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The Role of Chlorophyll in Photosynthesis

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The Role of Chlorophyll in Photosynthesis The Role of Chlorophyll in Photosynthesis The pigments of plants trap light energy and store it as chemical energy_ They do this by catalyzing an oxidation-reduction process in which hydrogen atoms are boosted from water to organic m,atter by Eugene t Rabinowitch and Govindjee y effort to understand the basis of multiplied by the molecular weight of J. A. Bassham; SCIENTIFIC AMERICAN, life on this planet must always the substances in question-in this case June, 1962]. The fog has also thinned X come back to photosynthesis: the carbohydrate and oxygen, out in other areas, and the day when process that enables plants to grow by When one of us first summarized the the entire sequence of physical and utilizing carbon dioxide (C02), water state of knowledge of photosynthesis 17 chemical events in photosynthesis will (H20) and a tiny amount of minerals, years ago, the whole process was still be well understood seems much closer. Photosynthesis is the one large-scale heavily shrouded in fog [see "Photo­ The photosynthetic process apparent­ process that converts simple, stable, in­ synthesis," by Eugene L Rabinowitch; ly consists of three main stages: (1) the organic compounds into the energy-rich SCIENTIFIC AMERICAN, August, 1958]. removal of hydrogen atoms from water combination of organic matter and oxy­ Five years later investigation had pene­ and the production of oxygen molecules; gen and thereby makes abundant life on b'ated the mists sufficiently to disclose (2) the transfer of the hydrogen atoms earth possible, Photosynthesis is the some of the main features of the proc­ from an intermediate compound in the source of all living matter on earth, and ess [see "Progress in Photosynthesis," first stage to one in the third stage, and of all biological energy, by Eugene L Rabinowitch; SCIENTIFIC (3) the use of the hydrogen atoms to The overall reaction of photosynthesis AMERICAU, November, 1953]. Since convert carbon dioxide into a carbohy­ can be summarized in the following then much new knowledge has been drate [see illustration on page 76]. equation: CO2 + H20 + light�(CH20) accumulated; in particular the sequence The least understood of these three + O2 + 112,000 calories of energy per of chemical steps that convert carbon stages is the first: the removal of hydro­ mole, (CH20) stands for a carbohydrate; dioxide into carbohydrate is now under­ gen atoms from water with the release �or example glucose: (CH2?)G' "Mole" stood in considerable detail [see "The of oxygen. All that is known is that it ;, , 1S short for gram molecule : one gram Path of Carbon in Photosynthesis," by entails a series of steps probably re­ quiring several enzymes, one of which contains manganese. The third stage­ the production of carbohydrates from carbon dioxide-is the best understood, thanks largely to the work of Melvin Calvin and his co-workers at the Univer­ sity of California at Berkeley. The sub­ ject of our article is the second stage: the transfer of hydrogen atoms from the first stage to the third, This is the energy-storing part of photosynthesis; in it, to use the words of Robert Mayer, a discoverer of the law of the conservation of energy, "the fleeting sun rays are fixed and skillfully stored for future use." he light energy to be converted into Tchemical energy by photosynthesis is first taken up by plant pigments, pri­ marily the green pigment chlorophyll. In photosynthesis chlorophyll functions CHLOROPLAST is the organelle in a plant cell within which photosynthesis takes place. as a photocatalyst: when it is in its en­ The chlorophyll is contained in the "grana," stacks of membranous sacs called lamellae, ergized state, which results from the seen here in cross section. A maize-cell chloroplast is enlarged 19,000 diameters in this absorption of light, it catalyzes an en­ electron micrograph made by A. E. Vatter of the University of Colorado Medical Center, ergy-storing chemical reaction. This 74 © 1965 SCIENTIFIC AMERICAN, INC PHOTOSYNTHETIC UNITS may be the small elements, looking part of another one are shadowed with chromium and enlarged somewhat like cobblestones, visible in this electron micrograph 175,000 diameters. Where the memhrane is torn away one can see made by Roderic B. Park and 10hn Biggins of the University of an ordered array of the units, which Park and Biggins call quanta· California at Berkeley. In the micrograph a single lamella and a somes and calculate could contain 230 chlorophyll molecules each. reaction is the primary photochemical photochemical process in photosynthesis tion-reduction. The hydrogen atom is process; it is followed by a sequence is the boosting of hydrogen atoms from transferred from a donor molecule (a of secondary "dark"-that is, nonphoto­ a stable association with oxygen in water "reductant") to an acceptor molecule chemical-reactions in