Metabolism of Carbohydrates: Notes

1. Glycolysis (Embden-Meyerhof Pathway)

• Location: Cytoplasm
• Purpose: Converts glucose (6C) → pyruvate (3C) to generate ATP and NADH.
• Phases:
  1. Preparatory Phase (Investment Phase)
     • Uses 2 ATP
     • Converts glucose → fructose-1,6-bisphosphate
  2. Payoff Phase (Energy Generation Phase)
     • Produces 4 ATP, 2 NADH, and 2 pyruvate
• Net Yield: 2 ATP, 2 NADH, 2 Pyruvate

2. TCA Cycle (Krebs Cycle, Citric Acid Cycle)

• Location: Mitochondrial matrix
• Purpose: Completes oxidation of acetyl-CoA → CO?, producing NADH and FADH?.
• Key Steps:
  1. Acetyl-CoA + Oxaloacetate → Citrate
  2. Decarboxylation → CO? release
  3. Energy capture as 3 NADH, 1 FADH?, 1 GTP (ATP equivalent) per cycle
• Net Yield (Per Glucose, i.e., 2 Turns of Cycle):
  • 6 NADH, 2 FADH?, 2 ATP (GTP), 4 CO?

3. Glyoxylate Cycle (Modified TCA in Plants & Bacteria)

• Location: Glyoxysomes (plants) & some bacteria
• Purpose: Enables conversion of fats → carbohydrates by bypassing CO? release steps.
• Key Features:
  • No CO? release (uses isocitrate lyase & malate synthase)
  • Acetate → Succinate → Glucose (via gluconeogenesis)
• Important in: Seed germination (conversion of stored fats to sugars).

4. Electron Transport Chain (ETC) & Oxidative Phosphorylation

• Location: Inner mitochondrial membrane
• Purpose: Uses NADH & FADH? to generate ATP via oxidative phosphorylation.
• Key Components:
  • Complex I (NADH dehydrogenase) – Transfers electrons from NADH to CoQ.
  • Complex II (Succinate dehydrogenase) – Transfers electrons from FADH? to CoQ.
  • Complex III (Cytochrome bc1 complex) – Transfers electrons from CoQ to cytochrome c.
  • Complex IV (Cytochrome c oxidase) – Transfers electrons to O?, forming H?O.
  • ATP Synthase (Complex V) – Uses proton gradient to synthesize ATP.
• Yield per Glucose:
  • Each NADH → 2.5 ATP
  • Each FADH? → 1.5 ATP
  • Total: ~32-34 ATP per glucose