Biology Forums - Study Force

During the light reactions of photosynthesis (utilizing non-cyclic electron transport)...

How many times are P680 chlorophyll molecules oxidized? How many times are P700 chlorophyll molecules oxidized?

each is oxidized only once, P680 gets oxidized and its lost electrons move to get accepted by P700 which also undergone oxidation, electrons lost from P700 goes to NADP+

Thanks a lot karim! I didn't know this one :-\

Remember, oxidized means to loss electrons, whereas reduced means to gain electrons. Photosynthesis begins when a photon strikes photosystem II and excites an electron of chlorophyll P680 (some sources say that P680 donates two electrons so I'm not totally sure if it oxidized once or twice). The excited electron is captured by the primary electron acceptor, pheophytin, and through a series of redox reactions, transferred to plastoquinone. A Z protein, associated with photosystem II and facing the thylakoid lumen, splits water into oxygen, hydrogen ions, and electrons. Two of these electrons are used to replace the missing electrons in chlorophyll P680. The same story is true for P700.

Is that for the production of one molecule of oxygen?

yes true 2 electrons are lost.. and so the 2 electrons are replaced by splitting of one water molecule into 2H+ and 1/2 O2 and 2 electrons.. so you don't get one molecule of O2... the oxygen atom produced unites with another similarly formed oxygen atom to form an O2 molecule.... the reaction centers ( P680, P700 ) can be re-utilized after they regain their lost electrons

this picture sums it all up

when i said each is oxidized only once i meant that each undergoes only one oxidation reaction throughout the process..... is it scientifically wrong to say " one oxidation reaction ( when there is loss of 2 electrons ) " ?

lol that's my dilemma. All resources say oxidized, but don't give the quantity. I would say once, but loses two electrons in the process.

I'd like to make a correction. As i was studying it turned out that the information i posted here are inaccurate and possibly wrong. Here is a correction.

2H2O ---> O2 + 4H+ + 4e-

The oxidation of two moles of water generates one mole of oxygen, four moles of protons, and four moles of electrons. It has been determined that only one PSII reaction center is involved in the release of a single oxygen molecule. Thus, in order to complete the oxidation of two water molecules, expel four protons, and produce a single molecule of O2, the PSII reaction center must be "closed" and then "opened" four times. This means that PSII must utilize the energy of four photons in order to evolve one molecule of O2.
The electron for reducing the oxidized P680 (after its excitation) is supplied by a cluster of four manganese ions associated with a small complex of proteins called OEC (oxygen-evolving complex) which is responsible for splitting (oxidation) of water and the consequent evolution of molecular oxygen. Experiments in which electron transport was driven by extremely short flashes of light -short enough to excite essentially one electron at a time- have demonstrated that the OEC has the capacity to store charges. Each excitation of P680 is followed by withdrawal of one electron from the manganese cluster, which stores the residual positive charge. When four positive charges have accumulated, the complex oxidizes two molecules of water and releases the product oxygen molecule.