Chapter 19: Light Reactions of Photosynthesis

Central to this process is the photoreceptor molecule chlorophyll a, a substituted tetrapyrrole containing a magnesium ion, which absorbs light and drives photoinduced charge separation within specialized reaction centers. The chapter contrasts the simpler cyclic electron flow found in photosynthetic bacteria—mediated by the P960 special pair and a cytochrome bc1 complex—with the more complex oxygenic photosynthesis found in green plants. In plants, two distinct photosystems operate in tandem, linked by the cytochrome bf complex in a pathway known as the Z scheme. Photosystem II (PSII) utilizes a P680 reaction center to extract electrons from water via a manganese-containing water-oxidizing complex, releasing oxygen as a byproduct and contributing to a proton gradient. Electrons are transferred through plastoquinone to the cytochrome bf complex, which pumps additional protons into the thylakoid lumen via the Q cycle before passing electrons to plastocyanin. Photosystem I (PSI), driven by a P700 reaction center, subsequently energizes these electrons to reduce ferredoxin, which ultimately powers the enzyme ferredoxin-NADP+ reductase to generate the reducing agent NADPH. The text further details how the accumulated proton-motive force across the thylakoid membrane drives the synthesis of ATP through the rotary mechanism of the CF0-CF1 ATP synthase, a process termed photophosphorylation that is mechanistically similar to oxidative phosphorylation in mitochondria. Additional topics include the role of accessory pigments like chlorophyll b and carotenoids in funneling energy via resonance energy transfer, the regulation of ATP synthase through redox conditions, and the alternative pathway of cyclic photophosphorylation which generates ATP without producing NADPH. The chapter concludes by examining the evolutionary origins of chloroplasts from cyanobacteria and the impact of herbicides such as diuron and paraquat on electron transport efficiency.