Carbohydrate Metabolism Quiz

Definition: The process of breaking down glucose into pyruvate, producing energy.
Location: Cytoplasm of the cell.
Phases:
- Energy Investment Phase: Consumes 2 ATP to phosphorylate glucose and its intermediates.
- Energy Payoff Phase: Produces 4 ATP, 2 NADH, and 2 pyruvate; net gain of 2 ATP.
Key Enzymes:
- Hexokinase (glucose → glucose-6-phosphate)
- Phosphofructokinase (PFK) (fructose-6-phosphate → fructose-1,6-bisphosphate)
- Pyruvate kinase (phosphoenolpyruvate → pyruvate)

Definition: The synthesis of glucose from non-carbohydrate precursors.
Location: Primarily in the liver and kidneys.
Key Precursors: Lactate, glycerol, and amino acids (especially alanine).
Key Enzymes:
- Pyruvate carboxylase (pyruvate → oxaloacetate)
- Phosphoenolpyruvate carboxykinase (oxaloacetate → phosphoenolpyruvate)
- Fructose-1,6-bisphosphatase (fructose-1,6-bisphosphate → fructose-6-phosphate)
Regulation: Stimulated by cortisol and glucagon; inhibited by insulin.

Definition: The breakdown of glycogen into glucose-1-phosphate and glucose.
Location: Liver and muscle cells.
Key Steps:
- Glycogen phosphorylase cleaves glycogen to release glucose-1-phosphate.
- Debranching enzyme is involved in removing branches of glycogen.
Regulation: Activated by glucagon and epinephrine; inhibited by insulin.

Definition: A metabolic pathway parallel to glycolysis that generates NADPH and ribose-5-phosphate.
Location: Cytoplasm.
Functions:
- Produces NADPH for reductive biosynthesis and antioxidant defense.
- Provides ribose-5-phosphate for nucleotide synthesis.
Key Enzymes:
- Glucose-6-phosphate dehydrogenase (G6PD) (glucose-6-phosphate → 6-phosphoglucono-δ-lactone)
- Transaldolase and transketolase (involved in sugar rearrangement)
Regulation: Activated by NADP+; inhibited by NADPH.

Role of Insulin:
- Secreted by beta cells of the pancreas in response to high blood glucose levels.
- Promotes glucose uptake by cells, especially muscle and adipose tissues.
- Stimulates glycogenesis (conversion of glucose to glycogen) and inhibits glycogenolysis and gluconeogenesis.
Key Effects:
- Lowering blood glucose levels.
- Enhancing storage of lipids and proteins.
Counter-Regulatory Hormones:
- Glucagon: Raises blood glucose by stimulating glycogenolysis and gluconeogenesis.
- Epinephrine: Increases blood glucose levels during stress or fight-or-flight response.
Feedback Mechanism: Blood glucose levels are tightly regulated by the balance between insulin and counter-regulatory hormones.

Breaks down glucose into pyruvate, yielding energy.
Occurs in the cytoplasm of cells.
Phases:
- Energy Investment Phase: Requires 2 ATP for phosphorylation reactions.
- Energy Payoff Phase: Produces 4 ATP, 2 NADH, and 2 pyruvate; results in a net gain of 2 ATP.
Key Enzymes:
- Hexokinase: Catalyzes conversion of glucose to glucose-6-phosphate.
- Phosphofructokinase (PFK): Converts fructose-6-phosphate to fructose-1,6-bisphosphate.
- Pyruvate kinase: Converts phosphoenolpyruvate to pyruvate.

Synthesizes glucose from non-carbohydrate sources.
Primarily occurs in the liver and kidneys.
Key Precursors: Includes lactate, glycerol, and amino acids (notably alanine).
Key Enzymes:
- Pyruvate carboxylase: Converts pyruvate to oxaloacetate.
- Phosphoenolpyruvate carboxykinase: Converts oxaloacetate to phosphoenolpyruvate.
- Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate to fructose-6-phosphate.
Regulation:
- Stimulated by cortisol and glucagon.
- Inhibited by insulin.

Breaks down glycogen into glucose-1-phosphate and glucose.
Occurs in liver and muscle cells.
Key Steps:
- Glycogen phosphorylase cleaves glycogen, producing glucose-1-phosphate.
- Debranching enzyme removes branches from glycogen.
Regulation:
- Activated by glucagon and epinephrine.
- Inhibited by insulin.

A pathway parallel to glycolysis that generates NADPH and ribose-5-phosphate.
Takes place in the cytoplasm.
Functions:
- Produces NADPH for biosynthesis and antioxidant defense.
- Provides ribose-5-phosphate for nucleotide synthesis.
Key Enzymes:
- Glucose-6-phosphate dehydrogenase (G6PD): Converts glucose-6-phosphate to 6-phosphoglucono-δ-lactone.
- Transaldolase and transketolase: Facilitate sugar rearrangements.
Regulation:
- Activated by NADP+; inhibited by NADPH.

Insulin:
- Secreted by pancreatic beta cells in response to high blood glucose levels.
- Facilitates glucose uptake in muscle and adipose tissues.
- Promotes glycogenesis and inhibits glycogenolysis and gluconeogenesis.
Key Effects:
- Lowers blood glucose levels.
- Promotes lipid and protein storage.
Counter-Regulatory Hormones:
- Glucagon: Raises blood glucose by stimulating glycogenolysis and gluconeogenesis.
- Epinephrine: Increases blood glucose during stress or fight-or-flight responses.
Feedback Mechanism: Ensures tight regulation of blood glucose through the balance of insulin and counter-regulatory hormones.