOSINE TRIPHOSPHATE mum yield from a single flash was one O2 molecule (ATP). per 2400 chlorophyll molecules present. However, Under certain conditions, electrons in PSI, instead it is now known that the two light-reaction mech- of going to NADP+,may return to an intermedi- anism of photosynthesis requires the transfer of ate (such as a cytochrome, plastoquinone, plasto- four electrons twice for every molecule of oxygen cyanin, or P700) and thus close the cycle. This type evolved. Thus there are at least eight photoacts lead- of cyclic electron flow, mediated by added cofactors ing to the evolution of one O2 molecule. There- and ADP and inorganic phosphate, also leads to the fore, the ratio of one O2 per 2400 chlorophylls production of ATP and has been termed cyclic phos- means one photoact per 300 chlorophyll molecules. phorylation; it has been shown to exist under certain By using spinach grown in moderate to high light experimental conditions in vivo. intensity, we know that there is one active PSII The mechanism of ATP formation is very differ- (P680) and one active PSI (P700) for a total of about ent from the electron transfer processes described 600 chlorophyll molecules present. If these chloro- earlier. Light produces a high-energy state, and then phyll molecules are equally divided in PSI and II, the actual phosphorylation occurs in the dark. André there is one reaction center per 300 chlorophyll Jagendorf and coworkers found that if chloroplasts molecules in each system. This is the commonly ac- are first suspended in an acidic medium and then cepted size of one physical unit in higher plants, transferred to an alkaline medium in the presence of algae, and cyanobacteria. ADP and Pi,ATP formation occurs without the need Accessory antenna pigments. In addition to chloro- of light. All of these experiments were explained in phyll a (the one pigment present in nearly all oxy- terms of a hypothesis proposed by Peter Mitchell, in genic photosynthetic organisms), there are other which light produces a H+ ion gradient (pH) across chlorophylls, such as chlorophyll b in the green algae the lamellar membranes in the chloroplasts, and the and higher plants. In brown algae, chlorophyll c re- energy dissipation of this H+ ion gradient via the places chlorophyll b.Inamarine cyanobacterium ATP synthase (sometimes called the coupling factor, Acaryochloris marina, most of Chl a is replaced by aprotein complex present in the lamellae) leads to Chl d. There are also nonchlorophyllous pigments phosphorylation. In addition, an electric field () belonging to two groups: (1) the carotenoids, so generated across the thylakoid membrane as a re- called because of similarity to the orange pigment of sult of the initial light-induced charge separation can carrots, are a variable assortment of pigments found also drive photophosphorylation. Reagents that dis- in all photosynthetic higher plants and in algae. sipate the electric field or the H+ gradient also inhibit (2) The phycobilins, or vegetable bile pigments, are phosphorylation. The two together are referred to as chemically related to animal bile pigments. The phy- proton motive force. cobilins are either red (phycoerythrin) or blue (phy- The ATP synthase is shown schematically in Fig. 5, cocyanin). Both types are present in special granules and its structure is shown in Fig. 3. It has two parts, a called phycobilisomes in red algae (Rhodophyta) and section that is embedded in the thylakoid membrane cyanobacteria: the red pigment in red algae, and and a section that protrudes into the stroma. Recent the blue pigment in cyanobacteria. Another phy- evidence demonstrates that the ATP synthase is a cocyanin called allophycocyanin is also present in rotary motor, and most of the membrane-associated cyanobacteria. All of these pigments are associated part rotates, while H+’s are translocated across the with specific proteins to form so-called antenna pig- membrane. Mechanical energy in this rotary motion ment proteins. The structures of many of these an- is coupled to the formation of high-energy phosphate tenna complexes are known, and the energy trans- bonds in ATP. The chemical mechanism of this pro- ferprocesses have been investigated using ultrafast cess is not yet well understood. laser techniques. See CAROTENOID; CHLOROPHYLL; Photosynthetic unit. The concentration of the spe- PHYCOBILIN. cial chlorophyll a molecules (P700 or P680) that en- Light absorbed by accessory pigments does con- gage in the chemical reactions is very low, only one tribute to energy storage in photosynthesis. This in several hundred chlorophyll molecules; energy ab- is simply demonstrated from measurements of the sorbed by other pigments is effectively transferred so-called action spectra of photosynthesis. In Photosynthesis 475 such measurements, photosynthesis is excited by tribute to photosynthesis by being transferred to monochromatic light, and the production of oxygen chlorophyll a.Bythis mechanism, red algae, grow- per incident quantum of light is measured as a func- ing relatively deep under the sea where only green tion of wavelength. The observed spectral variations light penetrates, can supply the energy of this light in the yield of photosynthesis can be related to the to chlorophyll a, which has very weak absorption in proportion of light absorbed at each wavelength by the green region of the spectrum. the different pigments in the cells. Measurements of Excitation energy is transferred efficiently in the this kind have led to the conclusion that quanta ab- chloroplasts from accessory pigments to chlorophyll sorbed by most carotenoids are 50–80% as effective a.Asimiliar transfer (often referred to as energy as those absorbed by chlorophyll a in contributing migration) occurs also between different chloro- energy to photosynthesis. An exception is fucoxan- phyll a molecules themselves. Excitation-energy thol, the carotenoid that accounts for the color of transfer among chlorophyll a molecules or among brown algae (Phaeophyta) and that of the diatoms; it phycobilin molecules, and excitation-energy trans- supplies light energy to photosynthesis about as ef- ferfrom accessory pigments (donor molecules) to fectively as chlorophyll a. The red and blue pigments chlorophyll a (acceptor molecules) or from vari- of the Rhodophyta and cyanobacteria are also highly ous short-wavelength forms of chlorophyll a to the effective. They can be as effective as chlorophyll or long-wavelength forms of chlorophyll a, has been somewhat less, depending, among other things, on demonstrated. The most widely accepted hypothe- the physiological status of the organism and the color sis, F¨orster’s hypothesis, is that energy transfer is pre- of the light to which they have become adapted. The ceded by thermal relaxation in the donor molecules. primary function of all these pigments is to harvest The efficiency of energy transfer depends upon three the light energy and transfer it to reaction-center basic factors: orientation of acceptor molecules with chlorophyll molecules. However, in addition, sev- respect to the donor molecule; overlap of the fluores- eral xanthophylls (violaxanthin, antheraxanthin, and cence spectrum of the donor molecule with the ab- zeaxanthin) and lutein are involved in photoprotect- sorption spectrum of the acceptor molecule; and the ing photosynthetic organisms against excess light. In distance between the two molecules. The function of many cases, excess light energy is lost as “heat” via most of the pigments (including most of the chloro- deexcitation of chlorophyll directly or via transfer to phyll a molecules) is to act as an antenna, harvest the zeaxanthin. energy, and transfer to very few (1 in 300) reaction- Energy transfer between pigment molecules in an- center Chl molecules, depending upon the pigment tenna system. Chlorophyll a in vivo is weakly fluo- system. Energy is thus trapped and used for photo- rescent, that is, some of the light quanta absorbed chemistry. See CHLOROPHYLL; PLANT PIGMENT. by it (up to 6%) are reemitted as light (Fig. 7, [Contributions of Rajni Govindjee to this article white curve). Observations of the action spectrum of are acknowledged.] Robert E. Blankenship; Govindjee chlorophyll a fluorescence in different oxygenic or- ganisms closely parallel the action spectrum of pho- Carbon Dioxide Fixation tosynthesis. In other words, fluorescence of chloro- The light-dependent conversion of radiant energy phyll a is excited also by light absorbed by the ac- into chemical energy as adenosine triphosphate cessory pigments. Excitation of chlorophyll a fluo- (ATP) and reduced nicotinamide adenine dinu- rescence by light quanta absorbed by phycoerythrin cleotide phosphate (NADPH) serves as a prelude to requires transfer of the excitation energy from the the utilization of these compounds for the reduc- excited phycoerythrin molecule to a nearby chloro- tive fixation of CO2 into organic molecules. Such phyll molecule (somewhat as in acoustic resonance, molecules, broadly designated as photosynthates, where striking one bell causes a nearby bell to ring). are usually but not invariably in the form of carbo- Therefore, light quanta absorbed by accessory pig- hydrates such as glucose polymers or sucrose, and ments, such as carotenoids and phycobilins, con- form the base for the nutrition of all living things, as well as serving as the starting material for fuel, fiber, animal feed, oil, and other compounds used by people. Collectively, the biochemical processes by 0.5 which CO is assimilated into organic molecules are fluorescence 2 ChI a known as the photosynthetic dark reactions, not be- 0.4 ChI b emission + carotenoids ChI a cause they must occur in darkness but because, in contrast to the photosynthetic light reactions, light 0.3 ChI b is not required. (We do recognize, however, that absorption 0.2 several enzymes need to be light-activated before

