P I and PII are proteic compounds present in the thylakoid membrane that contain about 300 chlorophyll molecules each.
Step I
First of all light with a wavelength of 690nm is absorbed by P II. The light excites an electron that leads to the photolysis of water(H2O--->2H+ +2e- + 1/2O2). The two electrons released go into the electron transport chain. The second compound in electron transport chain is plastoquinone that is reduced when it accepts electrons then oxidised when it loses them to P I. The next two compounds are cytocromes that make hydrogen protons go inside the thylakoid when they recieve the electrons.(I will mention what happens to the H+ in the second step). The next compound in the electron transport chain is a compound that contains Fe and S. The electrons finally reach the last step: Reduction of NADP+ to NADPH+H+.
Step II
In the second step is ATP synthesis. Hydrogen protons accumulate inside the thylakoid membrane. An electrochemical gradient is formed. The protons can leave the thylakoid according to this gradient through an enzyme.
When the protons flow through , it turns ADP into ATP.
To sumarize: the role of P II is O2 production through photolysis and ATP synthesis(indirectly) The role of P I is NADPH+H+ synthesis.
