Q.In non-cyclic (linear) electron flow, high-energy electrons ejected from the reaction-centre chlorophyll of PS II are ultimately used to reduce NADP+ to NADPH via PS I. The "electron hole" left behind in oxidised PS II is refilled with electrons obtained from:

PS II's oxidised reaction centre (P680+) is a powerful enough oxidant to split water at a manganese-containing water-splitting complex on the lumenal side of the thylakoid membrane: $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$. This refills PS II's electron hole, releases protons into the lumen (contributing to the proton gradient that drives ATP synthesis), and evolves the O2 that is photosynthesis's signature by-product -- confirming, as van Niel predicted, that photosynthetic O2 originates from water. The originally ejected electron travels dow …