Photostasis in Plants

Abstract

Life on earth is sustained by oxygenic photosynthesis, a process that involves the absorption and utilization of light energy from the sun. The chemical energy released by this endergonic process results in the oxidation of water molecules, the release of oxygen, and the generation of reductant (reduced ferredoxin and NADPH) and high-energy phosphate bond (ATP). This process takes place in the thylakoid membranes of chloroplasts. It requires coordinated interaction between a large number of electron carrier compounds and enzymatic proteins that facilitate the lateral transport of electrons in the thylakoid membrane from H2O molecules to ferredoxin and NADP+ and the transverse transport of H+ from the surrounding stroma to the enclosed space of the thylakoid lumen. The electron-transport components are localized in the thylakoid membrane and catalyze the transfer of electrons laterally in the plane of the membrane from the grana regions, where oxidation of H2O takes place, to the stroma-exposed thylakoid regions, where the reduction of ferredoxin and the generation of ATP take place (Andersson and Anderson, 1980; Anderson and Melis, 1983). Functionally, electron transport occurs from intermediate to intermediate in a sequential manner formulated as the Z-scheme of photosynthesis (Hill and Bendall, 1960). The overall process of electron transport from H2O to NADP+ is strongly endergonic and is realized through the input and utilization of light energy in two distinct steps. The absorption of light and the conversion of excitation energy to chemical energy takes place in photosystem II (PSII) and photosystem I (PSI) in the thylakoid membrane (Duysens et al., 1961). Light energy in PSII facilitates the generation of a strong oxidant capable of oxidizing H2O molecules. Light energy in PSI facilitates the generation of a strong reductant capable of reducing ferredoxin and NADP+.