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Quantum Efficiency of Photosynthetic Energy Conversion (Photosynthesis/Photophosphorylation/Ferredoxin) RICHARD K

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Quantum Efficiency of Photosynthetic Energy Conversion (Photosynthesis/Photophosphorylation/Ferredoxin) RICHARD K Proc. Natl. Acad. Sd. USA Vol. 74, No. 8, pp. 3377-81, August 1977 Biophysics Quantum efficiency of photosynthetic energy conversion (photosynthesis/photophosphorylation/ferredoxin) RICHARD K. CHAIN AND DANIEL I. ARNON Department of Cell Physiology, University of California, Berkeley, Berkeley, California 94720 Contributed by Daniel I. Arnon, June 7,1977 ABSTRACT The quantum efficiency of photosynthetic with cyclic and noncyclic photophosphorylation and a "dark," energy conversion was investigated in isolated spinach chlo- enzymatic phase concerned with the assimilation of CO2 (8). roplasts by measurements of the quantum requirements of ATP has established that the formation by cyclic and noncyclic photophosphorylation cata- Fractionation of chloroplasts (9) light lyzed by ferredoxin. ATP formation had a requirement of about phase is localized in the membrane fraction (grana) that is 2 quanta per 1 ATP at 715 nm (corresponding to a requirement separable from the soluble stroma fraction which contains the of 1 quantum per electron) and a requirement of 4 quanta per enzymes of CO2 assimilation (10). Thus, in isolated and frac- ATP (corresponding to a requirement of 2 quanta per electron) tionated chloroplasts, investigations of photosynthetic quantum at 554 nm. When cyclic and noncyclic photophosphorylation efficiency can be focused solely on cyclic and noncyclic pho-. were operating concurrently at 554 nm, a total of about 12 account the conversion of quanta was required to generate the two NADPH and three ATP tophosphorylation, which jointly for needed for the assimilation of one CO2 to the level of glu- photon energy into chemical energy without the subsequent cose. or concurrent reactions of biosynthesis and respiration that cannot be avoided in whole cells. Few areas of photosynthesis have received more intensive The present investigation was undertaken not to reactivate theoretical and experimental study and generated more con- old and now dormant controversies but to relate overall pho- troversy than the efficiency with. which photosynthetic cells tosynthetic quantum efficiency to the quantum efficiency of convert the electromagnetic energy of light into chemical en- cyclic and noncyclic photophosphorylation, the two energy ergy (for review, see refs. 1 and 2). Two different concepts, conversion reactions in plant photosynthesis that, for reasons never reconciled during the lifetimes of their main protagonists, discussed elsewhere (8, 11), can be investigated in isolated emerged from the many investigations. One concept, espoused chloroplasts but not in whole cells. In cyclic photophosphor- by Warburg et al. (3), was that photosynthetic quantum con- ylation (12-14), ATP is the sole product and no 02 is produced version has an efficiency of about 90%-i.e., that energy (Eq. 1), whereas in noncyclic photophosphorylation (15, 16), equivalent to that of 3 einsteins of red quanta (42 kcal each) is 02 evolution is coupled with the reduction of ferredoxin and sufficient to liberate 1 mol of 02 (corresponding to 1/6 mol of the formation of ATP (Eq. 2). glucose, for which AGO' = 686/6 = 114 kcal). In contrast, hp Emerson (4) and his followers (5) concluded that photosynthetic ADP + Pi -. ATP [1] efficiency was much lower, of the order of 8 to 12 quanta per 02, a range that is widely accepted today even though values 4 Fdo. +2 H20 +2 ADP +2 Pi less than 8 have, at times, been obtained by investigators (6, 7) who did not share Warburg's conclusions. ','4Fdred+02+2ATP+4H+ [2] Most studies of photosynthetic quantum efficiency were based on measurements of light-induced production of 02 in which Fd0, is oxidized ferredoxin and Fdred is reduced fer- (corrected for concurrent respiration) during complete pho- redoxin. tosynthesis by whole cells, usually unicellular algae of the Reduced ferredoxin, an iron-sulfur protein electron carrier Chlorella type. Discordant results were attributed to experi- with a reducing power (Eo' = -420 mV) equal to that of mo- mental variables such as errors in methods (usually manometric) lecular hydrogen (17), serves directly as a reductant in some of 02 measurement, variations in the concurrent 02 con- reactions but for CO2 assimilation in plants the reductant is sumption by respiration, participation of respiratory interme- NADPH (E ' = -320 mV) which is formed enzymatically (18, diates in photosynthesis, need for supplementary (catalytic) 19) with no further input of photon energy: illumination, and nutritional history, age, and physiological status of the cells (1-5). Left unchallenged, however, was the 4 FdreM + 2 NADP+ + 4 H+ main (and, to