Photosynthesis Flashcards
(50 cards)
What is primary electron transfer?
Where harvested excitation energy drives charge separation stabilised by secondary reactions
How does FRET happen between chlorophylls
They need to be <5nm apart
Non radiative transfer of energy from an excited donor to an acceptor
Spectral overlap between the donor and acceptor
How are gaps in chlorophyll absorbance accounted for?
Carotenoids plug gaps in absorbance
They move electrons from S0 to S2 to S1
How are chlorophylls structured?
Tetrapyrroles with a long hydrophobic isoprenoid tail
Alternate single and double bonds which forms π-electron system
Similar to hemes but less symmetrical and coordinate a central Mg2+
Absorbs blue and red light
How is proton motor force generated in chloroplast lumen?
Water is oxidised by PSII
Plastoquinol is oxidised by Cyt b6f
PQ/UQ has a relatively negative charge on the other side of the protons
Explain jablonski diagrams
S1 state is achieved by absorbing lower energy red photon
S2 state is achieved by absorbing higher energy blue photon
Electrons lose energy via internal conversions and fluorescence
What are billins and why are they useful?
Open chain tetrapyroles with conjugated π-electron system
in cyanobacteria antenna complexes
Quenches harmful excited states at 450-550nm and transfers remaining excitation energy to chlorophyll
Where does photosynthetic electron transport take place?
Thylakoid membrane. Folded up for high SA:V ratio
Stacked into grana connected by stronal lamellae
Why are antenna complexes necessary?
Despite pigment concentrations being low, increase electron transfer by 100 times.
Capture light and transfer it to the RC special pair
W=
What features make antenna complexes ideal for their function?
Wide spectral cross spectrum- contains lots of different pigments. π-electron system determines excited state.
High pigment concentration as these transfer energy to each other via FRET.
Modular- lots of antenna per RC in low light, vice versa
Why are purple bacteria favourable for studying photosynthesis?
Metabolically versatile
Easily genetically engineered
Easy to grow
Produce extensive internal membrane systems (intracytoplasmic membranes (ICMs)) to increase photosynthetic membrane under anaerobic conditions
How are intracytoplasmic membranes structured?
Lamellar (stacked discs like grana)
Vesicular
How can photosynthetic electron transfer become cyclic?
Proton can cycle from FNR to cyt B6f
Ferrodocxin can also donate an electron to reoxidise B6f
Describe anoxygenic photosynthesis
One type of reaction centre and light driven reaction
Cyclic electron transfer involving Cyt bc1 complex and Cyt C2
Uses ubiquinone and ubiquinol to generate pmf
NADPH is indirectly produced by reverse electron flow using external electron donors
Describe the antenna funnel system
Higher energy donor pigments transfer excitation energy to a lower energy acceptor via FRET
Carotenoids -> Chl b -> Chl a -> RC
No fixed physical arrangement and can be exchanged under certain light conditions
What type of antenna funnel systems are found in primary electron transfer systems?
Fused antenna: antenna and RCs bound to same polypeptide. Antenna not biochemically separated from RC
Core antenna: RC and antenna are formed from different polypeptides that interact with each other. Antenna and RC can biochemically separate
Describe Rba sphaeroides electron transfer chain?
LH2 surrounds the RC-LH1 which is ideal for energy transfer.
Electron moves from pigments with increasing wavelength, decreasing energy
PufX promotes dimerisation of LH1, preventing LH2 ring closing. Protein Y also prevents closing so UQ/UQH2 can move through
Describe Rba sphaeroides RC-LH1
BChls in LH1 are within 4nm of the special pair B870 which allows for efficient FRET
This is an energetically uphill transfer but it is driven by thermal energy in the environment
αβ are coupled such that the absorbance is red shifted and excitation energy can go from PSII to PSI
LH1 is formed by 14αβ pairs held open by protein Y and Puf X which dimerises it.
Bound to 2 carotenoids
What is present at the Rba sphaeroides LH1dimer interface?
PufX which creates a concave surface from TMs that have been pushed out
Lipids
How can RC-LH1 vary across species?
How open the LH1 rings are affects quinone/quinol diffusion
In complete rings, quinone/quinol moves through small pores in LH1 antenna
In open rings, such as Rba sphaeroides, this pore is covered by a carotenoid
14-17αβ pairs
3 or 4 RCs
Can bind Ca2+ to boost thermal stabiltiy
Describe T-tepidum LH1-RC
Monomeric
Forms completely closed ring
16 Ca2+ for thermal stability
This also redshifts LH1 for Qy transition
Has multiple pores as it has a carotenoid bound per αβ subunit
What is Rba sphaeroides RC made up of?
3 polypeptide chains:
L and M which form a heterodimer with pseudo C2 symmetry and binds electron transfer cofactors
H
Special pair P870
Electron that moves from Cyt C2 to UQb creates a relatively regative charge that contributes to the pmf
What makes electron transfer irreversible in the RC?
There is a large loss of redox potential as electron passes from P870* to quinone on Qb site
P870+ is too far from the Qa site for reverse electron flow
UQ to BPhe has such a large driving force it sits in the marcus inverted region, making it unfavourable
Why are back reactions dangerous?
Form triplet species in chlorophylls which causes ROS to damage RC proteins
