Glucose and other carbohydrates are synthesized in the carbon-fixing reaction of photosynthesis, often called the Calvin Cycle for Melvin Calvin, who performed much of the biochemical research. This phase of photosynthesis occurs in the stroma of the plant cell.
In the carbon-fixing reaction, an essential material is carbon dioxide, which is obtained from the atmosphere. The carbon dioxide is attached to a five-carbon compound called ribulose diphosphate. Ribulose diphosphate carboxylase catalyzes this reaction.
After carbon dioxide has been joined to ribulose diphosphate, a six-carbon product forms, which immediately breaks into two three-carbon molecules called phosphoglycerate. Each phosphoglycerate molecule converts to another organic compound, but only in the presence of ATP. The ATP used is the ATP synthesized in the energy-fixing reaction. The organic compound formed converts to still another organic compound using the energy present in NADPH. Again, the energy-fixing reaction provides the essential energy. The organic compounds that result each consist of three carbon atoms. Eventually, the compound interacts with one another and joins to form a single molecule of six-carbon glucose. This process also generates additional molecules of ribulose diphosphate to participate in further carbon-fixing reactions.
Glucose can be stored in plants in several ways. In some plants, the glucose molecules are joined to one another to form starch molecules. Potato plants, for example, store starch in tubers (underground stems). In some plants, glucose converts to fructose (fruit sugar), and the energy is stored in this form. In still other plants, fructose combines with glucose to form sucrose, commonly known as table sugar. The energy is stored in carbohydrates in this form. Plant cells obtain energy for their activities from these molecules. Animals use the same forms of glucose by consuming plants and delivering the molecules to their cells.
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