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Most higher plants contain about twice as much chlorophyll a as chlorophyll b. The bacterial chlorophylls differ only slightlyfrom the plant pigments (Fig. 18-36).Accessory Pigments Also Absorb LightIn addition to chlorophylls, the thylakoid membranes contain secondary light-absorbing pigments, together called the accessory pigments, the carotenoids and phycobilins.
Carotenoids may be yellow,red, or purple. The most important are j8-carotene (Fig. 18-36), ared-orange isoprenoid compound that is the precursor of vitamin A inanimals, and the yellow carotenoid xanthophyll. The carotenoid pig-Chapter 18 Oxidative Phosphorylation and Photophosphorylationments absorb light at wavelengths other than those absorbed by thechlorophylls (Fig. 18-37) and thus are supplementary light receptors.Phycobilins are linear tetrapyrroles that have the extended polyenesystem found in chlorophylls, but not their cyclic structure or centralMg2+. Examples are phycoerythrin and phycocyanin (Fig. 18-36).The relative amounts of the chlorophylls and the accessory pigments are characteristic for different plant species.
It is variation inthe proportions of these pigments that is responsible for the range ofcolors of photosynthetic organisms, which vary from the deep bluegreen of spruce needles, to the greener green of maple leaves, to thered, brown, or even purple color of different species of multicellularalgae and the leaves of some decorative plants.Experimental determination of the effectiveness of light of different colors in promoting photosynthesis yields an action spectrum(Fig. 18-38), often useful in identifying the pigment primarily responsible for a biological effect of light. By capturing light in a region of thespectrum not used by other plants, a photosynthetic organism canclaim its unique ecological niche.
For example, the phycobilins, presentonly in red algae and cyanobacteria, absorb in the region 520 to630 nm, allowing these organisms to live in niches where light of loweror higher wavelength has been filtered out by the pigments of otherorganisms living in the water above them, or by the water itself.577yellowviolet; blue! cyan ; greeni400500jorange600red700(a)100 -Chlorophyll Funnels Absorbed Energy to Reaction CentersThe light-absorbing pigments of thylakoid membranes are arranged infunctional sets or arrays called photosystems.
In spinach chloroplasts each photosystem contains about 200 molecules of chlorophyllsand about 50 molecules of carotenoids. The clusters can absorb lightover the entire visible spectrum but especially well between 400 to500 nm and 600 to 700 nm (Fig. 18-37). All the pigment molecules in aphotosystem can absorb photons, but only a few can transduce thelight energy into chemical energy. A transducing pigment consists ofseveral chlorophyll molecules combined with a protein complex alsocontaining tightly bound quinones; this complex is called a photochemical reaction center. The other pigment molecules in a photosystem are called light-harvesting or antenna molecules.
Theyfunction to absorb light energy and transmit it at a very high rate tothe reaction center where the photochemical reactions occur (Fig. 1839, p. 578), which will be described in detail later.The chlorophyll molecules in thylakoid membranes are bound tointegral membrane proteins (chlorophyll a/6-binding, or CAB, proteins) that orient the chlorophyll relative to the plane of the membraneand confer light absorption properties that are subtly different fromthose of free chlorophyll.
When isolated chlorophyll molecules in vitroare excited by light, the absorbed energy is quickly released as fluorescence and heat, but when chlorophyll in intact spinach leaves is excitedby visible light (Fig. 18-40 (step (T)), p. 579), very littlefluorescenceisobserved. Instead, a direct transfer of energy from the excited chlorophyll (an antenna chlorophyll) to a neighboring chlorophyll moleculeoccurs, exciting the second molecule and allowing the first to return toits ground state (step (2)). This resonance energy transfer is repeated to a third, fourth, or subsequent neighbor, until the chlorophyllat the photochemical reaction center becomes excited (step (3)).
In thisspecial chlorophyll molecule, an electron is promoted by excitation to ahigher-energy orbital. This electron then passes to a nearby electron400500600700Wavelength (nm)(b)Figure 18—38 Two ways to determine the actionspectrum for photosynthesis, (a) The results of aclassic experiment done by T.W. Englemann in 1882to determine what wavelength of light was mosteffective in supporting photosynthesis. Englemannplaced a filamentous, photosynthetic alga on a microscope stage and illuminated it with light from aprism, so that cells in one part of the filament received mainly blue light, another yellow, anotherred.
To determine which cells carried out photosynthesis most actively, bacteria known to migratetoward regions of high O2 concentration were alsoplaced on the microscope slide. The distribution ofbacteria showed highest O2 levels (produced by photosynthesis) in the regions illuminated with violetand red light, (b) A similar experiment using modern techniques for the measurement of O2 production yields the same result.
