How a chlorophyll molecule become able to give electron by getting light ?
Chlorophyll represents a very complex organelle, which is able to absorb light to start the reaction of water oxidation and reduction of `NADP^+` .
`2NADP^+ + H_2O -> 2H^+ + O_2 + 2NADPH`
The reaction begins with absorption of a light photon by a chlorophyll molecule. The electrons become excited, moving to a higher energy level, when chlorophyll a P700, located in photosystem I, absorbs the light photon. Since chlorophyll a P700 loses electrons, they being transfered to transferred to the electron acceptor `NADP^+` , it becomes a strong oxidizing agent. When NADP^+ receives electrons, `NADPH` is formed, as it is depicted below:
`NADP^+ + 2H^+ + 2F_d(reduced) -> NADPH + H^+ + 2F_d(oxidized)`
Chlorophyll a P700 becomes unstable, after electrons being transferred, and chlorophyll P680 fills up the electron difference. Since chlorophyll P680 loses it' electrons, it must oxidize water to gain electrons back, as it is depicted below:
`2H_2O -> O_2 + 4H^+ + 4e^-`