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Biochemistry: Carbohydrate Metabolism and Glucose Pathways

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  • What is the primary role of glucose in metabolism?

    Glucose is the major fuel for the body, converted to pyruvate and then acetyl-SCoA for ATP production.

  • Where does carbohydrate digestion begin and end?

    Digestion begins in the mouth with salivary α-amylase and ends in the small intestine where disaccharides are hydrolyzed to monosaccharides.

  • What enzyme catalyzes the hydrolysis of α-glycosidic bonds in starch and glycogen?

    α-Amylase catalyzes hydrolysis of α-glycosidic bonds, producing maltose and smaller polysaccharides.

  • Why is glucose phosphorylated to glucose-6-phosphate upon entering a cell?

    Phosphorylation traps glucose inside the cell and commits it to metabolism; phosphorylated glucose cannot cross the membrane.

  • Name the four main metabolic fates of glucose-6-phosphate.

    Glycolysis, glycogenesis (storage as glycogen), fatty acid synthesis, and entry into the pentose phosphate pathway.

  • What is the net result of glycolysis per glucose molecule?

    Conversion of glucose to two pyruvates, production of two ATPs, and two NADH molecules.

  • Which steps in glycolysis require ATP investment?

    Steps 1 and 3: phosphorylation of glucose to glucose-6-phosphate and fructose-6-phosphate to fructose-1,6-bisphosphate.

  • What is the role of phosphofructokinase in glycolysis?

    It catalyzes step 3, a major control point, activated by ADP/AMP and inhibited by ATP and citrate.

  • How is fructose-1,6-bisphosphate split in glycolysis?

    It is cleaved into two three-carbon sugars: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

  • What happens to pyruvate under anaerobic conditions in muscle?

    Pyruvate is reduced to lactate to regenerate NAD+, allowing glycolysis to continue.

  • Describe the pyruvate dehydrogenase complex function.

    Converts pyruvate to acetyl-SCoA, releasing CO2 and producing NADH, requiring multiple enzymes and cofactors.

  • What is the significance of the glycerol-phosphate shuttle?

    Transfers electrons from cytosolic NADH to mitochondrial FAD, producing 2 ATP per NADH during oxidative phosphorylation.

  • How does the malate-aspartate shuttle differ from the glycerol-phosphate shuttle?

    It transfers electrons from cytosolic NADH to mitochondrial NADH, yielding 3 ATP per NADH, and involves malate and aspartate transport.

  • What hormones regulate blood glucose levels?

    Insulin lowers blood glucose by promoting uptake and storage; glucagon raises glucose by stimulating glycogen breakdown and gluconeogenesis.

  • What metabolic changes occur during fasting/starvation?

    Glycogenolysis speeds up, gluconeogenesis increases, and ketone bodies are produced from acetyl-SCoA as fat metabolism rises.

  • What are ketone bodies and their role during starvation?

    Ketone bodies are produced from excess acetyl-SCoA and serve as alternative energy sources for brain and tissues.

  • What causes the symptoms of diabetes mellitus?

    Insufficient insulin or insulin resistance leads to high blood glucose, glucose in urine, and cellular glucose deprivation.

  • How does type I diabetes differ from type II diabetes?

    Type I is insulin-dependent due to pancreatic beta cell destruction; type II is insulin-resistant, often linked to obesity.

  • What is glycogenesis?

    The synthesis of glycogen from glucose-6-phosphate via glucose-1-phosphate and UDP-glucose.

  • How is glycogen broken down during glycogenolysis?

    Glycogen phosphorylase releases glucose-1-phosphate from glycogen, which is converted to glucose-6-phosphate.

  • What is gluconeogenesis?

    The synthesis of glucose from noncarbohydrate precursors like lactate, amino acids, and glycerol, mainly in the liver.

  • Why are some glycolysis steps bypassed in gluconeogenesis?

    Because steps 1, 3, and 10 are highly exergonic and irreversible, gluconeogenesis uses alternative enzymes and reactions.

  • What is the Cori cycle?

    The recycling of lactate produced by muscles during anaerobic glycolysis back to glucose in the liver.