In chlororespiration oxygen is consumed by a putative respiratory electron transfer system (ETS) within the thylakoid membrane of the chloroplasts and ATP is produced. It is a process that involves the interaction with the photosynthetic ETS in which NAD(P)H dehydrogenase transfers electrons to oxygen with the assistance of the photosynthetic plastoquinone (PQ), which acts as a non-photochemical redox carrier. Initially described in the unicellular alga Chlamydomonas reindhartdii, chlororespiration was highly disputed for years until the discovery of a NAD(P)H-dehydrogenase (NDH) complex (plastidic encoded) and plastid terminal oxidase (PTOX) (nuclear encoded) in higher-plant chloroplasts. PTOX is homologous to the plant mitochondrial alternative oxidase and has the role of preventing the over-reduction of the PQ pool while the NDH complexes provide a gateway for the electrons to form the ETS and consume oxygen. As a result of this process there is a cyclic electron flow around Photosystem I (PSI) that is activated under stress conditions acting as a photoprotection mechanism and could be involved in protecting against oxidative stress.
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