Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Light in Biology
Photosynthesis
Photophosphorylation

Let us recall the material from Chapter 11, which discussed that both NADPH and ATP are required in The Calvin Cycle to convert СО₂ into sugar. As far as we know, the reaction stoichiometry is defined by equation (11-16). In addition to the two molecules of NADPH required to reduce one molecule of СО₂, three more molecules of ATP are needed. It is pertinent to ask where they come from. The Z-scheme provides a simple answer to this. The drop in potential across the Electron Transport Chain connecting the "upper end" of Photosystem II with the "lower end" of Photosystem I is quite sufficient for ATP synthesis driven by electron transport. In all likelihood, only one molecule of ATP is synthesized for every pair of electrons passing through this carrier chain. Since, According to the stoichiometry of equation (11-16), there are 1¹/₂ molecules of ATP per molecule of NADPH, some additional mechanism for ATP synthesis must exist. Furthermore, METABOLISM/14.html">Chloroplasts undoubtedly harbor numerous other ATP-dependent processes, so the actual demand for ATP generated during Photosynthesis may be considerably higher.

As demonstrated by Arnon et al. [79f], additional ATP can be synthesized in chloroplasts via cyclic photophosphorylation: electrons residing at the "top" of photosystem I return to the cycle, which is closed by the dashed arrow indicated in Fig. 13-18. ATP synthesis utilizes an electron transport system that is either coupled to the Z-scheme transport chain or operates independently. In fact, Arnon et al. suggested that chloroplasts possess three Photosystems: photosystem I participates in cyclic photophosphorylation, whereas photosystem II consists of two parts that comprise the Components of the Z-scheme [80].

How do photosynthetic processes differ between plants (Fig. 13-18) and Bacteria? The answer is obvious: bacteria possess only photosystem I, while photosystem II, whose functioning releases О₂, is absent. Experiments have shown that the generation of reducing equivalents (reduced ferredoxin or NADPH) by photosynthetic bacteria requires approximately half as many light quanta as are needed by green plants, in which Н₂О must be split.