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You are watching: Clusters of light gathering pigments in a photosystem
Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.
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We now shift our attention tophotosynthesis, the second main process for synthesizing ATP. In plants,photosynthesis occurs in chloroplasts, large organelles found mainly in leaf cells.The principal end products are two carbohydrates that are polymers of hexose(six-carbon) sugars: the disaccharide sucrose (see Figure 2-10) and leaf starch, a large, insoluble glucose polymer (Figure 16-33). Leaf starch is synthesized andstored in the chloroplast. Sucrose is synthesized in the cytosol from three-carbonprecursors generated in the chloroplast and is transported from the leaf to otherparts of the plant. Nonphotosynthetic (nongreen) plant tissues like roots and seedsmetabolize sucrose for energy by the pathways described in the previous sections.Photosynthesis in plants, as well as in eukaryotic single-celled algae and inseveral photosynthetic prokaryotes (the cyanobacteria andprochlorophytes), also generates oxygen. The overall reactionof oxygen-generating photosynthesis,
Structure of starch. This large glucose polymer and the disaccharide sucrose (see Figure 2-10) are the principal endproducts of photosynthesis. Both are built of six-carbon sugars.
Our emphasis is on photosynthesis in plant chloroplasts, but we also discuss asimpler photosynthetic process that occurs in green and purple bacteria. Althoughphotosynthesis in these bacteria does not generate oxygen, detailed analysis oftheir photosynthetic systems has provided insights about the first stages inoxygen-generating photosynthesis — how light energyis converted to a separation of negative and positive charges across the thylakoidmembrane, with the simultaneous generation of a strong oxidant and a strongreductant. In this section, we provide an overview of the stages in photosynthesisand introduce the main components, including the chlorophylls, the principal lightabsorbing pigments.
Photosynthesis Occurs on Thylakoid Membranes
Chloroplasts are bounded by two membranes, which do not contain chlorophyll anddo not participate directly in photosynthesis (Figure 16-34). Of these two membranes, the outer one, like the outermitochondrial membrane, is permeable to metabolites of small molecular weight;it contains proteins that form very large aqueous channels. The inner membrane,conversely, is the permeability barrier of the chloroplast; it containstransporters that regulate the movement of metabolites into and out of theorganelle.
The structure of a leaf and chloroplast. The chloroplast is bounded by a double membrane: the outer membranecontains proteins that render it permeable to small molecules(MW < 6000); the inner membrane formsthe permeability barrier (more…)
Unlike mitochondria, chloroplasts contain a thirdmembrane — the thylakoidmembrane — that is the site of photosynthesis.In each chloroplast, the thylakoid membrane is believed to constitute a single,interconnected sheet that forms numerous small flattened vesicles, the thylakoids, which commonly arearranged in stacks termed grana (see Figure 16-34). The spaces within all the thylakoidsconstitute a single continuous compartment, the thylakoidlumen. The thylakoid membrane contains a number of integral membraneproteins to which are bound several important prosthetic groups andlight-absorbing pigments, most notably chlorophyll. Carbohydrate synthesisoccurs in the stroma, the soluble phase between the thylakoidmembrane and the inner membrane. In photosynthetic bacteria extensiveinvaginations of the plasma membrane form a set of internal membranes, alsotermed thylakoid membranes, or simplythylakoids, where photosynthesis occurs.
Three of the Four Stages in Photosynthesis Occur Only duringIllumination
It is convenient to divide the photosynthetic process in plants into four stages,each occurring in a defined area of the chloroplast: (1) absorption of light,(2) electron transport leading to the reduction of NADP+ toNADPH, (3) generation of ATP, and (4) conversion of CO2 intocarbohydrates (carbon fixation).All four stages of photosynthesis are tightly coupled and controlled so as toproduce the amount of carbohydrate required by the plant. All the reactions instages 1 – 3 are catalyzed by proteins in thethylakoid membrane. The enzymes that incorporate CO2 into chemicalintermediates and then convert it to starch are soluble constituents of thechloroplast stroma (see Figure 16-34).The enzymes that form sucrose from three-carbon intermediates are in thecytosol.
