Photosynthesis

Question 1

Principle behind light-induced electron transfer

Answer 1

• light shines on a pigment molecule and an electron is excited to a higher energy level
• as electron falls back to initial state it gives up energy and this energy translocates protons across a membrane
• creates a proton gradient to drive ATP synthesis through ATP synthase

Question 2

difference between primary and secondary photopigments

Answer 2

• secondary photopigments catch photons; the excitation energy moved to primary photopigments
• primary photopigments are chlorophylls that absorb at P680

Question 3

photon absorbance and transfer

Answer 3

• photon absorb by secondary pigment and electron promoted to excited state
• excited state energy transferred to primary pigment. no energy loss and no transfer of electrons
• excitation energy reaches primary pigment at reaction center
• at reaction center, pigment with excited electron is a strong electron donor; becomes strong electron acceptor after electron transfer
• the e- will be transferred through a series of carriers until it reaches a terminal electron acceptor
• strong electron acceptor needs to obtain an e- from a molecule with a smaller reduction potential to restore primary pigment to ground state.

Question 4

PS II

Answer 4

• P870 loses electrons to P870+
• two photons absorbed to produce two e- in rxn center; passed through the two carriers to the bound quinone to a lipid-soluble quinone and QH2 made
• no H+ translocated but QH2 results and diffuses to cytochrome bc1 complex
• two QH2, one Q, and 2 H+ used to produce a Q, QH2, 2 cyt c (red) and 4 H+ molecules.
• H+ translocated to inter membrane space
• 2 cyt c (red) diffuse to PS II and reduce P870+
• no net production of QH2 or NADPH

Question 5

PS I cyclic

Answer 5

• terminal reduced electron acceptor is ferredoxin or flavodoxin
• uses photons to generate e- and transfer to QH2; cytochrome bc1 complex uses QH2 to translocate 4H and reduce cytochrome c; the cytochrome c, diffuses to PS 1 and reduces P700+

Question 6

PS I noncyclic

Answer 6

• cytochrome bc1 complex not used
• PS I 700+ receives e- from H2S; no H+ translocated
• reduction that results in Fd (red) used to make NADPH
• 1 NADPH made for every 2 e-

Question 7

PS 1 + PS II as cycle

Answer 7

• PSII and cytochrome bf complex work together and produce a reduced pastocyanin (PC) that is used to provide reduce the primary pigment of PS 700+
• OEC extracts electrons from H2O and passes them to P680+; results in oxidation of H20 to yield 2H+ + 1/2O2
• produces 4 H+ and 1 NADPH

Question 8

PS 1 + PS II non-cyclic

Answer 8

• PS I, before NADPH made, Fd (red) used to make QH2 and QH2 shunted to cytochrome bf for production of PC and translocation of H+.

Question 9

Use of H2O for reduction of primary photopigments in PS II

Answer 9

• P680 receives energy from photon and oxidized to P680+ state
• P680+ gets e- from Tyr in OEC; Tyr gets from Mn
• e- occurs until Mn4+ reaches
• Mn4+ extracts 4 e- from 2 H20 and produces 4 H+ and O2

Question 10

Structure of thylakoid membrane in chloroplasts

Answer 10

• thylakoid located in inner membrane of chloroplast
• folded many times into grana
• PS I, PS II, cyt bf, and ATP synthase inside

Question 11

Orientation of ATP synthase with respect to PS I, PS II, cytochrome bf

Answer 11

• H+ translocated into human
• ATP synthase positioned facing the outside of the inner membrane in stroma space
• PS II and cytochrome bf located in grana
• PS I and larger numbers of ATP synthase located in stromal lamellae

Question 12

Co2 assimilation

Answer 12

• condensation with ribulose-1,5-bisphosphate into two molecules of 3 phosphoglycerate
• reduction to glyceraldehyde-3-phosphate uses 1 ATP and 1 NADPH
• 1 ATP to convert back to ribulose-1,5-bisphosphate

Question 13

Significance of reaction

Answer 13

• all reactions have to be working simultaneously to avoid accumulation of unusual carbon intermediates
• glyceraldehyde-3-phosphate can be used to synthesis of fructose-6-phosphate and synthesis of glucose and glucagon.

Question 14

Chloroplast transport

Answer 14

• inner membrane can’t pass charged and polar molecules but has transporters
• movement of materials (glucose) or movement of ATP
• ATP uses Pi-triose phosphate antiport system
• continuous circulating movement of the same molecule from the chloroplast to the cytoplasm
• can be used for transport of glyceraldehyde-3-phosphate for use in gluconeogenesis

Question 15

Starch and sucrose synthesis

Answer 15

• glyceraldehyde-3-phosphate used for glucose synthesis
• glucose used for glycogen synthesis
• starches made from glucose for energy storage inside chloroplasts.