Exam 3: 27 Photosynthesis I (COMPLETE) Flashcards
- In Photosystem II (PSII), light absorption of P680 triggers the formation of P680+Pheo.
Which of the following is the initial electron donor for P680+* ?
A) Water
B) Mn4CaO5
C) TyrD
D) TyrZ
D) TyrZ
Which of the following is the INITIAL electron ACCEPTOR for P680+*?
A) TyrZ
B) Pheophytin
C) H2O
D) Plastoquinol (PQH2)
B) Pheophytin
The initial electron acceptor of P680 during charge separation is pheophytin. When P680 absorbs a photon and becomes excited (P680*), the excited electron is rapidly transferred to pheophytin, a chlorophyll molecule lacking the central magnesium ion.
Which of the following is the INITIAL electron DONOR for P680+*?
A) TyrZ
B) Pheophytin
C) H2O
D) Plastoquinol (PQH2)
A) TyrZ
The initial electron donor to P680 during charge separation is TyrZ. After the excited electron from P680* is transferred to pheophytin, P680 becomes P680+, a strong oxidant. TyrZ then donates an electron to P680+, reducing it back to P680.
Which of the following is the FINAL electron DONOR for P680+*?
A) TyrZ
B) Pheophytin
C) H2O
D) Plastoquinol (PQH2)
C) H2O
The final electron donor in PSII is water (H2O). P680+, the strong oxidant created after the initial charge separation, is powerful enough to oxidize water, extracting electrons to replenish its electron deficit. This process is crucial for oxygenic photosynthesis, as it’s the step where water is split, releasing oxygen as a byproduct.
Which of the following is the FINAL electron ACCEPTOR for P680+*?
A) TyrZ
B) Pheophytin
C) H2O
D) Plastoquinol (PQH2)
D) Plastoquinol (PQH2)
The final electron acceptor in PSII is plastoquinol (PQH2). The electron received by pheophytin is passed along a chain of electron carriers within PSII, ultimately reducing plastoquinone (PQ) to plastoquinol (PQH2). PQH2 then leaves PSII and carries the electrons to the cytochrome b6f complex.
**2. In contrast to the photosynthetic linear electron transfer, the photosynthetic cyclic electron transfer generates ATP but not NADPH, fine-tuning the ATP/NADPH required for cellular metabolism.
Which protein complex in the cyclic electron transfer process contributes to the proton gradient establishment across the thylakoid membrane that drive ATP synthesis in cyclic electron transfer?
A) Ferredoxin: NADP+ oxidoreductase.
B) Water: Plastoquinone oxidoreductase
C) Plastoquinone: Plastocyanin oxidoreductase
D) Plastocyanin: Ferredoxin oxidoreductase
C) Plastoquinone: Plastocyanin oxidoreductase
Cyclic Electron Transport: The sources mention cyclic electron transport (CET) around PSI, which involves electron flow from PSI back to the cytochrome b6f complex via plastoquinone (PQ). While CET does not directly involve water splitting or NADPH formation, it does contribute to proton pumping by the cytochrome b6f complex, thus enhancing the proton gradient.
Cyclic Electron Transport (CET)
CET provides an alternative pathway for electron flow in photosynthesis. In this pathway, electrons excited at PSI are cycled back to the cytochrome b6f complex via PQ, bypassing PSII and NADP+ reduction. The cycle proceeds as follows:
Electron Excitation at PSI: Light energy absorbed by PSI excites electrons to a higher energy level.
Electron Transfer to Ferredoxin: The excited electrons are transferred to ferredoxin, as in the linear ETC.
Electron Transfer to PQ: Instead of reducing NADP+, ferredoxin donates electrons back to PQ, reducing it to PQH2.
Electron Transfer through Cytochrome b6f Complex: PQH2 delivers electrons to the cytochrome b6f complex, which translocates protons from the stroma to the lumen, contributing to the proton gradient.
Electron Return to PSI: Electrons from the cytochrome b6f complex are passed back to PSI via PC, completing the cycle.
- Without exceptions, all photosynthetic organisms contain a light-harvesting pigment-protein complex that capture light energy and transfer it to the reaction centers (RCs) where photochemistry takes place. Förster resonance energy transfer (FRET) is a mechanism describing energy transfer between two light sensitive molecules.
Which of the following statements on the excitation energy transfer between two pigments is correct?
A) The distance between the two pigments is the only factor governing the energy transfer efficiency between two molecules
B) The overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor must exist
C) The relative orientation of the pigments doesn’t play any roles for the rate of energy transfer.
D) Energy transfer happens between any two pigments
B) The overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor must exist
The rate of energy transfer from the FRET donor to the acceptor depends on several factors:
1) Distance between molecules (6th power)
2) Overlap between emission spectrum of the donor and the absorption spectrum of the acceptor
3) Relative orientation of the molecule
True or False:
Distance between molecules is a factor of rate energy transfer (FRET) between two pigments.
True
(leap frog)
True or False:
Overlap between emission spectrum is a factor of rate energy transfer (FRET) between two pigments.
True
(leap frog)
True or False:
Relative orientation of molecules is a factor of rate energy transfer (FRET) between two pigments.
True
(leap frog)
- Possible fates of an excited chlorophyll a can be:
A) Excitation energy transferring to other pigments.
B) Heat dissipation through non-photochemical quenching (NPQ)
C) Fluorescence
D) All of the above
D) All of the above
Possible fates of Chlorophyll a:
- Excitation transfer to other pigments
- Photochemistry -> photochemical quenching
-Heat-> Non-photochemical quenching
-Fluorescence
True or False:
Excitation transfer to other pigments is a possible fate of excited Chlorophyll a.
True
True or False:
Heat dissipation through non-photochemical quenching is a possible fate of excited Chlorophyll a.
True
True or False:
Florescence is a possible fate of excited Chlorophyll a.
True
True or False:
Photochemistry to photochemical quenching is a possible fate of excited Chlorophyll a.
True
