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The Role of in

The pigments of trap 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 , 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 , 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 pigments, pri­ marily the pigment chlorophyll. In photosynthesis chlorophyll functions

CHLOROPLAST is the organelle in a 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 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

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© 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 alway s 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

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© 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 differencede­ tron from water to carbon dioxide thus In the oxidation-reduction reactions pends not only on the 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 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). This calls for a massive lem in recent studies of the photosyn­ H transfer of electrons from water to investment of energy. The specific thetic process. As a matter of fact, it is carbon dioxide, converting the carbon amount needed is given by the differ- not yet definitely known what com­ pound releases electrons from the first stage, and what compound receives x them in the third; that is why these r------�()�------�)�I (CH20)I compounds are respectively labeled ZH 1[\ and X in the illustration at the left. About the donor, ZH, we have almost no -2 ------��------information; the following considera­ tions suggest the possible nature of the X. ::t:-' primary acceptor, I­ From the study of the mechanism of Z � 0 respiration we are familiar with an o 0.. important oxidation-reduction catalyst: Z nicotinamide adenine dinucleotide phos­ Q I- phate, or NADP (formerly known as tri­ � +.2 ------+------phosphopyridine nucleotide, or TPN). o w NADP has an oxidation-reduction po­ 0:: Z tential of about -.32 volt, thus in itself o it is not a strong enough reductant to � +.4 ------+------provide the -.4-electron-volt potential o needed to reduce carbon dioxide to car­ � bohydrate. NADP can achieve this feat, however, if it is supplied with additional energy in the form of the high-energy compound , or ATP. A molecule of ATP supplies about + .6 _---+--C #_LlGHT _ 10,000 calories per mole when its termi­ .8It�H z9 ll------4 - ¥------�� � 02 ;\ nal phosphate group is split off, and this yH is enough to provide the needed boost to the reducing power of NADP. Fur­ THREE STAGES of photosynthesis are the removal of hydrogen from water with the re­ lease of oxygen (bottom arrow), the transfer (vertical arrow) of the hydrogen by ener­ thermore, we know that NADP is re­ gy from light trapped by chlorophyll (color) and the use of the hydrogen to reduce carbon duced when cell-free preparations of dioxide+ to carbohydrate (top arrow). In tbis scheme the oxidation·reduction poten­ chloroplasts are illuminated. Put to­ tials involved are indicated by the scale at the left, and the hypothetical "primary gether, these two facts led to the now reductant" and "primary acceptor" intermediates are designated as ZH and X respectively. widely accepted hypothesis that the sec-

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© 1965 SCIENTIFIC AMERICAN, INC ond stage of photosynthesis manufac­ tures both ATP and reduced NADP and feeds them into the third stage.

