Photosystem 1 lecture 4 Flashcards
Overall reaction at photosystem 1
Pcred + Fdox -> Pcox + Fdred
What does photosystem 1 reduce and oxidise?
Plastocyanin is oxidised and Ferredoxin is reduced
What does PS1 function as?
Plastocyanin - Ferredoxin oxidoreductase
Why can’t P700+/P700 oxidise water?
It’s redox potential is too positive
What is Ferredoxin?
Powerful small soluble electron transfer protein
Can reduce NADP+ to NADPH
Binds 2Fe-2S cluster which acts as e- carrrier
What does P700*/P700+ reduce and why?
Reduces 2Fe-2S cluster in ferredoxin
It has a very negative redox potential
What is the efficiency of reaction at PS1?
44%
Free energy output = 76
Free energy input = 171 (per photon)
76/171 x 100 = 44%
What is plastocyanin?
Small soluble e- transfer protein
Copper ion bound at active site which acts as a e- carrier
Cu+ -> Cu2+ by PS1 and reduced by cytochrome b6f
Briefly describe electron transfer at PS1
- Light energy excites special pair P700 to P700+
- Charge separation occurs and e- transferred to ChlA then ChlA2
- e- transferred to phylloquinones then to iron-sulphur clusters finally to ferredoxin
- e- hole on P700 filled by e- from plastocyanin
(plastocyanin reduced by Cyt b6f)
What is charge recombination and why is this a problem in photosynthesis?
When separated charges caused by special chlorophyll pair excited recombine
Donated electron on ChlA- moves back to P680+ (PS2) or Chl1- to P700+ (PS1)
Undermines efficiency of reaction
How is charge separation avoided?
Electron carriers are spatially close and energetically close
e- quickly transferred to more negative carrier
Avoids e- hole on P700+ or P680+ being filled by that electron
What does Marcus theory dictate?
The ET (electron transfer) rate is optimal when driving force of free energy matches the reorganisation energy