An action spectrumdescribes the relative rate of photosynthesis for illumination with a constant number of photons ofdifferent wavelengths. Such an action spectrum isuseful because it suggests (by comparison with absorption spectra such as those in Fig. 18-37) whichpigments are able to channel energy into photosynthesis.578Part III Bioenergetics and MetabolismFigure 18-39 Organization of the photosystemcomponents in the thylakoid membrane, (a) Thedistribution of photosystems I and II, ATP synthase, and the cytochrome bf complex in the thylakoid membranes is not random. Photosystem I andATP synthase are almost completely excluded fromthe regions with tightly stacked membranes,whereas photosystem II and the cytochrome bf complex are enriched in these regions of tight packing.This separation of photosystems I and II preventsenergy absorbed by photosystem II from beingtransferred directly to photosystem I, and alsoplaces photosystem I in the regions most accessibleto NADP+ from the stroma.
(b) An enlargement ofa photosystem showing the reaction center, antennachlorophylls, and accessory pigments. Cytochromebf and ATP synthase of chloroplasts are describedlater in this chapter.Cytochrome bfPhotosystem IIPhotosystem IATP synthase•'!f T '(a)moleculesAntenna chlorophylls, Theseabsorb lightbound to proteinenergy,Carotenoids, other - transferring itaccessory pigments betweenmoleculesuntil itreaches thereactioncenter.Light,4aReaction centerPhotochemical reaction hereconverts the energy of a photoninto a separation of charge,initiating electron flow.(b)acceptor that is part of the electron transfer chain of the chloroplast,leaving the excited chlorophyll molecule with an empty orbital (an"electron hole") (step 0 ) . The electron acceptor thus acquires a negative charge.
The electron lost by the reaction-center chlorophyll is replaced by an electron from a neighboring electron donor molecule(step (5)), which becomes positively charged. In this way, excitation bylight causes electric charge separation and initiates an oxidationreduction chain. Coupled to the light-dependent electron flow alongthis chain are processes that generate ATP and NADPH.Chapter 18 Oxidative Phosphorylation and PhotophosphorylationAntennamoleculesReactioncenterchlorophyllLight excites an antenna molecule(chlorophyll or accessory pigment),raising an electron to a higherenergy level.The excited antenna moleculepasses energy to a neighboringchlorophyll molecule (resonanceenergy transfer), exciting it.*-•-This energy istransferred to areaction-center chlorophyll,exciting it.©ElectronacceptorThe excited reactioncenter chlorophyllpasses an electron toan electron acceptor.The electron hole in thereaction center isfilled by an electronfrom an electron donor.®ElectrondonorI(5)The absorption of a photon has causedseparation of charge in the reaction center.579Figure 18-40 A generalized scheme showing theconversion of energy from an absorbed photon intoseparation of charges at the photosystem reactioncenter.
The steps are further described in the text.Note that step (2) may be repeated a number oftimes between successive antenna molecules until areaction-center chlorophyll is reached. The asterisk(*) represents the excited state of an antenna molecule.Part III Bioenergetics and Metabolism580Light-Driven Electron Flowandreplacesit with e~taken from anelectron donor.Excitedreactioncenter loses e~to an electronacceptor...Thylakoid membranes have two different kinds of photosystems, eachwith its own type of photochemical reaction center and a set of antennamolecules.
The two systems have distinct and complementary functions. Photosystem I has a reaction center designated P700 and ahigh ratio of chlorophyll a to chlorophyll b. Photosystem II, with itsreaction center P680, contains roughly equal amounts of chlorophyll aand b and may also contain a third type, chlorophyll c. The thylakoidmembranes of a single spinach chloroplast have many hundreds ofeach kind of photosystem. All O2-evolving photosynthetic cells—thoseof higher plants, algae, and cyanobacteria—contain both photosystemsI and II; all other species of photosynthetic bacteria, which do notevolve O2, contain only photosystem I.It is between photosystems I and II that light-driven electron flowoccurs, producing NADPH and a transmembrane proton gradient.Light Absorption by Photosystem IIInitiates Charge SeparationFigure 18-41 Photochemical events following excitation of photosystems by light absorption.
(Thesteps shown here are equivalent to steps (4) and (5)in Fig. 18-40.) (a) Photosystem II (PSII). Z represents a Tyr residue in the Dl protein of PSII; Ph,pheophytin. (b) Photosystem I (PSD. Ao is a chlorophyll molecule near the reaction center of PSI; itaccepts an electron from P700 to become the powerful reducing agent Ao .
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