Absorption of Light
The initial step in photosynthesis is the absorption of light by chlorophyllsattached to proteins in the thylakoid membranes. Like cytochromes,chlorophylls consist of a porphyrin ring attached to a long hydrocarbon sidechain (Figure 16-35). They differfrom cytochromes (and heme) in containing a central Mg2+ion (rather than Fe atom) and having an additional five-membered ring. Theenergy of the absorbed light is used to remove electrons from an unwillingdonor (water, in green plants), forming oxygen,
and then to transfer the electrons to aprimary electron acceptor, a quinone designated Q,which is similar to CoQ.
The structure of chlorophyll a, theprincipal pigment that traps light energy. Chlorophyll b differs from chlorophylla by having a CHO group in place of theCH3 group (green). In the porphyrin ring, ahighly conjugated system, electrons are delocalized (more…)
Electrons move from the quinone primary electron acceptor through a chain ofelectron transport molecules in the thylakoid membrane until they reach theultimate electron acceptor, usually NADP+,reducing it to NADPH (see Figure16-4). The transport of electrons is coupled to the movement ofprotons from the stroma to the thylakoid lumen, forming a pH gradient acrossthe thyla-koid membrane(pHlumen < pHstroma),in much the same way that a proton-motive force is established across themitochondrial inner membrane during electron transport (see Figure 16-2).
Thus the overall reaction of stages 1 and 2 can be summarized as
Many photosynthetic bacteria do not use water as the donor of electrons.Rather, they use molecules such as hydrogen gas (H2) or hydrogensulfide (H2S) as the ultimate source of electrons to reduce theultimate electron acceptor (NAD+ rather thanNADP+).
Generation of ATP
Protons move down their concentration gradient from the thylakoid lumen tothe stroma through the F0F1 complex which couplesproton movement to the synthesis of ATP from ADP and Pi. This useof the proton-motive force to synthesize ATP is identical with the analogousprocess occurring during oxidative phosphorylation in the mitochondrion (seeFigures 16-28 and 16-30).
The ATP4− and NADPH generated by the second and thirdstages of photosynthesis provide the energy and the electrons to drive thesynthesis of polymers of six-carbon sugars from CO2 andH2O. The overall balanced equation is written as
The reactions that generate the ATP and NADPH used in carbon fixation aredirectly dependent on light energy; thus stages1 – 3 are called the lightreactions of photosynthesis. The reactions in stage 4 areindirectly dependent on light energy; they aresometimes called the dark reactions of photosynthesisbecause they can occur in the dark, utilizing the supplies of ATP and NADPHgenerated by light energy. However, the reactions in stage 4 are notconfined to the dark; in fact, they primarily occur during illumination.
Each Photon of Light Has a Defined Amount of Energy
Quantum mechanics established that light, a form of electromagnetic radiation,has properties of both waves and particles. When light interacts with matter, itbehaves as discrete packets of energy (quanta) calledphotons. The energy of a photon, ϵ, isproportional to the frequency of the light wave:ϵ = hγ,where h is Planck’s constant(1.58 × 10−34cal·s, or6.63 × 10−34J·s), and γ is the frequency of the light wave. It iscustomary in biology to refer to the wavelength of the light wave, λ,rather than to its frequency, γ. The two are related by the simpleequation γ = c÷ λ, where c is the velocity of light(3 × 1010 cm/s in a vacuum). Notethat photons of shorter wavelength have higherenergies.
Also, the energy in 1 mol of photons can be denoted by E= Nϵ, where N isAvogadro’s number(6.02 × 1023 molecules orphotons/mol). Thus
The energy of light is considerable, as we can calculatefor light with a wavelength of 550 nm(550 × 10−7 cm),typical of sunlight:
or about 52 kcal/mol, enough energy to synthesizeseveral moles of ATP from ADP and Pi if all the energy were used forthis purpose.
