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GOB Chemistry: Carbohydrate Metabolism and Glycolysis

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  • What is the first stage in carbohydrate catabolism?
    Digestion, which involves physical grinding and enzyme-catalyzed hydrolysis of carbohydrates into smaller molecules.
  • Which enzyme initiates carbohydrate digestion in the mouth?
    Salivary α-amylase catalyzes hydrolysis of glycosidic bonds in carbohydrates.
  • What happens to salivary α-amylase in the stomach?
    It continues to act for about an hour but is then inactivated by stomach acid; no further carbohydrate digestion occurs in the stomach.
  • Which enzyme continues carbohydrate digestion in the small intestine?
    Pancreatic α-amylase converts polysaccharides to maltose in the small intestine.
  • How are disaccharides hydrolyzed in the small intestine?
    Enzymes hydrolyze maltose to glucose, sucrose to glucose and fructose, and lactose to glucose and galactose.
  • What happens to glucose once it enters a cell from the bloodstream?
    It is immediately converted to glucose 6-phosphate, trapping it inside the cell and committing it to metabolism.
  • What are the main metabolic fates of glucose 6-phosphate?
    It can undergo glycolysis, enter the pentose phosphate pathway, be stored as glycogen, or be converted to acetyl-SCoA for energy or fat synthesis.
  • What are the two key products of the pentose phosphate pathway?
    NADPH for biochemical reactions and ribose 5-phosphate for nucleic acid synthesis.
  • What are the two stages of the pentose phosphate pathway?
    The oxidative stage produces ribulose 5-phosphate and NADPH; the nonoxidative stage produces ribose-5-phosphate and glycolytic intermediates.
  • What is the net result of glycolysis?
    Production of two pyruvate molecules, two ATP molecules (net), and two NADH/H+ molecules per glucose.
  • How many ATP molecules are invested and produced in glycolysis?
    Two ATP molecules are invested in early steps; four ATP molecules are produced later, yielding a net gain of two ATP.
  • Name the enzyme that catalyzes the phosphorylation of glucose to glucose 6-phosphate.
    Hexokinase or glucokinase transfers a phosphoryl group from ATP to glucose.
  • What enzyme converts glucose 6-phosphate to fructose 6-phosphate?
    Glucose 6-phosphate isomerase catalyzes this reversible isomerization.
  • Which enzyme catalyzes the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate?
    Phosphofructokinase-1 (PFK-1) uses ATP to add a second phosphoryl group.
  • What is the role of aldolase in glycolysis?
    It cleaves fructose 1,6-bisphosphate into two triose phosphates: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.
  • How are the triose phosphates interconverted?
    Triose phosphate isomerase rapidly interconverts dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.
  • Which enzyme catalyzes the oxidation and phosphorylation of glyceraldehyde 3-phosphate?
    Glyceraldehyde 3-phosphate dehydrogenase produces 1,3-bisphosphoglycerate and NADH.
  • What enzyme transfers a high-energy phosphoryl group from 1,3-bisphosphoglycerate to ADP?
    Phosphoglycerate kinase generates ATP and 3-phosphoglycerate.
  • What enzyme catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate?
    Enolase forms a high-energy enol phosphate intermediate.
  • What is the final step of glycolysis?
    Pyruvate kinase transfers a phosphoryl group from phosphoenolpyruvate to ADP, producing ATP and pyruvate.
  • How is fructose metabolized in glycolysis?
    In muscle, fructose is phosphorylated to fructose 6-phosphate; in liver, it is converted to glyceraldehyde 3-phosphate.
  • How is mannose metabolized in glycolysis?
    Mannose is phosphorylated to mannose 6-phosphate, then converted to fructose 6-phosphate to enter glycolysis.
  • How is galactose metabolized in glycolysis?
    Galactose is converted to glucose 6-phosphate via a five-step pathway.
  • What happens to pyruvate under aerobic conditions?
    It is converted to acetyl-SCoA by the pyruvate dehydrogenase complex and enters the citric acid cycle.
  • What happens to pyruvate under anaerobic conditions?
    It is reduced to lactate to regenerate NAD+ for glycolysis continuation.
  • What is alcoholic fermentation?
    In yeast, pyruvate is converted to ethanol and CO2 under anaerobic conditions.
  • What is the maximum theoretical ATP yield from one glucose molecule in humans?
    Approximately 30–32 ATP molecules, considering NADH and FADH2 contributions.
  • Which hormones regulate blood glucose levels?
    Insulin lowers blood glucose; glucagon raises blood glucose.
  • What is glycogenesis?
    The synthesis of glycogen from glucose when glucose levels are high.
  • What is glycogenolysis?
    The breakdown of glycogen to free glucose during energy demand or low blood glucose.
  • What is gluconeogenesis?
    The synthesis of glucose from noncarbohydrate sources like lactate, amino acids, and glycerol.