8885dc197253/1/041:59 PM Page725mac76mac76:385reb 19.7 Light Absorption 725 19.7 Light Absorption Chlorophylls Absorb Light Energy for Photosynthesis Visible light is electromagnetic radiation of wavelengths The most important light-absorbing pigments in the thy- 400 to 700 nm, a small part of the electromagnetic spec- lakoid membranes are the chlorophylls, green pigments trum(Fig. 19-39), ranging from violet to red. The en- with polycyclic, planar structures resembling the proto- ergy of a single photon(a quantum of light) is greater porphyrin of hemoglobin(see Fig 5-1), except that Mg at the violet end of the spectrum than at the red end; not Fe, occupies the central position (Fig. 19-40).The shorter wavelength(and higher frequency) correspond four inward-oriented nitrogen atoms of chlorophyll are to higher energy. The energy, E, in a"mole "of photons coordinated with the Mg. All chlorophylls have a long (1 einstein, or 6 x 10 photons)of visible light is 170 phytol side chain, esterified to a carboxyl-group sub- to 300 k, as given by the Planck equation stituent in ring I, and chlorophylls also have a fifth five- membered ring not present in heme. E=h The heterocyclic five-ring system that surrounds where h is Planck's constant(6.626 X 10 J-S)and v the Mg has an extended polyene structure, with al- is the wavelength. These amounts of energy are almost ternating single and double bonds. Such polyenes char an order of magnitude greater than the 30 to 50 kJ re- acteristically show strong absorption in the visible re quired to synthesize a mole of ATP from ADP and Pi. gion of the spectrum (Fig. 19-41); the chlorophylls have When a photon is absorbed, an electron in the ab- unusually high molar extinction coefficients(see Box sorbing molecule(chromophore) is lifted to a higher 3-1) and are therefore particularly well-suited for ab- energy level. This is an all-or-nothing event; to be ab- sorbing visible light during photosynthesis. sorbed, the photon must contain a quantity of energy (a Chloroplasts always contain both chlorophyll a and quantum) that exactly matches the energy of the ele chlorophyll b(Fig. 19-40a). Although both are green tronic transition. A molecule that has absorbed a photon their absorption spectra are sufficiently different (Fig is in an excited state, which is generally unstable. An 19-41) that they complement each other's range of electron lifted into a higher-energy orbital usually re- light absorption in the visible region. Most plants con- turns rapidly to its normal lower-energy orbital; the ex- tain about twice as much chlorophyll a as chlorophyll cited molecule decays to the stable ground state, b. The pigments in algae and photosynthetic bacteria giving up the absorbed quantum as light or heat or us- include chlorophylls that differ only slightly from the ing it to do chemical work. Light emission accompany- plant pigments ing decay of excited molecules(called fluorescence Chlorophyll is always associated with specific is always at a longer wavelength (lower energy)than binding proteins, forming light-harvesting com that of the absorbed light(see Box 12-2). An alterna- plexes (LiCs) in which chlorophyll molecules are tive mode of decay important in photosynthesis involves fixed in relation to each other, to other protein com direct transfer of excitation energy from an excited mol- plexes, and to the membrane. The detailed structure ecule to a neighboring molecule. Just as the photon is of one light-harvesting complex is known from x-ray a quantum of light energy, so the exciton is a quantum crystallography(Fig. 19-42) It contains seven mole- of energy passed from an excited molecule to another cules of chlorophyll a, five of chlorophyll b, and two molecule in a process called exciton transfer. of the accessory pigment lutein(see below) Type of Gamma rays X rays UV Infrared Microwaves Radio waves <1nm100n <1 millimeter 1 meter Thousands of mete Visible light Yellow Violet Blue Cyan Green Wavelength 380 00560600 Energy 240 170 (ke/einstein) FIGURE 19-39 Electromagnetic radiation. The spectrum of electromagnetic radiation, and the energy of photons in the visible range of the spectrum. One einstein is 6X 10 phe
