Is the answer 36, 38, 30, or 32 ATP per glucose?

All four are defensible depending on which convention your course uses. 36 or 38 come from the older, simpler P/O ratios (3 ATP per NADH, 2 per FADH2); 30 or 32 come from more accurate modern measurements (2.5 per NADH, 1.5 per FADH2). Within each pair, the lower number assumes the glycerol-3-phosphate shuttle for cytosolic NADH and the higher number assumes the malate-aspartate shuttle. Check which convention your professor or textbook uses before an exam.

Why does the cytosolic NADH from glycolysis need a shuttle at all?

NADH itself cannot cross the inner mitochondrial membrane. To get its electrons into the electron transport chain, a shuttle system transfers just the electrons across, regenerating NADH (malate-aspartate shuttle) or FADH2 (glycerol-3-phosphate shuttle) on the mitochondrial side — which is exactly why those 2 cytosolic NADH end up worth a different amount of ATP depending on which shuttle is used.

Why isn't the P/O ratio a clean whole number like 3 or 2?

ATP synthase requires roughly 4 protons to make 1 ATP (3 to turn the rotor, 1 to import phosphate), and measured proton-pumping stoichiometry per NADH or FADH2 oxidized doesn't divide evenly by 4. The modern 2.5 and 1.5 values come from that more precise proton accounting; the classic 3 and 2 values are the older, simplified textbook approximation.

ATP Yield Calculator — ATP per Glucose, Free

P/O ratio convention

Cytosolic NADH shuttle

Total ATP per glucose

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Stage	NADH	FADH₂	Substrate-level
Glycolysis	2 (cytosolic)	—	2 ATP
Pyruvate dehydrogenase (×2)	2	—	—
Citric acid cycle (×2 turns)	6	2	2 GTP
Total	10	2	4

How the total is actually built

Every glucose makes 4 ATP directly (substrate-level phosphorylation: 2 from glycolysis, 2 from the TCA cycle's GTP) with no oxygen required. Everything else comes from oxidative phosphorylation: the 10 NADH and 2 FADH₂ produced along the way feed electrons into the electron transport chain, and the proton gradient that creates drives ATP synthase. How much ATP each NADH or FADH₂ is "worth" — the P/O ratio — is where the textbook disagreement comes from.

2 of those 10 NADH are made in the cytosol during glycolysis, but NADH itself can't cross the inner mitochondrial membrane. A shuttle system moves the electrons across instead, and depending on which shuttle a tissue uses, those 2 NADH end up worth either a full NADH's worth of ATP (malate-aspartate shuttle) or only a FADH₂'s worth (glycerol-3-phosphate shuttle) — which is the second source of disagreement.

Related tools: Glycolysis pathway explorer · all biochem tools.

Worked example: modern accounting, malate-aspartate shuttle (the default)

Substrate-level ATP: 2 (glycolysis) + 2 (TCA GTP) = 4 ATP Mitochondrial NADH: 8 (2 from pyruvate dehydrogenase + 6 from the TCA cycle) 8 × 2.5 ATP/NADH = 20 ATP FADH2: 2 (from the TCA cycle) 2 × 1.5 ATP/FADH2 = 3 ATP Cytosolic NADH: 2 (from glycolysis), malate-aspartate shuttle → full NADH value 2 × 2.5 ATP/NADH = 5 ATP Total = 4 + 20 + 3 + 5 = 32 ATP

Switch the shuttle to glycerol-3-phosphate and those 2 cytosolic NADH drop to 1.5 ATP each instead of 2.5 — total falls to 30 ATP. Switch to classic P/O ratios (3.0/2.0) and the same two shuttle choices give 38 or 36 ATP instead. All four are "correct" — they're just different, equally real conventions.

Practice problems

1. Using classic P/O ratios (NADH = 3, FADH₂ = 2) and the glycerol-3-phosphate shuttle, what's the total ATP yield?

Show answer

Substrate-level: 4 ATP Mitochondrial NADH: 8 × 3 = 24 ATP FADH2: 2 × 2 = 4 ATP Cytosolic NADH (G3P shuttle → FADH2-equivalent): 2 × 2 = 4 ATP Total = 4 + 24 + 4 + 4 = 36 ATP

2. A muscle cell uses the glycerol-3-phosphate shuttle and modern P/O ratios. If glycolysis were somehow blocked entirely (no glucose breakdown past that point), how much of the 30 ATP total would be lost — and why is it more than just the 2 substrate-level ATP from glycolysis itself?

Show answer

Blocking glycolysis loses: the 2 substrate-level ATP AND the 2 cytosolic NADH (worth 2 × 1.5 = 3 ATP via the G3P shuttle) Total lost = 2 + 3 = 5 ATP out of 30 (not just 2) Glycolysis contributes ATP two ways — directly (substrate-level) and indirectly (its NADH feeds oxidative phosphorylation) — so losing it costs more than just its own direct ATP.

FAQ

Is the answer 36, 38, 30, or 32 ATP?
All four are defensible depending on convention. 36/38 use the older P/O ratios (3 and 2); 30/32 use modern measured values (2.5 and 1.5). Within each pair, the shuttle for cytosolic NADH picks the lower or higher number. Check which convention your course uses.

Why does cytosolic NADH need a shuttle?
NADH can't cross the inner mitochondrial membrane. A shuttle moves the electrons across instead, regenerating NADH (malate-aspartate) or FADH₂ (glycerol-3-phosphate) on the mitochondrial side — which is exactly why those 2 NADH end up worth a different amount depending on the shuttle.

Why isn't the P/O ratio a whole number?
ATP synthase needs roughly 4 protons per ATP, and the measured protons pumped per NADH or FADH₂ oxidized doesn't divide evenly by 4. The modern 2.5/1.5 values come from that more precise accounting; 3/2 is the older simplified approximation.