absorbance spectrum 0.1 they can function; see “Regulation of C3 cycle en-

(excitation at 440 nm) zymes” below.) CO2 fixation by photosynthetic or- elative fluorescence intensity elative fluorescence

r ganisms is an important mechanism by which this 400 450 500 550 600 650 700 “greenhouse” gaseous molecule is removed from wavelength, nm the atmosphere during carbon cycling on Earth. Ap- Fig. 7. Absorption spectrum of a maize (Zea mays) proximately 100 pentagrams of carbon (1 pentagram chloroplast suspension. Pigments responsible for specific 9 bands are shown. Also shown is the fluorescence emission equals 10 metric tons) as CO2 is assimilated annu- of chloroplasts from a maize chloroplast. ally into organic molecules by photosynthesis (about 476 Photosynthesis

3 CO the six (three-carbon) PGA molecules, are phospho- 3 ADP 3 RuBP 2 rylated by six molecules of ATP (releasing ADP to 3 6-C enzyme- bound unstable be used for photophosphorylation via the light reac- intermediate tions) to form six 1,3-bisphosphoglycerate (1,3-BP) 3 ATP molecules. The resulting compounds are reduced Carboxylation Sucrose 6 PGA 6 ATP (that is, in the reduction phase of the C3 cycle) by P the NADPH formed in photosynthetic light reactions 6 ADP to form six molecules of the three-carbon compound 6 1,3-BP several F6P phosphoglyceraldehyde (PGAL). PGAL is isomerized intermediates 6 NADPH to form another three-carbon compound, dihydroxy- Reduction P 6 NADP acetone phosphate (DHAP). PGAL, the aldehyde, and Regeneration DHAP, the ketone, are energetically equivalent, re- 1 PGAL TP PGAL duced compounds and can be considered the prod- 6 PGAL translocator ucts of the reductive phase of the C photosynthetic P P 3 cycle. PGAL and DHAP together form the triose phos- P 5 PGAL 1 DHAP phate (TP) pool of the chloroplast. The chloroplast TP pool is primarily composed of PGAL; the iso- F6P G6P merase responsible for PGAL:DHAP interconversion P CHLOROPLAST CYTOPLASM favors PGAL formation. Starch ATP The rest of the C3 photosynthetic cycle (the regen- eration phase) involves enzymatic steps that allow Fig. 8. Schematic outline of the C3 (Calvin-Benson-Bassham) CO2 assimilation cycle (the three phases are noted), showing the partitioning of assimilated carbon into starch within regeneration of RuBP, the initial carboxylation sub- the chloroplast [via the phosphorylated 6-C intermediates, fructose-6-phosphate (F6P) strate. One molecule of PGAL is made available and glucose-6-phosphate (G6P)], and the assimilate efflux (through the three-carbon triose phosphate [TP] transporter) across the chloroplast inner envelope to the cytoplasm for combination with DHAP isomerized from a sec- (in exchange for inorganic phosphate) leading to sucrose synthesis. For the reactions of ond PGAL (requiring a second “turn” of the Calvin- the C3 cycle, shown in the chloroplast, the relative numbers of molecules involved in each Benson-Bassham cycle wheel) to form a six-carbon specific step are shown to the left of the substrate and/or product. sugar. The other five PGAL molecules, through a complex series of enzymatic reactions, are rear- half of this amount is assimilated by photosynthetic ranged into three molecules of RuBP,which can again marine algae). be carboxylated with CO2 to continue the cycle. The route by which CO2 is assimilated had been It should be noted that the enzyme that incorpo- studied for over a century when it was discovered rates CO2 into an organic compound, RuBP carboxy- that photosynthesis leads to the accumulation of lase/oxygenase (rubisco), also allows oxygen (O2) sugars and starch. Details of the biochemical path- to react with RuBP, hence the “oxygenase” in the way leading to CO2 assimilation were worked out name. This reaction initiates the process called pho- in the 1950s, when the availability of paper chro- torespiration, which results in the release of one pre- 14 matographic techniques and CO2 allowed Melvin viously incorporated molecule of CO2 for every two Calvin, Andrew A. Benson, and James A. Bassham to molecules of O2 that are allowed to react. See PHO- develop the outline of the reductive pentose phos- TORESPIRATION. phate cycle (Calvin was awarded a Nobel Prize in Due to its low catalytic efficiency, rubisco Chemistry in 1961), now usually called the C3 cycle. (Fig. 9) can be up to half of the soluble protein in The name C3 cycle refers to the first stable product C3 chloroplasts, and most likely it is the most abun- generated from CO2, which is a three-carbon organic dant protein found in nature. Rubisco is a large and molecule. The C3 cycle forms the primary, or basic (with other, feeder pathways occurring in some plant types), route for the formation of photosynthate from CO2. C3 photosynthesis. The essential details of C3 pho- tosynthesis can be seen in Fig. 8. The entire cycle can be separated into three phases—carboxylation, reduction, and regeneration. Since the smallest inter- mediate in the cycle consists of three carbons, we will start with three molecules of CO2. During the initial carboxylation phase, these three molecules of CO2 are combined with three molecules of the five- carbon compound ribulose 1,5-bisphosphate (RuBP) in a reaction catalyzed by the enzyme RuBP carboxy- lase/oxygenase (rubisco) to form three molecules of an intermediate, unstable enzyme-bound six- carbon compound. These unstable molecules are hydrolyzed by the enzyme into six molecules of the three-carbon compound phosphoglyceric acid (PGA). These products of the carboxylation phase, Fig. 9. Structural model of Rubisco. Photosynthesis 477 complex enzyme, comprising eight large polypep- the reactions to synthesize more RuBP, the carboxy- tide subunits and eight small subunits. Interestingly, lation substrate. We have known for quite some time the small subunit polypeptide is produced (as a larger that the maximal measurable activities of fructose precursor form) in the cytoplasm from mRNA which 1,6-bisphophatase (FBPase) and sedoheptulose 1,7- is encoded in the nucleus. The precursor polypep- bisphosphatase (SBPase), two enzymes involved in tide is then transported across the chloroplast mem- RuBP regeneration, are not much greater than the brane (the mature form of this polypeptide can- rate of photosynthetic carbon assimilation and con- not be transported in this manner); processed into comitant carbon flow through the C3 cycle. Thus, the shorter, mature polypeptide; and then combined carbon flow through these enzymes might con- with large subunits (encoded in the chloroplast DNA tribute to rate limitation of photosynthetic carbon and produced in the stroma) to form the mature en- assimilation. Studies with transgenic plants overex- zyme. pressing these enzymes support this contention; in- The net product of two “turns” of the cycle, a six- creasing the amount of either enzyme led to a higher carbon sugar (G6P or F6P), is formed either within level of the carboxylation substrate RuBP as well as the chloroplast in a pathway leading to starch (a higher photosynthetic rates. polymer of many glucose molecules) or externally The autocatalytic nature of the cycle (that is, more in the cytoplasm in a pathway leading to sucrose substrate for initial carboxylation can be generated as (condensed from two six-carbon sugars, glucose and carbon flows through the steps of the pathway) can fructose). This partitioning of newly formed photo- be best understood by considering that the net prod- synthate leads to two distinct pools; starch is stored uct of one “turn” of the cycle (representing three in the photosynthesizing “source” leaf cells, and su- carboxylations), that is, a PGAL molecule, can be fed crose is available either for immediate metabolic re- back into the cycle. Thus the rate of carboxylation quirements within the cell or for export to “sinks” during an initial lag phase (as chloroplasts are ini- such as developing reproductive structures, roots, or tially illuminated) is dependent on the level of newly other leaves. Factors within the photosynthesizing formed RuBP.If all newly fixed carbon were fed back cell, such as energy requirements in different com- into the cycle, the level of RuBP would double after partments (mitochondria, cytoplasm, and chloro- five carboxylations. Since the rate of photosynthetic plasts), along with energy needs of the plant (such carbon fixation is initially dependent on the level of as increased sink requirements during different de- intermediates such as the substrate (RuBP) for the velopmental stages) and external, environmental fac- carboxylation reaction, the next five carboxylations tors (such as light intensity and duration) ultimately would occur in a shorter amount of time, resulting in regulate the partitioning of newly formed photosyn- an exponential increase in the rate of photosynthesis thetic product (PGAL) into starch or sucrose. See until factors other than intermediate levels become PLANT METABOLISM. limiting. This profound control of photosynthate partition- Regulation of C3 cycle enzymes. The photosynthetic ing is accomplished through regulation of PGAL ex- carbon assimilation cycle is regulated at a number of port from the chloroplast to the cytoplasm, as well enzymatic steps. The initial carboxylation catalyst, as by regulation of the enzymes that convert PGAL rubisco, as well as some of the enzymes involved in to sucrose in the cytoplasm and starch in the chloro- the regeneration phase, including glyceraldehyde-3- plast. Under conditions where sink demand is low phosphate dehydrogenase, phosphoribulose kinase, (and sucrose is not transported through the phloem SBPase, and FBPase, require activation. These en- away from source leaf cells), metabolic effectors ac- zymes are inactivated in the dark and activated in cumulate in the cytoplasm that lower the activities of the light. Several conditions are required for acti- the sucrose-forming enzymes and increase the activ- vation, including high concentrations of Mg2+, high ities of the starch-forming enzymes. This results in a pH, and a reductant (supplied in the chloroplast by condition that reduces PGAL export from the chloro- the enzyme thioredoxin). Thioredoxin is reduced by plast, and hence more PGAL is retained in the chloro- NADPH generated in the light. Thioredoxin acts as plast for starch formation. Also, under conditions aprotein disulfide oxidoreductase, converting disul- which cause low chloroplast PGAL levels (such as fide (S-S) bonds of the target proteins (all of the above low light), PGAL transport out of the chloroplast is re- enzymes except rubisco) to a reduced (−SH) form. stricted, resulting in decreased substrate for sucrose Rubisco activity is also modulated by a specific mech- formation, increasing the relative amount of starch anism involving another enzyme, called rubisco acti- production. The energy status of the cell affects su- vase. All of the aforementioned activating conditions crose formation (and therefore photosynthate par- within the chloroplast stroma are facilitated by light- titioning) because cytoplasmic uridine triphosphate dependent processes but are reversed in darkness. (used in the formation of sucrose) level is dependent This regulatory mechanism conveniently allows for on ATP generation, and also because PGAL export to the synthesis pathway to be “shut off,” preventing a the cytoplasm is coupled obligatorily to inorganic futile cycle during the night, when starch reserves phosphate (formed when ATP is metabolized in the are mobilized to meet cell energy requirements via cytoplasm) import into the chloroplast. intermediates which, if C3 cycle enzymes were acti- In addition to providing carbon skeletons for vated, would be reconverted to starch. starch and sucrose synthesis, PGAL is fed back into C4 photosynthesis. Initially, the C3 cycle was the C3 cycle to allow for the regenerative phase of thought to be the only route for CO2 assimilation, 478 Photosynthesis