� first it was assumed that NADP is identical with X, the primary ac­ ceptor in our scheme. Subsequent ex­ � .12 :::J periments by various workers-notably I- , Z Anthony San Pietro at Johns Hopkins « :::J v University and Daniel I. Arnon and his o colleagues at the University of Cali­ l- I fornia at Berkeley-suggested, however, (,9 that NADP is preceded in the "bucket :::i z 0::.09 1J I' Q brigade" of electron transfer by ferre­ w I­ D- D­ o:: doxin, a that contains iron. This (f) W o --.J (f) compound has an oxidation-reduction :::J (l) - « potential of about .42 volt; therefore U W --.J w if it is reduced in light it can bring about o > the reduction of NADP by a "dark" re­ 2 --.J� action requiring no additional energy Q.06 W 0:: supply. o --.J More recently Bessel Kok of the W >= Research Institute for Advanced Studies 2 :::J in Baltimore has found evidence sug­ I- gesting that compound X may be a still Z « stronger reductant, with a potential of ( \ \1\ about -.6 volt. If this is so, plants have 603 the alternatives of either applying this stronger reductant directly to the reduc­ tion of carbon dioxide or letting it re­ duce first and then NADP J '� and using reduced NADP to reduce o carbon dioxide. It seems a roundabout 450 500 550 600 650 700 750 procedure to create a reductant suffi­ WAVELENGTH (MILLIMICRONS) ciently strong for the task at hand, then to sacrifice a part of its reducing power " DROP," the drop in quantum yield (black curve) of oxygen in photosynthesis under long-wave illumination, is demonstrated in the green alga Chlorella pyrenoidosa. Peak ef­ and finally to use ATP to compensate ficiency is restored (broken line) by supplementary shorter-wave illumination. Absorption for the loss. It is not unknown, however, curves of a (solid color) and b(light color) are also shown. This illustration for nature to resort to devious ways in and the next two are based on data of the late Robert Emerson of the University of Illinois_ order to achieve its aims. For photosynthesis to be a self-con­ tained process the required high-energy �.09 :::J phosphate ATP must be itself manufac­ I- Z tured by photosynthesis. The formation « :::J of A TP has in fact been detected in il­ o luminated fragments of bacteria by Al­ l­ I bert W. Frenkel of the University of (,9 z Minnesota and in chloroplast fragments :::i.06�--�---4------+------�------1-\------+------�' Q 0:: I­ by Arnon and his co-workers [see "The w D­ D- o:: o Role of Light in Photosynthesis," by (f) (f) W [j] Daniel 1. Arnon: SCIENTIFIC AMERICAN, --.J :::J « November, 1960]. As a matter of fact, U w W ATP is needed not only to act as a --.J 2: o � booster in the reduction of an intelwe­ --.J �.03��------�------+---�----�------�---r��-r------4 W diate in the carbon cycle by reduced Q 0:: NADP but also for another step in the o --.J third stage of photosynthesis. According W >= to a sequence of reactions worked out 2 in 1951 by Andrew A. Benson and his :::J I- colleagues at the University of Califor­ � O ��-.���__ � ��.:__ '��____ � �____ � ��____ ��� ______� nia at Berkeley, carbon dioxide enters :::J o 450 500 550 600 650 700 750 photosynthesis by first reacting with WAVELENGTH (MILLIMICRONS)

a "carbon dioxide acceptor," a special QUANTUM YIELD is similarly affected by long-wave illumination in the red alga Por­ phosphate called ribulose diphos­ phyridium cruentum (black curve). Yield drops to less than half of its maximum when phate. It turns out that the production absorption by (solid-color curve) is at its peak. Absorption of the pigments of this compound from its precursor- (light-color curve) and (broken-color curve) are also shown.

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© 1965 SCIENTIFIC AMERICAN, INC 2.5 .------..,.------,------,

2 �------���------�------�

r=- z w ::2: w u z « I .5 z Z Q w t- 0.- LL 0: 0 0 (/) Q 0 ro t- « « e;2.5 w 2: t- t- U « w -' LL W LL 0: w 2 z 0 (/) 0: w

�1.5

.5r------r------���----�

o L4:-:------: �=------::-:!-::------4 00 500 600 -:=700"..! 00 500 600 700 WAVELENGTH (MILLIMICRONS)

EMERSON EFFECT is shown for Chlorella (top left), the blue· the supplementary illumination is varied. The curve of the action green alga Anacystis nidulans (top right), Porphyridium (bottom spectrum turns out to be parallel to the absorption curves (color) left) and the Navicula minima (bottom right). In each case of the various accessory pigments: in Chlorella, phy. the black curve shows the of the Emerson effect, or cocyanin in Anacystis, phycoerythrin in Porphyridium and fuco· the degree of enhancement in quantum yield as the wavelength of xanthol (solid color) and (light color) in Navicula.

r=- r------r------r------, Z 2 w ::2: w u z �1.5�--��-----�----��----�------4 Q I t­ z O.- W 0: LL I o o (/) , ro o I « I W � I 2: e; I t­ I � I -' U W 5 0: ti:. f------,-I--+--I-TJL---.-.:\--\-+------l t:::i I z ,/ o -"," K? o '------'------'------I W 600 650 700 750 600 700 750 � WAVELENGTH (MILLIMICRONS)

DETAILED ACTION SPECTRA of the Emerson effect reveal the peaks of chlorophyll a 670 (solid color) as well as of chlorophyll presence of chlorophyll a 670 in Chlorella (left) and Navicula bin Chlorella and chlorophyll c in Navicula (broken curves). The (right). The Emerson·effect peaks coincide with the absorption chlorophyll a absorption curve is also shown (light-color curve).

