Cellular Respiration: Pyruvate Modification and Acetyl-CoA Formation

Glycolysis Review

• A six-carbon glucose molecule is split into two three-carbon pyruvate molecules.
• Electrons from the bonds holding the glucose molecule together are transferred to NAD^+, forming NADH (the reduced form).
• Glycolysis yields approximately two molecules of ATP per glucose molecule.

Pyruvate Modification

• Pyruvate must be modified before entering the citric acid cycle.
• The modification process involves:
  • Decarboxylation: One carbon atom is removed from pyruvate, releasing it as CO_2. With the removal of one carbon the pyruvate molecule which originally had 3 carbons now has 2 carbons.
  • Electron Liberation: Breaking a carbon-carbon bond releases electrons, which are then loaded onto NAD^+.
    • NAD^+ (oxidized form) gains electrons and is reduced to NADH.
    • NADH acts as an electron carrier, transporting electrons to the mitochondrial membrane.
  • Coenzyme A (CoA) addition: A two-carbon molecule receives a "molecular decoration" in the form of CoA.
    • The resulting molecule, with two carbons attached to CoA, is called acetyl-CoA.
    • Acetyl-CoA serves as the starting material (substrate) for the citric acid cycle.

Citric Acid Cycle Location

• The citric acid cycle takes place inside the mitochondrion, specifically in the mitochondrial matrix.
• The mitochondrion consists of:
  • An outer membrane.
  • A highly folded inner membrane forming cristae.
  • The mitochondrial matrix: the inner compartment enclosed by the inner membrane, containing enzymes that catalyze the reactions of the citric acid cycle.

Role of Citric Acid Cycle

• The citric acid cycle produces a small amount of ATP.
• Its primary role is to extract the remaining electrons from the original glucose molecule (now in the form of acetyl-CoA) and transfer them to electron carriers such as NAD^+ and FAD inside the mitochondrial matrix.
• These electron carriers subsequently donate electrons for further energy production.