# How many ATP are produced in oxidative phosphorylation?
> Oxidative phosphorylation makes about 26 to 28 ATP per glucose, or 2.5 ATP per NADH and 1.5 per FADH2. Proton counting explains the fractions and why older textbooks say 34.
Oxidative phosphorylation produces about **26 to 28 ATP per glucose**, roughly 87% of the 30 to 32 ATP that complete aerobic respiration yields in human cells. Each NADH that feeds it supports about 2.5 ATP and each FADH₂ about 1.5. The fractions come from counting protons: electrons from NADH drive about 10 protons across the inner mitochondrial membrane, electrons from FADH₂ about 6, and making and exporting one ATP costs about 4.
Older textbooks put the same step at 32 to 34 ATP because they assumed whole numbers, 3 ATP per NADH and 2 per FADH₂. Within the modern range, whether the answer is 26 or 28 depends on how the two NADH made in glycolysis pass their electrons into the mitochondrion.
## How many ATP does oxidative phosphorylation produce per glucose?
Breaking down one glucose molecule fills 12 electron carriers: 10 NADH and 2 FADH₂. Two NADH are made in [glycolysis](/guides/how-many-nadh-are-produced-in-glycolysis) in the cytosol and two when pyruvate is converted to acetyl-CoA. The [Krebs cycle](/guides/how-many-atp-are-produced-in-the-krebs-cycle) makes the other six NADH and both FADH₂.
At 2.5 ATP per NADH and 1.5 per FADH₂, the eight NADH made inside the mitochondrion support 20 ATP and the two FADH₂ support 3. The two cytosolic NADH add 5 or 3, depending on the shuttle that carries their electrons inward, so oxidative phosphorylation yields 28 or 26 ATP per glucose.
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Glycolysis and the Krebs cycle also make four ATP directly, by substrate-level phosphorylation, which brings [cellular respiration](/guides/how-many-atp-are-produced-in-cellular-respiration) as a whole to 30 or 32 ATP per glucose. Molecular Biology of the Cell gives about 30, counting each glycolytic NADH at 1.5 ATP.
## How many ATP are produced per NADH and FADH₂?
One NADH oxidized in the mitochondrion supports about **2.5 ATP**, and one FADH₂ about 1.5. These values are P/O ratios: ATP made for each pair of electrons that reaches an oxygen atom. Hinkle reviewed the main measurements made since 1937 and found that about 2.5 for NADH-linked substrates and 1.5 for succinate, which donates electrons at the FADH₂ level, fit most reports once their systematic errors are explained.
The fractions follow from proton counting. Electrons from NADH enter the respiratory chain at complex I and, on their way to oxygen, drive about 10 protons out of the mitochondrial matrix. Electrons from FADH₂ enter further along, at ubiquinone, skip complex I and move about 6. ATP synthase lets protons flow back and spends them on ATP, and bringing in phosphate and ADP while exporting ATP costs about one more proton per ATP.
Textbooks round that cost to 4 protons per ATP, which gives 10 ÷ 4 = 2.5 and 6 ÷ 4 = 1.5. Watt and colleagues later found that the rotor of bovine ATP synthase is a ring of eight c subunits, so one full turn moves 8 protons and makes 3 ATP. That is 2.7 protons per ATP, or 3.7 with transport, and it puts the ceilings at about 2.7 ATP per NADH and 1.6 per FADH₂.
Because c-subunit sequences are nearly identical across vertebrates, the same price probably applies to human mitochondria. Mookerjee and colleagues used it to calculate maximum P/O ratios of 2.73 for NADH-linked substrates and 1.64 for succinate. Table 1 sets these values beside the older and standard ones.
Table 1. ATP per NADH and FADH₂ under four accountings. Protons per ATP include about one proton for transporting phosphate, ADP and ATP; electrons from NADH and FADH₂ move about 10 and 6 protons. The 10-subunit row is the yeast ring structure Hinkle considered. Sources: Hinkle (2005), Mookerjee et al. (2017), Watt et al. (2010), Beignon et al. (2022).
Accounting
Protons per ATP
ATP per NADH
ATP per FADH₂
Older textbooks, whole numbers
Not derived from protons
3
2
10-subunit rotor ring
4.3
2.3
1.4
Standard modern values
4
2.5
1.5
Mammalian 8-subunit rotor ring
3.7
2.7
1.6
Cytosolic NADH is the exception to a single value: it is worth 2.5 or 1.5 ATP depending on the shuttle, and that choice is the whole difference between 26 and 28.
## Why do some sources say 26 ATP and others 28?
The inner mitochondrial membrane is impermeable to NADH, so the two NADH from glycolysis pass only their electrons inward, through one of two shuttles. The malate-aspartate shuttle carries the electrons in as malate, which is oxidized in the matrix to regenerate NADH. That NADH enters at complex I and keeps its value of about 2.5 ATP, 5 for the pair, so oxidative phosphorylation reaches 28.
The glycerol 3-phosphate shuttle hands the electrons to ubiquinone instead, the same entry point FADH₂ uses. The pair then drives 12 protons across the membrane rather than 20, each NADH is worth about 1.5 ATP, and the total falls to 26. Which shuttle dominates depends on the tissue. In the brain, the malate-aspartate shuttle is considered the main route, particularly in neurons.
## Why do older textbooks say 34 ATP?
The 34 comes from whole-number P/O ratios of 3 ATP per NADH and 2 per FADH₂. Ten NADH at 3 and two FADH₂ at 2 give 34 ATP from oxidative phosphorylation, or 32 if the glycerol 3-phosphate shuttle drops the two glycolytic NADH to 2 each. Adding the four ATP made directly gives the 38 and 36 still quoted for respiration as a whole.
Those integers were long read from early experiments as exact. Hinkle traced the apparent contradictions between studies to systematic measurement errors and concluded that the fractional values follow from the coupling ratios of proton transport.
The structure of ATP synthase now sets a ceiling as well. With the eight-subunit ring, Mookerjee and colleagues calculated that complete oxidation of glucose can yield at most 33.45 ATP with the malate-aspartate shuttle and 31.81 with the glycerol 3-phosphate shuttle. A total of 38 is more than that structure allows, so a 34 for oxidative phosphorylation in a recent source signals old accounting rather than different biology.
## Does the electron transport chain produce ATP?
The electron transport chain is often credited with the 26 to 28 ATP, but it makes none itself. Complexes I, III and IV use the energy of electron transfer to pump protons across the inner membrane of [mitochondria](/guides/how-many-mitochondria-are-in-a-cell). ATP synthase then makes the ATP as protons flow back into the matrix. Together, electron transport and ATP synthesis make up oxidative phosphorylation.
Oxygen is the final electron acceptor: it takes the electrons at complex IV and combines with protons to form water. Without oxygen the chain stops, the proton gradient runs down, and a cell falls back on the 2 ATP of glycolysis, regenerating NAD⁺ through [fermentation](/guides/how-many-atp-are-produced-in-fermentation).
## What sets the yield in a living cell
The count has three inputs: how many electron carriers a fuel fills, how many protons each carrier's electrons move, and how many protons each ATP costs. Only the first depends on the fuel. That is why the same 2.5 and 1.5 can price fat as well as sugar, and why [beta oxidation](/guides/how-many-atp-does-beta-oxidation-produce) makes one palmitate worth several times as much ATP as one glucose.
What a cell actually recovers is lower. Some protons leak back across the inner membrane without passing through ATP synthase, so part of the oxygen a cell consumes makes no ATP at all. Every figure on this page, 26 to 28 included, is a theoretical yield for complete, tightly coupled oxidation rather than a count of ATP measured from each glucose.