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3-Phosphoglycerate (3PG)

By tracing the steps in this manner, they soon discovered a cycle that "fixes" CO2 in a larger molecule, produces a carbohydrate, and regenerates the initial CO2 acceptor. This cycle was appropriately named the Calvin-Benson cycle (Figure 8.13).

The initial reaction in the Calvin-Benson cycle adds the one-carbon CO2 to a receptor, the five-carbon compound ribulose 1,5-bisphosphate (RuBP). The product is an inter mediate six-carbon compound, which quickly breaks down and forms two three-carbon molecules of 3PG (as Calvin and colleagues observed; Figure 8.14). The enzyme that catalyzes this fixation reaction, ribulose bisphosphate carboxylase/ oxygenase (rubisco), is the most abundant protein in the world, comprising about 20 percent of all the protein in every plant leaf.

The Calvin-Benson cycle is made up of three processes

The Calvin-Benson cycle uses the high-energy coenzymes made in the thylakoids during the light reactions (ATP and NADPH) to reduce CO2 to a carbohydrate. There are three processes that make up the cycle:

► Fixation of CO2. As we saw, this reaction is catalyzed by rubisco, and its product is 3PG.

► Reduction of 3PG to form a carbohydrate, glyceralde-hyde 3-phosphate (G3P). This series of reactions involves a phosphorylation (using the ATP made in the light reactions) and a reduction (using the NADPH made in the light reactions).

8.13 The Calvin-Benson Cycle The Calvin-Benson cycle uses CO2 and the ATP and NADPH + H+ generated in the light reactions to produce glucose. This diagram shows only the key steps; the values given are those necessary to make one molecule of glucose, which requires six "turns" of the cycle.

What Are The Steps Light Reaction

3PG is reduced to G3P in a two-step reaction requiring ATP and NADPH + H+.

| The remaining five-sixths of the G3P is processed in the complex reactions that produce RuMP.

3PG is reduced to G3P in a two-step reaction requiring ATP and NADPH + H+.

| The remaining five-sixths of the G3P is processed in the complex reactions that produce RuMP.

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