This is in reply to JackBean's post of Tue Oct 13, 2009 2:36 am, to which jwalin responded on Tue Oct 13, 2009 3:50 am.
First, on JackBean's discussion on "the 'provided/provider, ... it is provided by the 'light,' etc.," I am unclear as to whether you're suggesting the existence of two interesting possibilities.
As I wrote on Dec. 2, 2010, the Calvin cycle does not exist, because carbon reduction is a light reaction according to the original papers by Melvin Calvin and Francis K. Fong and their co-workers. However, had the Calvin cycle existed in plant photosynthesis, and carbon reduction occurs in the dark by the NADPH produced "in the light," i.e., the 3-PGA is reduced to the triose (PGL) oxidation level by the NADPH, are you suggesting the two possible pathways as follows?
First, by "hydrogen provider," do you mean to suggest that, in the dark, the reduction can occur by "hydrogen tunneling" through an activation barrier; and, second, by "electron provider," do you mean by "electron tunneling" followed by proton transfer to keep the "electroneutrality"?
In reality, the reduction of the PGA to PGL, though simple on paper, is mechanistically quite complex. To my knowledge, no one has done a calculation on which of the above-described pathways is more probable. [The reduction is from an acid carbonyl to an aldehyde carbonyl, a difficult, many-body problem, I think.] If I were to guess, I'd go for the "hydrogen tunneling" rather than the two-step electron tunneling followed by proton transfer.
For an introduction to the chemical rate processs involving thermal activation barriers, please refer to Francis K. Fong, Acct. Chem. Res. 9, 433-438 (1976). For a more detailed account, see, Francis K. Fong, "Theory of Molecular Relaxation. Application in Chemistry anbd Biology, Wiley Interscience, 1975, 314 pages.
But then, of course, photosynthesis does not occur in the dark; and the Calvin cycle does not exist. What occurs in plants, most probably, can be described by a "photochemical complex model" described by Dennis J. Diestler and Francis K. Fong, J. Am. Chem. Soc. 100, 1992-1996 (1977).
Last edited by fkfong on Wed Dec 08, 2010 5:27 pm, edited 3 times in total.
I'm sorry, but I missed seeing your reply dated Dec 02, 2010 6:32 pm, asking me for a brief reply, "in one or two simple sentences."
Sentence One: In the Calvin cycle, the NADPH and ATP produced from the "light reactions," i.e., the Z scheme, are used to reduce, in the dark, one of the two PGA's from splitting of the 6-C addition intermediate to yield one triose molecule, PGL, which condenses to yield sucrose.
Sentence Two: But the original papers by Melvin Calvin and Francis K. Fong and their co-workers show that photosynthetic carbon fixation by the RuBP yields the 6-C intermediate, which splits into one molecule of PGA and another of PGL, i.e., the carbon dioxide uptake by the RuBP in the light is reduced directly to triose (without the intervention of ATP and NADPH), which condenses to yield sucrose.
The original papers by Calvin and Fong were cited in my second Dec. 2, 2010 post. For a more detailed explanation, Google "the Calvin cycle website." Of the 108,000 results, click on the very first one, "The Official Calvin Cycle Website - Blogspot." I hope I am not spamming by referring you to this result.
For a succinct schematic representation of this discussion, by Purdue University Chemistry Professor John B. Grutzner, see, NSFfunding.com's webpage, "The Calvin Cycle Website."
Last edited on Dec. 8, 2010 to modify Sentence One, in order to be explicit on the 6-C intermediate's splitting into two PGA's, in the dark, as compared to the experimentally observed light reaction (Sentence Two), in which the 6-C intermediate splits into one molecule of PGA and another of PGL.
Last edited by fkfong on Wed Dec 08, 2010 5:25 pm, edited 1 time in total.
I apologize for this activity, but there is one more comment needed to complete this discussion stream.
On Tue Oct 13, 2009 12:43 am you wrote jwalin, "And last, but not least this triose called glyceraldehyde phosphate is primarily recycled, that's why is it called The Calvin CYCLE ..."
My question is, are you saying that the triose PGL (phosphoglyceraldehyde), instead of undergoing condensation to make sucrose, is recycled to form RuBP in the Calvin cycle? I don't think so. This is because, in photosynthesis, according to the original papers by Calvin et al, the PGL is the product of direct photoreduction of 1/2 of the 6-C addition intermediate splitting, which condenses to form sucrose.
In the work of Francis K. Fong et al, one of the two 3-C fragments from the 6-C RuBP addition intermediate is likewise directly reduced to the PGL in the light. The other 3-C fragment remains unreduced as the PGA. The NADPH and ATP from the light reactions presumably reduce some of this PGA to PGL, which then undergoes cyclic condensation to regenrate the 5-C RuBP. However, when the light is turned off, the entire photosynthetic apparatus decays exponentially to zero in the dark. See, Fong, Francis K. and Butcher, Karen A. (1988) Biochem. Biophys. Res. Commun., 150, 399-404. See, also, "The Official Calvin Cycle Website" at Fig. 2.
P.S. If the [url] is on for this posting, why is my giving the url for this reference "spam"?
Last edited, once, on 12-15-10, to make corrections for a faithful interpretation of the original papers by Calvin, Fong and their co-workers.
Who is online
Users browsing this forum: No registered users and 2 guests