Carbon fixation reactions

Carbon fixation reactions - three turns, introducing a...

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Carbon fixation reactions Calvin cycle . This is the basic cycle of photosynthesis, and it consists of three stages:  1. Fixation:  CO 2  diffuses into the  stroma  of the  chloroplast  where the ATP generated  earlier in Step One is used to chemically bind CO 2  to  ribulose 1,5-bisphosphate (RuBP) , a  5-carbon sugar. The 6-carbon molecule produced is unstable and immediately separates  into two, 3-carbon molecules of  3-phosphoglycerate (PGA)  with the assistance of the  enzyme  ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 2. Reduction:  PGA, in a two-step process, is reduced (using the NADPH produced  during PsI) to  3-phosphoglyceraldehyde (PGAL) 3. Regeneration:  the PGAL molecule is rearranged and re-creates RuBP, the starting  substance to which CO 2  binds.  To produce glucose, the cycle has to make 
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Unformatted text preview: three turns, introducing a total of 3 atoms of carbon, one at each turn. To fix each requires 3 ATP energy molecules and 2 NADPH reducing power molecules for a final total of 9 ATP (3 molecules x 3 turns of the cycle) and 6 NADPH (2 molecules x 3 turns of the cycle). The material that leaves the Calvin cycle is not the 6-carbon glucose, but is a 3-carbon (triose) sugar, PGAL (glyceraldehyde 3-phosphate) , much of which moves from the chloroplast into the cytosol (the cytoplasm of the cell). Here the molecules in a series of reactions build the final product, sucrose. Not all of the PGAL migrates into the cytosol, however. Some remains in the chloroplast where it is converted into starch as an energy reserve of the cell....
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