There are two photosystems which intervene successively in the photosynthesis, the photosystem II intervening before the photosystem I:
Photosystem II:
The photosystem II, which is located in the grana, has as a reaction center a pair of chlorophyll molecules to P680 (absorbing light of wavelength less than or equal to 680 nm). At this point, the energy accumulated by the reaction center releases an energetic electron that is transported on a chain of electron acceptors. The electron passes through a plastoquinone then by the cytochrome b6f complex by driving the pumping of a H + proton towards the lumen of the thylakoid. This gradient of proton concentration around the thylakoid membrane is at the origin of the electrochemical gradient allowing ATP synthase to phosphorylate ADP into ATP.
Photosystem I:
The electron then passes to Photosystem I, which is located in the intergranular chloroplast thylakoids. It consists of a pair of P700 chlorophyll molecules absorbing light of wavelength less than or equal to 700 nm. Under the action of light, it releases a new energetic electron that, passing through a ferredoxin, allows it to reduce NADP + to NADPH. In cyclic photophosphorylation, this electron does not reduce NADP + but returns to plastoquinone.
The chlorophyll molecules of both photosystems are identical but are associated with different proteins.
In the case of Photosystem II, the electrons necessary for the reduction of chlorophyll P680 are provided by oxidation of water (the term of photolysis of water is not correct) which produces protons (H +) for reduction. NADP. According to the reaction 2H2O → 4H + + O2 + 4e-
In the case of photosystem I, it is the electrons from the photophosphorylation chains that reduce the chlorophyll molecule P700.
