Order of pathways: Choose the correct sequence for complete aerobic oxidation of glucose in eukaryotic cells.

Introduction / Context:
Pathway order determines where ATP and reducing equivalents are generated and how carbon flows to CO2. Getting the sequence right is essential for interpreting metabolic data and inhibitor effects.

Given Data / Assumptions:

• Standard eukaryotic metabolism under aerobic conditions.
• Glucose is the starting substrate; oxygen is available.
• Pyruvate dehydrogenase links glycolysis to the Krebs cycle.

Concept / Approach:
Glycolysis occurs first in the cytosol, producing pyruvate (and ATP/NADH). Pyruvate is converted to acetyl-CoA, which enters the Krebs cycle in the mitochondrial matrix, generating more NADH and FADH2. Finally, the electron transport chain uses these reduced cofactors to drive ATP synthesis.

Step-by-Step Solution:
Start: Glycolysis (glucose → pyruvate) with substrate-level ATP production.Link: Pyruvate oxidation (PDH) forms acetyl-CoA.Next: Krebs cycle oxidizes acetyl-CoA to CO2, producing NADH/FADH2.Final: Electron transport chain harnesses NADH/FADH2 to synthesize ATP.

Verification / Alternative check:
Experimental inhibition (e.g., rotenone, antimycin, cyanide) stops ATP gain after NADH formation, showing ETC is downstream of glycolysis and the Krebs cycle.

Why Other Options Are Wrong:

• Any sequence starting with Krebs or ETC ignores the need for glycolysis-derived carbon entry and reduced cofactors.

Common Pitfalls:
Forgetting the PDH step between glycolysis and the Krebs cycle; although not named in options, it is implied in the correct order.

Final Answer:
Glycolysis → Krebs cycle → Electron transport chain