The Calvin-Benson Cycle.

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Introduction. After the light dependent reactions, the ATP and the NADPH produced in the light reactions goes to the stroma of the chloroplast for the Calvin cycle. The Calvin cycle fixes carbon dioxide and the adds energy and hydrogen ions to the resulting molecules to yield a three carbon phosphate sugar called PGAL or G3P. Surplus molecules of this sugar are harvested by the plant for the manufacture of glucose and other carbohydrates.

Step 1. Carbon fixation. Each of 3 Carbon dioxide molecules are combined with each of three 5 carbon starting molecules called RuBP (Ribulose biphosphate). This reaction is catalysed by an enzyme called rubisco. Notice that at the start there are three five carbon molecules (3 X 5 =15). After the carbon dioxides are incorporated into the cycle or fixed then you have 3 X 6 carbons = 18.

Step 2. Production of PGA. Each of the three 6 carbon products of step one is unstable and breaks down into two three carbon molecules called 3-PGA or 3-phosphoglyceric acid.

Step 3. Rearrangement of molecular structure. Energy from ATP provided by the light reaction is used to rearrange the threee carbon structure to form an intermediate molecule ready to receive electrons and hydrogens from the NADPH produced in the light reaction.

Step 4. Redox reactions add energy from NADPH. The NADPH gives up its electrons and hydrogens to the three carbon intermediate compound. Since there are six of these three carbon molecules the result is 6 molecules of a three carbon molecule(note total carbons still = 18!) called glyceraldehyde 3-phosphate or G3P. In some texts this is called phosphoglyceraldehyde and abbrieviated PGAL.

Step5. Harvesting of one surplus G3P(PGAL). The chloroplast then harvests one of the six G3P's as the real end product of photosynthesis. The plant then uses the harvested G3P(PGAL) to make a wide variety of carbohydrates including glucose that is needs for its existence. It is this surplus G3P that provides the energy for most of the rest of the Earth's creatures.

Step 6. Regeneration of the RuBP starting material. The remaining five G3P's (5 X 3 = 15 carbons) are rearranged and phoshorylated using 3ATP's to produce 3 new 5 carbon molecules of RuBP. Now these are recycled into the Calvin-Benson cycle and are ready to participate in step one, carbon fixation again!

pgd revised 2/17/01

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