![]() ![]() 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 5.15) can be organized into three basic stages: fixation, reduction, and regeneration. These energy-carrying molecules travel into the stroma where the Calvin cycle reactions take place. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery, Andrew Benson ( Figure 5.14).įigure 5.14 Light-dependent reactions harness energy from the sun to produce ATP and NADPH. ![]() The reactions are named after Melvin Calvin, the scientist who discovered them, and reference the fact that the reactions function as a cycle. In plants, carbon dioxide (CO 2) enters the leaf through the stomata and diffuses into the mesophyll cells and into the stroma of the chloroplast-the site of the Calvin cycle reactions where sugar is synthesized. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The carbohydrate molecules made will have a backbone of carbon atoms.
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