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© 1965 SCIENTIFIC AMERICAN, INC ribulose monophosphate-calls for a green plants: chlorophyll a and chloro­ 1943 by H. J. Dutton, W. H. Manning molecule of ATP. phyll b. Only chlorophyll a absorbs light and B. B. Duggar at the University of ATP is produced both in chloroplasts at wavelengths longer than 680 milli­ Wisconsin; later the finding was extend­ and in mitochondria, the tiny intracel­ microns; the absorption of chlorophyll b ed to other organisms by L. N. M. Duy­ lular bodies that are the site of the en­ rises to a peak at 650 millimicrons and sens of the University of Leiden. Known ergy-liberating stage of respiration in becomes negligible at about 680 milli­ as sensitized , the phenome­ animals as well as plants. The mito­ microns. Emerson found that the quan­ non indicates that the initial absorber chondria produce ATP as their main tum yield of photosynthesis at the far­ has transferred its energy of excitation , exporting it as packaged en­ red end of the spectrum beyond 680 to chlorophyll a; the transfer is effected ergy for many life processes. The chloro­ millimicrons can be brought to the full by a kind of resonance process. Careful plasts, on the other hand, make ATP efficiency of 12 percent by simultane­ measurements have shown that certain only as an auxiliary source of energy ously exposing the plant to a second accessory pigments-chlorophyll b, phy­ for certain internal purposes. The beam of light with a wavelength of 650 coerythrin, phycocyanin and fucoxan­ energy of the light falling on the chloro­ millimicrons. In other words, when light thol-pass on to chlorophyll a between plasts is stored mostly as oxidation­ primarily absorbed by chlorophyll a was 80 and 100 percent of the light quanta reduction energy by the uphill transfer supplemented by light primarily ab­ they absorb. For some other accesso­ of electrons. Only a relatively small sorbed by chlorophyll b, both beams ry pigments-for example -the fraction is diverted to the formation of gave rise to oxygen at the full rate. This transfer is less efficient. ATP, and this fraction too ultimately relative excess in photosynthesis when a This puts accessory pigments back in becomes part of the oxidation-reduction plant is exposed to two beams of light the role of being mere adjuncts to chlo­ energy of the final products of photo­ simultaneously, as compared with the rophyll a. True, they can contribute, by synthesis: oxygen and carbohydrate. yield produced by the same two beams means of resonance transfer, light en­ separately, is known as the Emerson ergy to photosyntheSiS, thereby improv­ Ist us now consider the uphill transport effect, or enhancement. ing the supply of energy in regions of of electrons in greater detail. Recent On the basis of his discovery Emerson the spectrum where chlorophyll a is a investigations have yielded considerable concluded that pl}otosynthesis involves poor absorber. Chlorophyll a, however, information about this stage. Appar­ two photochemical processes: one using collects all this energy before it is used ently the pumping of the electrons is a energy supplied by chlorophyll a, the in the primary photochemical process. two-step affair,and among the most im­ other using energy supplied by chloro­ vVhy, then, the enhancement effect? portant intermediates in it are the cata­ phyll b or some other "accessory" pig­ Why should chlorophyll a need, in lysts called cytochromes. ment. Experimenting with various com­ order to give rise to full-rate photo­ The idea of a two-step electron-trans­ binations of a constant far-red beam synthesis, one "secondhand" quantum fer process grew from a consideration with beams of