Light-harvesting complex 2 (LH2) contains two groups of bacteriochlorophyll (BChL) pigments, B800 and B850. Light energy absorbed by B800 is transferred to B850 (blue arrows). It is then passed to another group of BChLs (B880) in light-harvesting complex 1 (LH1), and finally into the reaction centre (RC). The excited RC fully reduces ubiquinone molecules (Q), which leave the LH1–RC complex and pass into the membrane, transferring electrons (e−) to the cytochrome bc1 complex; this transfer forms part of a cyclic electron-transport pathway (red arrows) that drives photosynthesis. This pathway is completed by a soluble protein, cytochrome c2 (Cyt c2). The periplasm is the region between the cell membrane and the cell wall of the bacterium.