Which molecule acts as the primary electron carrier produced during glycolysis and the citric acid cycle?

• A. NADP+
• B. NAD+
• C. FAD
• D. NADH

Answer: (D)

The key idea is how cells shuttle high‑energy electrons to the electron transport chain. The main electron carrier produced in glycolysis and the citric acid cycle is NADH. In glycolysis, the step catalyzed by glyceraldehyde‑3‑phosphate dehydrogenase reduces NAD+ to NADH as glyceraldehyde‑3‑phosphate is oxidized, capturing electrons that will be carried to the mitochondrial membrane to drive ATP production. In the citric acid cycle, several dehydrogenase reactions transfer electrons to NAD+, forming NADH for each turn of the cycle. NADH is the reduced form that donates electrons to the electron transport chain, whereas NAD+ is the oxidized form that accepts electrons. FADH2 also carries electrons but is produced in fewer steps and contributes differently to ATP yield, so it’s not the primary carrier in these pathways. NADP+ is involved mainly in photosynthesis and certain biosynthetic reactions, not in glycolysis or the citric acid cycle.