Glycolysis and the Citric Acid Cycle

Hw from chpt 6&7

Each pyruvate molecule is converted into acetyl coenzyme A (acetyl CoA).
This conversion occurs in the matrix of the mitochondria.
Key events during this conversion for each pyruvate molecule:
- A molecule of carbon dioxide (CO_2) is released (decarboxylation).
- A molecule of nicotinamide adenine dinucleotide (NADH) is formed, indicating an oxidation step occurred.

Entry of Acetyl CoA into the Cycle:
- The two-carbon acetyl portion of acetyl CoA is transferred to a four-carbon molecule (oxaloacetate).
- This reaction produces a six-carbon compound (citrate).
- The coenzyme A (CoA) is released, ready to participate in another pyruvate conversion.
First Decarboxylation and Oxidation:
- Carbon dioxide (CO_2) is released from the six-carbon molecule, resulting in a five-carbon compound.
- A hydrogen atom is removed and transferred to NAD^+ (nicotinamide adenine dinucleotide), forming NADH.
Second Decarboxylation and Oxidation (and ATP Production):
- A second oxidation and decarboxylation event takes place.
- More NADH and carbon dioxide (CO_2) are produced.
- Crucially, a molecule of adenosine triphosphate (ATP) is directly generated at this stage (substrate-level phosphorylation).
- As a result, a four-carbon molecule is formed.
Regeneration of the Four-Carbon Molecule:
- The four-carbon molecule formed is further oxidized.
- Hydrogen atoms removed during this oxidation are used to form additional NADH and flavin adenine dinucleotide (FADH2).
- This final step regenerates the original four-carbon molecule that initially reacted with acetyl CoA, completing the cycle.

Each glucose molecule yields two pyruvate molecules during glycolysis.
Each of these two pyruvate molecules is subsequently converted into acetyl CoA.
Therefore, for every single glucose molecule, the citric acid cycle must complete two full circuits to completely break down both acetyl CoA molecules derived from the original glucose.