Describe the events that occur after the special pair of PbRC absorbs a photon.

The reaction center of Purple Bacteria is called PbRC. It has three hydrophobic subunits H, L, and and also eleven transmembrane helices. PbRC has a special pair of bacteriophylls ${\mathrm{P}}_{\mathrm{L}}/{\mathrm{P}}_{\mathrm{M}}$and also contains bacteriopheophytins ${\mathrm{H}}_{\mathrm{L}}/{\mathrm{H}}_{\mathrm{M}}$, accessory bacteriochlorophylls ${\mathrm{B}}_{\mathrm{L}}/{\mathrm{B}}_{\mathrm{M}}$, and ubiquinones.

The symmetry of PbRC is two-fold. Bacterial chlorophyll a has a maximum absorption wavelength of 870 nanometers (P870) and bacterial chlorophyll b has a maximum absorption wavelength of 960 nanometers (P960).

When the first photon is absorbed by a specific pair in the L subunit of the PbRC, the electron is excited, and P960 enters an excited state. Bacterial pheophytin b accepts an electron from the excited P960. The electron then migrates to Q_A, from where it is transferred to $Q_B$. The $Q_{\mathrm{A}}$does not totally reduce in this process, and it is further reduced by another photon that occurs after the first occurrence. After the second photon has completed the reduction, $Q_{\mathrm{A}}$transfers one additional electron to $Q_B$. $Q_B$becomes $Q_B$2- and gains two hydrogen ions from the cytoplasm after obtaining two electrons. After that, the two electrons are returned to the special pair. Protons cross the cell membrane as a result of this electron transfer.