Light Reactions Flashcards
Where is the photosynthetic electron transport chain found?
Across the thylakoid membranes in chloroplasts.
Why is photosynthesis endothermic?
A lot of energy is needed to oxidise water.
What are the two components of the proton motive force?
Membrane potential and pH difference
What is the proton motive force dominated by in photosynthesis?
pH gradient
What are the products of photosynthesis that are later used by the Calvin cycle?
ATP and NADPH
What are the cofactors found in PSII?
P680, Pheophytin, Qa, Qb, Mn cluster and Tyr
What does PSII do?
Uses 4 photons of light to remove 4 electrons from water, generating a proton gradient.
What is the net effect of PSII activity?
To transport 4 reducing equivalents across the membrane.
How many electrons are carried by plastoquinone?
2 electrons
Where is plastoquinone reduced and oxidised?
Reduced by PSII. Moves out of PSII to Cytochrome b6f (it is membrane soluble). Oxidised in Cytochrome b6f during the Q cycle.
Describe the structure of cytochrome b6f.
Homodimer where each dimer consists of cytochrome f, cytochrome b6, Rieske centre and Subunit IV.
What does cytochrome f contain?
Heme c
What does cytochrome b6 contain?
Two heme b molecules.
What does the Rieske centre in cytochrome b6f contain?
An iron-sulfur cluster - [2Fe-2S]
What is the net effect of cytochrome b6f actvity?
Translocation of 8 protons - contributes to proton motive force.
Reduction of two PC per quinol from PSII
What enzyme is cytochrome b6f homologous to and why?
Complex III in mitochondria - both use a Q cycle, oxidise a quinone and reduce a mobile electron carrier.
What happens to the plastoquinone molecules produced in cytochrome b6f?
One diffuses back to PSII and one is used to reduce plastocyanin.
What is the Q cycle in cytochrome b6f needed?
To mediate electron transfer between PSII and PSI
How many protons are pumped across the membrane by PSII and cytochrome b6f?
12 in total - 4 by PSII, 8 by cytochrome b6f.
What is plastocyanin?
A single electron carrier, where Cu(I) is reduced to give Cu(II). Plastocyanin is soluble and acts as a mobile electron carrier between cytochrome b6f and PSI.
What is the result of the distorted tetrahedral coordination in plastocyanin?
Gives copper an unusually high potential compared to an isolated copper ion.
What happens if cyanobacteria or green algae are copper starved?
Plastocyanin is replaced with cytochrome c6 (contains heme c groups).
Describe the structure of PSI in cyanobacteria.
Trimeric
Describe the structure of PSI in eukaryotes.
Monomeric
What type of reaction centre is present in PSI?
Type 1 - terminal electron acceptor is an FeS cluster.
What cofactors are found within PSI?
chlorophylls, accessory chlorophylls, 2 quinones, 3[4Fe-4S] centres
What is the active pigment in PSI?
P700 - consisting of two chlorophyll a molecules.
Why is PSI unable to oxidise water?
The P700 pigment absorbs a longer wavelength than the P680 pigment in PSII, providing less energy - not enough to oxidise water.
What type of chemistry is performed by PSI?
1 electron chemistry
Give an overview of the electron transfer steps that occur in PSI.
- Electron transfer from plastocyanin to P700
- Electron transfer from P700 to chlorophyll a
- Electron transfer from chlorophyll a to quinone bound at position A1
- Electron transfer from the quinone to FeS clusters
- Electron transfer from FeS to ferredoxin
What is ferredoxin?
A soluble single electron carrier. Contains an Fe2-S2 cluster.
What are the major pathways for the electrons from reduced ferredoxin to go to?
Ferredoxin-NADP reductase (FNR), Cyclic electron flow, flavin-thioredoxin reductase.
What are the minor pathways for the electrons from reduced ferredoxin to go to?
Nitrate reductase, nitrogenase, sulfite reductase and glutamate synthase.
What cofactor is used by ferredoxin-NADP reductase?
FAD
Where is FAD found and what does it do?
Located spatially close to the FeS of ferredoxin. Acts as a sequential 2 electron carrier:
FAD -> FADH* -> FADH2
What is NADPH and when is it used?
2 electron carrier, also essentially a hydride carrier. Used in anabolic reactions and acts as a source of reductants in the Calvin cycle.
What is meant by the Z-scheme?
Non-cyclic electron flow from water to NADP+
What happens during cyclic electron flow?
Ferredoxin/NADP are used to reduce plastoquinone and this is catalyses by ferredoxin-plastoquinone reductase. Still results in proton translocation, but no net generation of NADPH. Only contributes to the proton motive force.
When does cyclic electron flow happen?
When there are already sufficient reducing equivalents available for use in the Calvin cycle.
Why is the proton motive force mainly dominated by pH in the thylakoids?
H+ ion movement into thylakoid is counterbalanced by Mg2+ ions moving out of the thylakoid membranes, this means there is neutral charge across the membrane.
What happens to plastoquinol (delivered from PSII) in cytochrome b6f?
It is reoxidised to give plastoquinone.
How are reaction centre cofactors bound to PSI?
Within 11 TM helices.
Why is PSI considered to be simpler than PSII, despite the fact that they are homologous enzymes?
PSI performs 1 electron chemistry which is simpler than the 4 electron chemistry performed by PSII.
What is catalysed by nitrogenase and where is it found?
Catalyses the reduction of nitrogen to give ammonia, requiring lots of reducing electrons. Found in some cyanobacteria.
Why is NADPH more useful than reduced ferredoxin?
Ferredoxin is only a one electron carrier, and is therefore not useful in driving many reactions. NADPH is a two electron carrier and can be more widely used.
When is NAD(H) used?
During catabolic reactions
How does photoactivation of P680 in PSII allow the removal of electrons from water?
Activation of P680 removes one electron from the pigment. This makes the pigment electronegative enough to remove an electron from water. 4 photons of light are therefore needed to remove all 4 electrons.
Give an overview of non-cyclic photosynthetic electron transport.
Light absorbed by antenna pigments of PSII and PSI
Absorbed energy is then transferred to the reaction centre pigments - P680 in PSII, P700 in PSI.
Activation of P680 removes one electron from the pigment - allows removal of e from water.
Electron is transferred from PSII to Cytb6f by plastoquinol (PQ)
Generates a proton gradient – used to drive ATP synthase
Electron transferred from Cytb6f to PSI by plastocyanin (PC)
Activation of P700 excites the electron to a higher redox potential to reduce ferredoxin (Fd)
Fd reduces FAD which is used to reduce NADP+
Generates ATP and NADPH which are used in the Calvin cycle
What is the function of ATP synthase?
Uses the proton gradient to drive ATP synthesis.
How many protons need to be translocated in order to produce one molecule of ATP?
3-4
