ATP is also used in the regeneration of RuBP.įigure 3. The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C 6H 12O 6). The molecules of ADP and NAD +, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized. A reduction is the gain of an electron by an atom or molecule. This type of reaction is called a reduction reaction, because it involves the gain of electrons. This process is called carbon fixation, because CO 2 is “fixed” from its inorganic form into organic molecules.ĪTP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. RuBisCO catalyzes a reaction between CO 2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. RuBP has five atoms of carbon and a phosphate group on each end. In the stroma, in addition to CO 2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). The Calvin cycle reactions (Figure 2) can be organized into three basic stages: fixation, reduction, and regeneration. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery (Figure 1). The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle. In plants, carbon dioxide (CO 2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast-the site of the Calvin cycle reactions where sugar is synthesized. These energy-carrying molecules travel into the stroma where the Calvin cycle reactions take place. Light-dependent reactions harness energy from the sun to produce ATP and NADPH.
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