How many ATP molecules can be generated from one glucose molecule during aerobic cellular respiration? (Draw the pathway including enzymes and intermediates, and choose one of shuttle systems and show your calculations)
The Correct Answer and Explanation is:
Correct Answer:
Up to 30 or 32 ATP molecules can be generated from one glucose molecule during aerobic cellular respiration, depending on the shuttle system used to transfer NADH from the cytosol into the mitochondria.
Pathway Overview of Aerobic Respiration (with key enzymes and intermediates):
- Glycolysis (Cytoplasm)
- Enzymes: Hexokinase, Phosphofructokinase, Pyruvate kinase
- Intermediates: Glucose → Glucose-6-phosphate → Fructose-1,6-bisphosphate → Glyceraldehyde-3-phosphate → Pyruvate
- ATP gain: 2 ATP (net), 2 NADH
- Pyruvate Oxidation (Mitochondrial matrix)
- Enzyme: Pyruvate dehydrogenase
- Intermediate: Pyruvate → Acetyl-CoA
- NADH gain: 2 NADH (1 per pyruvate)
- Citric Acid Cycle (Krebs Cycle)
- Enzymes: Citrate synthase, Isocitrate dehydrogenase, α-Ketoglutarate dehydrogenase, Succinate dehydrogenase
- Intermediates: Acetyl-CoA + Oxaloacetate → Citrate → α-Ketoglutarate → Succinyl-CoA → Malate → Oxaloacetate
- ATP and electron carriers: 2 ATP (GTP), 6 NADH, 2 FADH₂
- Electron Transport Chain and Oxidative Phosphorylation (Inner mitochondrial membrane)
- Complexes: I to IV, ATP synthase
- NADH and FADH₂ donate electrons to the chain
- Proton gradient powers ATP synthesis
**Calculation using the Malate-Aspartate Shuttle:
(more efficient shuttle system found in liver, heart, kidneys)
- From glycolysis: 2 NADH × 2.5 ATP = 5 ATP
- From pyruvate oxidation: 2 NADH × 2.5 ATP = 5 ATP
- From Krebs cycle:
- 6 NADH × 2.5 ATP = 15 ATP
- 2 FADH₂ × 1.5 ATP = 3 ATP
- 2 ATP (GTP) from substrate-level phosphorylation
Total ATP = 5 + 5 + 15 + 3 + 2 = 30 ATP
If the Glycerol-3-phosphate shuttle is used (less efficient, common in muscle and brain), cytosolic NADH yields 1.5 ATP each, giving a total of 32 ATP.
Explanation
Aerobic cellular respiration is the process by which cells extract energy from one glucose molecule using oxygen. This complex process occurs in stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
Glycolysis begins in the cytoplasm, where glucose is broken down into two molecules of pyruvate. This step generates two ATP molecules directly and produces two NADH molecules, which must be shuttled into the mitochondria.
Next, each pyruvate enters the mitochondrial matrix and is converted to acetyl-CoA, producing two more NADH molecules. These acetyl-CoA molecules then enter the citric acid cycle. For each acetyl-CoA, the cycle generates three NADH, one FADH₂, and one GTP (which is equivalent to ATP). Since two acetyl-CoA are produced per glucose, this results in six NADH, two FADH₂, and two ATP.
The high-energy electrons from NADH and FADH₂ are then passed through the electron transport chain in the inner mitochondrial membrane. This movement of electrons drives the production of ATP via chemiosmosis.
The total ATP yield depends on which shuttle system transfers the NADH from glycolysis into the mitochondria. The malate-aspartate shuttle, used in high-efficiency organs, allows cytosolic NADH to yield 2.5 ATP each. In this case, the total yield is about 30 ATP. The glycerol-3-phosphate shuttle results in only 1.5 ATP per cytosolic NADH, increasing the total to around 32 ATP.
Thus, the range of ATP produced from one glucose molecule during aerobic respiration is typically between 30 and 32, depending on the shuttle system used.
