A closeup of a photosynthetic primary reaction

The 3-dimensional structure of the reaction center and earlier kinetic and spectroscopic results have provided a detailed picture of the primary photosynthetic reactions.

The reactions illustrated above are repeated when another light quantum is absorbed except that this time the Q_B-molecule is reduced to hydroquinone. The hydroquinone leaves the reaction center and is replaced by a new Q_B-molecule from the membrane. The electrons eventually return to the reaction center (to the cytochrome) via other electron carriers. Details in the protein environment around chlorophyll and pheophytin explain why only the right half (the L-subunit branch) of the reaction center is active.