Bioenergetic Pathways in the Chloroplast: Photosynthetic Electron Transfer

Abstract

In this review, we address bioenergetic pathways in the chloroplast of Chlamydomonas reinhardtii, with a focus on photosynthetic electron transfer. The conversion of solar energy into chemical energy by oxygenic photosynthesis, as performed by plants, green algae and cyanobacteria, supports the life on this planet. The production of oxygen (O2) and the assimilation of carbon dioxide (CO2) into organic matter determine, to a large extent, the composition of our atmosphere. Plant photosynthesis is conducted by a series of reactions that occur mainly in the chloroplast, resulting in light-dependent H2O oxidation, NADP+ reduction and ATP formation. NADPH and ATP, produced by linear electron flow (LEF), are required for carbon fixation via the Calvin-Benson-Bassham (CBB) cycle. Besides, photosynthetic electron transfer may operate in a cyclic electron flow (CEF) mode to satisfy the cellular ATP demand. Electrons derived from LEF may also be diverted to various other metabolic pathways, e.g. via ferredoxin (FDX). In addition, photosynthesis evolved to maximize its outcome while minimizing photooxidative stress. In this regard, mechanisms such as non-photochemical quenching (NPQ) and state transitions regulate energy influx at different light availabilities, which feedback to the proton-motive force (pmf) and the redox state of the plastoquinone/plastoquinol (PQ) pool, thereby also regulating LEF and CEF. To overcome possible limitations in electron transfer at the acceptor side of photosystem (PSI), alternative electron transfer pathways evolved, including flavodiiron proteins (FDPs), allowing safe utilization of O2 as alternative electron acceptor, as well as the hydrogenase, which utilizes two electrons and two protons to produce H2. Nevertheless, reactive oxygen species (ROS) may be formed, e.g. via the Mehler reaction at the acceptor side of PSI, which is why photosynthetic electrons are also utilized in detoxification mechanisms to prevent excessive damage. In conclusion, photosynthetic electron transfer is interwoven in a regulatory network that is aimed at adjusting ATP and NADPH production in a way that electron transfer is not harmful to the cell.