19.7 Light Absorption Visible light is electromagnetic radiation of wavelengths 400 to 700 nm, a small part of the electromagnetic spectrum (Fig. 19–39), ranging from violet to red. The energy of a single photon (a quantum of light) is greater at the violet end of the spectrum than at the red end; shorter wavelength (and higher frequency) corresponds to higher energy. The energy, E, in a “mole” of photons (1 einstein, or 6 1023 photons) of visible light is 170 to 300 kJ, as given by the Planck equation: E h where h is Planck’s constant (6.626 1034 J s) and is the wavelength. These amounts of energy are almost an order of magnitude greater than the 30 to 50 kJ required to synthesize a mole of ATP from ADP and Pi . When a photon is absorbed, an electron in the absorbing molecule (chromophore) is lifted to a higher energy level. This is an all-or-nothing event; to be absorbed, the photon must contain a quantity of energy (a quantum) that exactly matches the energy of the electronic transition. A molecule that has absorbed a photon is in an excited state, which is generally unstable. An electron lifted into a higher-energy orbital usually returns rapidly to its normal lower-energy orbital; the excited molecule decays to the stable ground state, giving up the absorbed quantum as light or heat or using it to do chemical work. Light emission accompanying decay of excited molecules (called fluorescence) is always at a longer wavelength (lower energy) than that of the absorbed light (see Box 12–2). An alternative mode of decay important in photosynthesis involves direct transfer of excitation energy from an excited molecule to a neighboring molecule. Just as the photon is a quantum of light energy, so the exciton is a quantum of energy passed from an excited molecule to another molecule in a process called exciton transfer. Chlorophylls Absorb Light Energy for Photosynthesis The most important light-absorbing pigments in the thylakoid membranes are the chlorophylls, green pigments with polycyclic, planar structures resembling the protoporphyrin of hemoglobin (see Fig. 5–1), except that Mg2, not Fe2, occupies the central position (Fig. 19–40). The four inward-oriented nitrogen atoms of chlorophyll are coordinated with the Mg2. All chlorophylls have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV, and chlorophylls also have a fifth fivemembered ring not present in heme. The heterocyclic five-ring system that surrounds the Mg2 has an extended polyene structure, with alternating single and double bonds. Such polyenes characteristically show strong absorption in the visible region of the spectrum (Fig. 19–41); the chlorophylls have unusually high molar extinction coefficients (see Box 3–1) and are therefore particularly well-suited for absorbing visible light during photosynthesis. Chloroplasts always contain both chlorophyll a and chlorophyll b (Fig. 19–40a). Although both are green, their absorption spectra are sufficiently different (Fig. 19–41) that they complement each other’s range of light absorption in the visible region. Most plants contain about twice as much chlorophyll a as chlorophyll b. The pigments in algae and photosynthetic bacteria include chlorophylls that differ only slightly from the plant pigments. Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in which chlorophyll molecules are fixed in relation to each other, to other protein complexes, and to the membrane. The detailed structure of one light-harvesting complex is known from x-ray crystallography (Fig. 19–42). It contains seven molecules of chlorophyll a, five of chlorophyll b, and two of the accessory pigment lutein (see below). 19.7 Light Absorption 725 380 Violet Green Blue Cyan Yellow Orange Red Wavelength (nm) Energy (kJ/einstein) 300 430 500 560 600 650 750 240 200 170 Wavelength Type of radiation Gamma rays Visible light X rays UV Infrared Microwaves Radio waves 1 nm 100 nm 1 millimeter 1 meter Thousands of meters FIGURE 19–39 Electromagnetic radiation. The spectrum of electromagnetic radiation, and the energy of photons in the visible range of the spectrum. One einstein is 6 1023 photons. 8885d_c19_725 3/1/04 1:59 PM Page 725 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page726 6mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation O in chlorophyll b Saturated bond CH3 YI>CH2CH phytol side chain CI I>CH CH CH CHs0 O CO CO0 CH3 2 phycocyanobilin H CHs CH2 CH2 CH3 CH, CH ycocyanobil CHs CHs Hs CH CHI CHs CH HC、C CHs CHs CH. H3C CHs CHa tein(xanthophyll) FIGURE 19-40 Primary and secondary photopigments. (a) Chloro- Carotene (a carotenoid) and (d) lutein(a xanthophyll) are accessory phylls a and b and bacteriochlorophyll are the primary gatherers of pigments in plants. The areas shaded pink are the conjugated systems ight energy()Phycoerythrobilin and phycocyanobilin(phycobilins) (alternating single and double bonds)that largely account for the ab- are the antenna pigments in cyanobacteria and red algae. (c)B- sorption of visible light Cyanobacteria and red algae employ phycobilins central Mg. Phycobilins are covalently linked to spe- such as phycoerythrobilin and phycocyanobilin (Fig. cific binding proteins, forming phycobiliproteins 19-40b)as their light-harvesting pigments. These open- which associate in highly ordered complexes called phy- chain tetrapyrroles have the extended polyene system cobilisomes(Fig. 19-43) that constitute the primary found in chlorophylls, but not their cyclic structure or light-harvesting structures in these microorganisms
Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19–40b) as their light-harvesting pigments. These openchain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or central Mg2. Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phycobilisomes (Fig. 19–43) that constitute the primary light-harvesting structures in these microorganisms. 726 Chapter 19 Oxidative Phosphorylation and Photophosphorylation A CH2 N Mg H O C I II IV III G D B M 0 ; ; H H CH2 CH3 CH2 CH3 CH CH3 N N N D CH3 O CH3 O CH2 B D O C O CH2 CH3 CH3 CH3 CH3 G CH3 phytol side chain b-Carotene O C CH3 in bacteriochlorophyll CHO in chlorophyll b Saturated bond in bacteriochlorophyll A J G CH3 CH3 CH3 CH3 H3C CH3 CH3 CH3 CH3 CH3 Phycoerythrobilin A A A A COO N CH3 CH3 CH2 COO CH2 CH3 CH2 CH2 N H N H CH3 CH O A CH3 N H CH3 CH O Unsaturated bond in phycocyanobilin B in phycocyanobilin CH2 CH3 Chlorophyll a G G (a) (b) (c) Lutein (xanthophyll) CH OH 3 H C3 H C3 H3C CH3 CH3 CH3 HO CH3 CH3 CH3 (d) FIGURE 19–40 Primary and secondary photopigments. (a) Chlorophylls a and b and bacteriochlorophyll are the primary gatherers of light energy. (b) Phycoerythrobilin and phycocyanobilin (phycobilins) are the antenna pigments in cyanobacteria and red algae. (c) - Carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants. The areas shaded pink are the conjugated systems (alternating single and double bonds) that largely account for the absorption of visible light. 8885d_c19_690-750 3/1/04 11:32 AM Page 726 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page727 6mac76:385 Chlorophyll b Phycoerythrin FIGURE 19-41 Absorption of visible light by photopigments. Plants The relative amounts of chlorophylls and accessory pigments are are green because their pigments absorb light from the red and blue characteristic of a particular plant species Variation in the proportions regions of the spectrum, leaving primarily green light to be reflected of these pigments is responsible for the range of colors of photosyn- or transmitted. Compare the absorption spectra of the pigments with thetic organisms, from the deep blue-green of spruce needles, to the the spectrum of sunlight reaching the earth's surface; the combination greener green of maple leaves, to the red, brown, or purple color of of chlorophylls(a and b) and accessory pigments enables plants to some species of multicellular algae and the leaves of some foliage rvest most of the energy available in sunlight. plants favored by gardeners 480-570nm 550-650nm transfer Thylakoid FIGURE 19-42 A light-harvesting complex, LHCIL. The functional FIGURE 19-43 A phycobilisome. In these highly structured assem- unit is an LHC trimer, with 36 chlorophyll and 6 lutein molecules. blies found in cyanobacteria and red algae, phycobilin pigments hown here is a monomer, viewed in the plane of the membrane, with bound to specific proteins form complexes called phycoerythrin(PE), its three transmembrane a-helical segments, seven chlorophyll a phycocyanin(PC), and allophycocyanin(AP). The energy of photons molecules(green), five chlorophyll b molecules(red), and two mole- absorbed by PE or PC is conveyed through AP (a phycocyanobilin. cules of the accessory pigment lutein (yellow), which form an intern binding protein) to chlorophyll a of the reaction center by exciton cross-brace transfer, a process discussed in the text