some cases the two cell types, mesophyll and bun- Mesophyll Cell dle sheath, are not necessarily adjacent (sedges are

− an example). Exotic plants have now been found that HCO CO 3 2 perform C4 photosynthesis in single cells in which the chloroplasts that initially combine carbon diox- oxaloacetate PEP (3-C) ide with PEP are spatially separated from the chloro- pyrophosphate plasts where the carbon dioxide is reassimilated via P ATP NADPH + AMP the C3 cycle. C4 metabolism is classified into three types, de- + NADP P ATP pending on the primary decarboxylation reaction 2 ADP used with the four-carbon acid in the bundle sheath 4-C acid pyruvate cells. The majority of C species (exemplified by (3-C) 4 sugarcane, maize, crabgrass, and sorghum) are of type 1 (see below), and employ NADP-malic en- zyme (NADP-ME) for decarboxylation. NAD-malic en- zyme (NAD-ME) C4 plants (type 2) include amaran- pyruvate 4-C acid (3-C) thus, atriplex, millet, pigweed, and purslane. Type 3C4 types use phosphoenol pyruvate carboxyki- NADP nase (PCK) for decarboxylation and include Panicum grasses. The decarboxylases are also located in dif- NADPH C cycle ferent intracellular compartments as indicated: RuBP 3 PGAL CO2 starch sucrose 1. NADP-ME type, NADP-malic enzyme (chloroplasts) Bundle Sheath Cell NADP+ + malic acid −−−−−−−−−−−−−−−−−−−→

pyruvic acid + CO2 + NADPH (6) Fig. 10. Schematic outline of the C4 carbon dioxide assimilation process in the two cell types of a NADP-ME-type plant. 2. NAD-ME type,

NAD-malic enzyme (mitochondria) NAD+ + malic acid −−−−−−−−−−−−−−−−−−−→ although it was recognized by plant anatomists that pyruvic acid + CO + NADH (7) some rapidly growing plants (such as maize, sugar- 2 cane, and sorghum) possessed an unusual organi- 3. PCK type, zation of the photosynthetic tissues in their leaves phosphoenolpyruvate carboxykinase (cytosol) (Kranz morphology). Work by Hugo Kortschak, Con- Oxaloacetic acid + ATP −−−−−−−−−−−−−−−−−−−−−−−−−→ stance Hartt, and colleagues in Hawaii as well as PEP + CO2 + ADP (8) that of M. D. (Hal) Hatch and Roger Slack in Aus- tralia demonstrated that plants having the Kranz In addition to differing decarboxylation reactions, anatomy utilized an additional CO2 assimilation route the particulars of the CO2 fixation pathway in NAD- now known as the C4-dicarboxylic acid pathway ME and PCK plant types differ from those depicted in (Fig 10). Carbon dioxide enters a mesophyll cell Fig. 10 with respect to the three-carbon compound where it is combined (in the form of bicarbonate) transported from bundle sheath to mesophyll cells. with the three-carbon compound phosphoenolpyru- With NAD-ME types, the three-carbon compound vate (PEP) via the enzyme PEP carboxylase to form a can be either pyruvic acid or alanine, and in PCK four-carbon acid, oxaloacetate, which is reduced to types this compound is PEP.Therefore, the three vari- malic acid or transaminated to aspartic acid. The four- ations in the C4 pathway necessarily predicate differ- carbon acid moves into bundle sheath cells where ent energy (ATP and NADPH) usage in the two cell the acid is decarboxylated and the CO2 reassimi- types. The generation of ATP from ADP,and NADPH lated via the C3 cycle. The resulting three-carbon from NADP via noncyclic electron flow through pho- compound, pyruvic acid, moves back into the mes- tosystem I (PSI) and photosystem II (PSII), is tightly ophyll cell and is transformed into PEP (at the cost coupled: neither compound can be produced with- of 2 ATP molecules) via the enzyme pyruvate phos- out sufficient substrate for both. Therefore, the dif- phate dikinase located in the mesophyll chloroplasts ferent usage of ATP and NADPH in the mesophyll to complete the cycle. The net effect of this cycle is and bundle sheath chloroplasts of the three C4 plant to increase the CO2 concentration around rubisco, types (due both to variations in the pathway of car- thereby reducing photorespiration via the compet- bon flow in the photosynthetic cycle and to vari- ing oxygenase activity of this enzyme. ations in partitioning of portions of the pathway As depicted in Fig. 10, extensive transport of between cell types) is supported by variations in metabolites must occur between the two cell types the photochemical apparatus which allow for dif- that are found in most C4 plants. The diffusion of fering ability to produce ATP without concomitant metabolites between two cell types is facilitated NADPH production. These alternative pathways of by the presence of plasmodesmata connecting the ATPproduction (which result in different ratios of cells to form a cytoplasmic continuum. However, in ATP:NADPH produced) are cyclic and pseudocyclic Photosynthesis 479

