Figure 1.

Schematic representation of the photosynthetic apparatus in the intracytoplasmic membrane of purple bacteria. The RC (red) is surrounded by the light-harvesting complex I (LH-I, green) to form the LH-I–RC complex, which is surrounded by multiple light-harvesting complexes LH-II (green), forming altogether the PSU. Photons are absorbed by the light-harvesting complexes and excitation is transferred to the RC initiating a charge (electron-hole) separation. The RC binds quinone Q_B, reduces it to hydroquinone Q_BH₂, and releases the latter. Q_BH₂ is oxidized by the bc₁ complex, which uses the exothermic reaction to pump protons across the membrane; electrons are shuttled back to the RC by the cytochrome c₂ complex (blue) from the ubiquinone–cytochrome bc₁ complex (yellow). The electron transfer across the membrane produces a large proton gradient that drives the synthesis of ATP from ADP by the ATPase (orange). Electron flow is represented in blue, proton flow in red, and quinone flow, likely confined to the intramembrane space, in black.