Q.How many ATPs are produced by one molecule of FADH2?
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Aerobic Respiration Energy Yield
The Intuition: Why Bother with Oxygen?
Think of glucose as a tightly packed bundle of energy — like a compressed spring. To get that energy out, you have to "unwind" the glucose molecule step by step. If you just set it on fire, you get all the energy at once as heat, which is useless for a cell. The cell needs to capture that energy in a usable form: ATP.
Aerobic respiration is the most efficient way to do this. Why? Because oxygen is the ultimate electron hog. It pulls electrons through the chain of reactions with enormous force, allowing the cell to extract the maximum possible energy from each glucose molecule. Without oxygen, you leave a lot of energy on the table — that's why anaerobic respiration yields only 2 ATP per glucose, while aerobic respiration yields around 36–38.
The Precise Statement
The complete aerobic oxidation of one molecule of glucose (C6H12O6) yields a theoretical net maximum of 36 or 38 ATP molecules, depending on the cell type and the shuttle system used to move electrons from the cytoplasm into the mitochondria.
The net yield is what matters — you must subtract the ATP consumed in the process (e.g., 2 ATP used in glycolysis). The gross production is higher, but the cell's investment is already accounted for in the net figure.
Where Does the ATP Come From? A Stage-by-Stage Breakdown
Aerobic respiration happens in four main stages. Here is the ATP tally for each, assuming one glucose molecule:
| Stage | Location | ATP (substrate-level) | NADH | FADH2 |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | 2 (net) | 2 | 0 |
| Pyruvate oxidation | Mitochondrial matrix | 0 | 2 | 0 |
| Krebs cycle | Mitochondrial matrix | 2 | 6 | 2 |
| Total before ETC | — | 4 | 10 | 2 |
Now, the real ATP bonanza comes from the electron transport chain (ETC) and oxidative phosphorylation. Each NADH donates electrons that pump enough protons to generate roughly 3 ATP. Each FADH2 generates roughly 2 ATP (because it enters the chain at a later, lower-energy point).
So from the ETC:
- 10 NADH × 3 ATP = 30 ATP
- 2 FADH2 × 2 ATP = 4 ATP
Add the 4 ATP from substrate-level phosphorylation: 30 + 4 + 4 = 38 ATP.
The 3 ATP per NADH and 2 ATP per FADH2 are approximate values. Modern measurements suggest the actual numbers are closer to 2.5 and 1.5, respectively, giving a more realistic yield of about 30–32 ATP. But for most Indian exam contexts (CBSE, NEET, etc.), the classical 36/38 figure is still the expected answer.
The 36 vs. 38 Mystery
Why do some textbooks say 36 and others 38? It comes down to the cost of moving NADH from the cytoplasm into the mitochondria. …
FADH2 and NADH feed electrons into the electron transport chain at different points, which changes how much ATP each yields. …
Each FADH2 produced during aerobic respiration (e.g. in the Krebs cycle, from succinate dehydrogenase) yields 2 ATP via oxidative phosphorylation.
During the electron transport chain (ETC) stage of aerobic respiration, reduced coenzymes generated in glycolysis and the Krebs cycle donate their electrons to the ETC, driving ATP synthesis by chemiosmosis:
- NADH donates electrons at Complex I, pumping protons at three points along the chain, and is conventionally counted as yielding 3 ATP per molecule. …
- CBSE 2026Set ANNUAL1 markQ.How many ATPs are produced by one molecule of FADH2?
›Reveal solutionSolution
Each FADH2 produced during aerobic respiration (e.g. in the Krebs cycle, from succinate dehydrogenase) yields 2 ATP via oxidative phosphorylation.
During the electron transport chain (ETC) stage of aerobic respiration, reduced coenzymes generated in glycolysis and the Krebs cycle donate their electrons to the ETC, driving ATP synthesis by chemiosmosis:
- NADH donates electrons at Complex I, pumping protons at three points along the chain, and is conventionally counted as yielding 3 ATP per molecule. …
- CBSE 2026Set ANNUAL1 markMCQQ.The number of ATP molecules produced by complete oxidation of one glucose molecule is(a) 2(b) 18(c) 36(d) 38
›Reveal solutionSolution
Complete oxidation of one glucose gives a maximum of 38 ATP, so the answer is (D).
Counting the theoretical maximum ATP yield from one glucose molecule:
- Glycolysis: net 2 ATP + 2 NADH.
- Link reaction and Krebs cycle: 2 ATP (GTP) + several NADH and FADH2.
- Oxidative phosphorylation (ETC): each NADH gives about 3 ATP and each FADH2 about 2 ATP. …
- CBSE 2023Set ANNUAL1 markMCQQ.The end product of aerobic respiration are :(a) sugar and oxygen(b) CO2 and energy(c) water and energy(d) CO2, H2O and energy
›Reveal solutionSolution
Aerobic respiration yields CO2, H2O and energy.
In aerobic respiration glucose is fully oxidised in the presence of oxygen to give carbon dioxide, water and energy (stored as ATP), summarised as C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy. So the end products are CO2, H2O and energy. …
- CBSE 2022Set ANNUAL1 markMCQQ.In end products of aerobic respiration are:(a) Sugar and Oxygen(b) CO2 and energy(c) Water and energy(d) CO2, H2O and Energy
›Reveal solutionSolution
Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP) → (d).
In aerobic respiration glucose is completely oxidised:
- Carbon is released as CO₂, hydrogen combines with oxygen to form H₂O, and energy is trapped as ATP (with heat). …
- CBSE 2020Set ANNUAL1 markMCQQ.In aerobic respiration total number of ATP molecules formed from 1 glucose molecule is :(a) 32(b) 34(c) 38(d) 20
›Reveal solutionSolution
Complete aerobic respiration of one glucose molecule (glycolysis + link reaction + Krebs cycle + oxidative phosphorylation via the electron transport chain) yields a net total of 38 ATP, as taught in the NCERT scheme.
Breaking it down as per NCERT: glycolysis nets 2 ATP (+2 NADH), the link reaction/pyruvate oxidation produces 2 NADH, the Krebs cycle (run twice per glucose) yields 2 ATP directly plus 6 NADH and 2 FADH2, and oxidation of all these NADH/FADH2 through the electron transport chain generates …
- CBSE 2018Set ZOOLOGY1 markQ.Write down the chemical equation involved in aerobic respiration.
›Reveal solutionSolution
Aerobic respiration is the complete breakdown of glucose using oxygen, producing carbon dioxide, water, and energy.
Aerobic respiration is the process by which glucose (a 6-carbon sugar) is completely oxidised in the presence of oxygen (through glycolysis, the Krebs cycle, and the electron transport chain) to release carbon dioxide, water, and a large amount of usable energy in the form of ATP (up to about 38 ATP per glucose molecule, theoretically).
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