photophosphorylation, with the cyclic pathway con- the leaf. Since the C4 plant is more efficient at fixing sidered the major pathway of uncoupled ATP pro- CO2 than C3 plants, more CO2 is incorporated per duction in chloroplasts, and the pseudocyclic path- unit water lost. C4 plants have a greater efficiency way possibly acting as a “fine-tuning” modulator. of nitrogen usage. Since rubisco is produced only in Variations in the photochemical apparatus that bundle sheath cells in C4 plants, only 10–35% of the indicate enhanced cyclic photophosphorylation ca- leaf nitrogen is tied up in this enzyme, as opposed pacity (utilizing only PSI) are a high chlorophyll a/b to 40–60% in C3 plants. Since C4 plants have to “ex- ratio, low Chl/P700 ratio, and a low PSII reaction. pend” less carbon on producing the protein rubisco, These characteristics are found in bundle sheath they have higher rates of sugar formation, which can chloroplasts of NADP-ME-type plants, indicating that facilitate the rapid growth rates seen in such C4 plants the primary function of the photochemical appara- as maize, sugarcane, sorghurn, and crabgrass. Other tus in these chloroplasts is the generation of ATP. differences in response to the environment between NADPH is supplied via the decarboxylation of malic C3 and C4 plants are as follows: C4 plants exhibit a acid to support the C3 cycle activity (PGA conver- nonsaturating response curve of leaf photosynthesis sion to PGAL) in these chloroplasts. Assays of chloro- to light levels found in nature. In addition, C4 plants phyll a/b ratio, Chl/P700 ratio, and PS II activity indi- tolerate more salinity and higher temperatures than cate that NAD-ME mesophyll chloroplasts also have a do C3 plants. The higher energy requirements of C4 primary role of cyclic photophosphorylation, while plants (2 ATPs per CO2 assimilated) are also reflected NAD-ME bundle sheath chloroplasts have a primary by the fact that quantum yields of photosynthesis for role of noncyclic electron flow. In PCK-type plants, C3 plants are higher than for those possessing the mesophyll chloroplasts appear to have a photochem- auxiliary C4 system. At 2% oxygen partial pressure ◦ ical apparatus similar to C3 chloroplasts, while bun- and 30 C, maximum quantum yield for C3 plants is dle sheath chloroplasts appear to have a low PSII about 0.073 mole CO2 assimilated per absorbed Ein- activity. The enhanced ability of PCK bundle sheath stein (an Einstein is a mole of photons) of light, while chloroplasts to produce ATP via cyclic photophos- for C4 plants the maximum quantum yield is 0.054. phorylation supplies the extra ATP needed to con- However, at normal O2 partial pressures (21% O2), vert pyruvic acid to PEP. These variations in the C4 quantum yields are almost identical. This is due to pathway and photochemical apparatus among the the presence of high photorespiration in C3 plants, C4 plant types demonstrate the close relationship and thus represents a net quantum yield rather than a that has evolved between light reactions and the bio- true photosynthetic yield. See PHOTORESPIRATION. chemical processes of carbon dioxide assimilation, Crassulacean acid metabolism photosynthesis. and show the highly integrated cooperation between Under arid and desert conditions, where soil water the cell types involved. is in short supply, transpiration during the day Benefits of C4 cycle. The concentration of CO2 in when temperatures are high and humidity is low air is presently about 0.037% by volume (and in- may rapidly deplete the plant of water, leading to creasing with time due to burning of fossil fuels), desiccation and death. By keeping stomata closed a concentration that does not fully saturate the C3 during the day, water can be conserved, but the cycle when it is operating at capacity due to the uptake of CO2, which occurs entirely through the low affinity of rubisco for CO2.Itwould be neces- stomata, is prevented. Desert plants in the Cras- sary to have about 0.1% CO2 to saturate photosynthe- sulaceae, Cactaceae, Euphorbiaceae, and 15 other sis in C3 plants, which can be achieved only under families have evolved, apparently independently controlled conditions (CO2-enriched greenhouses or of C4 plants, a similar strategy of concentrating growth chambers). Leaf photosynthesis in C4 plants, and assimilating CO2 by which the CO2 is taken however, is fully saturated at air CO2 concentrations. in at night when the stomata open; water loss Thus, C4 photosynthesis may be considered to be is low because of the reduced temperatures and an evolutionary adaptation to current-day CO2 levels correspondingly higher humidities. Although these in air. During the C4 cycle, CO2 is rapidly captured succulent plants with thick, fleshy leaves were via biochemical reactions in mesophyll chloroplasts known since the nineteenth century as being un- and released near rubisco in bundle sheath chloro- usual, the biochemical understanding of the process plasts. This serves to increase the CO2 concentra- did not occur until the 1960s and 1970s when the tion around the enzyme, increasing its catalytic effi- details of C4 photosynthesis were being worked out. ciency and decreasing its reaction with O2 and thus It was first studied in plants of the Crassulaceae; photorespiration; thus the ambient concentration of thus, the process has been called crassulacean acid CO2 in air is not rate-limiting. The spatial compart- metabolism (CAM). mentalization of portions of CO2 assimilation into In contrast to C4, where two cell types usually the two cell types not only allows C4 plants to assim- cooperate, the entire CAM process occurs within ilate air CO2 rapidly with minimal photorespiration, an individual cell; the separation of C4 and C3 is but also partly explains other physiological charac- thus temporal rather than spatial. At night, CO2 teristics and responses to the external environment combines with PEP through the action of PEP car- of C4 plants. C4 plants have a higher efficiency of boxylase, resulting in the formation of oxaloacetic water use. Water vapor exits from leaves through acid and its conversion into malic acid. The PEP is the same stomatal pores through which CO2 enters formed from starch or sugar via the glycolytic route 480 Photosynthesis

