Link Between Glycolysis and the TCA Cycle The end product of glycolysis is pyruvate. Before entering the citric acid cycle, pyruvate is converted to which activated molecule?

Introduction / Context:
Glycolysis occurs in the cytosol and generates pyruvate. To feed the TCA cycle, pyruvate must be transported into mitochondria and converted to an activated two-carbon donor. Identifying the correct intermediate clarifies central metabolism and bioenergetics.

Given Data / Assumptions:

• Pyruvate dehydrogenase complex (PDC) resides in the mitochondrial matrix.
• Coenzymes include TPP, lipoamide, FAD, CoA, and NAD+.

Concept / Approach:
PDC catalyzes oxidative decarboxylation of pyruvate to acetyl-CoA, releasing CO2 and producing NADH. Acetyl-CoA then condenses with oxaloacetate to form citrate, beginning the TCA cycle. Acetaldehyde formation is a yeast fermentation step, not the aerobic link to TCA; acetic acid is not the activated thioester needed for entry.

Step-by-Step Solution:

Verification / Alternative check:
PDC deficiency causes lactic acidosis and neuro symptoms due to impaired acetyl-CoA supply, underscoring its central role.

Why Other Options Are Wrong:

• Acetaldehyde: fermentation intermediate in yeast.
• Acetic acid: lacks CoA activation; not used directly by citrate synthase.
• Oxaloacetate: a TCA intermediate formed from other routes, not directly from pyruvate for TCA entry under standard conditions (except via pyruvate carboxylase for gluconeogenesis/anaplerosis).

Common Pitfalls:
Confusing anaerobic fermentative pathways with aerobic respiration steps.

Final Answer:
Acetyl-CoA