Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A. N. Ogurtsov 2011

Molecular Mechanisms of Photosynthesis
Linear electron transport chain in oxygenic photosynthetic organisms

Unidirectional (linear) electron transport in METABOLISM/14.html">Chloroplasts, in which an electron is transferred from a Water molecule to NADP⁺, involves both Photosystems, PS1 and PS2.

The process begins at photosystem PS2 with the absorption of the first photon, resulting in The transfer of an electron from chlorophyll a P₆₈₀ (near the luminal surface of PS2) to plastoquinone Q_B (on the stromal surface of PS2), forming a semiquinone (Figure 184).

The oxidized P⁺₆₈₀ generated by this electron transfer abstracts the missing electron from a water molecule. This produces intermediates that ultimately lead to the synthesis of O₂, as well as protons that remain in the thylakoid lumen and contribute to the Generation of the proton-motive force.

Next, a second photon is absorbed by chlorophyll a P₆₈₀, causing the semiquinone to accept a second electron and take up two protons from the stroma, forming dihydroplastoquinone QH₂. This molecule then dissociates from PS2 and diffuses within the thylakoid membrane to the cytochrome bf complex, where QH₂ binds to the Q₀ center of the cytochrome bf complex on the luminal side. The cytochrome bf complex is structurally and functionally analogous to the cytochrome bc₁ complex of purple Bacteria and the CoQH₂-cytochrome c reductase in Cell/35.html">Mitochondria. Similarly, the Q-cycle takes place within the cytochrome bf complex coupled with the PS2 reaction center, which enhances the proton-motive force generated by electron transport.

In photosystem PS1, the absorption of a photon drives the transfer of an electron from chlorophyll a P₇₀₀ through a chain of intraprotein acceptors to the water-soluble protein ferredoxin, which is bound to the stromal side of PS1. The oxidized chlorophyll P⁺₇₀₀ is reduced by an electron transferred from the cytochrome bf complex by the copper-containing water-soluble protein plastocyanin located in the thylakoid lumen.

The bacterial counterpart of PS1 is shown in Figure 178.

Upon receiving electrons from QH₂, the cytochrome bf complex passes them one by one to the Cu²⁺ form of plastocyanin, reducing it to the Cu⁺ state. The reduced plastocyanin then diffuses through the thylakoid lumen to PS1, carrying a single electron per protein molecule. Upon binding to PS1, the electron is transferred to chlorophyll, returning it to its initial state.

Ferredoxin, which transports electrons via its built-in Fe-S iron-sulfur cluster, passes the electron to the enzyme ferredoxin-NADP reductase; using FAD and a proton taken up from the stroma, this enzyme reduces NADP⁺ to NADPH.

F₀F₁-complexes on the thylakoid membrane utilize the proton-motive force established during linear electron transport to synthesize ATP within the thylakoid stroma. Thus, linear electron transport results in The production of NADPH and ATP in the stroma, where they are subsequently used for CO₂ fixation.