few CAM species use NAD-ME for decarboxylation. Vacuole Since the stomata are closed most of the day, de- carboxylation of the stored malate (or oxaloacetate) results in an elevation of its concentration around ru- tonoplast malic acid bisco. The table summarizes the major physiological storage pool differences between C3,C4, and CAM plants. Other CO2 assimilation mechanisms. Both the C4 malate cycle and CAM involve the synthesis of oxaloacetic acid, which is also one of the intermediates in malate the tricarboxylic acid (TCA) cycle of respiration. In oxaloacetate the late 1960s a light-driven reversal of the TCA cycle was discovered. This CO2 fixation cycle, called the reductive carboxylic acid cycle, results in the net Chloroplast PEP − HCO3 synthesis of pyruvic acid via the reversal of the three pyruvate decarboxylation steps in the TCA cycle (pyruvic acid CO2 or PEP starch pool CO2 to acetyl coenzyme A, isocitric acid to α-ketoglutaric acid, and succinyl CoA to succinic acid). The path- C3 photosynthesis way has been detected in some photosynthetic bac- teria. See CITRIC ACID CYCLE. DAY NIGHT In most photosynthetic bacteria, the C3 cycle is functional despite some differences in detail. The Fig. 11. Scheme for the flow of CO2 within a single crassulacean acid metabolism (CAM) green sulfur bacteria, however, carry out C3 pho- cell over a day, showing initial dark CO2 fixation, malic acid storage in the vacuole at night, followed by decarboxylation and the C3 cycle the next day. tosynthesis poorly or not at all. Chlorobium thio- sulfatophilum (alternate name: Chlorobium limi- cola), lacking the key enzyme rubisco, utilizes a of respiration. Thus, there is a daily reciprocal re- reductive carboxylic acid cycle in which reduced lationship between starch (a storage product of C3 ferredoxin drives the TCA cycle in reverse, result- photosynthesis) and the accumulation of malic acid ing in carboxylation reactions much like those of (the terminal product of nighttime CO2 assimilation; the reductive carboxylic acid cycle. Heterocysts of Fig. 11). cyanobacteria do not have a functional C3 cycle As in C4 plants, there may be variations in the de- because, in contrast to the normal cells of these carboxylase that provides the CO2 for assimilation via bacteria, the heterocyst cell (implicated in nitrogen the C3 cycle. In some CAM plants (such as pineapple) fixation) lacks the key enzyme rubisco. Here, CO2 fix- PCK is used, while in others (cactus) the decarboxy- ation in heterocysts may occur through PEP carboxy- lase is the NADP-malic enzyme (NADP-ME type). A lase as in C4 and CAM photosynthesis. Guard cells in

Some characteristics of the three major plant groups

Characteristics C3 C4 CAM

Leaf anatomy in cross Diffuse distribution of Layer of bundle sheath Spongy, often lacking section organelles in cells around vascular palisade cells; mesophyll mesophyll and tissue with a high cells have large vacuoles palisade cells with less concentration of chloroplasts in bundle chloroplasts; layers of sheath cells if present mesophyll cells around bundle sheath Theoretical energy 1:3:2 1:5:2 1:6.5:2 requirement for net CO2 fixation (CO2:ATP:NADPH) Carboxylating enzyme Rubisco PEP carboxylase, then Darkness: PEP carboxy- rubisco lase; light: mainly rubisco CO2 compensation 30–70 0–10 0–5 in dark concentration, ppm CO2 Transpiration ratio, g H2O/ 450–950 250–350 50–55 g dry weight increase Maximum net photo- 15–40 40–80 1–4 synthetic rate, mg CO2/ (dm2 leaf)(h) Photosynthesis sensitive to Yes No Yes high O2 Photorespiration detectable Yes Only in bundle sheath Difficult to detect Leaf chlorophyll a/b ratio 2.8± 0.4 3.9± 0.6 2.5–3 Maximum growth rate, g dry 0.5–2 4–5 0.015–0.018 wt/(dm2 leaf)(day) Optimum temperature for 15–25° C (59–77° F) 30–40° C (86–104° F) About 35° C (95° F) photosynthesis Photosynthesis 481

C3 plants, which regulate the opening of stomatal in the dark by using respiration to utilize organic pores for gas exchange in leaves, also lack rubisco compounds from the environment. They are ther- and apparently use PEP carboxylase exclusively to mophilic bacteria found in hot springs around the fix CO2. world. They also distinguish themselves among the Contributions of the late Martin Gibbs to this arti- photosynthetic bacteria by possessing mobility. An cleare acknowledged. example is Chloroflexus aurantiacus. Gerald A. Berkowitz; Archie R. Portis, Jr.; Govindjee 4. Heliobacteria (Heliobacteriaceae). These are strictly anaerobic bacteria that contain bacterio- Bacterial Photosynthesis chlorophyll g. They grow primarily using organic Certain bacteria have the ability to perform photo- substrates and have not been shown to carry out synthesis. This was first noticed by Sergey Vinograd- autotrophic growth using only light and inorganic sky in 1889 and was later extensively investigated substrates. An example is Heliobacterium chlorum. by Cornelis B. Van Niel, who gave a general equa- Like plants, algae, and cyanobacteria, anoxygenic tion for bacterial photosynthesis. This is shown in photosynthetic bacteria are capable of photophos- reaction (9). phorylation, which is the production of adeno- sine triphosphate (ATP) from adenosine diphosphate bacteriochlorophyll 2H2A + CO2 + light −−−−−−−−−→ {CH2O}+2A + H2O (ADP) and inorganic phosphate (Pi) using light as the enzymes (9) primary energy source. Several investigators have suggested that the sole function of the light reaction where A represents any one of a number of reduc- in bacteria is to make ATP from ADP and Pi. The hy- tants, most commonly S (sulfur). drolysis energy of ATP (or the proton-motive force Photosynthetic bacteria cannot use water as the that precedes ATP formation) can then be used to hydrogen donor and are incapable of evolving oxy- drive the reduction of CO2 to carbohydrate by H2A gen. They are therefore called anoxygenic photo- in reaction (9). synthetic bacteria. The prokaryotic cyanobacteria Photochemical apparatus. Photosynthetic bacteria (formerly called blue-green algae) are excluded in do not have specialized organelles such as the chloro- this discussion of bacterial photosynthesis, since plasts of green plants. Electron micrographs of cer- their photosynthetic system closely resembles that tain photosynthetic bacteria show tiny spherical found in eukaryotic algae and higher plants discussed sacs, with double-layered walls, as a result of invagi- above. Anoxygenic photosynthetic bacteria can be nations which form stacks of membranes (Fig. 12a). classified in four major groups: Other photosynthetic bacteria have invaginations 1. Proteobacteria.Two groups with somewhat which form thylakoids (Fig. 12b). These intracyto- different properties are known. plasmic membranes, often called chromatophores, (A) Nonsulfur purple bacteria (Rhodospirillaceae). contain the photosynthetic apparatus and can be In these bacteria, H2Aisusually an organic H2 donor, isolated easily by mechanical disruption of bacte- such as succinate or malate; however, these bacteria riafollowed by differential centrifugation. Isolated can be adapted to use hydrogen gas as the reductant. chromatophores are often used for biochemical and They require vitamins for their growth and usually biophysical studies of bacterial photosynthesis. grow anaerobically in light, but they can also grow Reaction centers. The pigment bacteriochloro- aerobically in the dark by using respiration to utilize phyll (BChl) is a necessary component for bacterial organic compounds from the environment. They are thus facultative photoheterotrophs. Examples of this µ group are Rhodospirillum rubrum and Rhodobac- 0.25 m ter sphaeroides. (B) Sulfur purple bacteria (Chromatiaceae). These cannot grow aerobically, and H2Aisaninorganic sul- fur compound, such as hydrogen sulfide, H2S; the carbon source can be CO2. These bacteria are called (a) obligate photoautotrophic anaerobes. An example is Chromatium vinosum (alternate name: Allochro- matium vinosum). 2. Green sulfur bacteria (Chlorobiaceae). These bacteria are capable of using the same chemicals as Chromatiaceae but, in addition, use other organic H2 donors. They may then be called photoautotrophic and photoheterotrophic obligate anaerobes. An ex- (b) ample of the green sulfur bacteria is Chlorobium tepidum. Fig. 12. Photosynthetic bacteria. (a) Electron micrograph of Rhodobacter sphaeroides with vesicle-like invaginations 3. Green gliding bacteria (Chloroflexaceae) [also (from T. W. Goodwin, ed., Biochemistry of Chloroplasts, known as filamentous anoxygenic phototrophs, vol. 1, Academic Press, 1966). (b) Pictorial representation of FAP]. These are primarily photoorganotrophic bac- a stacked invagination in a photosynthetic bacterium; at left is a longitudinal section and at right is a transverse teria which can grow under anaerobic conditions section (after R. Whittenbury and A. G. McLee, Archiv. fur ¨ in light by photosynthesis or in aerobic conditions Mikrobiologie, 59:324–334, 1967). 482 Photosynthesis

