Study behavioral objective 40 and read the pages indicated in the text by E-21. 

The reduction of carbon occurs in the stroma of the chloroplast by means of a series of reactions known as the Calvin cycle or the light independent reactions (dark reactions), named after its discoverer, Melvin Calvin, who received a Nobel Prize for his work on the elucidation of this pathway. This was the same pathway you studied in the previous web page. The light independent Calvin cycle is analogous to the Krebs cycle in that, by the end of each turn of the cycle, the starting compound is regenerated. The starting (and ending) compound in the Calvin cycle is a five-carbon sugar with two phosphate groups -- ribulose 1,5-bisphosphate (RuDP). The process begins when carbon dioxide enters the cycle and is "fixed" (bonded covalently) to RuDP. The resultant six-carbon compound immediately splits to form two molecules of 3-phosphoglycerate, or PGA. (Each PGA molecule contains three carbon atoms; hence the designation of the Calvin cycle as the C3 or three-carbon, pathway. The six-carbon intermediate has never been isolated.) 

RuDP carboxylase (commonly called "Rubisco"), the enzyme catalyzing this crucial initial reaction, is quite abundant in chloroplasts, making up more than 15 percent of the total chloroplast protein. (It is said to be the most abundant protein in the world. Can you say why?) 

As in the Krebs cycle, each step is catalyzed by a specific enzyme. At each full turn of the cycle, a molecule of carbon dioxide enters the cycle and is reduced, and a molecule of RuDP is regenerated. Six revolutions of the cycle, with the introduction of six atoms of carbon, are necessary to produce a six-carbon sugar, such as glucose The overall equation for the production of a molecule of glucose is: 

The immediate product of the cycle is glyceraldehyde 3-phosphate (PGAL), the primary molecule transported from the chloroplast to the ground substance of the cell. This same triose-phosphate ("triose" means a three-carbon sugar) is formed when the fructose 1,6-bisphosphate molecule is split at the fourth step in glycolysis, and it is interconvertible with another triose-phosphate, dihydroxyacetone phosphate. Using the energy provided by hydrolysis of the phosphate bonds, the first four steps of glycolysis can be reversed to form glucose from glyceraldehyde 3- phosphate. 

Although glucose commonly is represented as a product of photosynthesis in summary equations, in reality, very little free glucose is generated in photosynthesizing cells. Rather, most of the fixed carbon is converted either to sucrose, the major transport sugar in plants, or to starch, the major storage carbohydrate in plants. 

Much of the glyceraldehyde 3-phosphate (PGAL)  produced by the Calvin cycle is exported to the ground substance, where it can be rapidly converted to glucose 1-phosphate and fructose 6- phosphate. The glucose 1-phosphate then is converted to the nucleotide-sugar UDP-glucose (uridine diphosphate-glucose), which is linked with the fructose 6-phosphate to form sucrose 6- phosphate, the immediate precursor of sucrose. Removal of the phosphate by hydrolysis leaves sucrose. 

Most of the glyceraldehyde 3-phosphate (PGAL) that remains in the chloroplast is converted to starch, which is stored temporarily during the light period as grains in the stroma. Through a series of reactions, the glyceraldehyde 3-phosphate is converted to glucose 1-phosphate, which then is used to produce the nucleotide-sugar ADP-glucose, the immediate precursor of starch. The glucose is added directly to the growing starch grain by the enzyme starch syntheses. At night, sucrose is produced from starch for export from the leaf. Carbon derived from starch appears to be transported from the chloroplast into the ground substance as glucose rather than triose- phosphate. 

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