which no further molecules to a much less stable one (an "oxidant"); after the reaction the energy is stored. with carbon in organic matter. The donor is said to be oxidized and the ac­ Once it was thought that in photo­ oxygen atoms "left behind" combine ceptor to be reduced. The transfer of synthesis the primary photochemical into oxygen molecules, an association an electron can often substitute for the process is the decomposition of carbon also much less stable than the one be­ transfer of a hydrogen atom: in an aque­ dioxide into carbon and oxygen, fol­ tween oxygen and hydrogen in water. ous system (such as the interior of the lowed by the combination of carbon and The replacement of stable bonds (be­ living cell) there are always hydrogen water. More recently it has been sug­ tween oxygen and hydrogen) by looser ions (H+), and if such an ion combines gested that the energy of light serves bonds (between oxygen and oxygen and with the electron acceptor, the acquisi­ primarily to dissociate water, presum­ between hydrogen and carbon) obvious­ tion of an electron becomes equivalent ably into hydroxyl radicals (OH) and ly requires a supply of energy, and it to the acquisition of a hydrogen atom hydrogen atoms; the hydroxyl radicals explains why energy is stored in photo­ (electron + H+ ion � H atom). would then react to form oxygen mole­ synthesis. The chain of oxidation-reduction re­ cules. It is better than either of these The transfer of hydrogen atoms from actions in photosynthesis has some links two formulations to say that the primary one molecule to another is called oxida- that involve electron transfers and 75 © 1965 SCIENTIFIC AMERICAN, INC others that involve hydrogen-atom trans­ dioxide to carbohydrate and forming a ence between the oxidation-reduction fers. For the sake of simplicity we shall proportionate amount of oxygen? potentials of the two couples involved in speak of electron transfers, with the Oxidation-reduction energy can con­ the reaction: oxygen-water and carbon­ understanding that in some cases what veniently be measured in terms of elec­ dioxide-carbohydrate. The oxygen-water is actually transferred is a hydrogen trochemical potential. Between a given potential is about +.8 volt; the carbon­ atom. Indeed, the end result of the donor of electrons and a given acceptor dioxide-carbohydrate potential, about reactions undoubtedly is the transfer of there is a certain difference of oxidation­ -.4 volt. The transfer of a single elec­ hydrogen atoms. reduction potentials. This difference de­ tron from water to carbon dioxide thus In the oxidation-reduction reactions pends not only on the nature of the two requires +.8 minus -.4, or 1.2, electron of photosynthesis the electrons must be reacting substances but also on the na­ volts of energy. For a molecule of car­ pumped "uphill"; that is why energy ture of the products of the reaction; it bon dioxide to be reduced to CH20- must be supplied to make the reaction is characteristic of the two oxidation­ the elementary molecular group of a go. The tiny chlorophyll-containing reduction "couples." For example, when carbohydrate-four electrons (or hydro­ chloroplasts of the photosynthesizing oxygen is reduced to water (H20) its gen atoms) must be transferred; hence plant cell act as chemical pumps; they potential is +.81 volt, but when it is the total energy needed is 4.8 electron obtain the necessary power from the reduced to hydrogen peroxide (H202) volts. This works out to 112,000 calories absorption of light by chlorophyll (and the potential is +.27 volt. The more per mole of carbon dioxide reduced and to some extent from absorption by other positive the potential, the stronger is the of oxygen liberated. In short, the pump­ pigments in the chloroplast). It is im­ oxidative power of the couple; the more ing of electrons in the second stage of portant to realize that the energy is negative the potential, the stronger is photosynthesis entails the storage of stored in the two products organic mat­ its reducing power. 112,000 calories of energy per mole for ter and free oxygen and not in either of When two oxidation-reduction couples each set of four electrons transferred. them separately. To release the energy are brought together, the one contain­ We know the identity of the primary by the combustion of the organic matter ing the stronger oxidant tends to oxidize electron donor in photosynthesis (water) (or by respiration, which is slow, en­ the one containing the stronger reduc­ and of the ultimate electron acceptor zyme-catalyzed combustion) the two tant. In photosynthesis, however, a weak (carbon dioxide), but what are the in­ products must be brought together oxidant (C02) must oxidize a weak re­ termediates involved in the transfer of again. ductant (H20), producing a strong oxi­ electrons from the first stage to the dant (02) and a strong reductant (a third? This has become the focal prob­ ow much energy is stored in the carbohydrate).
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