photosynthesis. There are specialized BChl absorbed photon) must be very high (close to 1.0). molecules in bacteria which engage in the primary (3) The primary light reaction should occur at very ◦ ◦ chemical reactions of photosynthesis. In addition low temperatures, down to 1 K (−460 For−273 C). to these specialized molecules, there are 40– (4) The above photochemical reaction should be ex- 50 BChl molecules referred to as antenna pigments, tremely fast, that is, in the picosecond range. whose sole function is to harvest light energy and All the above criteria are fulfilled by P870, and thus transfer it to reaction center molecules. This is it is the reaction center of bacterial photosynthesis similar to the photosynthetic unit of plants, algae, in Rhodobacter sphaeroides. Among other reaction and cyanobacteria. Each reaction center contains a centers that have been identified and studied exten- special pair (dimer) of BChl molecules that engage sively are P890 in Chromatium vinosum, P960 in in chemical reactions after they trap the absorbed Rhodopseudomonas viridis (also called Blastochlo- light energy. They are also called the energy traps of ris viridis), P840 in Chlorobium limicola and P798 bacterial photosynthesis. Helicobacterium chlorum. Each species of bacteria The energy trap in Rhodobacter sphaeroides has has only one type of reaction center, unlike plants, been identified as P870. Such identification is carried algae, and cyanobacteria, which utilize two types out with a difference (absorption) spectrophotome- of reaction centers, PSI and PSII, that include P680 ter. In this instrument a weak monochromatic mea- and P700, respectively. The reaction centers from suring beam monitors the absorption of the sample; oxygenic photosynthetic organisms have been iden- abrief but bright actinic light given at right angles to tified by means similar to those used for bacterial the measuring beam initiates photosynthesis. When reaction centers. Reaction centers have been iso- photosynthesis occurs, changes in absorption take lated as pure proteins, which has served the im- place. Figure 13a shows the absorption spectrum of portant function of providing a well-defined system reaction centers isolated from R. sphaeroides. These in which primary reactions of photosynthesis can changes are measured as a function of the wave- be studied. A milestone in bacterial photosynthesis length of measuring light. A plot of the change in- wasreached in the early 1980s by the crystallization duced in R. sphaeroides reaction centers by an ac- and determination of the three-dimensional struc- tinic light flash, as a function of the wavelength of ture of Rhodopseudomonas viridis reaction centers measuring light, is the difference absorption spec- by Hartmut Michel, Johann Deisenhofer, and Robert trum (Fig. 13b). This spectrum is due largely to the Huber, who received the 1988 Nobel Prize in Chem- photooxidation of the BChl dimer, P870. istry for their work. These crystals enabled an atomic If P870 is the energy trap, then the following cri- resolution of the molecular structure of the reaction teria must be met: (1) It must undergo a reduction center to be obtained. or oxidation reaction, since this is the essential reac- Although isolated reaction centers are able to ab- tion of photosynthesis. The decrease in absorption at sorb light and convert it to chemical energy, the 870 nm (Fig. 13) is an oxidation reaction since chem- antenna pigment system in chromatophores (or in ical oxidants cause a similar change. (2) The quan- whole cells) absorbs most (>90%) of the light. The tum yield (number of trap molecules oxidized per antenna transfers this energy to the reaction center. Antenna BChl molecules are bound to protein in a specific manner; this binding and pigment-pigment 400 Bchl, Bpheo interactions modify the properties of the pigment Bchl 1 and define the absorption maxima and the width of − the absorption band. An example is B800 (B repre- cm

1 200 − Bpheo Bchl sents BChl, and the number indicates the wavelength of one of the absorption peaks in nanometers) found mM Bchl , ∋ Bpheo in Rhodobacter sphaeroides (Fig. 14). 0 Components of photosynthetic bacteria. These bac- (a) 400 600 800 1000 teria contain the usual components of living ma- terial: proteins, lipids, carbohydrates, deoxyribonu-

1 50 − cleic acid (DNA), ribonucleic acid (RNA), and

cm various metals. However, the specific components of 1 0 − interest to the electron transport system of bacterial

mM −50 ∋ , photosynthesis are quinones, pyridine nucleotides,

∆ −100 and various iron-containing proteins (cytochromes, ferredoxins, Rieske iron-sulfur centers, and others) (b) 400 600 800 900 1200 in addition to the photosynthetic pigments which 1000 wavelength, nm capture light energy. In contrast to plastoquinones found in plants, Fig. 13. Plots of (a) absorption spectrum and (b) the light-induced absorption changes in it, as occurring in bacteria contain substituted benzoquinones called reaction centers isolated from carotenoidless mutant R-26 ubiquinones (UQ or coenzyme Q) and substituted of Rhodobacter sphaeroides.Ina, bands attributed to naphthoquinones called menaquinones (MK or vita- bacteriochlorophyll and bacteriopheophytin are labeled BChl and BPheo, respectively. The ordinate in a is the min K2) which act as electron acceptors. The pur- millimolar extinction coefficient; in b,itisthe differential ple bacteria have a pool of UQ (about 25 UQ per extinction coefficient. (After R. K. Clayton, Photosynthesis: Physical Mechanisms and Chemical Patterns, Cambridge reaction center) which mediates transfer of elec- University Press, 1980) trons and protons between protein complexes in the Photosynthesis 483

0.8 ria utilizes Bchl a. Here, there is an “uphill” energy B transfer from the antenna to its reaction center. The green bacterium Chlorobium sp. contains a 0.6 B small amount of BChl a but a large quantity of an- other type of chlorophyll called chlorobium chloro- C phylls, BChl c, d or e, depending on the species. C 0.4 C B The BChl a has been shown to be associated with B the reaction center, and some antenna complexes

absorbance while the BChl c acts only as antenna. It is located in 0.2 B

0 400 600 800 wavelength, nm

Fig. 14. Absorption spectrum of chromatophores from the bacterium Rhodobacter sphaeroides. Absorption bands attributed to BChl a are labeled as B, and those attributed to carotenoids as C. (After R. K. Clayton, Photosynthesis: Physical Mechanisms and Chemical Patterns, Cambridge University Press, 1981) chromatophore membrane. However, MK is found only in some bacteria, sometimes in a smaller quan- tity (about 1–2 MK molecules per reaction center) than the more plentiful UQ. In these organisms, menaquinone’s function is probably limited to elec- tron transfer within the reaction center. Other or- (a) ganisms contain only menaquinone. In contrast to plants which contain NADP, the major pyridine nu- cleotide in bacteria is nicotinamide adenine dinu- cleotide (NAD); it is present in large quantities and M Mg MgA seems to be active in photosynthesis. Among the var- MgB ious cytochromes, the c-type cytochromes and the 4 ps b-type cytochromes are the important ones for bac- terial photosynthesis. Pigments. Most photosynthetic bacteria contain

