Excitation energy
travels down a energy gradient from B800 to B850 bacteriochlorophyll a pigments
absorbing at 800 and 850 nm in the LH2 complex (blue). Energy is transfered to a
LH1 complex (green) which is eventually is trapped by the Reaction Centre
(yellow). 
This process provides chemical energy in the form of ATP by converting solar energy into a concentration gradient of protons across the cell membrane. The flow of protons through the memebrane is the source of protomotive force that drives the reaction which generates ATP from ADP + Pi.
Light Trapping
Light is trapped by peripheral Light Harvesting Complex II (LH2) which contains bacteriochlorophyll a and carotenoids as the absorbing molecules. The excitation energy is passed from one complex to another until it enters the core complex comprising a Reaction Centre (RC) surounded by Light Harvesting Complex I (LH1).
The light energy captured by LH2 and LH1 complexes is used to release electron in the Reaction Centre. These are used in the reduction of ubiquinone ( UQ ) molecules to ubiquinol ( UQH2 ). The cytochrome b/c1 complex oxidises UQH2 and at the same time pumps protons out of the cell (green arrow) while returning electrons to the Reaction Centre via Cytochrome c2 (pink arrow). FoF1-ATP synthetase uses the proton gradient across themembrane to generate energy rich ATP molecules.