Chlorophyll a Is Present in Both Components of aPhotosystem
The absorption of light energy and its conversion into chemical energy occurs inmultiprotein complexes, called photosystems, located in thethylakoid membrane. A photosystem has two closely linked components, anantenna containing light-absorbing pigments and areaction center comprising a complex of proteins and twochlorophyll a molecules. Each antenna (named by analogywith radio antennas) contains one or more light-harvestingcomplexes (LHCs). The energy of the light captured by LHCs isfunneled to the two chlorophylls in the reaction center, where the primaryevents of photosynthesis occur.
Found in all photosynthetic organisms, both eukaryotic and prokaryotic,chlorophyll a is the principal pigment involved inphotosynthesis, being present in both antennas and reaction centers. In additionto chlorophyll a, antennas contain other light-absorbingpigments: chlorophyll b in vascular plants, andcarotenoids in both plants and photosynthetic bacteria(Figure 16-36). The presence ofvarious antenna pigments, which absorb light at different wavelengths, greatlyextends the range of light that can be absorbed and used for photosynthesis.
The structure of β-carotene, a pigment that assists inlight absorption by chloroplasts. β-Carotene, which is related to the visual pigment retinal(see Figure 21-47), is oneof a family of carotenoids containing long hydrocarbon chains (more…)
One of the strongest pieces of evidence for the involvement of chlorophylls andβ-carotene in photosynthesis is that the absorptionspectrum of these pigments is similar to the actionspectrum of photosynthesis (Figure16-37). The latter is a measure of the relative ability of light ofdifferent wavelengths to support photosynthesis.
Photosynthesis at different wavelengths. (a) The action spectrum of photosynthesis in plants; that is, theability of light of different wavelengths to support photosynthesis.(b) The absorption spectra for three photosynthetic pigments:chlorophyll (more…)
When chlorophyll a (or any other molecule) absorbs visiblelight, the absorbed light energy raises the chlorophyll a to ahigher energy state, termed an excited state. This differs fromthe ground (unexcited) state largely in the distribution of electrons around theC and N atoms of the porphyrin ring (see Figure16-35). Excited states are unstable, and will return to the groundstate by one of several competing processes. For chlorophyll amolecules dissolved in organic solvents, such as ethanol, the principalreactions that dissipate the excited-state energy are the emission of light(fluorescence and phosphorescence) and thermal emission (heat). The situation isquite different when the same chlorophyll a is bound to theunique protein environment of the reaction center.
Light Absorption by Reaction-Center Chlorophylls Causes a Charge Separationacross the Thylakoid Membrane
The absorption of a quantum of light of wavelength ≈680 nm causes achlorophyll a molecule to enter the first excitedstate. The energy of such photons increases the energy ofchlorophyll a by 42 kcal/mol. In the reaction center, thisexcited-state energy is used to promote a charge separation across the thylakoidmembrane: an electron is transported from a chlorophyll molecule to the primaryelectron acceptor, the quinone Q, on the stromal surface of themembrane, leaving a positive charge on the chlorophyll close to the luminalsurface (Figure 16-38). The reducedprimary electron acceptor becomes a powerful reducing agent, with a strongtendency to transfer the electron to another molecule. The positively chargedchlorophyll, a strong oxidizing agent, will attract an electron from an electrondonor on the luminal surface. These potent biological reductants and oxidantsprovide all the energy needed to drive all subsequent reactions ofphotosynthesis: electron transport, ATP synthesis, and CO2fixation.
