Q.The correct sequence of flow of electrons in the light reaction is
a. PS II, plastoquinone, cytochrome, PS I, ferredoxin.
b. PS I, plastoquinone, cytochrome, PS II ferredoxin.
c. PS II, ferredoxin, plastoquinone, cytochrome, PS I.
d. PS I, plastoquinone, cytochrome, PS II, ferredoxin.

Electron transport in the chloroplast moves electrons, released either from water (via PS II) or directly from an excited PS I reaction centre, through a defined sequence of membrane-bound carriers, generating both ATP and (in the non-cyclic route) NADPH+H+ along the way. In non-cyclic transport, the correct order of carriers is: water is split at the Oxygen Evolving Complex on PS II, releasing electrons that excite P680; these electrons pass through pheophytin, then the mobile carrier plastoquinone (PQ), then the cytochrome b6-f complex, then the mobile carrier plastocyanin (PC), and are accepted by the excited P700 reaction centre of PS I; from P700, electrons pass to the primary acceptor ferredoxin-reducing substance (FRS), then to ferredoxin, and finally reduce NADP+ to NADPH+H+. Because the electron's energy level rises at each photosystem (on absorbing a photon) and falls in between (while passing through the carrier chain), a plot of this sequence traces a zig-zag shape resembling the letter Z, giving non-cyclic transport its alternative name, the Z-scheme. ATP is generated specifically during the downhill PS II-to-PS I segment of this chain (and, in the separate cyclic pathway, as electrons return from ferredoxin back to P700 via plastoquinone, cytochrome b6-f and plastocyanin) - not by the electron transport itself acting directly on ADP, but by the proton gradient this transport builds across the thylakoid membrane, which ATP synthase then taps to phosphorylate ADP into ATP (the chemiosmotic mechanism). Releasing one electron costs two quanta of light in non-cyclic transport (one for water-to-PS I transport, one for PS I-to-NADP+ transport), so producing the two NADPH+H+ molecules needed per CO2-fixation cycle requires four electrons and 8 quanta of light overall - the same 8-quanta cost as evolving one molecule of oxygen.