19.7 Light Absorption 727 Absorption 300 Sunlight reaching the earth Wavelength (nm) 400 500 600 700 Chlorophyll b -Carotene Phycocyanin Chlorophyll a Phycoerythrin Lutein 800 FIGURE 19–41 Absorption of visible light by photopigments. Plants are green because their pigments absorb light from the red and blue regions of the spectrum, leaving primarily green light to be reflected or transmitted. Compare the absorption spectra of the pigments with the spectrum of sunlight reaching the earth’s surface; the combination of chlorophylls (a and b) and accessory pigments enables plants to harvest most of the energy available in sunlight. The relative amounts of chlorophylls and accessory pigments are characteristic of a particular plant species. Variation in the proportions of these pigments is responsible for the range of colors of photosynthetic organisms, from the deep blue-green of spruce needles, to the greener green of maple leaves, to the red, brown, or purple color of some species of multicellular algae and the leaves of some foliage plants favored by gardeners. FIGURE 19–42 A light-harvesting complex, LHCII. The functional unit is an LHC trimer, with 36 chlorophyll and 6 lutein molecules. Shown here is a monomer, viewed in the plane of the membrane, with its three transmembrane -helical segments, seven chlorophyll a molecules (green), five chlorophyll b molecules (red), and two molecules of the accessory pigment lutein (yellow), which form an internal cross-brace. FIGURE 19–43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilinbinding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text. PE PE PC 550–650 nm PC AP AP Thylakoid membrane Exciton transfer Chlorophyll a reaction center 480–570 nm Light 8885d_c19_690-750 3/1/04 11:32 AM Page 727 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page728 6mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Accessory Pigments Extend the Range Chlorophyll Funnels the Absorbed Energy to Reaction Centers by Exciton Transfer In addition to chlorophylls, thylakoid membranes con- The light-absorbing pigments of thylakoid or bacterial tain secondary light-absorbing pigments, or accessory membranes are arranged in functional arrays called pigments, called carotenoids. Carotenoids may be yel- photosystems. In spinach chloroplasts, for example low, red, or purple. The most important are B-carotene, each photosystem contains about 200 chlorophyll and which is a red-orange isoprenoid, and the yellow 50 carotenoid molecules. All the pigment molecules in a carotenoid lutein(Fig. 19-40c, d). The carotenoid pig- photosystem can absorb photons, but only a few chloro- ments absorb light at wavelengths not absorbed by the phyll molecules associated with the photochemical re- chlorophylls(Fig. 19-41) and thus are supplementary action center are specialized to transduce light into light receptors. chemical energy. The other pigment molecules in a Experimental determination of the effectiveness of photosystem are called light-harvesting or antenna light of different colors in promoting photosynthesis molecules. They absorb light energy and transmit ields an action spectrum (Fig. 19-44), often useful rapidly and efficiently to the reaction center (Fig. 19-45) in identifying the pigment primarily responsible for a bi- The chlorophyll molecules in light-harvesting com- ological effect of light. By capturing light in a region of plexes have light-absorption properties that are subtly the spectrum not used by other organisms, a photosyn- different from those of free chlorophyll. When isolated thetic organism can claim a unique ecological niche For chlorophyll molecules in vitro are excited by light, the example, the phycobilins in red algae and cyanobacte- absorbed energy is quickly released as fluorescence and ria absorb light in the range 520 to 630nm (Fig. 19-41), heat, but when chlorophyll in intact leaves is excited by allowing them to occupy niches where light of lower or visible light(Fig. 19-46, step 1), very little fluores higher wavelength has been filtered out by the pigments cence is observed. Instead, the excited antenna chloro- of other organisms living in the water above them, or by phyll transfers energy directly to a neighboring chloro- the water itself phyll molecule, which becomes excited as the first molecule returns to its ground state(step (2).This transfer of energy, exciton transfer, extends to a third fourth, or subsequent neighbor, until one of a special pair of chlorophyll a molecules at the photochemical re- action center is excited(step 3). In this excited chloro- phyll molecule, an electron is promoted to a higher- energy orbital. This electron then passes to a nearby electron acceptor that is part of the electron-transfer chain, leaving the reaction-center chlorophyll with a FIGURE 19-44 Two ways to determine the action spectrum for pho- (a) tosynthesis.