Under aerobic conditions that are likely to prevail in chloroplasts in vivo, the optimal concentration of ferredoxin for cyclic photophosphorylation was found to be equal to that required for NADP reduction and about onetenth of that needed for cyclic photophosphorylation under anaerobic conditions. In the presence of ferredoxin and NADP, cyclic photophosphorylation operated concurrently with noncyclic photophosphorylation, producing an ATP: NADPH ratio of about 1.5. The effective operation of ferredoxin-catalyzed cyclic photophosphorylation by itself required a curtailment of the electron flow from water which was accomplished experimentally by the use of either an inhibitor or far-red monochromatic light. An unexpected discovery was that the operation of cyclic photophosphorylation by itself was also regulated by a back reaction of NADPH and ferredoxin with two components of chloroplast membranes, component C550 and cytochrome b559. The significance of these findings to photosynthesis in vivo is discussed.Solar energy is first converted into biologically useful chemical energy by photosynthetic phosphorylation (photophosphorylation), the process by which the photosynthetic apparatus transforms the electromagnetic energy of sunlight into phosphate bond energy of ATP, the universal energy carrier of living cells. The energy of the photochemically generated ATP and reducing power is conserved through the biosynthesis of organic compounds from CO2. When these are later degraded by fermentation and respiration, reducing power and ATP are regenerated to drive the multitude of endergonic reactions and activities of living cells.Chloroplasts, the photosynthetic organelles of green plants, have two types of photophosphorylation, cyclic and noncyclic, names devised to denote the coupling of ATP formation to a light-induced electron flow that is either of a closed (cyclic) type that yields only ATP or of a unidirectional (noncyclic) type that yields not only ATP but also NADPH as reducing power (1). Ferredoxin plays a key role in both. In cyclic photophosphorylation ferredoxin is the endogenous catalyst (2, 3) and in noncyclic photophosphorylation ferredoxin is the electron acceptor (4) that in turn reduces NADP by an enzymatic reaction that is independent of light (5).In this paper we report the concurrent operation of ferredoxin-dependent cyclic and noncyclic photophosphorylation under conditions that are likely to exist in chloroplasts in vivo. These experiments led to the discovery that noncyclic electron flow provides a regulatory mechanism that permits cyclic photophosphorylation to operate either concurrently with the noncyclic type (and thereby to increase the ratio of ATP to NADPH as needed for photosynthetic CO2 assimilation) or to operate by itself and produce only ATP for those endergonic reactions that do not require photochemically generated reducing power. Illumination. Incident monochromatic illumination was provided by a 250 W tungsten-halogen lamp (General Electric type EHN) and appropriate inte...

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A monomeric form of the isolated cytochrome b6f complex from spinach chloroplast membranes has been isolated after treatment of the dimeric complex with varying concentrations of Triton X-100. The two forms of the complex are similar as regards electron transfer components and subunit composition. In contrast to a previous report (Huang et al. (1994) Biochemistry 33: 4401-4409) both the monomer and dimer are enzymatically active. However, after incorporation of the respective complexes into phospholipid vesicles, only the dimeric form of the cytochrome complex shows uncoupler sensitive electron transport, an indication of coupling of electron transport to proton translocation. The absence of this activity with the monomeric form of the cytochrome complex may be related to an inhibition by added lipids.

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The quantum efficiency of photosynthetic energy conversion was investigated in isolated spinach chloroplasts by measurements of the quantum requirements of ATP formation by cyclic and noncyclic photophosphorylation catalyzed by ferredoxin. ATP formation had a requirement of about 2 quanta per 1 ATP at 715 nm (corresponding to a requirement of 1 quantum per electron) and a requirement of 4 quanta per ATP (corresponding to a requirement of 2 quanta per electron) at 554 nm. When cyclic and noncyclic photophosphorylation were operating concurrently at 554 nm, a total of about 12 quanta was required to generate the two NADPH and three ATP needed for the assimilation of one CO2 to the level of glucose.Few areas of photosynthesis have received more intensive theoretical and experimental study and generated more controversy than the efficiency with. which photosynthetic cells convert the electromagnetic energy of light into chemical energy (for review, see refs. 1 and 2). Two different concepts, never reconciled during the lifetimes of their main protagonists, emerged from the many investigations. One concept, espoused by Warburg et al. (3), was that photosynthetic quantum conversion has an efficiency of about 90%-i.e., that energy equivalent to that of 3 einsteins of red quanta (42 kcal each) is sufficient to liberate 1 mol of 02 (corresponding to 1/6 mol of glucose, for which AGO' = 686/6 = 114 kcal). In contrast, Emerson (4) and his followers (5) concluded that photosynthetic efficiency was much lower, of the order of 8 to 12 quanta per 02, a range that is widely accepted today even though values less than 8 have, at times, been obtained by investigators (6, 7) who did not share Warburg's conclusions.Most studies of photosynthetic quantum efficiency were based on measurements of light-induced production of 02 (corrected for concurrent respiration) during complete photosynthesis by whole cells, usually unicellular algae of the Chlorella type. Discordant results were attributed to experimental variables such as errors in methods (usually manometric) of 02 measurement, variations in the concurrent 02 consumption by respiration, participation of respiratory intermediates in photosynthesis, need for supplementary (catalytic) illumination, and nutritional history, age, and physiological status of the cells (1-5). Left unchallenged, however, was the main (and, to us, dubious) premise underlying these studies with whole cells-namely, that the photoproduction of 1 mol of 02 always corresponds to the assimilation of 1 mol of CO2 to the level of glucose and that, therefore, 02 evolution is a reliable measure of the total amount of chemical energy stored.A different perspective and experimental approach to the question of photosynthetic quantum efficiency emerged from studies of photosynthesis by isolated chloroplasts in which the process was physically separated into a light phase concerned with cyclic and noncyclic photophosphorylation and a "dark," enzymatic phase concerned with the assimilation of CO2 (8).Fractiona...