Glycolysis and the citric acid cycle (also known as the Krebs cycle or TCA cycle) are fundamental metabolic pathways that cells use to extract energy from glucose. These pathways generate high-energy molecules like NADH and FADH2, which are then used in the electron transport chain to produce ATP, the cell’s energy currency.

This article examines key statements about ATP production related to glycolysis and the citric acid cycle, clarifies their accuracy, and explains where ATP synthesis actually occurs within the cell.

Statement A: Electrons Released During Glycolysis and Citric Acid Cycle Produce 10 NADH and 2 FADH2 per Glucose

This statement is correct.

• For each molecule of glucose, glycolysis produces 2 NADH molecules.

• The conversion of 2 pyruvate molecules (from glycolysis) to 2 acetyl-CoA molecules produces 2 NADH.

• The citric acid cycle turns twice per glucose molecule (once per acetyl-CoA), producing 6 NADH and 2 FADH2.

• Total NADH = 2 (glycolysis) + 2 (pyruvate oxidation) + 6 (citric acid cycle) = 10 NADH.

• Total FADH2 = 2 (citric acid cycle).

These electron carriers then feed into the electron transport chain to generate ATP.

Statement B: Electrons Released Produce 20 NADH and 4 FADH2 per Glucose

This statement is incorrect.

• The numbers given here are double the actual values.

• The total NADH and FADH2 molecules produced per glucose are 10 and 2, respectively, not 20 and 4.

Statement C: The Coenzymes Produced Are Oxidized by the Electron Transport Chain

This statement is correct.

• NADH and FADH2 donate electrons to the electron transport chain (ETC) located in the inner mitochondrial membrane.

• As electrons pass through the ETC, energy is released and used to pump protons, creating a proton gradient.

• This gradient drives ATP synthesis via ATP synthase.

Statement D: Conversion of ADP and Pi to ATP Takes Place in the Intermembrane Space of Mitochondria

This statement is incorrect.

• ATP synthesis occurs in the mitochondrial matrix, not the intermembrane space.

• The electron transport chain pumps protons from the matrix into the intermembrane space, creating a proton gradient.

• Protons flow back into the matrix through ATP synthase, driving the conversion of ADP and inorganic phosphate (Pi) into ATP.

Summary Table of Statements

Correct Combination of Statements

Based on the above analysis, the correct combination is:

(4) A and C

How ATP Is Produced in Cellular Respiration

1. Glycolysis (cytosol):

   • Glucose → 2 pyruvate + 2 ATP (net) + 2 NADH.

2. Pyruvate Oxidation (mitochondrial matrix):

   • 2 pyruvate → 2 acetyl-CoA + 2 NADH + 2 CO2.

3. Citric Acid Cycle (mitochondrial matrix):

   • 2 acetyl-CoA → 6 NADH + 2 FADH2 + 2 ATP (or GTP) + 4 CO2.

4. Electron Transport Chain and Oxidative Phosphorylation (inner mitochondrial membrane):

   • NADH and FADH2 donate electrons.

   • Proton gradient formed across inner membrane.

   • ATP synthase uses gradient to convert ADP + Pi → ATP in the matrix.

Conclusion

The production of ATP during glucose metabolism involves multiple steps and compartments. For each glucose molecule, 10 NADH and 2 FADH2 molecules are generated during glycolysis and the citric acid cycle. These coenzymes are oxidized by the electron transport chain, which drives ATP synthesis in the mitochondrial matrix—not the intermembrane space.

Correct answer: (4) A and C