Pathway hand-off — In what chemical form does the end product of glycolysis enter the tricarboxylic acid (TCA) cycle under aerobic conditions?

Introduction:
Glycolysis ends with pyruvate, but the TCA cycle begins with acetyl-CoA condensing with oxaloacetate to form citrate. This question examines your knowledge of the crucial linking reaction that enables aerobic oxidation of carbohydrate-derived carbon in mitochondria.

Given Data / Assumptions:

• Aerobic conditions allow mitochondrial respiration.
• Pyruvate can be metabolized via several fates depending on oxygen and cell type.
• The TCA cycle accepts acetyl units as acetyl-CoA.

Concept / Approach:
The pyruvate dehydrogenase (PDH) complex in the mitochondrial matrix converts pyruvate to acetyl-CoA by oxidative decarboxylation, producing NADH and CO2. The resulting acetyl-CoA enters the TCA cycle by combining with oxaloacetate to form citrate, enabling full oxidation to CO2 and maximum ATP yield via oxidative phosphorylation.

Step-by-Step Solution:

Verification / Alternative check:
PDH deficiency or thiamine deficiency (cofactor for PDH) leads to lactic acidosis and impaired aerobic energy production, highlighting the indispensability of acetyl-CoA formation from pyruvate.

Why Other Options Are Wrong:

• Pyruvate: must be converted to acetyl-CoA to enter the TCA cycle.
• NADH: electron carrier, not the carbon substrate for TCA entry.
• Glucose: upstream of glycolysis; does not enter TCA directly.
• Oxaloacetate: TCA acceptor molecule, not the glycolysis product entering the cycle.

Common Pitfalls:
Confusing pyruvate’s identity as the end product of glycolysis with the actual TCA entry molecule acetyl-CoA.

Final Answer:
AcetylCoA.