Refer to the figure. When the [NADPH]/[NADP+] is very high, photophosphorylation
ID: 257424 • Letter: R
Question
Refer to the figure. When the [NADPH]/[NADP+] is very high, photophosphorylation is predominately cyclic.
A. What is the primary function of the cyclic pathway? What ultimately is produced as a result of the cyclic pathway?
767 20.2 Photochemical Reaction Centers ge call s the dir ntials of P7O0 Ao Fe-S Pheo Fd PO Cyclic Fd:NADP oxidoreductase Noncycli Cyt Light NADP complex NADPH Plastocyanin Light P700 Proton gradient PQA Plastoquinone PQB Second quinone Ao Electron acceptor chlorophyl H,0)} evolving complex 1.0 ?O2 P680 A1 Phylloquinone IGURE 20-16 Integration of photosystems I and Ilin chloroplasts. bis 7 scheme shows the pathway of electron transfer from H,0 (lower Protons move across the thylakoid membrane during the water-splttin el to NADP* (far right) in noncyclic photosynthesis. The position on reaction and during electron transfer through the cytochrome b&i; con high-energy electrons flow"downhill through the carrier chains as shown vertical ntial To raise the energy of electrons derived from H,0 to the en- alternative path of electrons is cyclic electron transfer, in which elect level required to reduce NADP to NADPH, each electron must be move from ferredoxin back to the cytochrome buf complex, instead twice (heavy arrows) by photons absorbed in PSl and PSI. One ducing NADP* to NADPH. The cyclic pathway produces more ATI s required per electron in each photosystem. After excitation, the less NADPH than the noncyclic scale of each electron carrier reflects its standard reduction plex, producing the proton gradient that is essential to ATP formation.Explanation / Answer
A. In cyclic electron transfer pathway, only photosystem I not PS II is involved, the electrons moves from ferrodoxin back to cytochrome b6f complex rather than passing to NADP oxidoreductase. Cyclic electron flow is not accompanied by NADPH formation or evolution of oxygen. Rather, it is accompanied by pumping of protons into the thylakoid lumen by cyt b6f which creates a proton motive force that drives ATP synthesis when protons passes through ATP synthase. Cyclic pathway produces more ATP relative to NADPH.
B. NADPH is required in carbon assimilation reactions and other biosynthetic processes. Hence reduced ratio of NADPH/NADP+ upregulates noncyclic electron transfer pathway to ensure that enough NADPH is available for carbon fixation reaction.