During non-cyclic electron transport of photosynthesis of higher plants,  A is the ultimate electron donor and B is the ultimate electron acceptor. 

Identify A and B are, respectively,

In  non-cyclic electron transport of photosynthesis of higher plants, the electrons of water ultimately reach ${\mathrm{NADP}}^{+}$ to form NADPH after passing through Mn, tyrosine or ${\mathrm{Y}}_{\mathrm{z}}$, P 680, pheo, $Q_{\mathrm{A}}, {\mathrm{Q}}_{\mathrm{B}}$, PQ, Fe.S protein, cytochrome f, plastocyanin, P700,  ${\mathrm{A}}_0, {\mathrm{A}}_1, {\mathrm{F}}_{\mathrm{x}}, {\mathrm{F}}_{\mathrm{A}/\mathrm{B}}$, ferredoxin, and FNR enzyme.