Ch3 H3c Ch3

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) ß-

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.

Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19-40b) as their light-harvesting pigments. These open-chain 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 phy-cobilisomes (Fig. 19-43) that constitute the primary light-harvesting structures in these microorganisms.

Chlorophyll Special Pair Image

300 400 500 600 700 800

Wavelength (nm)

300 400 500 600 700 800

Wavelength (nm)

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 photosyn-thetic 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.

Chlorophyll Special Pair Image

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 a-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 19-42 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 a-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.

Light

480-570 nm 550-650 nm

Exciton transfer

Light

480-570 nm 550-650 nm

Exciton transfer

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 phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text.

Thylakoid membrane

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 phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text.

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 photosyn-thetic organism can claim a unique ecological niche. For example, the phycobilins in red algae and cyanobacte-ria 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 (!) ), 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 ©). 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 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 photosynthesis. (a) Results of a classic experiment performed by T. W. En-glemann 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.

Antenna chlorophylls, bound to protein

Carotenoids, other accessory pigments

These molecules absorb light energy, transferring it between molecules until it reaches the reaction center.

Antenna chlorophylls, bound to protein

Carotenoids, other accessory pigments

A A4 —d

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).

missing electron (an "electron hole," denoted by + in Fig. 19-46) (step @). 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 ©), which thereby becomes positively charged. In this way, excitation by light causes electric charge separation and initiates an oxidation-reduction chain.

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.

Reaction-center chlorophyll

Light excites an antenna molecule (chlorophyll or accessory pigment), raising an electron to a higher energy level.

The excited antenna molecule passes energy to a neighboring chlorophyll molecule (resonance energy transfer), exciting it.

This energy is transferred to a reaction-center chlorophyll, exciting it.

Reaction-center chlorophyll

The excited reaction-center chlorophyll passes an electron to an electron acceptor.

The electron hole in the reaction center is filled by an electron from an electron donor.

Electron acceptor

Electron donor

The absorption of a photon has caused separation of charge in the reaction center.

SUMMARY 19.7 Light Absorption

■ Photophosphorylation in the chloroplasts of green plants and in cyanobacteria involves electron flow through a series of membrane-bound carriers.

■ In the light reactions of plants, absorption of a photon excites chlorophyll molecules and other (accessory) pigments, which funnel the energy into reaction centers in the thylakoid membranes. In the reaction centers, photoexcitation results in a charge separation that produces a strong electron donor (reducing agent) and a strong electron acceptor.

19.8 The Central Photochemical Event: Light-Driven Electron Flow

Light-driven electron transfer in plant chloroplasts during photosynthesis is accomplished by multienzyme systems in the thylakoid membrane. Our current picture of photosynthetic mechanisms is a composite, drawn from studies of plant chloroplasts and a variety of bacteria and algae. Determination of the molecular structures of bacterial photosynthetic complexes (by x-ray crystallography) has given us a much improved understanding of the molecular events in photosynthesis in general.

Bacteria Have One of Two Types of Single Photochemical Reaction Center

One major insight from studies of photosynthetic bacteria came in 1952 when Louis Duysens found that illumination of the photosynthetic membranes of the purple bacterium Rhodospirillum rubrum with a pulse of light of a specific wavelength (870 nm) caused a temporary decrease in the absorption of light at that wavelength; a pigment was "bleached" by 870 nm light. Later studies by Bessel Kok and Horst Witt showed similar bleaching of plant chloroplast pigments by light of 680 and 700 nm. Furthermore, addition of the (nonbiological) electron acceptor [Fe(CN)6]3— (ferricyanide) caused bleaching at these wavelengths without illumination. These findings indicated that bleaching of the pigments was due to the loss of an electron from a photochemical reaction center. The pigments were named for the wavelength of maximum bleaching: P870, P680, and P700.

Photosynthetic bacteria have relatively simple pho-totransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical studies have revealed many of the molecular details of reaction centers of bacteria, which therefore serve as prototypes for the more complex phototransduction systems of plants.

The Pheophytin-Quinone Reaction Center (Type II Reaction Center) The photosynthetic machinery in purple bacteria consists of three basic modules (Fig. 19-47a): a single reaction center (P870), a cytochrome bc1 electron-transfer complex similar to Complex III of the mito-chondrial electron-transfer chain, and an ATP synthase, also similar to that of mitochondria. Illumination drives electrons through pheophytin and a quinone to the cy-tochrome bc1 complex; after passing through the complex, electrons flow through cytochrome c2 back to the reaction center, restoring its preillumination state. This light-driven cyclic flow of electrons provides the energy for proton pumping by the cytochrome bc1 complex. Powered by the resulting proton gradient, ATP synthase produces ATP, exactly as in mitochondria.

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors: two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome.

The extremely rapid sequence of electron transfers shown in Figure 19-48b has been deduced from physical studies of the bacterial pheophytin-quinone centers, using brief flashes of light to trigger phototransduction and a variety of spectroscopic techniques to follow the flow of electrons through several carriers. A pair of bacteriochlorophylls—the "special pair," designated (Chl)2—is the site of the initial photochemistry in the bacterial reaction center. Energy from a photon absorbed by one of the many antenna chlorophyll molecules surrounding the reaction center reaches (Chl)2 by exciton transfer. When these two chlorophyll molecules—so close that their bonding orbitals overlap—absorb an ex-citon, the redox potential of (Chl)2 is shifted, by an amount equivalent to the energy of the photon, converting the special pair to a very strong electron donor. The (Chl)2 donates an electron that passes through a neighboring chlorophyll monomer to pheophytin (Pheo). This produces two radicals, one positively charged (the special pair of chlorophylls) and one negatively charged (the pheophytin):

(Chl)2 + Pheo-> '(Chl)+ + 'Pheo— (charge separation)

The pheophytin radical now passes its electron to a tightly bound molecule of quinone (QA ), converting it to a semiquinone radical, which immediately donates its

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