Chlorophyll- electrons

[tomaszp]

Hi

I'm sorry if I make mistakes, but I'm not 100 % familiar with the English language .

In most of the books I can found only information that chlorophyll is used to absorb the quantum of electromagnetic radiation and transfer electrons to the reaction center. Nowhere I can't find any information on that, I hope some of you may know:

-molecule of chlorophyll- for me is a cation, cuz of Mg2+ in the center- whence chlorophyll takes electrons?

-when a photon hits the magnesium, whence Mg gets the electron and on what principle the electron moves from one chlorophyll molecule to another?

-when the photone strikes an electron and take it from chlorophyllcan I say that chlorophyll is a radical ?

-can it be that the electron is transported by a molecule of chlorophyll on the principle of resonance (in total chlorophyll as a structure is strongly coupled)

-How does the electron is transferred from molecule to molecule?

 

[iRNAblogger]

So I think the source of your confusion is that you missed a critical step of the light reactions: the splitting of water. The initial source of "free" electrons in the system is the water molecule which is split by photosystem II. Once the electrons are "freed," I THINK that they are essentially "absorbed" by chlorophyll, which when excited by a photon, is able to channel them through the electron transport chain in order to produce ATP. This explanation is how I think about the light reactions, but I'm definitely not an expert.

[CharonY]

That is not quite the correct way to think about it. The charge separation is photon-driven and is best understood using the Kok model (and denoted as S-states). The primary electron donor is P680 and during each photon-driven charge separation event one electron is extracted from the Mn4CaO5 cluster and transferred towards a mobile plastiquinone over a series of intermediate acceptors.

Water binding occurs during the transition between S2 to S3 (i.e. when the four Mn oxidation states go from +3 +4 +4 +4 to +4 +4 +4 +4). After one additional excitation the transient S4 state is reached and O-O bonding occurs and an additional H2O binds to move to S0.

The role of the pigment is essentially to collect the energy from the photons to allow the oxidation of the reaction center to occur. You may want to look up resonance energy transfer for details on that.

 

In other words, the electron transfer is not primarily through the chlorophylls and other pigments, but they facilitate it. For the more general question about electron transfer between molecules, most models indicate facilitated tunneling processes (i,.e. classic Marcus theory), during which donor and acceptor sites are brought in close proximity.

[iRNAblogger]

Sorry! I guess I misspoke. The electrons are not freed and then absorbed: is rather that the chlorophyll is oxidizing the water (taking the electrons directly from the water). Is this more correct?

 

Also I noticed thomaszp that you were interested in the the way the electron is carried and transported. You mentioned resonance, which is important for the photon absorption (this is because the organization of the double bonds creates a conjugated system which is easily excited by light). However, would it be accurate to say that it is actually the Mg atom which carries and transfers the electron (going by its changes in oxidation states, mentioned by CharonY)?

[CharonY]

I would use more precise language here as the chlorophylls themselves are not doing the water-splitting or providing electrons. The whole PSII complex is required, with the components playing interconnected roles. The release of electrons occurs at the oxygen evolving complex, for example at the manganese (not magnesium) core. I.e. oxygen release does not happen on the chlorophylls at all. A more detailed explanation:

Dark adapted PSII is in S1, first charge separations leads to oxidized P680 and reduced plastoquinone PQB (i.e. P680+/PQB-). Then electron from the OEC reduces P680 and the becomes to the above mentioned +3 +4 +4 +4 +4 manganese configuration (from +3 +3 +4 +4 at S1).

 

Thus, during the first charge separation the oxidation state of P680 only changes for short amount of of time, whereas the OEC remains until the next step.

[vampares]

Electrons are the most functional form of energy. (we don't have computers running on light, heat or gravity)

 

The issue is making the electrons go. So, chlorophyll is provided an electron. From where I do not know. When solar energy hit the chlorophyll, the chlorophyll is deformed. The deformation is opposed, stoichiometricly and the loose electron bears the difference. The electron is then accepted across a membrane and chemically stored along with the higher energy. This side of the membrane then must eject a electron. This electron will be lower energy. In this scenario we would be looking for something equivalent to a peroxide to be evolved.

[CharonY]

Pretty much no.