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Synthesis, Carbon Dioxide Reduction Is a Series of Enzyme Reactions, None of Which Needs Light

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Synthesis, Carbon Dioxide Reduction Is a Series of Enzyme Reactions, None of Which Needs Light CHLOROPHYLL ENERGY LEVELS AND ELECTRON FLOW IN PHOTOSYNTHESIS* BY WILLIAM ARNOLD AND J. R. Azzi BIOLOGY DIVISION, OAK RIDGE NATIONAL LABORATORY, OAK RIDGE, TENNESSEE Communicated July 16, 1968 At the Second International Conference on Photosensitization in Solids,' Dr. R. C. Nelson presented data on the absolute values of the energy levels of chlorophyll as a solid. These new values, together with measurements of "glow curves" that we have been making, allow us to give now a somewhat more real- istic picture of the operation of the photosynthetic apparatus in green plants. The work of Calvin and associates has shown2 that in the process of photo- synthesis, carbon dioxide reduction is a series of enzyme reactions, none of which needs light. This reduction cycle (the Calvin cycle) is driven by electrons at -0.4 v and by ATP. The discovery by Hill' that chloroplasts could produce oxygen in the light if they had an electron acceptor present makes it possible to study the photoreaction part of photosynthesis divorced from carbon reduction. We now know that chloroplasts in the light produce ATP and electrons at -0.4 v, just as they should if they are to drive the Calvin cycle. Over the years since the discovery of the Hill reaction, the work of a great many different people has led to a theory as to how the photosynthetic apparatus works. This theory is known colloquially as the "Z-scheme." It started from a suggestion of Hill and Bendall4 and is elaborated in its most detailed form in scheme 6 of Witt's paper.' A simplified version of the Z-scheme is shown in Figure 1. Light absorbed by System II chlorophyll lifts an electron from the level of H20 at +0.8 v to 0 v, the electron then flows through an electron transport system to +0.4 v, and ATP is produced by this flow. (Two kinds of chlorophyll are required by the phe- nomenon of enhancement,6 and we use the nomenclature of Duysens,7 System I and System II, and the redox scale in which the reducing end is negative.) Light absorbed by System I chlorophyll lifts the electron from the level of cytochrome f at +0.4 v to the level of ferredoxin at -0.4 v, from which it flows to the Calvin cycle. This theory has been useful in explaining many of the observations on photo- synthesis. Nevertheless, for other observations it is inadequate and we feel that it must be changed. ELECTRONS TO -0.4- B THE CALVIN CYCLE FIG. 1.-A schematic representation for the _ Z-scheme, showing electron flow through Sys- ° o SYSTEM tem II to System I to the Calvin cycle with z V the production of ATP by the electron trans- I port chain. f+04 SYSTEM MJ\ XATP +0.8 H20 29 Downloaded by guest on September 25, 2021 30o BOTANY: ARNOLD AND AZZI PROC. N. A. S. We now give four objections to the Z-scheme: (1) Some time ago, it was found that green plants undergoing photosynthesis emit delayed light.8 This light has the same emission spectrum as the fluores- cence of chlorophyll in the living plant,9 but lasts far too long to be fluorescence. On the Z-scheme, since each quantum stores only +0.8 ev of the 1.83 ev of the absorbed quantum, presumably a thermal fluctuation must furnish 1 ev of energy. A simple calculation (using the frequency factor that we discuss below) shows that the Z-scheme predicts the intensity of the delayed light to be approximately 106 times too small. (2) Bertsch, West, and Hill10 have recently found that the delayed light from chloroplasts (measured 1 msec after illumination) is increased several times when ferricyanide is added to start the electron flow. When the delayed light. was first found, everyone assumed that photosynthesis (or electron flow, in the Hill reaction) must compete with delayed light for the stored energy. But this new observation suggests that two quanta cooperate to give both delayed light and electron flow. In order for the two quanta to be able to cooperate, they must be absorbed in the same chlorophyll system, not one in System I and the other in System II. (3) Jones'1 has shown that the intensity of delayed light made by a flash, on completely relaxed plants, is proportional to the square of the energy of the excit- ing flash. This means that two quanta cooperated to make the delayed light, and this is in agreement with the experiment of Bertsch et al. (4) We know that chlorophyll is the pigment that absorbs the energy used in photosynthesis. From the fact