Glycolysis and the Krebs Cycle

Conversion of Glucose to Pyruvate

• At the end of glycolysis, one glucose molecule (6 carbons) is converted into two molecules of pyruvate (3 carbons each).
• This conversion accounts for all 6 carbons initially present in glucose.

Bridge Step: Pyruvate to Acetyl-CoA

• Bridge Step: Converts pyruvate to acetyl-CoA, not part of glycolysis or the Krebs cycle.
• Each pyruvate (3 carbons) loses one carbon:
  • The carbon is fully oxidized to CO2.
  • This occurs twice for each glucose molecule, resulting in 2 CO2 molecules.
• NAD+ to NADH: This conversion reduces NAD+ to NADH, storing energy for ATP production.
• Aerobic Process: Requires oxygen.

Differences in Eukaryotic and Prokaryotic Cells

• Eukaryotic Cells:
  • Glycolysis occurs in the cytoplasm.
  • Acetyl-CoA shuttled into the mitochondrial matrix for the Krebs cycle.
  • The electron transport chain is in the inner mitochondrial membrane.
• Prokaryotic Cells:
  • Lack mitochondria, so processes occur in the cytoplasm.
  • Electron transport chain proteins are in the plasma membrane.

Krebs Cycle Overview

• Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons).
• The cycle regenerates oxaloacetate to process another acetyl-CoA.

Key Steps and Reactions

• Decarboxylation Reactions: Remove carbons as CO2:
  • Pyruvate to Acetyl-CoA (bridge step)
  • Isocitrate to alpha-ketoglutarate
  • Alpha-ketoglutarate to succinyl-CoA
• Energy Production:
  • NAD+ reduced to NADH for ATP synthesis.
  • FAD reduced to FADH2 for ATP synthesis.
  • GTP (equivalent to ATP) formation.

Energy Yield

• NADH: Yields 3 ATP per molecule in the electron transport chain.
• FADH2: Yields 2 ATP per molecule.

Complete Oxidation of Glucose

• Each glucose undergoes two cycles, producing 6 CO2 molecules:
  • 2 from the bridge step
  • 4 from Krebs cycle (2 per cycle)
• All carbons from glucose end as CO2.
• The energy from glucose is extracted and stored in NADH and FADH2, used later for ATP production in the electron transport chain.

Summary

• The Krebs cycle involves decarboxylation and multiple oxidation-reduction reactions.
• For every glucose, 2 ATP (or GTP), 6 NADH, and 2 FADH2 are produced.
• The cycle continues as long as there's a supply of acetyl-CoA and oxaloacetate.
• The processes in the mitochondria are critical for energy extraction from glucose.