Figure 1.

Photosynthetic and respiratory electron transport and their interactions. Linear photosynthetic electron transport (bottom) involves excitation of reaction centers, photosystem I and II, (PSI, PSII) and extraction of electrons from H₂O by the PSII O₂ evolving complex. Extracted electrons pass through PSII reaction centers, the plastoquinone (PQ) pool, Cytochrome b₆f (Cytb₆f), plastocyanin (PC) and to PSI where they are used to generate reduced ferredoxin (FDX) and NADPH through the activity of Ferredoxin NADP⁺ reductase (FNR); the NADPH and the ATP synthesized by the ATP synthase (fueled by the proton gradient across thylakoid membranes) and the NADPH are used to reduce CO₂ and drive metabolic processes in the cells. Reducing electrons generated on the acceptor side of PSI or PSII can be routed through AEF pathways: Cyclic electron flow occurs through both PGR5/PGRL1, and the NDA2 pathways. In the Mehler reaction, PSI-derived electrons are used to reduce O₂, and the ROS generated (O₂⁻ and H₂O₂) can be converted to H₂O through superoxide dismutase and catalase/ascorbate peroxidase. In pseudocyclic electron flow (PCEF), electrons from PSI/FDX are transferred to the diiron proteins (FLV) to reduce O₂ to H₂O. The plastoquinol terminal oxidase (PTOX) catalyzes the reduction of O₂ on the acceptor side of PSII. In chloroplast-to-mitochondria electron flow (CMEF), electrons are exported from the chloroplast to the mitochondrion through the function of OAA (or 2-oxoglutarate)/malate redox shuttles on both the chloroplast and mitochondria envelopes; reductant is shuttled between the compartments through the interconversion of malate/NAD(P)⁺ to OAA/NAD(P)H. ***, Transporting triose-P out of chloroplasts is another potential avenue for delivering reductant to mitochondria. However, since there is still little reported evidence for that route of delivery, the extent to which it provides reductant to power mitochondrial respiration is uncertain. The electrons released from these reductants are used to drive oxidative phosphorylation in mitochondria (top) through either cytochrome oxidase (complex IV) or alternative oxidases (AOXs), generating additional ATP. ICEM, inner chloroplast envelope membrane; OCEM, outer chloroplast envelope membrane; IMM, inner mitochondrion membrane; OMM, outer mitochondrion membrane. Representation of the photosynthetic electron transport chain was modified from Grossman in [18]. Created with BioRender.com.