|MadSci Network: Botany|
Green algae perform photosynthesis in the same way that higher plants do, and have a similar set of dark reactions, called the Calvin-Benson Cycle. The carboxylation phase of the Calvin cycle is where carbon dioxide is "fixed", being converted from the inorganic form to an organic form by combination with ribulose 1,5-bisphosphate to form two molecules of 3- phosphoglycerate. The reduction phase of the Calvin cycle then produces glyceraldehyde 3-phosphate (GAP), by reducing the organic acid to an aldehyde. It is here that the regeneration phase starts, and it is here that your question arises. In actuality, glucose is not part of the Calvin cycle. Glucose can be made through gluconeogenesis from a host of precursors, including pyruvate, 3- phosphoglycerate, GAP, and fructose 6-phosphate. We've seen that 3- phosphoglycerate and GAP are products of the Calvin cycle. GAP can also be converted into dihydroxyacetone phosphate (DHAP). When GAP and DHAP condense, the product is the sugar fructose 1,6-biphosphate. This can be converted into glucose. It can also be converted into fructose 6-phosphate, which can also be converted into glucose. Fructose 6-phosphate is central to the regeneration phase of the Calvin cycle. What is being regenerated? Ribulose 1,5-bisphosphate, the starting material of the carboxylation phase of the Calvin cycle. There are a number of sugar conversion involved in converting fructose 6-phosphate to ribulose 1,5-bisphosphate. GAP can condense with fructose 6-phosphate to produce xylulose 5-phosphate and erythrose 4-phosphate. DHAP then condenses with the erythrose to produce sedoheptulose, which then condenses with another molecule of GAP to make xylulose and ribose. Both the these later compounds can be converted to ribulose. (I've left off some of the phosphorylation steps here). So, the simple answer to your question is that glucose is not directly involved in the Calvin cycle or carbon fixation. PGAL is what I am calling GAP here. GAP contributes to the formation of fructose, erythrose, xylulose, sedoheptulose, and ribose. Note however that these processes do not occur in isolation. Just as the intermediates of the Krebs cycle are used in biosynthetic reactions, giving rise to amino acids and nucleic acids and not just an energy-producing cycle, so too the intermediates in the Calvin cycle are integrated into biosynthetic pathways. Could the GAP generated in the Calvin cycle contribute to glycolysis? Yes. Could the GAP generated in the Calvin cycle contribute to gluconeogenesis? Yes. Could the carbon fixed into GAP via the Calvin cycle eventually be respired through the Krebs cycle and re-released as carbon dioxide? Yes, it happens all the time. Metabolism is an integrated whole. Pathways only help our understanding of these complex systems. They are artificial constructs and do not adequately express the true nature of living organisms. Hope this helps. Cheers, Dr. Mark Schneegurt
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