Photosystems - (Oct/29/2007 )

Hi,

Just wondering where do the electrons that are excited to a higher state via absorbtion of a photon come from in photosystem I? Are they just "there" in the reaction centre (cholorophyll pigments?)

Could someone please say if I have gotten this correctly...put real briefly..

The electron from PSI goes to higher state...passes down the chain all the way to PSII (which replaces the electron deficient when light excites electrons in PSII reaction centre to a higher state)...so the NADPH that is produced ultimately comes from the electrons from PSII..I feel this is correct (but who knows )...

Also when the deficients are regained by the electrons say from water (for PSII) and from PSI (for PSII) what happens to these electrons-do they just stay there, waiting to be targeted by the light?

Any help will be moooost appreciated as I have an exam REALLY soon and need help!!

Thanks in advance,
b_06er

Think of the electron movement more like a domino cascade. One electron bumps another, then it another, then it another, etc. down your chain. So the electron driving the NADPH formation is not necessarily the SAME electron that was expelled from the photon-absorbing chlorophyll molecule.

To help you better understand the stoichiometry of it all, think of the whole process as a chemical reaction. Complicated thought the machinery may be, they really are just catalysts to drive the reaction forward. First, take your basic reaction:
6H20 + 6CO2 --> C6H12O6
and look where each constituent part comes into play. Each bond formation, for example, frees up an electron and consumes energy (thus, a low-energy electron). Second, realize that this is a NET reaction, and that the molecular location of events (hydrolysis, eg.) can vary within a plant, a leaf, or an individual cell.

Best of luck.