Question

You have learned that proton motive force is important in movement of ions across membranes. Both proton motive force and protons in general are crucial in the function of light reactions. Please explain the source of protons and how they used in the light reactions.

Answer 1

Source of protons during light reaction are

1. Due to photolysis of water molecules .

2. Due to electron transport through photosystems ie(from PSII to PSI )

1. PHOTOLYSIS OF WATER : When light ( photons ) falls on PSII 2 electrons are dislodged and accepted by the Primary electron acceptor (PEA). To compensate this loss of electron simultaneous splitting of water occurs :

Electrons are taken up by PSII , but the protons are accumalated within the lumen of thyllakoids because splitting of H₂O takes place on the inner side of the membrane

2. Proton motive force is also created due to transport of electrons during their transport from one  photosystem to the other .

The PEA (which is located on the outer side of the membrane) while transporting electrons, transport it to a proton carrier rather than to an electron carrier and so when electrons are transported form PEA to Cyt b3 to Cyt f , a proton is released into the lumen of thyllakoid from the stroma .

3. Further increase in the PMF is due to reduction of protons in the stroma when NADP reductase enzyme ( Located on the outer side of the thyllakoid membrane reduces NADP to NAPDH⁺+ H⁺. These protons are also removed from the stroma.

As a result inside the chloroplast, protons decrease in the stroma and increase inside the lumen of the thyllakoid. Why is this gradient essential ? Because it is breakdown of this gradient which cause movement of protons through Fo( Transmembrane channel ) of the ATPase enzyme . Enough energy is liberated during this transport which causes conformational change in the F1 particle of ATPase enzyme ( Facing the stroma) and thus catalyse formation of ATP molecules .