shorter wavelength, and obtained by resonance transfer from an­ of the energy economy of photosynthe­ using four different types of other absorber in addition to the one sis. Precise measurements, particularly (green, red, blue-green and brown), Em­ quantum it had absorbed itself? those made by the late Robert Emerson erson's group found that the strongest A better understanding of this para­ and his co-workers at the University of enhancement always occurred when the dox resulted from the discovery that Illinois, showed that the reduction of second beam was absorbed mainly by there apparently exist in the cell not one molecule of carbon dioxide to carbo­ the most important only chlorophyll a and chlorophyll b hydrate, and the liberation of one mole­ (the green pigment chlorophyll b in but also two forms of chlorophyll a. cule of oxygen, requires a minimum of green cells, the red pigment phycoery­ These two forms have different light­ eight quanta of light energy. The maxi­ thrin in , the blue pigment absorption characteristics, and they mum quantum yield of photosynthesis, phycocyanin in blue- and the probably also have different photochem­ defined as the number of oxygen mole­ reddish pigment fucoxanthol in brown ical functions. cules that can be released for each quan­ algae). Such results suggested that these In the living cell chlorophyll a ab­ tum of light absorbed by the plant cell, other pigments are not mere accessories sorbs light most strongly in a broad is thus 1/8, or 12 percent. Since the of chlorophyll a but have an important band with its peak between 670 and transfer of four electrons is involved in function of their own in photosynthesis 680 millimicrons. In our laboratory at the reduction of one carbon dioxide [see top illustration on opposite pagel. the University of Illinois we undertook molecule, it was suggested that it takes to plot the Emerson effect more care­ two light quanta to move each electron. ertain findings concerning the be- fully than before as a function of the Emerson and his colleagues went on C havior of pigments in living plant wavelength of the enhancing light. We to determine the quantum yield of pho­ cells, however, seemed to make this con­ found that for green and brown cells the tosynthesis in monochromatic light of clusion untenable. Illuminated plant resulting curve showed, in addition to different wavelengths throughout the cells fluoresce; that is, pigment mole­ peaks corresponding to strong absorp­ visible spectrum. They found that the cules energized by the absorption of tion by the accessory pigments, a peak yield, although it remained constant at light quanta reemit some of the ab­ at 670 millimicrons that must be due to about 12 percent in most of the spec­ sorbed energy as fluorescent light. The chlorophyll a itself [see bottom illustra­ trum, dropped sharply near the spec­ source of fluorescence can be identiRed, tion on opposite pagel. It was this Rnd­ trum's far-red end [see illustrations on because each substance has its own ing that suggested the existence of two page 77l. This decline in the quantum characteristic fluorescence spectrum. forms of chlorophyll a. The form that yield, called the "red drop," begins at a The main fluorescing pigment in plants absorbs light at the longer wavelengths­ wavelength of 680 millimicrons in green always proves to be chlorophyll a, even mainly above 680 millimicrons-seemed plants and at 650 millimicrons in red when the light is absorbed by another to belong to one pigment system, now algae. pigment. This had first been shown for often called System I. The form that There are two chlorophylls present in brown algae in a study conducted in absorbs at 670 millimicrons seemed to

