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Natural Science Forum / Biology / Botany / January 2005Yet more glucose
“Take a little sunshine. Take some carbon dioxide molecules out of the air. Take some hydrogen atoms out of water. Shuffle them around inside a bunch of little green chloroplasts inside green plant cells. What do you get? That's right. It's GLUCOSE and OXYGEN!†Back in September, as one of the comments about this sort of stuff, Jim Perry wrote “I have seen the page proofs of the new chapter on photosynthesis for the new version of Biology of Plants by Raven, Evert and Eichhorn. Glucose is no longer the formulaic end product, but rather C3H6O3†C3H6O3 may be preferable to glucose but, (not having seen the proofs of ‘Biology of Plants’ myself, I wonder, yet again, about the overall equation itself? If it is written 3CO2 +3H2O → C3H6O3+ 3O2 there will remain the implication that some of the oxygen evolved must be derived from carbon dioxide. If additional water is added on the left to avoid this, as in 3CO2 +6H2O → C3H6O3+ 3O2 +3H2O it poses the question of what the additional water is doing there when it then reappears on the right. In reality, if we add together all of the partial reactions of the Benson-Calvin Cycle we get 3 CO2+ 2H2O + H3PO4 → CH2OH.CO.CH2OPO(OH)2 + 3O2 but if this is too much for students to handle why don’t we all avoid unnecessary confusion by saying that the principal end-product of photosynthetic carbon assimilation is sucrose? This is undoubtedly where much of the triosephosphate ends up as the carbon source for much of plant metabolism. Of course sucrose isn't made in the chloroplast but then neither is glucose. At least I know of no firm evidence that free glucose is either made, or is an important metabolite within, chloroplasts. Am I right? Regards David Walker See also:- <http://www.digitalpublisher.co.uk/Oxygraphics/ohwhat.htm > Sucrose is certainly a good choice as a major product of photosynthesis for many species. Starch is probably an even better choice. Even better is to mention both because some species produce little starch in their leaves. I think a C3 product is at least better than saying glucose is the product. Historically, starch was recognized as the first visible product of photosynthesis by Julius Sachs in the 1860s. Starch grains accumulate in chloroplasts of many species during photosynthesis. Other good reasons for using starch as the major product of photosynthesis is that students can easily identify starch in leaves using the iodine test. It really helps students remember concepts if they demonstrate the concept to themselves, and they can easily test for starch in leaves. They cannot easily test for C3 products of photosynthesis. I believe that the reasons why glucose is so often used as the major product of photosynthesis in textbooks is to make the summary equation for photosynthesis simpler and the exact reverse of the summary equation for cellular respiration. C6H12O6 as the product makes the summary equation easier to balance than with starch or sucrose. It is an oversimplication without telling students that it is an oversimplication or explaining why. Ganong (1908) does not have the term glucose in his index or mention it in his lengthy discussion on photosynthesis. He refers to C6H12O6 in his photosynthesis summary equation as "photosynthate" and states "the chemical composition of the photosynthate approximates to C6H12O6, which may be taken as a conventional formula for the photosynthate in general." Even he, the leading botany teacher of his era, didn't fully explain the apparent contradiction that he wrote C6H12O6 in the summary equation but emphasized starch formation during photosynthesis: "The student is here in contact with one of the most significant facts in all organic nature, viz., the appearance of starch in lighted green leaves." David R. Hershey dh321@excite.com Reference Ganong, W.F. 1908. A Laboratory Course in Plant Physiology. New York: Henry Holt. |
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