What is photosynthetic apparatus.
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The primary reactions of photosynthesis are mediated by three protein complexes embedded in the thylakoid membranes of chloroplasts. These complexes are PSII, the cytochrome b6f complex (Cytb6f), and PSI, which are connected in series through the photosynthetic electron transport chain. Light energy captured by the light-harvesting systems of PSII (LHCII) and PSI (LHCI) is transferred to the reaction center chlorophylls to create a charge separation across the membrane. This leads to the formation of a strong oxidant on the donor side of PSII capable of splitting water into molecular oxygen, protons, and electrons. The electrons are transferred stepwise from PSII to the plastoquinone pool, Cytb6f, plastocyanin, and PSI where a second charge separation creates a strong reductant capable of reducing ferredoxin that subsequently reduces NADP+ to NADPH. Besides this linear electron pathway there is a cyclic pathway in which electrons are cycled around PSI through the Cytb6f complex. The electron transfer reactions are coupled to proton pumping into the lumen space of the thylakoids and the resulting pH gradient drives the ATP synthase for ATP production. Ultimately NADPH and ATP are used for CO2 assimilation by the Calvin-Benson cycle. Thylakoid membranes of land plants are organized in two domains, the grana stacks, comprising 80% of the membrane, and the stromal nonappressed membranes that connect different grana stacks. The photosynthetic complexes with exception of Cytb6f, are not distributed evenly through the thylakoid membrane system. Whereas PSII is mostly confined to the grana regions, PSI and ATP synthase are located in the stroma lamellae. Electron transfer between these complexes is mediated by the diffusible electron carriers plastoquinone and plastocyanin.
A characteristic feature of the photosynthetic complexes is that they all consist of numerous chloroplast and nucleus-encoded subunits. In the case of PSII, PSI, and Cytb6f these complexes contain in addition pigments such as chlorophylls and xanthophylls as well as hemes, quinones, and iron-sulfur (Fe-S) centers that act as redox cofactors. Hence the biogenesis of the photosynthetic apparatus involves a concerted interplay between the chloroplast and nucleocytosolic genetic systems as well as a tight coordination between protein and pigment synthesis and insertion into the thylakoid membranes. Analysis of numerous mutants affected in photosynthetic activity in Chlamydomonas reinhardtii, Arabidopsis (Arabidopsis thaliana), and Zea mays has provided many new insights into the assembly of the photosynthetic complexes
Answer:
The photosynthetic apparatus is a complex machinery consisting of several large protein–pigment complexes whose components are encoded by both nuclear and chloroplast genes. Thus, the biosynthesis of this system involves two distinct genetic systems which act in a coordinate manner.