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© 1965 SCIENTIFIC AMERICAN, INC belong to another pigment system: Sys­ millimicrons (at 668 millimicrons) and this as it may, the important implication tem II. In the second system the form of another band at 683 millimicrons [see of the new finding is that photosynthe­ chlorophyll a that absorbs at 670 milli­ bottom illustration on page 78]. sizing cells possess two light-absorbing microns is strongly assisted by accessory systems, one containing a form of chlo­ pigments, probably by resonance trans­ f chlorophyll a is extracted from living rophyll a absorbing around 683 milli­ fer of their excitation energy. Careful I plants, there is only one product; we microns and the other a form absorbing analysis of the absorption band of chlo­ must therefore assume that in the living around 670. The latter system includes rophyll a by C. Stacy French of the cell the two forms differ in the way chlorophyll b (in green-plant cells) or Carnegie Institution of Washington's molecules of chlorophyll a are clumped other accessory pigments (in brown, red Department of Plant , and also together, or in the way they are asso­ and blue-green algae). Further investi­ in our laboratory, confirmed that the ciated with different chemical partners gation-particularly of red algae-has band is double, with one peak near 670 (, lipids or other substances). Be suggested, however, that the distribu-

-.6

-.4

� LlGH1

-.2

� --- --' ---I-----

+.4

+.6

+.8

HYDROGEN TRANSFER in photosynthesis is now conceived of trons are passed "downhill" to cytochrome J, synthesizing adeno· as a two·step process involving two pigment systems. Hydrogen sine triphosphate (ATP) in the process. Energy from System I (pri­ atoms (or electrons) from the donor (ZH) are hoosted to cyto­ marily chlorophyll a, with some accessory pigments), trapped by chrome bG hy energy collected in System II and trapped hy a hypo­ pigment 700 (P 700), boosts the electrons to a receptor (X), whence thetical "pigment 680" (P 680). The pigments of System II include they move via ferredoxin (Fd) to nicotinamide adenine dinucleo· chlorophyll (I670 and such accessory pigments as chlorophyll b or tide phosphate (NADP). Energy from ATP helps to move the elec· c, phycoerythrin or phycocyanin, depending on the plant. The elec· trons to phosphoglyceric acid (PGA) and into the carbon cycle.

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© 1965 SCIENTIFIC AMERICAN, INC tion of these two components in the two systems may be less clear-cut. In red algae a large fraction of the chlorophyll a absorbing at 670 millimicrons seems to belong to System I rather than Sys­ tem II. In all likelihood the two systems pro­ vide energy for two different photo­ chemical reactions, and efficiency in photosynthesis requires that the rates of the two reactions be equal. What are these reactions? This question brings us to another significantfinding, which suggested the participation of cyto­ chromes in photosynthesis. Cytochromes are proteins that carry an iron atom in an attached chemical group. They are found in all mitochon­ dria, where they serve to catalyze the reactions of respiration. Robert Hill and his co-workers at the University of Cam­ bridge firstfound that chloroplasts also contain cytochromes-two kinds of them. 25 REPRESENTATIVE PROJECT One, which they named cytochrome t, � has a positive oxidation-reduction poten­ tial of about .4 volt. The other, which PLANNING RESEARCH C ORPORATION is a publicly owned, they named cytochrome be, has a poten­ vigorously expanding professional service organization. Its tial of about 0 volt. In 1960 Hill, to­ expertise is in the fields of engineering, mathematics, data gether with Fay Bendall, proposed an processing, economics, business administration, and the physical ingenious hypothesis as to how the and social sciences. Its professional staff uses the sharpest tool yet two cytochromes might act as inter­ devised against complex problems confronting governments mediate carriers of electrons and con­ and industry-the multidisciplined team. If you have a degree and nect the two photochemical systems [see two to five years of experience or more, why not appraise illustration on opposite page J. They sug­ the Planning Research environment, the challenges offered in the gested that cytochrome be receives an U.S. and Europe, and the remuneration? electron by a photochemical reaction The Corporation's working atmosphere, one of absorbing from the electron donor ZH; the electron interest, contains a degree of stimulation and excitement usually is then passed on to cytochrome t by a found only in academic surroundings. Twenty-eight separate "downhill" reaction requiring no light disciplines are represented on the professional staff, ranging from energy. (The oxidation-reduction poten­ Astronautical Engineering to Zoology. There is no problem tial of cytochrome t is much more posi­ anywhere in the free world, susceptible to by the tive than that of cytochrome be-) A sec­ multidisciplined calculus employed by Planning Research, ond photochemical reaction moves the which the Corporation will not undertake to solve . electron uphill again, from cytochrome t In the past eleven years more than seven hundred projects have to the electron-acceptor X in the third been completed. Sixty-one projects are now in progress. stage of photosynthesis. In this sequence the photochemical reactions store en­ Opportunities are continually opening in th,,: Corporation's ergy and the reaction between the two five divisions: Economics; Computer Systems; Management cytochromes releases energy. Some of Control Systems; Operations and Cost Analysis; and Systems the released energy, however, can be Engineering. General capabilities required are those which can be salvaged by the formation of an ATP applied against conceptual design, cost effectiveness, reliability molecule; this occurs in the transfer of evaluation, and implementation of space, military, and command electrons among cytochromes in respira­ systems; transportation; traffic analysis; land-use; logistics; tion. In this way ATP is obtained with­ undersea technology; and world marketing. out spending extra light quanta on its formation, which the tight energy econ­ *Send for this informative brochure which describes the work omy of photosynthesis does not allow. of the Corporation, including thirty-two representative projects. Experiments by Duysens and his asso­ Address: Director of Technical Personnel. Please include resume. ciates confirmedthis hypothesis, by showing that the absorption of light by System I causes the oxidation of a cyto­ chrome, whereas the absorption of light PLANNING RE SEARCH CORPORA TION by System II causes its reduction. This An Equal Opportunity Employer is exactly what we would expect. The Home Office:1100 Glendon Avenue, Los Angeles, California 90024 illustration on the opposite page shows Washington, D.C. • Honolulu, Hawaii. Paris, France St. Petersburg, Fla .• Norfolk, Va .• st. Louis, Mo .• Englewood Cliffs, N.J. that the light reaction of System II