BChl a,atetrahydroporphyrin. The chlorophyll of H-HB H-HA green plants, algae, and cyanobacteria, by contrast, 2 S is a dihydroporphyrin. In diethyl ether, BChl a has 70 ms 150 ps absorption maxima at 365, 605, and 770 nm. The infrared band of various antenna BChl a has maxima Q Q at 800 (B800), 850 (B850), or 890 nm (B890). These B Fe A antenna absorption bands in the bacterial cell are due to the formation of complexes of BChl a with 0.3 ms different proteins. See CHLOROPHYLL. (b) The reaction center protein (composed of L, M, Fig. 15. Reaction centers of bacteria. (a) Structure of the and H subunits) from Rhodobacter sphaeroides reaction center of the purple nonsulfur bacterium binds four BChl a and two bacteriopheophytin (BPh; Rhodopseudomonas viridis as determined by x-ray analysis similar to BChl but does not contain magnesium). of the crystalline preparation. In this diagram, the left side shows the complete structure in a space-filling Twoofthe BChl form the energy trap P870. An- representation. On the right side, the protein has been other BChl and a BPh are involved in the transfer of removed for clarity and only the components of the electron transport chains are shown (after R. E. electrons within the protein. The exact locations of Blankenship, Molecular Mechanisms of Photosynthesis, these chromophores in the reaction center protein Blackwell Science, Oxford, 2002). (b)Asimplified wasfirst established in the crystals of Rhodopseu- representation of the donor-acceptor complex based on the x-ray data and on spectroscopic data for Rhodobacter domonas viridis reaction centers (Fig. 15a). Sim- sphaeroides. The blocks define the aromatic ring systems ilar information is now available for Rhodobacter of bacteriochlorophyll (M-Mg and Mg), bacteriopheophytin sphaeroides reaction centers (Fig. 15b). (H-H), the quinones (Q), which are ubiquinone and menaquinone, and Fe2+. M-Mg is the primary electron The bacterium Rhodopseudomonas viridis uti- donor, a dimer of bacteriochlorophyll a (Rhodobacter lizes an antenna with an infrared absorption band sphaeroides)orb (Rhodopseudomonas viridis). Subscripts A and B label the two potential electron transfer pathways, at 1015 nm. The isolated BChl from this species has of which only pathway A appears active. The arrows show absorption maxima at 368, 582, and 795 nm in di- the various electron transfer reactions with their half-times. ethyl ether, and has been designated BChl b. The Note that QB is absent in the crystal of Rhodopseudomonas viridis (after J. F. Norris and G. Van Brakel, Photosynthesis, reaction center of R. viridis, P960, uses BChl b and in Govindjee, J. Amesz, and D. C. Fork, eds., Light Emission BPh b much in the same way as P870 in other bacte- by Plants and Bacteria, Academic Press, 1986). 484 Photosynthesis

chlorosome complexes, which are appressed to the pigments to the energy traps within this time for cytoplasmic side of the cell membrane and contain efficient photosynthesis to occur. In reaction center about 200,000 molecules of chlorobium chlorophyll. preparations, it takes only 3 ps to create a definitively The second group of pigments is the carotenoids, stable charge separation (see below) after the absorp- which have absorption peaks from 450 to 550 nm. tion of light. Moreover, the lifetime of the physical The carotenoids of photosynthetic bacteria are of state or states preceding P870 oxidation is <3 ps. great variety and include some which are found Thus, it appears that within a few picoseconds of in green plants, for example, the lycopenes. How- receiving excitation energy, the reaction center has ever, some are typical only of bacteria: γ -carotene, converted the absorbed light energy into chemical which is found in large quantities in green sulfur energy. Similar reactions occur in plants, algae, and bacteria, and spirilloxanthol, which is found mainly cyanobacteria. in purple bacteria. Carotenoids function to prevent Mechanisms of electron transport. The first act of photooxidation and destruction of antenna bacteri- photosynthesis is the absorption of light by various ochlorophyll. They also function in bacterial pho- pigments. As discussed above, light energy absorbed tosynthesis by transferring their absorbed energy by the carotenoids B800 and B850 is transferred to to bacteriochlorophyll. Similar roles are found for B875 and finally to the reaction centers, where the carotenoids in plants and cyanobacteria. primary reaction occurs: the oxidation of the reac- Transfer of excitation energy. Light energy absorbed tion center BChl dimer leads to bleaching of P870 by the carotenoids is transferred to BChl with vary- and reduction of an acceptor (Fig. 16). In the cur- ing efficiency (30–90%), as demonstrated by the rent model, P (short for P870 and so on) is oxidized method of sensitized fluorescence. (Similar meth- to P+ and an intermediate I is reduced to I− within ods have been used for demonstrating energy trans- afew picoseconds; I includes a BChl monomer and ferfrom carotenoids, chlorophyll b, and phyco- a BPh molecule. The reduced I− transfers the elec- bilins to chlorophyll a in oxygenic photosynthesiz- tron to an iron-quinone complex, reducing the pri- ers.) When light energy is absorbed by carotenoids, mary quinone (QA)toasemiquinone within 100–200 only the fluorescence of bacteriochlorophyll (B875) ps. For most anoxygenic bacteria, QA is ubiquinone, is observed. By the same method, energy trans- though for those containing both menaquinone and fer with efficiencies approaching 100% has been ubiquinone the menaquinone functions as QA. Al- demonstrated from B800 to B850 to B875. The though an iron atom is in this complex and is within high quantum yield (almost 1.0) of P870 oxida- 0.5–1.0 nm of the quinone, its presence is not nec- tion, when bacteria are excited in the antenna pig- essary for the reduction of QA, nor does the iron un- ments, is a clear demonstration of an extremely effi- dergo redox changes. The function of this nonheme cient excitation energy transfer by antenna pigments iron in the reaction center is unknown. In plants, and trapping in reaction centers. algae, and cyanobacteria, PSII contains QA, which is The lifetime of the excited state of antenna BChl in a bound plastoquinone; the function of the iron there the bacterial cell is of the order of 30–50 ps. The ex- is also unknown. citation energy must be channeled from the antenna The photooxidized donor BChl dimer, P+, can be

hn + - - nH e e H+ Cyt c

energy reaction center QH2 cytochrome proton antenna e- Q bc1 complex channel

+ H ATP synthase

+ ADP Pi

nH+ ATP

Fig. 16. Electron and proton transport in purple photosynthetic bacteria. For details and explanation of symbols, see the text. The shapes of the proteins are largely hypothetical. Photosynthesis 485 re-reduced by a cytochrome c in 1–30 µs, thus oxi- cyt c cyt c+ dizing the cytochrome. In Rhodobacter sphaeroides h␯ PQ Q P+Q–Q PQ–Q and a number of other species, this cytochrome A B A B A B is soluble Cyt c2.Inother bacteria (for example, Rhodopseudomonas viridis) the cytochrome that + UQH2 donates electrons to P is an integral part of the re- – PQAQB (11) action center. The photochemical reactions and the UQ electron transfers in the reaction center are summa- H+ rized in reaction (10). After this set of reactions, the h␯ – – + + – – + – + PQAQB H2 PQAQB (H )PQAQB (H )PQAQB (H ) h␯ 3 ps 200 ps + – PIQA P*IQA P I QA cyt c+ cyt c H+ cyt c cyt c+

