MadSci Network: Biochemistry

Re: The fate of the electron in cyclic photophosphorylation in photosynthesis

Date: Wed Sep 26 12:06:08 2001
Posted By: Maggie Guo, Grad student, Plant Physiologu and Molecular Biology Program, Dept.of Plant Biology, UIUC
Area of science: Biochemistry
ID: 1000581379.Bc

In photosynthetic electron transport, every facilitating protein compound on 
the thylakoid can not accept electron (be reduced) unless it just 
lost(excited, be oxidized) one of its own. So the excited electron from one 
compound may not be the exact same one that just accepted. This is also the 
case in cyclic electron transport: The electron back to P700 may not be the 
same original one from P700. 

So the process is like: PSI pigments are excited by one photon and cause the 
excitation and removement of an electron from P700. Then oxidized P700 can 
receive one electron from reduced PC and form reduced P700. Electrons from 
reduced PSI now can move to mobile ferrodoxins (this movement and all the 
following "movement" and "transport" are due to the oxidation-reduction 
process), from there to cytb6 complex and plastoquinone. The full reduction 
of plastoquinone takes two protons from stroma side of thylakoid (not from 
PSII) besides accepted electrons. These two protons will be moved to 
thylacoid lumen in later transport, thus facilitate the ATP synthesis; and 
the electrons will be transported via Fe-S, cytf and PC, then back to PSI. 
So you are right, the electrons will go to another "hole", but another 
electron will be removed later from the "hole" thus form a electron 
transport chain.

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