MCB Lec27 Photosynthesis Light Reactions

6CO2 + 6H2O --> C6H12O6 + 6O2

- Polyphyrin head (N and Mg2+) + long hydrocarbon phytol tail
- Losing Mg2+ gives pheophytin
- Modifications of porphyrin head cause absorption shift
- Insoluble in water
- found in chloroplast membrane lipids

Beta-carotene

phycoerythrobilin

- Contains mobile electron carriers including PQ and PC

thylakoid membrane

- All cofactors in a proteinaceous environment
- Final electron donor is H2O
- Initial electron Donor: Tyrz
- Final electron acceptor of PSII is PQH2

Model that explains pattern of O2 evolution in flashing light
- shows how water splits

- Molecular nature of S has different oxidation states of the Mn complex in
PSII
- Know the general composition of the manganese cluster center Mn4CaO5 and its redox
nature equivalent (KnowT Flashcards)
- Most reduce during the catalytic cycle is S0 and the most oxidized is S4

Two fates
- When a absorbs light, it is excited from its ground state to its singlet excited state
- a is the ground state and Chl a* is the excited state

- The distance between molecules
- The overlap between the emission spectrum of the donor and the absorption
spectrum of the acceptor
- The relative orientation of the molecules
○ Excitation energy transfer can only occur when donor and acceptor have an
energy state in common (conservation of energy)

PS I: 96 Chla
PSII: 35 Chla

- electrons are transferred from plastocyanin to ferradoxin
- P700+ formed in PSI photochemistry has a redox potential of 0.5V, which is lower than that of P680+ --> PSI cannot oxidize water

Electron flow follows a cyclic pattern that is probably identical to the Q cycle in
mitochondrial Complex III

a one-electron carrier by cycling between the Cu+ and Cu2+ oxidation state

- 4 electrons are released by the oxygen-evolving complex of Photosystem II
- 4 PQH2 oxidation release 8 H+, resulting in 2PQH2 and 2 PQ
- 4 electrons transferred to PQ forming 2PQH2

PQH2 + 2PC oxi + 2H+ --> PQ + 2PC red + 2H+

- the net result of Q cycle is two e- from QH2 reduce 2 molecules of
cytochrome c
- 4 protons are translocated to the intermembrane space, 2 from QH2 in the first round of
the Q cycle and 2 from QH2 in the second round

● Constantly working pump, as the proton gradient drives ATP synthesis, but not NADPH
● Cyclic electron transport around PSI results in a constantly working pump