Electrons in photosystem II

[biology_06er]

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

[Jammerz]

First off, you have the order mixed up a little. It's the opposite of how they're numbered; electrons move from PSII to PSI, not the other way around. Counterintuitive, I know.

Anyway, these electrons come from water. At PSII, a molecule of water is split; one oxygen, two hydrogens; one electron is taken from each hydrogen, yielding two electrons.

The electrons (still in PSII) are excited by light, accepted by an electron carrier, and taken to PSI, where they are again excited to a high energy state, and finally accepted by NADP+, reducing it to NADPH (each NADP+ can carry two electrons... it's "full name" is actually NADPH+H). These electrons are carried over and they are used to reduce what will eventually become glucose and ATP. In other words, these electrons become part of the final product.

The oxygen from the original step (at PSII) will be combined with another oxygen that was made by breaking apart another water molecule to form oxygen gas; this is why plants give off O2.

New electrons come from another molecule of water, and the cycle repeats. The same electrons are not kept in PSII or PSI; new ones are constantly put in. This is called "noncyclic electron flow".

It is possible... if there's no water, for the same electrons to cycle through PSII and PSI over and over; this is called "cyclic electron flow" and is much less efficient.

[mith]

the PS numbering system is order in which they were discovered, not the electron transfer order..

[mcar]

Biochemical usually takes place. The solar energy has its effect on the electrons and these are activated. Electrons are subatomic particles which play a role in certain chemical reactions--and so these becomes excited. The effect is the transformation of the solar energy into chemical energy which is utilized by the plant for its next special processes.