Photosynthesis electron transport chains Flashcards
Give an example of a photosynthetic bacterium that is used as a model for cyclic electron transport
Rhodobacter
What are the two types of electron flow through an electron transport chain?
cyclin and non-cyclic
What type of electron flow is mostly used in higher plants?
non-cyclic electron flow
What are the reaction centres special pair of chlorophylls of the two photosystems in the electron transport chain of higher plants?
Photosystem I = reaction centre is P700
Photosystem II = reaction centre is P680
What is the reaction centre?
A photosensitive pair of chlorophyll molecules
Describe the non-cyclic electron flow in higher plants
1) PSII reaction centre P680 absorbs photon and electron excited to a higher energy level and passed to pheophytin, P680 now oxidised
2) electron passed through plastiquinone A to reduce plastiquinone B, which is released into the PQ/PQH2 membrane pool
3) cytochrome bf complex oxidises 2 PQH2 to pump 4H+ into the thylakoid space and the electrons to plastocyanin
4) the electrons pass from plastocyanin to the P700 chlorophylls of PSI
5) P700 absorbs photon and electron excited and passed to the 4Fe-4S centre of Ferredoxin
6) electron then passes to Ferredoxin-NADP reductase to catalyse the reduction of NADP+ to NADPH
What happens to the redox potential of chlorophyll following excitation of an electron to a higher energy?
It becomes more negative, hence why the electron is able to flow to the next complex
How are is the oxidised P680 reaction centre restored to its original state?
PSII has a Manganese coordinated, which hydrolyses water and allows for the sequential delivery of electrons to reduce P680
What is the equation for the splitting of water?
H2O —-> 2H+ + 1/2 O2 + 2e-
How many electrons are pumped by the non-cyclic electron flow in higher plants? Where do these electrons come from?
6 H+
4 H+ come from the Q cycle of cytochrome bf complex
2 H+ come from the hydrolysis of water on the P side of the membrane
When may cyclic electron flow occur in a higher plant?
when the cell is in a low energy state and doesn’t have sufficient ATP to carry out the Calvin cycle
What part of the chloroplast contains the photosystems?
thylakoid membrane
What part of the chloroplast contains the ATP synthase?
lamellae
What happens in cyclic electron flow in plants? about 4 stages
1) P700 absorbs light and is oxidised as electron passes to ferredoxin
2) ferredoxin passes electrons to the PQ/PQH2 membrane pool to reduce plastoquinone
3) plastoquinone passes electron through cytochrome b6f complex, pumps 4H+ and reduces plastocyanin, which returns the electrons back to P700.
4) the additional H+ pumped allow for the production of more ATP via ATP synthase
What type of electron flow occurs in photosynthetic bacteria?
Cyclic
Describe the cyclic electron flow in photosynthetic bacteria
1) P870 absorbs photon, excited electron passed to bacteriopheophytin then to quinone B via quinone A
2) Quinone B is reduced and released into the membrane Q/QH2 pool
3) cytochrome bc1 oxidises 2 QH2 to pump 4H+ to the P side and electrons to cytochrome C2, which transfers them back to the P870 chlorophyll molecules
What part of the photosynthesis electron transport chain (in plants and that in bacteria) is analogous to complex III of the respiratory chain? why?
Plants = cytochrome bf complex Bacteria = Cytochrome bc1
All carry out the Q cycle and pump 4H+ from N to P side
What is the reaction centre special pair in Rhodobacter?
P870
What is photoinduced charge separation? How does the structure of the reaction centre facilitate this?
absorbed light excites electron from ground state and the electron is transferred to the N side of the membrane to separate the charge (electron moved to N side, positive/oxidised chlorophyll on P side)
The redox centres are held in fixed positions within the reaction centre by transmembrane domains of polypeptides: special pair are held on the P side, and the quinone B (bacteria)/ plastoquinone B (plants) is held on the P side - this ensures the flow of electron from more negative to more positive results in a charge separation.
Which is used by bacteria, plants, or mitochondria?
1) ubiquinone/ubiquinol
2) plastoquinone/plastoquinol
3) quinone/quinol
1) mitochondria
2) plants
3) bacteria
What are light harnessing complexes?
surround reaction complex and act as antennae to collect more photons and transfer the energy to the reaction centre.
What is delocalisation excitation coupling and resonance energy transfer that occurs in LH complexes?
Delocalisation excitation coupling = energy transfer/sharing between close pigments in the same LH complex
Resonance energy transfer = energy transfer to other LH complexes and eventually to the reaction centre
