Glycolysis and the Bridge Step

Glycolysis Overview

• Conversion of glucose (6 carbons) into 2 molecules of pyruvate (3 carbons each).
• Accounts for all 6 carbons from the original glucose.

Bridge Step: Pyruvate to Acetyl-CoA

• Occurs in the presence of oxygen.
• Not part of glycolysis or the Krebs cycle; acts as a transition between them.
• Pyruvate (3 carbons) is converted into Acetyl-CoA (2 carbons).
• One carbon from pyruvate is fully oxidized to CO2.
• Produces NADH by reducing NAD+.
• NADH is used in the electron transport chain to produce ATP.

Krebs Cycle (Citric Acid Cycle)

Location

• In eukaryotic cells: in the matrix of the mitochondria.
• In prokaryotic cells: occurs in the cytoplasm.

Process

• Acetyl-CoA combines with oxaloacetate (4 carbons) to form citrate (6 carbons).
• Involves a series of oxidation reactions leading back to oxaloacetate.
• Each cycle results in the production of:
  • 2 CO2 (per cycle)
  • 3 NADH
  • 1 FADH2
  • 1 ATP (or GTP)

Key Reactions

• Decarboxylation reactions remove carbons as CO2.
• Energy from oxidation is stored in NADH and FADH2.
• NADH and FADH2 enter the electron transport chain to produce ATP.

Carbon Accounting

• For one glucose molecule, two cycles of the Krebs cycle are required.
• Each glucose molecule results in 6 CO2 molecules (3 from each pyruvate).

Energy Production

NADH and FADH2

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

Summary

• The Krebs cycle completes the oxidation of glucose to CO2.
• Electron transport chain extracts remaining energy from NADH and FADH2.
• Reduced molecules (NADH, FADH2) have more energy.

Additional Notes

• Acetyl-CoA in eukaryotes enters mitochondria, aiding in combining with oxaloacetate to form citric acid.
• Overview includes the loss of CO2 in the bridge step and Krebs cycle per glucose.