(a) Results of a classic experiment performed by T.W.En- glemann in 1882 to determine the wavelength of light that is most ef- fective in supporting photosynthesis. Englemann placed cells of a filamentous photosynthetic alga on a microscope slide and illuminated them with light from a prism, so that one part of the filament received mainly blue light, another part yellow, another red. To determine which algal cells carried out photosynthesis most actively, Englemann also placed on the microscope slide bacteria known to migrate toward re- gions of high O2 concentration. After a period of illumination, the dis. tribution of bacteria showed highest O2 levels (produced by photo- synthesis)in the regions illuminated with violet and red light (b) Results of a similar (an oxygen electrode) for the measurement of O2 production. An ac- ion spectrum (as shown here) describes the relative rate of photo- synthesis for illumination with a constant number of photons of dif- ferent wavelengths. An action spectrum is useful because, by comparison with absorption spectra(such as those in Fig. 19-41),it b) suggests which pigments can channel energy into photosynthesis
Accessory Pigments Extend the Range of Light Absorption In addition to chlorophylls, thylakoid membranes contain secondary light-absorbing pigments, or accessory pigments, called carotenoids. Carotenoids may be yellow, red, or purple. The most important are -carotene, which is a red-orange isoprenoid, and the yellow carotenoid lutein (Fig. 19–40c, d). The carotenoid pigments absorb light at wavelengths not absorbed by the chlorophylls (Fig. 19–41) and thus are supplementary light receptors. Experimental determination of the effectiveness of light of different colors in promoting photosynthesis yields an action spectrum (Fig. 19–44), often useful in identifying the pigment primarily responsible for a biological effect of light. By capturing light in a region of the spectrum not used by other organisms, a photosynthetic organism can claim a unique ecological niche. For example, the phycobilins in red algae and cyanobacteria absorb light in the range 520 to 630 nm (Fig. 19–41), allowing them to occupy niches where light of lower or higher wavelength has been filtered out by the pigments of other organisms living in the water above them, or by the water itself. Chlorophyll Funnels the Absorbed Energy to Reaction Centers by Exciton Transfer The light-absorbing pigments of thylakoid or bacterial membranes are arranged in functional arrays called photosystems. In spinach chloroplasts, for example, each photosystem contains about 200 chlorophyll and 50 carotenoid molecules. All the pigment molecules in a photosystem can absorb photons, but only a few chlorophyll molecules associated with the photochemical reaction center are specialized to transduce light into chemical energy. The other pigment molecules in a photosystem are called light-harvesting or antenna molecules. They absorb light energy and transmit it rapidly and efficiently to the reaction center (Fig. 19–45). The chlorophyll molecules in light-harvesting complexes have light-absorption properties that are subtly different from those of free chlorophyll. When isolated chlorophyll molecules in vitro are excited by light, the absorbed energy is quickly released as fluorescence and heat, but when chlorophyll in intact leaves is excited by visible light (Fig. 19–46, step 1 ), very little fluorescence is observed. Instead, the excited antenna chlorophyll transfers energy directly to a neighboring chlorophyll molecule, which becomes excited as the first molecule returns to its ground state (step 2 ). This transfer of energy, exciton transfer, extends to a third, fourth, or subsequent neighbor, until one of a special pair of chlorophyll a molecules at the photochemical reaction center is excited (step 3 ). In this excited chlorophyll molecule, an electron is promoted to a higherenergy orbital. This electron then passes to a nearby electron acceptor that is part of the electron-transfer chain, leaving the reaction-center chlorophyll with a 728 Chapter 19 Oxidative Phosphorylation and Photophosphorylation (a) 400 20 Relative rate of photosynthesis Wavelength (nm) 40 60 80 100 0 500 600 700 (b) FIGURE 19–44 Two ways to determine the action spectrum for photosynthesis. (a) Results of a classic experiment performed by T. W. Englemann in 1882 to determine the wavelength of light that is most effective in supporting photosynthesis. Englemann placed cells of a filamentous photosynthetic alga on a microscope slide and illuminated them with light from a prism, so that one part of the filament received mainly blue light, another part yellow, another red. To determine which algal cells carried out photosynthesis most actively, Englemann also placed on the microscope slide bacteria known to migrate toward regions of high O2 concentration. After a period of illumination, the distribution of bacteria showed highest O2 levels (produced by photosynthesis) in the regions illuminated with violet and red light. (b) Results of a similar experiment that used modern techniques (an oxygen electrode) for the measurement of O2 production. An action spectrum (as shown here) describes the relative rate of photosynthesis for illumination with a constant number of photons of different wavelengths. An action spectrum is useful because, by comparison with absorption spectra (such as those in Fig. 19–41), it suggests which pigments can channel energy into photosynthesis. 8885d_c19_690-750 3/1/04 11:32 AM Page 728 mac76 mac76:385_reb:
