Entry of glycolytic products into mitochondria In aerobic respiration, which compound produced in the cytosol typically enters the mitochondrion before being converted to acetyl-CoA by the pyruvate dehydrogenase complex?

Introduction / Context:
Linking glycolysis to the citric acid cycle requires transport of carbon from the cytosol into mitochondria. Identifying which metabolite crosses the mitochondrial membranes clarifies the organization of central metabolism.

Given Data / Assumptions:

• Glycolysis occurs in the cytosol and produces pyruvate.
• Pyruvate is transported into the mitochondrial matrix by the mitochondrial pyruvate carrier.
• Inside the matrix, pyruvate dehydrogenase converts pyruvate to acetyl-CoA, CO2, and NADH.

Concept / Approach:
Acetyl-CoA is generated inside mitochondria; it does not cross the inner membrane efficiently. Therefore, the transported species is pyruvate, not acetyl-CoA. Other glycolytic intermediates, like glyceraldehyde-3-phosphate (phosphoglyceraldehyde), remain in the cytosol during glycolysis.

Step-by-Step Solution:
Cytosolic glycolysis ends with pyruvate formation.Pyruvate crosses the inner membrane via the pyruvate carrier.Matrix conversion: pyruvate + CoA + NAD+ → acetyl-CoA + CO2 + NADH.

Verification / Alternative check:
Tracer studies show rapid mitochondrial uptake of pyruvate; acetyl-CoA accumulation requires intact pyruvate dehydrogenase, supporting intra-mitochondrial generation.

Why Other Options Are Wrong:
Acetyl-CoA: generally cannot exit/enter across the inner membrane; it is formed inside.Phosphoglyceraldehyde: remains a cytosolic glycolytic intermediate.Oxaloacetate has limited permeability and is typically generated inside depending on shuttles.Lactate is converted to pyruvate (via LDH) before mitochondrial oxidation in most tissues.

Common Pitfalls:
Assuming acetyl-CoA is transported directly; overlooking the dedicated pyruvate carrier.

Final Answer:
Pyruvate.