us, dubious) premise underlying these studies with Fd-NADP whole cells-namely, that the photoproduction of 1 mol of 02 - 4Fdox +2NADPH+2H+ [3] always corresponds to the assimilation of 1 mol of CO2 to the reductase level of glucose and that, therefore, 02 evolution is a reliable ATP is a product of both cyclic and noncyclic photophos- measure of the total amount of chemical energy stored. phorylation (Eq. 1 and Eq. 2) which, according to recent evi- A different perspective and experimental approach to the dence, may proceed concurrently in isolated chloroplasts (20, question of photosynthetic quantum efficiency emerged from 21). Thus, the quantum efficiency of light-induced ATP for- studies of photosynthesis by isolated chloroplasts in which the mation by isolated chloroplasts provides a direct and reliable process was physically separated into a light phase concerned index of the efficiency with which photon energy is converted into chemical energy during photosynthesis. An measurements of The costs of publication of this article were defrayed in part by the argument against photosynthetic payment of page charges from funds made available to support the quantum efficiency in isolated chloroplasts is that isolation research which is the subject of the article. This article must therefore procedures may damage the photosynthetic apparatus and be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. Abbreviation: DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea. 3377 Downloaded by guest on September 24, 2021 3378 Biophysics: Chain and Arnon Froc. Natl. Acad. Sci. USA 74 (1977) result in low efficiency. That these and other experimental was also confirmed through measurements of light intensity by hazards do exist is illustrated by the wide range of values for the chemical actinometry (at 420-nm) with the potassium fer- quantum efficiency of photosynthetic phosphorylation reported rioxalate actinometer of Hatchard and Parker (24). from different laboratories. The values (expressed henceforth The fraction of incident monochromatic illumination ab- as quantum requirements; minimal quantum requirements are sorbed by chloroplasts was measured in each quantum effi- equivalent to maximal quantum efficiencies) varied from 3 to ciency experiment with a large Ulbricht integrating sphere 200 quanta of absorbed light per molecule of ATP formed (see (50-cm diameter) in a manner similar to that described by review, ref. 22). Warburg and Krippahl (25). An RCA 6217 photomultiplier The present investigation was prompted by recent findings tube served as the light detector inside the sphere; the photo- (20, 21) that further documented the role of ferredoxin as the multiplier output was measured outside the sphere with a digital native catalyst of cyclic photophosphorylation (13, 14) and voltmeter (Hewlett-Packard model 3440A). Monochromatic characterized the optimal experimental conditions under which illumination to the sphere was provided by a light beam that high quantum efficiencies (low quantum requirements) of was isolated by using the same interference filters used in ferredoxin-catalyzed photophosphorylations would most likely providing actinic monochromatic illumination to the reaction be found. Low quantum requirements were indeed observed: mixtures. 2 quanta per ATP under far-red monochromatic illumination Analytical Procedures and Reagents. Chlorophyll and (715 nm) that supported only cyclic photophosphorylation and NADPH were determined as described (23, 26). ATP was 4 quanta per ATP under short-wavelength monochromatic il- measured by the method of Hagihara and Lardy (27). Ferre- lumination (554 nm) that supported both cyclic and noncyclic doxin was isolated either from spinach leaves (28) or from the photophosphorylation. The theoretical implications of these blue-green alga Spirulina maxima (29). NADP+ and ADP were values and their relationship to measurements of quantum re- purchased from the Sigma Chemical Co. (St. Louis, MO). quirements in whole cells are discussed. RESULTS METHODS Quantum Requirements of Cyclic Photophosphorylation Chloroplasts. Chloroplasts were isolated from spinach leaves at 715 nm. Cyclic photophosphorylation in vivo occurs in (Spinacia oleracea var. High Pack) grown in a greenhouse in chloroplast lamellae that are in contact with the stroma fluid a nutrient solution culture (23) and freshly harvested before that contains dissolved 02 but in vitro the process traditionally each experiment. The chloroplast preparations were "broken" has been investigated under anaerobic conditions, for reasons chloroplasts, prepared as described (20) except that an addi- that are discussed elsewhere (8, 20). Recently (20), when fer- tional low-speed centrifugation step was added to remove any redoxin-catalyzed cyclic photophosphorylation by isolated residual large fragments. The broken chloroplasts used consisted chloroplasts
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