Respiration ATP ledger: glycolysis, Krebs cycle, electron transport, fermentation and RQ

What it does

Cells get the energy in food by respiration: they take glucose apart step by step and store the energy as ATP. This page follows one glucose molecule through the four stages and keeps a running account, like a bank statement:

• Glycolysis in the cytoplasm: glucose → 2 pyruvate, 2 ATP (net) and 2 NADH.
• Link reaction in the mitochondrion: 2 pyruvate → 2 acetyl CoA, 2 CO₂ and 2 NADH.
• Krebs cycle (two turns): 4 CO₂, 6 NADH, 2 FADH₂ and 2 ATP.
• Electron transport system: NADH and FADH₂ give up their electrons, oxygen takes them at the end, and most of the ATP is made here.

A bar shows where the ATP comes from. Tick No oxygen to see fermentation (yeast making alcohol or muscle making lactic acid), which stops after glycolysis with just 2 ATP. Switch between the older textbook count (3 ATP per NADH, 2 per FADH₂: 38 in all) and the newer count (2.5 and 1.5: about 32).

The second box works out the respiratory quotient (RQ), the CO₂ given out divided by the O₂ taken in, for any food written as a formula of carbon, hydrogen and oxygen.

It fits Class 10 Science (life processes: aerobic and anaerobic respiration) and Class 11 Biology (respiration in plants: glycolysis, fermentation, the TCA cycle, the electron transport system, the respiratory balance sheet and RQ). See where the sugar came from in calvin and C4, and how ATP is made by a flow of H⁺ in the chloroplast too in z-scheme stepper.

How to use it

1. Press Next stage to move through the four stages, and Back to go back. The big line names the stage and where it happens; the text explains it.
2. Read the running totals: ATP made directly, NADH, FADH₂, CO₂, O₂ and the total ATP so far.
3. Slide Glucose molecules to scale everything up.
4. Tick No oxygen and pick yeast or muscle to compare fermentation.
5. In Respiratory quotient, pick a food or type a formula such as C18H34O2.

Worked example

After all four stages, one glucose has given 4 ATP directly, 10 NADH and 2 FADH₂, released 6 CO₂ and used 6 O₂. With the textbook count, the electron transport system turns these into 10 × 3 + 2 × 2 = 34 ATP, so the total is 4 + 34 = 38 ATP. With the newer count it is 4 + 25 + 3 = 32 ATP. Without oxygen the same glucose gives only 2 ATP, 19 times less.

For RQ, glucose C₆H₁₂O₆ needs 6 + 12/4 − 6/2 = 6 O₂ and gives 6 CO₂: RQ = 1. The fat tripalmitin, C₅₁H₉₈O₆, needs 51 + 98/4 − 6/2 = 72.5 O₂ for 51 CO₂: RQ = 0.703. Oxalic acid, C₂H₂O₄, needs only 0.5 O₂ for 2 CO₂: RQ = 4.

Tips

• NADH and FADH₂ are like cheques: the ATP is only “cashed” in the last stage, and only if oxygen is there.
• The 38 is a maximum. Real cells make less, because some energy is used to move things in and out of the mitochondrion.
• An RQ below 1 means the food has little oxygen of its own, so it needs more O₂ from outside: fats.

FAQ

Why do books give different totals (38, 36, 32, 30)? The older count uses 3 ATP per NADH and 2 per FADH₂. Newer measurements give about 2.5 and 1.5, and the NADH from glycolysis can be worth less depending on how it enters the mitochondrion.

Is CO₂ made in glycolysis? No. In aerobic respiration it comes from the link reaction and the Krebs cycle; in alcoholic fermentation it is released when pyruvate becomes ethanol.

What is the RQ when there is no oxygen? With no O₂ taken in, the RQ cannot be worked out (it would be infinite), as in yeast fermenting sugar.

Last updated 2026-11-09.