The primary event in photosynthesis. After a photon of light of wavelength ≈680 nm isabsorbed by one of the many chlorophyll molecules in one of thelight-harvesting complexes (LHCs) of an antenna (only one isshown), some of the absorbed energy (more…)
The significant features of the primary reactions of photosynthesis aresummarized in the following model, in which P represents the chlorophylla in the reaction center, and Q represents the primaryelectron acceptor:
According to this model, the ground state of the reaction-center chlorophyll, P,is not a strong enough reductant to reduce Q; that is, an electron will not movespontaneously from P to Q. However, the excited state of the reactioncenterchlorophyll, P*, is an excellent reductant and rapidly (in about10−10 seconds) donates an electron to Q, generatingP+ and Q−. Thisphotochemical electron movement, which depends on theunique environment of both the chlorophylls and the acceptor within the reactioncenter, occurs nearly every time a photon is absorbed. The acceptor,Q−, is a powerful reducing agent capable oftransferring the electron to still other molecules, ultimately toNADP+. The powerful oxidant P+ canremove electrons from other molecules to regenerate the original P. In plants,the oxidizing power of four molecules of P+ is used, by wayof intermediates, to remove four electrons from H2O to formO2:
Chlorophyll a also absorbs light at discrete wavelengths shorterthan 680 nm (see Figure 16-37b). Suchabsorption raises the molecule into one of several higher excited states, whichdecay within 10−12 seconds (1 picosecond, ps) to the firstexcited state P*, with loss of the extra energy as heat. Photochemical chargeseparation occurs only from the first excited state of the reaction-centerchlorophyll a, P*. This means that the quantumyield — the amount of photosynthesis perabsorbed photon — is the same for allwavelengths of visible light shorter than 680 nm.
The chlorophyll a molecules within reaction centers are capableof directly absorbing light and initiating photosynthesis. However, even at themaximum light intensity encountered by photosynthetic organisms (tropicalnoontime sun, ≈1.2 × 1020photons/m2/s), each reaction-center chlorophylla absorbs about one photon per second, which is not enoughto support photosynthesis sufficient for the needs of the plant. To increase theefficiency of photosynthesis, especially at more typical light intensities,organisms utilize additional light-absorbing pigments.
Light-Harvesting Complexes Increase the Efficiency of Photosynthesis
As noted earlier, each reaction center is associated with an antenna, whichcontains several light-harvesting complexes (LHCs), packed with chlorophylla and, depending on the species, chlorophyllb and other pigments. LHCs promote photosynthesis byincreasing absorption of 680-nm light and by extending the range of wavelengthsof light that can be absorbed (see Figure16-37).
Photons can be absorbed by any of the pigment molecules in each LHC. The absorbedenergy is then rapidly transferred (in <10−9seconds) to one of the two chlorophyll a molecules in theassociated reaction center, where it promotes the primary photosynthetic chargeseparation (see Figure 16-38). Within anLHC are several transmembrane proteins whose role is to maintain the pigmentmolecules in the precise orientation and position that are optimal for lightabsorption and energy transfer, thereby maximizing the very rapid and efficientprocess known as resonance transfer of energy from antennapigments to reaction-center chlorophylls. As depicted in Figure 16-39a, some photosynthetic bacteria contain twotypes of LHCs: the larger type (LH1) is intimately associated with a reactioncenter; the smaller type (LH2) can transfer absorbed light energy to an LH1.Figure 16-39b shows the structure ofthe subunits that make up the LH2 complex in Rhodopseudomonasacidophila. Surprisingly, the molecular structures of plantlight-harvesting complexes are completely different from those in bacteria, eventhough both types contain carotenoids and chlorophylls in a clustered geometricarrangement within the membrane.
Light-harvesting complexes from the photosynthetic bacteriumRhodopseudomonas acidophila. (a) Schematic depiction of the cylindrical LHCs and the reactioncenter as viewed from above the plane of the membrane. Each LH2complex consists of nine subunits (more…)
Although antenna chlorophylls can transfer absorbed light energy, they cannotrelease an electron. As we’ve seen already, reaction-centerchlorophylls are able to release an electron after absorbing a quantum of light.To understand their electron-releasing ability, we examine the structure andfunction of the reaction center in bacterial and plant photosystems in the nextsection.
In stage 3, movement of protons down theirelectrochemical gradient through F0F1 complexespowers the synthesis of ATP from ADP and Pi.
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