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© 1965 SCIENTIFIC AMERICAN, INC HYPOTHETICAL ARRANGEMENT of pigments in a chloroplast layers might contain the compounds responsible for the transport lamella would have System I in a monomolecular layer at the top of hydrogen atoms (or electrons). The water·to·oxygen cycle would and System II at the bottom. The space between the two pigment then be linked to System II and the carbon cycle to System I.

should flood the intermediates between later supported by various other obser­ about one molecule per unit. Further­ the two photochemical reactions with vations. The pigment molecules are more, experiments suggest that pigment electrons taken from ZH; the light re­ packed so closely in the unit that when 700 is oxidized by light absorbed in action of System I should drain these one of them is excited by light it readily System I and reduced by light absorbed electrons away, sending them up to the transfers its excitation to a neighbor by in System II. It has an oxidation-reduc­ acceptor X and into the third stage of resonance. The energy goes on traveling tion potential of about +.4 volt. All photosynthesis. through the unit, rather as the steel ball these properties fit the role we have in a pinball machine bounces around assigned pigment 700 in our scheme: his, then, describes in a general way among the pins and turns on one light collecting energy from a 300-molecule Tthe oxidation-reduction process by after another. Eventually the migrating unit in System I, using it to transfer which the chloroplasts store the energy energy quantum arrives at the entrance an electron to the acceptor X and re­ of light in photosynthesis. Several other to an enzymatic "conveyor belt," where covering the electron from cytochrome f investigators have contributed evidence it is trapped and utilized either to load [see illustration on page 80 J. for the two-step mechanism; notable an electron onto the belt or to unload One suspects that there should be a among them are French, Kok, Arnon, one from it. (The steel-ball analogy counterpart of pigment 700 in System Horst Witt of the Max-Vollmer Institute should not be taken literally; the migra­ II, but so far none has been convincing­ in Berlin and their colleagues. In detail tion of energy is a quantum-mechanical ly demonstrated. We believe, however, the process probably is much more com­ phenomenon, and the quantum's loca­ that a pigment we have tentatively plex than our scheme suggests. Its tion can only be definedin terms of named pigment 680-from the antici­ "downhill" central part seems to include, probability; its entrapment depends on pated position of its absorption band­ in addition to the two cytochromes, cer­ the probability of finding it at the en­ does serve as an energy trap in System tain compounds of the group known as trance to the conveyor belt.) II. Its existence is supported by the dis­ and also plastocyanin, a pro­ How is the quantum trapped? The covery of a new fluorescent emission tein that contains copper. trap must be a pigment molecule with band of chlorophyll at 693 millimicrons, What is known of the submicroscopic what is called a lower excited state; the which is compatible with absorption at structure in which the reactions of the migrating quantum can stumble into 680 millimicrons. This band is emitted second stage of photosynthesis take such a molecule but cannot come out of by certain algae when they are exposed place? There is much evidence that the it. Kok has found evidence that System I to strong light of the wavelengths ab­ photosynthetic apparatus consists of contains a small amount of a special sorbed by System II. "units" within the chloroplasts, each unit form of chlorophyll called pigment 700 containing about 300 chlorophyll mole­ because it absorbs light at a wavelength hat is the spatial organization of cules. This picture first emerged from of 700 millimicrons; this pigment could Wthe pigment systems in the electron­ experiments conducted in 1932 by Em­ serve as a trap for the quantum bounc­ boosting mechanism of the second stage erson and William Arnold on photo­ ing around in System I. There seems to of photosynthesis? It seems that the two synthesis during flashesof light; it was be a proper amount of pigment 700: systems may be arranged in two mono-

