Carbon Dioxide Enters the Cycle Carbon dioxide molecules enter the Calvin cycle from the atmosphere. An enzyme in the stroma of the chloroplast combines these carbon dioxide molecules with 5-carbon compounds that are already present in the organelle, producing 3-carbon compounds that continue into the cycle. For every 6 carbon dioxide molecules that enter the cycle, a total of twelve 3-carbon compounds are produced. Other enzymes in the chloroplast then convert these compounds into higher-energy forms in the rest of the cycle. The energy for these conversions comes from ATP and high-energy electrons from NADPH.

Sugar Production At midcycle, two of the twelve 3-carbon molecules are removed from the cycle. This is a very special step because these molecules become the building blocks that the plant cell uses to produce sugars, lipids, amino acids, and other compounds. In other words, this step in the Calvin cycle contributes to all of the products needed for plant metabolism and growth.

The remaining ten 3-carbon molecules are converted back into six 5-carbon molecules. These molecules combine with six new carbon dioxide molecules to begin the next cycle.

Summary of the Calvin Cycle The Calvin cycle uses 6 molecules of carbon dioxide to produce a single 6-carbon sugar molecule. The energy for the reactions that make this possible is supplied by compounds produced in the light-dependent reactions. As photosynthesis proceeds, the Calvin cycle works steadily, removing carbon dioxide from the atmosphere and turning out energy-rich sugars. The plant uses the sugars to meet its energy needs and to build macromolecules needed for growth and development, including lipids, proteins, and complex carbohydrates such as cellulose. When other organisms eat plants, they, too, can use the energy and raw materials stored in these compounds.

The End Results The two sets of photosynthetic reactions work together—the light-dependent reactions trap the energy of sunlight in chemical form, and the light-independent reactions use that chemical energy to produce stable, high-energy sugars from carbon dioxide and water. And, in the process, animals, including ourselves, get plenty of food and an atmosphere filled with oxygen. Not a bad deal at all!

In Your Notebook What happens to the NADP ⁺, ADP, and sugars produced by the Calvin cycle?

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