8885dc19690-7503/1/0411:32 AM Page729 6mac76:385 19.7 Light Absorption 729 Ante Reaction-center These molecules absorb light energy, Light excites a antenna molecule (chlorophyll or Carotenoids, other molecul Light molecule passes energy transfer) Photochemical reaction here inverts the energy of a photon into a separation of charge, initiating electron flow. FIGURE 19-45 Organization of photosystems in the thylakoid mem- This energy is brane. Photosystems are tightly packed in the thylakoid membrane, Electron with several hundred antenna chlorophylls and accessory pigments rrounding a photoreaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (black arrow). Also embedded in the thylakoid mem- brane are the cytochrome b,f comp 19-52) missing electron (an"electron hole, denoted by in Fig. 19-46)(step4). The electron acceptor acquires a negative charge in this transaction. The electron lost y the reaction-center chlorophyll is replaced by an electron from a neighboring electron-donor molecule (step 5)), which thereby becomes positively charged In this way, excitation by light causes electric charge separation and initiates an o.midation-reduction The electro Electron chain filled by al from FIGURE 19-46 Exciton and electron transfer. This scheme shows conversion of the energy of an absorbed photon into paration of charges at the reaction center. The steps are further de- ribed in the text. Note that step (1 may repeat between succes- sive antenna molecules until the exciton reaches a reaction-center chlorophyll. The asterisk ()represents the excited state of an antenna The absorption of a photon has caused molecule
19.7 Light Absorption 729 These molecules absorb light energy, transferring it between molecules until it reaches the reaction center. Antenna chlorophylls, bound to protein Carotenoids, other accessory pigments Light Reaction center Photochemical reaction here converts the energy of a photon into a separation of charge, initiating electron flow. FIGURE 19–45 Organization of photosystems in the thylakoid membrane. Photosystems are tightly packed in the thylakoid membrane, with several hundred antenna chlorophylls and accessory pigments surrounding a photoreaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (black arrow). Also embedded in the thylakoid membrane are the cytochrome b6f complex and ATP synthase (see Fig. 19–52). 1 2 3 4 5 The absorption of a photon has caused separation of charge in the reaction center. Antenna molecules Reaction-center chlorophyll * * * Electron acceptor Electron donor – + – + Light The electron hole in the reaction center is filled by an electron from an electron donor. The excited reactioncenter chlorophyll passes an electron to an electron acceptor. This energy is transferred to a reaction-center chlorophyll, exciting it. The excited antenna molecule passes energy to a neighboring chlorophyll molecule (resonance energy transfer), exciting it. Light excites an antenna molecule (chlorophyll or accessory pigment), raising an electron to a higher energy level. FIGURE 19–46 Exciton and electron transfer. This generalized scheme shows conversion of the energy of an absorbed photon into separation of charges at the reaction center. The steps are further described in the text. Note that step 1 may repeat between successive antenna molecules until the exciton reaches a reaction-center chlorophyll. The asterisk (*) represents the excited state of an antenna molecule. missing electron (an “electron hole,” denoted by in Fig. 19–46) (step 4 ). The electron acceptor acquires a negative charge in this transaction. The electron lost by the reaction-center chlorophyll is replaced by an electron from a neighboring electron-donor molecule (step 5 ), which thereby becomes positively charged. In this way, excitation by light causes electric charge separation and initiates an oxidation-reduction chain. 8885d_c19_690-750 3/1/04 11:32 AM Page 729 mac76 mac76:385_reb: