Passage
Upon entering muscle cells, glucose is immediately phosphorylated by hexokinase to produce glucose 6-phosphate (G6P) . This phosphorylation event prevents glucose from exiting the cell. G6P may then enter one of three metabolic pathways, depending on the needs of the cell (Figure 1) .
Figure 1 Three potential fates of G6P, determined by the needs of the cellUnder low ATP conditions, G6P enters glycolysis, where it is converted to two pyruvate molecules. The energy released in this process produces a net total of two ATP molecules per glucose molecule consumed. The resulting pyruvate molecules can then enter the mitochondria, where they participate in additional metabolic reactions that produce more ATP.When the cell experiences oxidative stress, G6P enters the pentose phosphate pathway (PPP) and produces NADPH, a reducing agent. During this process, G6P loses one carbon atom in the form of CO₂ and is converted to ribose 5-phosphate (R5P) , which is required for nucleotide synthesis. In addition to low NADPH conditions, G6P may also enter the PPP during replication, when nucleotides are needed for DNA synthesis. If nucleotides are not needed but energy is, R5P can be converted back to glycolytic intermediates and reenter glycolysis. Each glucose molecule that is converted to R5P prior to glycolysis yields an average of 1.67 ATP molecules.When all the cell's metabolic requirements are met, additional G6P can enter glycogenesis and produce glycogen, which stores glucose for later use; this process consumes ATP. When signal molecules bind G protein-coupled receptors on muscle cells, glycogenolysis is activated to release individual monomers from glycogen, allowing them to enter the appropriate pathway.
-When the cell is in need of glucose, glycogenolysis is upregulated, beginning with the activation of glycogen phosphorylase. Glycogen phosphorylase catalyzes the production of which of the following molecules?

Question

Passage
Upon entering muscle cells, glucose is immediately phosphorylated by hexokinase to produce glucose 6-phosphate (G6P) . This phosphorylation event prevents glucose from exiting the cell. G6P may then enter one of three metabolic pathways, depending on the needs of the cell (Figure 1) .

Figure 1 Three potential fates of G6P, determined by the needs of the cellUnder low ATP conditions, G6P enters glycolysis, where it is converted to two pyruvate molecules. The energy released in this process produces a net total of two ATP molecules per glucose molecule consumed. The resulting pyruvate molecules can then enter the mitochondria, where they participate in additional metabolic reactions that produce more ATP.When the cell experiences oxidative stress, G6P enters the pentose phosphate pathway (PPP) and produces NADPH, a reducing agent. During this process, G6P loses one carbon atom in the form of CO₂ and is converted to ribose 5-phosphate (R5P) , which is required for nucleotide synthesis. In addition to low NADPH conditions, G6P may also enter the PPP during replication, when nucleotides are needed for DNA synthesis. If nucleotides are not needed but energy is, R5P can be converted back to glycolytic intermediates and reenter glycolysis. Each glucose molecule that is converted to R5P prior to glycolysis yields an average of 1.67 ATP molecules.When all the cell's metabolic requirements are met, additional G6P can enter glycogenesis and produce glycogen, which stores glucose for later use; this process consumes ATP. When signal molecules bind G protein-coupled receptors on muscle cells, glycogenolysis is activated to release individual monomers from glycogen, allowing them to enter the appropriate pathway.
-When the cell is in need of glucose, glycogenolysis is upregulated, beginning with the activation of glycogen phosphorylase. Glycogen phosphorylase catalyzes the production of which of the following molecules?

Quizplus · Accepted Answer

11ecba2d_0e0c_14ea_a3bf_774971fc19b4_MD0008_00 Glycogen phosphorylase breaks the bond between two adjacent glucose molecules in a glycogen chain by phosphorolysis, or the addition of a phosphate molecule across a bond. The bond broken by this reaction is an α(1→4) linkage, so the phosphate must be added to either carbon 1 or carbon 4 of a glucose unit. The glucose unit that is released was attached by carbon 1, and it is to this carbon atom that phosphate is added, producing glucose 1-phosphate (G1P). (Choice A) UDP-glucose is produced by the combination of G1P with UTP along with the release of pyrophosphate. This reaction, catalyzed by UDP-glucose pyrophosphorylase, is involved in glycogenesis but not glycogenolysis. (Choice B) UDP-galactose is an intermediate of galactose metabolism. It is converted to UDP glucose and later G1P, but is not itself a product of glycogenolysis. (Choice D) Glucose 6-phosphate (G6P) is the final product of glycogenolysis, produced when phosphoglucomutase moves the phosphate group from carbon 1 to carbon 6. It is not produced by glycogen phosphorylase. Educational objective: Glycogen is degraded by phosphorolysis, or the addition of phosphate across a bond, to produce glucose 1-phosphate (G1P). G1P is subsequently transformed into glucose 6-phosphate for use in glycolysis or the pentose phosphate pathway.

Passage Upon Entering Muscle Cells, Glucose Is Immediately Phosphorylated by Hexokinase