+ – – to the cytochrome b-c complex (an integral mem- P IQA PIQA (10) 1–30 ␮s brane protein) which contains two b cytochromes, a c cytochrome, a Rieske iron-sulfur center, and two − electron is transferred from QA to QB (a bound UQ), quinone-binding sites. Plants also contain a similar − producing QAQB.Inasubsequent absorption of a complex, where cytochrome b is replaced by cy- − − photon, the QAQB state is created, which is followed tochrome b6, and cytochrome c is replaced by cy- − − by electron transfer from QA to QB,forming QBH2 tochrome f. The mechanism is strikingly similar, on a with the uptake of two protons. The bound quinol molecular level, to that of noncyclic electron transfer (QBH2)isreplaced by a UQ molecule. This cycle is from photosystem II to plastocyanin via the plasto- known as the two-electron gate and is summarized quinone pool and the cytochrome b-f complex. In all in reaction (11) [omitting the early photochemical likelihood, it includes a pathway called a Q cycle by steps illustrated in reaction (10)]. The same cycle Mitchell; this cycle incorporates two different redox- occurs in photosystem II, except that the electron linked pathways for the electrons. For each quinol + donor to P is tyrosine Z, and QBH2 is another plas- oxidized by this complex, two molecules of Cyt c are toquinol instead of ubiquinol. The molecular detail reduced, two protons are removed from the quinol, is so similar in oxygenic and anoxygenic photosyn- an additional two protons are removed from the cyto- thesizers that many of the herbicides which act to plasm, and these four protons are released into the − inhibit PSII electron transfer from QA to QB are also intermembrane space. Absorption of two photons − potent inhibitors of electron transfer from QA to QB leads to the translocation of four protons across the in photosynthetic bacteria. However, one difference membrane. Structural and mechanistic data on AT- − + involves a unique role of CO2/HCO3 in reaction (11) Pases indicate that 3–5 H ’s are needed to make an in PSII of plants and cyanobacteria, but not in pho- ATP(Fig. 16). tosynthetic bacteria. Bicarbonate has been shown to The mechanism described here for the generation be bound on the electron acceptor side of PSII, but of ATP from light energy is largely from studies on not in photosynthetic bacteria. Rhodobacter sphaeroides and is generally valid for Protons are taken up from the cytoplasm at the other purple photosynthetic bacteria. same time as the electrons reduce the quinones. The mechanisms for oxidizing the reduced sub- The first proton does not bind directly to the strate H2A[reaction (9)] are known in much less de- − − semiquinone (QA or QB), but instead it binds to a tail than those for photophosphorylation. Most sub- protonatable amino acid of the reaction center. The strates feed electrons into the quinone pool, and the net result from the absorption of two photons is the resulting quinol can be used by the cytochrome b-c formation of a ubiquinol in the membrane, the oxi- complex. An example is succinate, which reduces dation of two Cyt c, and the removal of two protons quinone via a succinate dehydrogenase. In bacte- from the cytoplasm of the bacterial cell. In plants ria that have a low potential cytochrome c bound and cyanobacteria, the quinol is plastoquinol, and it to the reaction center (such as Cyt c551 in Chro- is water that is ultimately oxidized. matium vinosum), electrons from some substrates The doubly reduced ubiquinone (QH2, quinol) can possibly be fed into the reaction center through through a cyclic pathway serves to re-reduce the this cytochrome. The electrons for the reduction of oxidized cytochrome (cyt c+). This cyclic reaction NAD+ in purple photosynthetic bacteria are from the (Fig. 16) is coupled to the production of ATP via quinone pool, but these electrons require additional the creation of a proton gradient (more accurately energy gained from the hydrolysis of ATP or action aproton motive force) across the membrane. Just of the proton motive force. as in plants, the proton motive force (which in- Alternatively, especially in some green bacteria, cludes two components: a membrane potential, and the primary stable acceptor of electrons in the aproton gradient) is used to drive ATP synthesis. reaction center may not be a quinone but an ac- Protons move down the potential gradient through ceptor with a negative enough oxidation-reduction the ATPase to contribute energy to drive the ADP + potential to directly reduce NAD+.Inseveral green Pi → ATPreaction. bacteria, this electron acceptor has been shown to This overall mechanism is consistent with P. be an iron-sulfur (Fe·S) center instead of a quinone. Mitchell’s chemiosmotic theory. The quinol pro- The midpoint redox potential of this Fe·S center is duced by the two-electron gate mechanism binds much lower than that for the quinone acceptor in 486 Photosphere

the purple photosynthetic bacteria. This Fe·S cen- a high current condition. See PHOTOELECTRIC DE- ter can then directly reduce a ferredoxin, and this VICES. W. R. Sittner can drive the NAD+ → NADH reaction. The reduced ferredoxin may also feed electrons into a cytochrome b complex from which a soluble Cyt c could be Phototube reduced, thus allowing cyclic electron transfer to occur. This scheme is very reminiscent of PSI-driven An electron tube comprising a photocathode and an reactions in oxygenic photosynthesis. However, not anode mounted within an evacuated glass envelope all green photosynthetic bacteria follow the above through which radiant energy is transmitted to the pattern, but instead they resemble more the purple photocathode. A gas phototube contains, in addi- photosynthetic bacteria. tion, argon or other inert gas which provides ampli- The reduced pyridine nucleotide NADH and the fication of the photoelectric current by partial ion- ATP made in the light reactions are then utilized ization of the gas. The photocathode emits electrons to convert carbon sources into carbohydrates. The when it is exposed to ultraviolet, visible, or near- pathway of carbon in anoxygenic photosynthetic infrared radiation. The anode is operated at a posi- bacteria involves a reversed tricarboxylic acid tive potential with respect to the photocathode. See (Krebs) cycle or another cycle called the hydrox- ELECTRICAL CONDUCTION IN GASES; ELECTRON TUBE. yproprionate cycle. See BACTERIAL PHYSIOLOGY AND Characteristics. A phototube responds to radiation METABOLISM. over a limited range of the spectrum that is deter- Govindjee; Robert E. Blankenship; R. J. Shopes mined by the photocathode material. Radiant sen- Bibliography. R. E. Blankenship, Molecular Mecha- sitivity, shown in the illustration as a function of nisms of Photosynthesis, Blackwell Science, Oxford, wavelength, is the photoelectric current emitted per 2002; B. Demmig-Adams et al. (eds.), Photoprotec- unit of incident monochromatic radiant power. Sen- tion, Photoinhibition, Gene Regulation, and Envi- sitivity on the short-wavelength side of the curves ronment, Springer, Dordrecht, 2005; P.G. Falkowski is limited by the transmittance of the glass enve- and J. A. Raven, Aquatic Photosynthesis,2ded., lope. Electron affinity of the photocathode deter- Princeton University Press, Princeton, 2007; J. H. mines the long-wavelength threshold of sensitivity. Golbeck (ed.), Photosystem I: The Light-Driven Plas- See PHOTOEMISSION. tocyanin: Ferredoxin Oxidoreductase, Springer, Typical phototube characteristics are summarized Dordrecht, 2006; Govindjee et al. (eds.), Discover- in the table. Quantum efficiency, or photoelectron ies in Photosynthesis, Springer, Dordrecht, 2005; yield, is the number of electrons emitted per inci- B. R. Green and W.W.Parson (eds.), Light-Harvesting dent photon. It is tabulated at the wavelength of Antennas in Photosynthesis, Springer, Dordrecht, maximum response. For photometric applications 2003; B. Grimm et al. (eds.), Chlorophylls and Bac- a useful parameter is luminous sensitivity: the teriochlorophylls: Biochemistry, Biophysics, Func- photoelectric current per lumen incident from a tions and Applications, Springer, Dordrecht, 2006; specified source of light. A source commonly used B. Ke, Photosynthsis: Photobiochemistry and Pho- tobiophysics, Springer, Dordrecht, 2001; A. W. D. 100 Larkum et al. (eds.), Photosynthesis in Algae, 80 S-17 Springer, Dordrecht, 2003; R. C. Leegood et al. 60 (eds.), Photosynthesis: Physiology and Metabolism, 40 S-11 Springer, Dordrecht, 2000; G. Papageorgiou and S-13 Govindjee (eds.), Chlorophyll a Fluorescence: A S-4 20 Signature of Photosynthesis, Springer, Dordrecht, 2005; R. R. Wise and J. K. Hoober (eds.), The Struc- S-20 10 ture and Function of Plastids, Springer, Dordrecht, 8 2006; T. Wydrzynski and K. Satoh (eds.), Photosys- 6 tem II: The Light-Driven Water-Plastoquinone Oxi- 4 doreductase, Springer, Dordrecht, 2005. S-5 2 S-1 Phototransistor 1 radiant sensitivity, mA/watt radiant sensitivity, A semiconductor device with electrical character- 0.8 istics that are light-sensitive. Phototransistors differ 0.6 from photodiodes in that the primary photoelectric 0.4 current is multiplied internally in the device, thus S-8 S-3 increasing the sensitivity to light. For a discussion of 0.2 this property see TRANSISTOR. Some types of phototransistors are supplied with 0.1 a third, or base, lead. This lead enables the pho- 200 400 600 800 1000 1200 totransistor to be used as a switching, or bistable, wavelength, nm device. The application of a small amount of light Curves of the average spectral sensitivity characteristics of causes the device to switch from a low current to some typical phototubes.