Reaction centres

Pigments in the photosynthetic reaction centres absorb light, in the form of photons yielded from light harvesting complexes, promoting an electron to a higher energy level within the pigment. The free energy created is used to reduce an electron acceptor and is critical for the production of chemical energy during photosynthesis.

Green plants have two reaction centres known as photosystem I and photosystem II and the structures of these centres are complex involving a multisubunit protein. The reaction centre found in Rhodopseudomonas bacteria is currently better understood since it has fewer proteins than the examples in green plants.

Currently displayed is the photosynthetic reaction centre from the bacterium Rhodopseudomonas viridis

This pair of chlorophyll molecules, often called the "special pair", absorbs photons at roughly 960nm, and thus is called P960 (with P standing for "pigment"). Once P960 absorbs a photon it ejects an electron, which is transferred through another molecule of bacteriochlorophyll (the ) to the in the . This initial charge separation yields a positive charge on P960 and a negative charge on the bacteriopheohytin (creating Bph-). This process takes place in 10 picoseconds (10^-11 seconds).

The charges on the P960⁺ and the BPh^- could undergo charge recombination in this state. This would waste the high-energy electron and convert the absorbed light energy in to heat. Several factors of the reaction centre structure serve to prevent this. First the transfer of an electron from BPh^- to P960⁺ is relatively slow compared to two other redox reactions in the reaction centre. The faster reactions involve the transfer of an electron from BPh^- (BPh^- is oxidised to BPh) to the electron acceptor (Q_A) and the transfer of an electron to P960⁺ (P960⁺ is reduced to P960) from a in the subunit above the reaction centre.

in on the reaction centre

The high-energy electron which resides on the tightly bound quinone molecule Q_A is transferred to an exchangeable quinone molecule Q_B. Two of the high-energy electrons are required to fully reduce Q_B to QH₂ taking up two protons from the cytoplasm in the process. The reduced quinone QH₂ diffuses through the membrane to another protein complex (cytochrome bc₁-complex) where it is oxidised. In the process the reducing power of the QH₂ is used to pump protons across the membrane to the periplasmic space. The electrons from the cytochrome bc₁-complex are then transferred through a soluble cytochrome c intermediate, called cytochrome c₂, in the periplasm to the cytochrome subunit. Thus, the flow of electrons in this system is cyclical.