Aerobic Respiration and the Citric Acid Cycle

Overview of Aerobic Respiration

• Process used by cells to produce energy.
• Involves several pathways:
  • Glycolysis
  • Pyruvate oxidation
  • Citric acid cycle (TCA cycle/Krebs cycle)
  • Electron transport
• Sugars, fats, and proteins can be broken down in these pathways to produce ATP.

Focus on Citric Acid Cycle (Krebs Cycle)

• Takes place in the matrix of the mitochondrion, where mitochondrial DNA and fatty acid breakdown occur.
• Involves eight chemical reactions.
• Key molecules:
  • Acetyl CoA
  • Oxaloacetate
• Produces:
  • Carbon dioxide
  • NADH
  • ATP
  • FADH2
• NADH and FADH2 act as electron carriers for the electron transport chain.

Steps of the Citric Acid Cycle

1. Formation of Citrate
   • A 2-carbon molecule (acetyl CoA) combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate).
2. Biochemical Changes
   • The 6-carbon molecule undergoes changes to regenerate the original 4-carbon molecule.
   • Two carbon dioxides are released during this conversion.

Origin of 2-Carbon Molecule (Acetyl CoA)

• Derived from pyruvate, a product of glycolysis.
• Pyruvate transported to mitochondrial matrix and oxidized to acetyl CoA by pyruvate dehydrogenase.
• Each pyruvate oxidation produces:
  • 1 NADH
  • 1 carbon dioxide
• Acetyl CoA is the starting point for the citric acid cycle.

Detailed Reaction Steps

1. Citrate Formation
   • Acetyl group from acetyl CoA is transferred to oxaloacetate, forming citrate.
2. Enzymatic Reactions Leading to Succinate
   • Series of four reactions produce:
     • 2 carbon dioxides
     • 2 NADHs
     • 1 ATP
3. Recycling of Succinate to Oxaloacetate
   • Three additional reactions produce:
     • FADH2
     • 1 more NADH

Importance of Energy Molecules

• Energy molecules (NADH, FADH2, ATP) are essential for cell function.
• Errors in these pathways can lead to life-threatening diseases.