Photosynthesis Flashcards
What is the chemical equation for photosynthesis?
6CO2 + 6H2O —> C6H12O6 + 6O2
What does light driven electron transport chains generate?
- ATP
- NADPH
- both needed to reduce CO2 to sugar
What are the two different types of photosynthesis?
Anoxygenic - electron donor is H2S (SO42-) - better e- donor - need only one photosystem to reduce NADP+ Oxygenic - electron donor is H2O (O2) - bad e- donor - needs 2 photosystems in series to reduce NADP+
Why are chlorophylls good at absorbing light?
- high extinction coefficient
Where is the gap in chlorophyll?
Huge absorbance gap where visible light is, hence why it does not absorb green and instead reflects
How does an organism that has chlorophyll fill the gap?
organism uses other pigments to fill the gap
- pigments phycocyanin and phycoerythrin in the 500-600nm wavelength region
- pigment carotenoids use for 400-500nm region
What happens when chlorophyls absorb light?
- becomes excited
- shifts redox potential to more negative value
What can bacteriochlorophylls do that normal chlorophylls cannot do?
- absorb in the infrared
What is the structure of the Phycobilin pigment?
- linear tetrapyrrole
- no Mg
- light harvesting complexes
What are the two functions of Carotenoids?
- protection from photooxidation
- light harvesting: not perfect but do fill gap
What happens once light is absorbed by a pigment?
- energy transfer from electron to the chlorophyll in the photosystem
- works better if the two electrons are closer together (<0.5nm)
(get 100% quantum yield)
How can you prevent fluorescence from happening?
- having a faster energy transfer than competing reactions
- photon emitted doesn’t have a longer wavelength than the photon absorbed
Name different energy transfer mechanisms from fastest to slowest?
fastest - delocalised exciton coupling
fast - resonance transfer (resonance between dipoles)
slowest - inductive resonance
What is the overall organisations of the light harvesting complexes?
- reaction centre in the middle containing light harvesting complex 1
- 2 chlorophylls - light harvesting complexes surround the reaction centre
- have multiple chlorophylls that orientate themselves in each light harvesting complex to absorb the most energy and get a fast transfer
- energy transferred onto the reaction centre
What are the different photosynthetic organism groups that undergo photosynthesis?
anoxygenic phototrophs - purple bacteria (proteobacteria) have photosystem 2 - green bacteria have photosystem 1 oxygenic phototrophs - cyanobacteria have both photosystems
What are the characteristics of the purple bacteria?
- use photosystem 2
- have chlorophyll A
- Thylakoid membrane
- Calvin-Benson cycle for CO2 fixation
- has reverse electron transfer
What are the two types of purple bacteria (Proteobacteria)?
Purple sulphur bacteria - H2S and S as electron donor Purple non-sulphur bacteria - alpha-proteobacteria - names begin with Rhodo - can use a wide range of fermentation products as electron donors - can grow anaerobically
How does the photosystem 2 work in purple bacteria?
- special pair exist in the reaction centre
- 2 bacteria chlorophylls
- they absorb a photon or get it passed on by light harvesting complexes
- one of the chlorophyll will get oxidised once excited
- donates an e- to another chlorophyll
- this passes on the e- to bacteriopheophytin
- passed e- to quinone
- passes e- to a quinone pool in the membrane
- there is a symmetry present between 2 branches
What is the chlorophyll in the special pair in photosystem 2?
- P870
What happens when pigment P870 gets excited? Redox explanation
redox potential shifts from +500 to -900
- it then gets oxidised (loses electron) passing it onto bacteriopheophytin and Quinones
- quinone is at +100 mV so 1V is lost v quickly so that electron flows at one direction
- can be passed from the Cytbc1 complex
- then passes onto now oxidised chlorophyll
- cycle can happen and it can get reduced again back to -900
What does the cyclic electron transfer produce?
PMF that can be used to make ATP
How is NADPH created during cyclic electron transfer?
- do a reverse electron transfer
- use NADH dehydrogenase
- put 4 protons in to the cell to transfer electron from UQ to NADH
- redox potential of NADH and NADPH is similar, but NADPH is less
- moves electron from NADH to NADPH using transhydrogenase
Why is an external electron donor needed in photosystem 2?
- to replenish the electrons given to NADH by UQ when trying to make NADPH
- electron donors can be H2S, H2, Succinate2-
What are the two types of green bacteria?
green sulphur bacteria
- uses H2S and S as an electron donor
- have chlorosomes (light harvesting organelles)
- using reductive TCA cycle for CO2 fixation
chloroflexus
- filamentus bacteria
- can glide across surfaces
- found in hot springs
- 3-hydroxypropionate cycle for CO2 fixation
What are chlorosomes?
vesicles that touch the membranes
- stuffed fill with bacteria chlorophylls
- able to grow at very low light intensities
How does the photosystem 1 work in chloroplast?
- special pair exist in the reaction centre
- 2 chlorophylls
- they absorb a photon or get it passed on by light harvesting complexes
- one of the chlorophyll will get oxidised once excited
- donates an e- to another chlorophyll
- this passes on the e- to phylloquinone
- passed e- to quinone
- passes e- to iron sulfur complexes
- e- end up in the level of ferredoxin
- there is a symmetry present between 2 branches (only go down one path)
What is the chlorophyll in the special pair in photosystem 1?
P840
What happens when pigment P840 gets excited? Redox explanation
redox potential shifts from +300 to -1200
- it then gets oxidised (loses electron) passing through FeS clusters and Quinones, ending up at Ferredoxin
- can be passed to the Cytbc1 complex
- goes back to the chlorophyll at +300 to P840
- cycle can happen and it can get reduced again back to -1200
Why is no reversed electron transfer needed in photosystem 1?
Because Ferredoxin is at the same level of NADPH and therefore electrons can be passed easier
- Ferredoxin can be used directly to reduce CO2
What are the characters of cyanobacteria?
- have chlorophyll a
- use H2O as electron source, generating Oxygen
- uses both photosystems to create a Z system
- uses Calvin-Benson cycle for CO2 fixation
What are physcobilisomes?
light-harvesting complexes in cyanobacteria and red algae
What is the difference between the individual photosystems and the Z system?
- redox potential of photosystem 2 (P680) is red not infrared and is more positive (+1000) than the redox potential of water (+880)
- can take electrons from water via the Mn4 cluster
- needs 2 photons
What happens when pigment P680 gets excited in Z-scheme? Redox explanation
In PSII:
redox potential shifts from +1000 to -700
- it then gets oxidised (loses electron) passing through Pheo and PQ, ending up at Ferredoxin
In PSI:
end up with electrons to reduce ferredoxin
- from that they can reduce NADPH
- e- goes to water and then PQ and can be passed to the Cytbf complex
- cycle can happen and it can get then go to P700 in PSI.
Why is no reversed electron transfer needed in Z-scheme?
- using 2 photosystems, can shift redox potential to use h2o as an electron donor and ends up with NADPH
How is ATP made?
Through cyclic e- transfer in PSI
- protomotive force made by e- going from Ferredoxin to PQ then Cytbf
- used to make ATP
