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18-33). In the light reactions, chlorophyll and otherpigments of the photosynthetic cells absorb light energy and conserveit in chemical form as the two energy-rich products ATP and NADPH;simultaneously, O2 is evolved. In the carbon fixation reactions, ATPand NADPH are used to reduce CO2 to form glucose and other organicproducts. The formation of O2, which occurs only in the light, and thereduction of CO2, which does not require light, thus are distinct andseparate processes.
In this chapter we are concerned only with thelight reactions; the reduction of CO2 is described in Chapter 19.In photosynthetic eukaryotic cells, both the light and carbon fixation reactions take place in the chloroplasts (Fig. 18-34). When solarenergy is not available, mitochondria in the plant cell generate ATP byoxidizing carbohydrates originally produced in chloroplasts in thelight.Chloroplasts may assume many different shapes in different species, and they usually have a much larger volume than mitochondria.Outer membraneInner membraneThylakoidsGrana (thylakoids)Stroma(a)(b)Chapter 18 Oxidative Phosphorylation and Photophosphorylation573They are surrounded by a continuous outer membrane, which, like theouter mitochondrial membrane, is permeable to small molecules andions.
An inner membrane system encloses the internal compartment,in which there are many flattened, membrane-surrounded vesicles orsacs, called thylakoids, which are usually arranged in stacks calledgrana. The thylakoid membranes are separate from the inner chloroplast membrane. Embedded in the thylakoid membranes are the photosynthetic pigments and all the enzymes required for the primarylight reactions.
The fluid in the compartment surrounding the thylakoids, the stroma, contains most of the enzymes required for the carbon fixation reactions, in which CO2 is reduced to form triose phosphates and, from them, glucose.Our discussion here focuses on the nature of the light-absorbingsystems and their roles in photosynthesis.Light Produces Electron Flow in ChloroplastsHow do the pigment molecules of the thylakoid membranes transduceabsorbed light energy into chemical energy? The key to answering thisquestion came from a discovery made in 1937 by Robert Hill, a pioneerin photosynthesis research.
He found that when leaf extracts containing chloroplasts were supplemented with a nonbiological hydrogen acceptor and then illuminated, evolution of O2 and simultaneous reduction of the hydrogen acceptor took place, according to an equation nowknown as the Hill reaction:2H2O + 2Alight2AH2 + O 2where A is the artificial hydrogen acceptor. One of the nonbiologicalhydrogen acceptors used by Hill was the dye 2,6-dichlorophenolindophenol, now called a Hill reagent, which in its oxidized form (A) isblue and in its reduced form (AH2) is colorless.
When the leaf extractsupplemented with the dye was illuminated, the blue dye became colorless and O2 was evolved. In the dark neither O2 evolution nor dyereduction took place. This was the first specific clue to how absorbedlight energy is converted into chemical energy: it causes electrons toflow from H2O to an electron acceptor. Moreover, Hill found that CO2was not required for this reaction, nor was it reduced to a stable formunder these conditions. He therefore concluded that O2 evolution canbe dissociated from CO2 reduction.
Several years later it was foundthat NADP+ is the biological electron acceptor in chloroplasts, according to the equation2H2O + 2NADP+light> 2NADPH + 2H+ + O2This equation shows an important distinction between mitochondrial oxidative phosphorylation and the analogous process in chloroplasts: in chloroplasts electrons flow from H2O to NADP + , whereas inmitochondrial respiration electrons flow in the opposite direction, fromNADH or NADPH to O2, with the release of free energy. Because lightinduced electron flow in chloroplasts is in the reverse or "uphill" direction, from H2O to NADP + , it cannot occur without the input of freeenergy; this energy comes from light. To understand how this occurs,we must first consider the effects of light absorption on molecularstructure.OHNHiVOHOxidized form(blue)JOHReduced form(colorless)DichlorophenolindophenolAbsorption of Light Excites MoleculesVisible light is electromagnetic radiation of wavelengths 400 to700 nm, a small part of the electromagnetic spectrum (Fig.
