AP BIOLOGY

Detailed explanation-1: -Excited P680 is a good electron donor and can transfer its excited electron to the primary electron acceptor, pheophytin.

Detailed explanation-2: -2 Cyclic Photophosphorylation. This process begins in PSI as follows: the electron passes from chlorophyll P680 to ferredoxin (Fd, the primary acceptor), then to pheophytin (Ph), then to complex b6-f, and then to plastocyanin (Pc), before returning to chlorophyll P680, and then the cycle is repeated.

Detailed explanation-3: -Excitation of the reaction center, via light absorption by chlorophylls (Chl) in the antenna, drives electron transfer from the cluster of four chlorophylls bound to the D1-and D2-proteins, to the pheophytin (Pheo) acceptor leading to the radical pair P680•+ P680•−.

Detailed explanation-4: -A photon of light energy travels until it reaches a molecule of chlorophyll. The photon causes an electron in the chlorophyll to become “excited.” The energy given to the electron allows it to break free from an atom of the chlorophyll molecule.

Detailed explanation-5: -The electron released by reaction center of photosystem II i.e., P680 moves uphill and reduces pheophytin, a nonmagnesium chlorophyll a molecule. From pheophytin, electrons are accepted by plastoquinone which requires two electrons for complete reduction.