Hatrack River Forum: Light Reactions Photosynthesis:

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Posted by Phanto (Member # 5897) on :

I made the statement that the light reactions of photosynthesis recycle all the electrons. Someone told me I was wrong. I am confused. Can someone quickly resolve the issue?

thanks ^^.

Posted by Will B (Member # 7931) on :

Electrons can't be created or destroyed by any chemical reaction, photosynthesis included. So you were right, although it's a strange way to put it.

Posted by BannaOj (Member # 3206) on :

The light reactions of photosynthesis, take photons of light energy and convert them into chemical energy, which moves electrons. I would call it a conversion to "chemical" energy rather than "electric" energy because it isn't creating direct electrical current for the most part.

AJ

Posted by Tarrsk (Member # 332) on :

Well, it actually depends. Photosystem I *can* recycle electrons- that is, the high-energy electrons are passed from carrier to carrier as usual, but are then returned to chlorophyll for another cycle. This reaction produces ATP, but no NADPH. The usual photosystem I reaction, on the other hand, reduces NADP to NADPH as its final step, and the electron is not recovered.

Photosystem II, the first "half" of the light reactions, does not recycle electrons at all, but instead transfers its electrons to Photosystem I.

Posted by James Tiberius Kirk (Member # 2832) on :

quote:
Photosystem II, the first "half" of the light reactions, does not recycle electrons at all, but instead transfers its electrons to Photosystem I.

If I remember correctly, photosystem I then transfers the electrons to another molecule, right? So in that way, you could argue that the electrons are "passed on" rather than recycled.

--j_k, who edited this when he remembered that it might not be O2

Posted by rivka (Member # 4859) on :

Electrons are recycled in cyclic phosphorylation. By definition, they are not in noncyclic phosphorylation (which is involved in the light reactions). As j_k said, they are "passed on"; and Tarrsk was right about the final receiver being NADPH.

The original electron source is water, which is consumed by the light reactions and not regenerated by them.

Watch! [Big Grin]

[edited becuz apparently last night I couldn't spell [Razz]

]

[ December 19, 2006, 11:40 AM: Message edited by: rivka ]

Posted by Tarrsk (Member # 332) on :

quote:
Originally posted by James Tiberius Kirk:

quote:
Photosystem II, the first "half" of the light reactions, does not recycle electrons at all, but instead transfers its electrons to Photosystem I.
If I remember correctly, photosystem I then transfers the electrons to another molecule, right? So in that way, you could argue that the electrons are "passed on" rather than recycled.

--j_k, who edited this when he remembered that it might not be O2

Yes, like I said, Photosystem I normally transfers the electron to NADP, to form NADPH. But the Photosystem I pathway contains an alternate last step that does actually recycle the electron to the chlorophyll molecule.

Edit: That's what I get for not reading the whole thread before responding. Rivka, of course, is correct. [Smile]

Cyclic phosphorylation is the "alternate" pathway to which I was referring.

Posted by BannaOj (Member # 3206) on :

I agree with all of the above as far as Photosystem I and II go. However, the distinction I was trying to draw I would not call the cascading electron system an electrical "current" in the classical sense like a battery although there is research trying to do exactly that.

(I had a summer internship at the Center for the Early Events in Photosynthesis at Arizona State U.)

Posted by rivka (Member # 4859) on :

AJ, I still don't see where anyone in this thread called it an electrical current. I agree that it is not, for several reasons. Even nerve activity only approximates an electrical current; photosynthesis doesn't even do that.

Posted by BannaOj (Member # 3206) on :

I'm sorry... that was the idea I seemed to think the first and second posts were discussing ...although they really didn't now that I re-read.

AJ

Posted by rivka (Member # 4859) on :

Ok. [Smile]

I wondered if there was some post that had been deleted that you were responding to. Misreading something I totally relate to. [Big Grin]