18-35),ranging from violet to red. The energy of a single photon (a quantumof light) is greater at the violet end of the spectrum than at the red end.The energy in a "mole" of photons (one einstein; 6 x 1023 photons) is170 to 300 kJ, about an order of magnitude more than the energy required to synthesize a mole of ATP from ADP and Pi (about 30 kJunder standard conditions). The energy of an einstein in the infraredor microwave regions of the spectrum is too small to be useful in thekinds of photochemical events that occur in photosynthesis.
UV lightand x rays, on the other hand, have so much energy that they damageproteins and nucleic acids and induce mutations that are often lethal.Figure 18-35 The spectrum of electromagneticradiation, and the energy of photons in the visiblerange of the spectrum. One einstein is 6 x 1023photons.lType ofradiationGamma raysX rays ! UV!I< 1 nm 100 nmWavelengthInfrared1 Microwaves< 1 millimeterRadio waves1 meter Thousands of metersVisible lightYellowVioletBlueCyanGreenOrangeRed:iWavelength 380(nm)Energy(kJ/einstein)430300500240560600200650750170The ability of a molecule to absorb light depends upon the arrangement of electrons around the atomic nuclei in its structure.
When aphoton is absorbed, an electron is lifted to a higher energy level. Thishappens on an all-or-none basis; 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 photonis in an excited state, which is generally unstable.
The electronslifted into higher-energy orbitals usually return rapidly to their normal lower-energy orbitals; the excited molecule reverts (decays) to thestable ground state, giving up the absorbed quantum as light or heator using it to do chemical work. The light emitted upon decay of excitedmolecules, called fluorescence, is always of a longer wavelength(lower energy) than the absorbed light. Excitation of molecules by lightand their fluorescent decay are extremely fast processes, occurring inabout 10~15 and 10~12 s, respectively. The initial events in photosynthesis are therefore very rapid.Chapter 18 Oxidative Phosphorylation and Photophosphorylation575Chlorophylls Absorb Light Energy for PhotosynthesisThe most important light-absorbing pigments in the thylakoid membranes are the chlorophylls, green pigments with polycyclic, planarstructures resembling the protoporphyrin of hemoglobin (see Fig.
7 18), except that Mg 2+ , not Fe 2 + , occupies the central position (Fig. 1836). Chlorophyll a, present in the chloroplasts of all green plant cells,contains four substituted pyrrole rings, one of which (ring IV) is reduced, and a fifth ring that is not a pyrrole.
All chlorophylls have a longphytol side chain, esterified to a carboxyl-group substituent in ring IV.The four inward-oriented nitrogen atoms of chlorophyll a are coordinated with the Mg 2+ .Cin bacteriochlorophyllSaturated bond inbacteriochlorophyllChlorophyll a/3-Carotenecoo~ cooIQHUnsaturated bondin phycocyaninPhycoerythrinFigure 18-36 Structures of the primary photopigments chlorophylls a and b and bacteriochlorophyll,and of the accessory pigments /3-carotene (a carotenoid) and phycoerythrin and phycocyanin (phycobil-ins).
The areas shaded pink represent conjugatedsystems (alternating single and double bonds),which largely account for the absorption of visiblelight.576Part III Bioenergetics and MetabolismThe heterocyclic five-ring system that surrounds the Mg2+ has anextended polyene structure, with alternating single and double bonds.Such polyenes characteristically show strong absorption in the visibleregion of the spectrum (Fig. 18-37); the chlorophylls have unusuallyhigh molar absorption coefficients (see Box 5-1) and are therefore particularly well-suited for absorbing visible light during photosynthesis.Figure 18-37 Absorption of visible light by photopigments shown in Fig. 18-36.
Plants are greenbecause their pigments absorb light from the redand violet regions of the spectrum, leaving primarily green light to be reflected or transmitted. Compare the absorption spectra of the pigments withthe spectrum of sunlight reaching the earth's surface; the combination of chlorophylls (chl a and chlb) and accessory pigments enables plants to harvestmost of the energy available from sunlight.PhycoerythrinPhycocyanin300400500600700800Wavelength (nm)Chloroplasts of higher plants always contain two types of chlorophyll. One is invariably chlorophyll a, and the second in many speciesis chlorophyll b, which has an aldehyde group instead of a methylgroup attached to ring II (Fig. 18-36). Although both are green, theirabsorption spectra are slightly different (Fig. 18-37), allowing the twopigments to complement each other's range of light absorption in thevisible region.
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