that chlorophyll in the plant fluoresces, we know that chlorophyll exists in the excited state for some time. It would seem that any theory of photosynthesis must involve the absolute energy levels of chlorophyll. This the Z-scheme does not do. Absolute Energy Levels of Chlorophyll. Nelson did not make his measurements on chlorophyll but rather on ethyl chlorophyllide a and b. Since there is no reason to expect any large difference in the levels, and since the precision of his measurement exceeds that of our glow curves, we will use his levels as if they were those of chlorophyll. He measured the energy necessary to extract an electron from the solid; for a metal, this would be called the work function. He also measured the electron affinity of the solid, that is, the energy given up by an electron to form chl- in the solid. In Figure 2, we show energy levels of chlorophyll as found by Nelson. We give both the absolute scale, where an electron at infinity in a vacuum is assigned zero energy, and the redox scale. To find the relation between the two scales, we used the absolute values -4.80 of the Fe++-Fe+++ level (given in Fig. 43 of Gurney's Ions in Solution'2). We compared this with the redox level (+0.77). A simple algorithm for obtaining the redox level is to add 4.00 to the absolute value and change the sign of the sum. Glow Curves.-Two years ago we showed that the method of glow curves,'3 used for many years to study the storage of energy in inorganic crystals, can also Downloaded by guest on September 25, 2021 VOL. 61, 1968 BOTANY: ARNOLD AND AZZI 31 ETHYL CHLOROPHYLLIDES -1.0- Chi a -2.96 --3.0 ChI o Chi -& -0.8- -3.1 Chlb* -3.2 Chi 6* -0.6- 3.3 --3.4 "-04~~~~~~~~ FIG. 2.-Nelson's energy levels for ethyl chlorophyllides a and b plotted against ,-Q2--. --3.8 both the redox scale in volts and the abso- X 0 --4.04 lute energy scale in electron volts. For the Cn0 transfer of one electron, the energy differ- O-4.2 0~~~~~~~~~~~~~~~~~~~, ence between two redox levels is in electron ca volts. r +0.4- --44O +0.6- -4.64 +0.8 Chl 0+ --4.8 +1.0- --5.0 Chi b+ +1.2- Chl b -5.16.6 --52 be used in the study of the light reaction in photosynthesis. Here we will give only a short description and some new results. A sample of green plant material (algae, leaf plugs, or chloroplasts) is held in the dark at room temperature until the effects of previous illumination have dis- appeared. We say that the sample is relaxed. The sample is frozen to some low temperature, between -10C and -196oC, illuminated, and heated at a constant rate from the low temperature to + 100'C. During the heating, a photomultiplier measures the light emission from the sample. With green plants five different light emissions are observed: four are spikes of emission that occur at certain temperatures during heating, and the fifth is a general emission between +30° and +100'C. The intensity of this light does not depend upon the rate of heating, as a glow curve should, but it does depend upon the presence of oxygen. In the glow curve, the four peaks of emission, which we have called Z, A, B, and C, occur at -155°, -6°, +30°, and +520C when we use a heating rate of 30/sec. We feel that the Z peak has nothing to do with photosynthesis, since one must use blue light to excite it and since it is present in plant material that had been previously heated at 1000C for 5 min. The three peaks A, B, and C give every indication of being intimately involved with the light reaction of photosynthesis. All three peaks disappear if the sample is heated to 550C for 5 min before making the experiment. In the presence of DCMU, only the B peak is found. Bishop's Scenedesmus Mutant #8, in which System I is inactive, gives all three peaks; Mutant #11, in which System II is inactive, gives none.'4 That the melting of the ice does not play an essential role is shown by the fact that we obtain the same three peaks, although at slightly different temperatures, when chloroplasts are suspended in 66 per cent glycerol with a melting point at -47°C. At the time the 1966 paper was written, we thought that the activation energy given by these glow curves represented the sum of the energy needed to return an electron to chlorophyll plus the energy needed to return a hole to chlorophyll Downloaded by guest on September 25, 2021 32 BOTANY: ARNOLD AND AZZI PROc. N. A. S. Nelson's results make this unlikely, since we now know that an electron returned to chlorophyll goes to the chl- level, 0.2 ev above the level of excited chlorophyll, and that a hole returned to chl b+ level is 0.23 ev below the ground state of chloro- phyll a.
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