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 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. (Heliobacteriaceae). These are strictly anaerobic bacteria that contain bacterio- Bacterial Photosynthesis 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 , 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 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 (). 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. .Two groups with somewhat which form thylakoids (Fig. 12b). These intracyto- different properties are known. plasmic membranes, often called chromatophores, (A) Nonsulfur (). 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 . Examples of this µ group are Rhodospirillum rubrum and Rhodobac- 0.25 m ter sphaeroides. (B) Sulfur purple bacteria (). 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 vinosum (alternate name: Allochro- matium vinosum). 2. (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 tepidum. Fig. 12. Photosynthetic bacteria. (a) Electron micrograph of sphaeroides with vesicle-like invaginations 3. Green gliding bacteria (Chloroflexaceae) [also (from T. W. Goodwin, ed., Biochemistry of Chloroplasts, known as filamentous anoxygenic , 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 . 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 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

complexes, which are appressed to the pigments to the energy traps within this time for cytoplasmic side of the 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.), 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.