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© 1965 SCIENTIFIC AMERICAN, INC molecular layers, with a protein layer between them containing the enzymatic conveyor belt [see Wustration on oppo­ site page]. The chloroplasts are known from electron microscope studies to consist of a set of lamellae: thin alter­ nating layers of protein and fatty mate­ rial piled one atop the other. Each layer appears to consist of particles arrayed rather like cobblestones in a pavement. The particles were first observed in elec­ tron micrographs made by E. Steinmann of the Technische Hochschule in Zurich; subsequently Roderic B. Park and John Biggins of the University of California at Berkeley made clearer micrographs of the particles and named them quan­ tasomes [see illustration on page 75]. The units comprising Systems I and II the earnest camera may operate independently or they may be sufficiently close together to ex­ change energy by resonance, when for people in earnest such exchange is needed to maintain a balanced rate of operation by the two systems. about photography The picture of the energy-storing sec­ ond stage of photosynthesis presented in this article is, of course, still only a working hypothesis. Alternative hypoth­ eses are possible, one of which we shall briefly describe. For many years the late James Franck, who shared the See it at your Nikon dealer, or write: Nikon Inc. 623 Stewart Ave., Garden City, N. Y. 11533 Nobel prize in physics for 1925, tried to Subsidiary of Ehrenreich Photo-Optical Industries, Inc., N.Y. City Showroom: III Fifth Ave. develop a plausible physicochemical mechanism of photosynthesis. In 1963 he proposed, together with Jerome L. SCIENTIFIC Rosenberg of the University of Pitts­ To preserve you r copies of burgh, a concept according to which AMERICAN the two consecutive photochemical steps occur in one and the same energy trap. � A choice of handsome and durable library files-or binders-for your copies of

In other words, according to Franck, the SCIENTIFIC AMERICAN. same chlorophyll molecule that takes � the electron away from the initial donor Both styles bound in dark green library fabric stamped in gold . ZH and transfers it to a cytochrome then � Files-and binders-prepacked for sale in pairs only. supplies energy for the transfer of the Index for entire year in December issue." electron from the cytochrome to the acceptor X. In the first transfer, Franck suggested, the chlorophyll a molecule functions in the short-lived "singlet" excited state (in which the valence elec­ trons have opposite spins); in the sec­ ond transfer it functions in the long­ lived "triplet" state (in which the valence electrons have parallel spins). Franck's hypothesis avoids certain diffi­ culties of the "two trap" theory, but new FILES difficulties arise in their place. On bal­ Hold 6 issues in each. Hold 6 issues in each. ance the two-trap picture seems to us Single copies easily accessible. Copies open flat. the more plausible one at present. Price: $3.00 for each pair (U.S.A. only). Price: $4.00 for each pair (U.S.A. only). Address your order, enclosing check or No doubt this picture will change as Address your order, enclosing check or money order, to: Department 6F money order, to: Department 68 more information emerges. It is merely a first effort to penetrate the inner sanc­ New York City residents please add 4% Sales Tax tum of photosynthesis, the photocata­ ':'Supply of back copies of SCIENTIFIC AMERICAN is limited. To replace missing lytic laboratory in which the energy of copies, mail request with your order for filesor binders. sunlight is converted into the chemical SCIENTIFIC AMERICAN, 415 Madison Ave., New York 17, N.Y. energy of life.

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© 1965 SCIENTIFIC AMERICAN, INC