Chapter 4: Plastids: Structure & Function

Considerable attention is given to the composition of these membranes, noting that the thylakoid is rich in galactolipids and embedded with chlorophylls, carotenoids, and protein complexes necessary for energy transduction. The summary details the physiology of the light reactions, where solar energy is captured by light-harvesting complexes and converted into chemical energy (ATP and NADPH) through the Z-scheme of electron transport involving Photosystems II and I, the cytochrome b/f complex, and mobile carriers like plastoquinone and plastocyanin. The description explains the physics of light absorption, excitation energy transfer, and the role of accessory pigments in broadening the action spectrum. Furthermore, the chapter elucidates the mechanism of photophosphorylation, describing how a proton gradient established across the thylakoid membrane drives ATP synthesis via a chemiosmotic coupling factor (CF0-CF1 complex). The narrative then shifts to the dark reactions (the Calvin cycle), occurring in the stroma, where the enzyme RubisCO catalyzes the carboxylation of ribulose-1,5-biphosphate to fix atmospheric carbon dioxide into carbohydrates. The text contrasts standard C3 photosynthesis with adaptive mechanisms found in other plants, such as the C4 pathway, which utilizes spatial separation in mesophyll and bundle sheath cells to overcome photorespiration, and Crassulacean Acid Metabolism (CAM), which employs temporal separation to conserve water in arid environments. Additionally, the regulation of membrane stacking via LHC-II phosphorylation and the dynamic movement of complexes to balance excitation energy between photosystems are explored. The chapter concludes by discussing the translocation of metabolites via specific inner-membrane translocators, such as the phosphate translocator, and reviews the developmental biology of nongreen plastids, including amyloplasts for starch storage, chromoplasts for pigmentation, and etioplasts containing prolamellar bodies formed in the absence of light.