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Respiratory Balance Sheet: Total ATP from One Glucose

Biology · Respiration in Plants · NEET

The respiratory balance sheet adds up all the ATP made when one glucose is fully broken down in aerobic respiration. NCERT gives a net gain of 38 ATP per glucose. The rule to remember: each NADH gives 3 ATP and each FADH2 gives 2 ATP. Memory hook: "8 NADH from before ETS, 2 FADH2, plus 2+2 direct ATP" all add up to 38.
Respiratory Balance Sheet: 1 Glucose = 38 ATPGlycolysis2 ATP (direct)2 NADHLink reaction(x2 pyruvate)2 NADHKrebs (x2)2 ATP + 6 NADH2 FADH2ETSNADH x3FADH2 x2Direct ATP = 4 | 10 NADH x 3 = 30 | 2 FADH2 x 2 = 4Total net gain = 4 + 30 + 4 = 38 ATP
Stage-by-stage respiratory balance sheet. Add 4 direct ATP, 30 ATP from 10 NADH (x3), and 4 ATP from 2 FADH2 (x2) to reach the NCERT net gain of 38 ATP per glucose.

Your doubts, answered

How does one glucose give a net gain of 38 ATP? Show the full count.

Count each stage. Glycolysis: 2 ATP direct (net) + 2 NADH. Link reaction (pyruvate to acetyl CoA, 2 pyruvate): 2 NADH. Krebs cycle (runs twice): 2 ATP (from GTP) + 6 NADH + 2 FADH2. So the totals before ETS are: 4 ATP direct, 10 NADH, 2 FADH2. In the ETS, each NADH gives 3 ATP (10 x 3 = 30) and each FADH2 gives 2 ATP (2 x 2 = 4). Add: 4 + 30 + 4 = 38 ATP net gain per glucose.

Is the answer 36 ATP or 38 ATP for NEET?

For NEET, follow NCERT, which clearly states a net gain of 38 ATP per glucose. The 36 ATP figure appears in some other books because in those the 2 NADH made in the cytoplasm during glycolysis must be shuttled into the mitochondria, and one shuttle costs energy. NCERT does not use that subtraction, so write 38 ATP unless a question quotes the shuttle. Trust the NCERT number in the exam.

Why does NADH give 3 ATP but FADH2 gives only 2 ATP?

NADH enters the electron transport system at Complex I, so its electrons pass through more proton pumps (Complexes I, III, IV) and push more protons across the inner membrane. FADH2 enters later at Complex II, skipping Complex I, so it pumps fewer protons. More protons pumped means more ATP made by ATP synthase. That is why NADH gives 3 ATP and FADH2 gives 2 ATP. NEET has tested this exact reversal several times.

How many NADH and FADH2 are made in total from one glucose?

Total NADH = 10 (2 from glycolysis + 2 from the link reaction + 6 from the two turns of the Krebs cycle). Total FADH2 = 2 (both from the Krebs cycle, one per turn). These are the 'reducing equivalents' that carry electrons to the ETS. NCERT phrases this as 8 NADH + 2 FADH2 for the mitochondrial part alone, plus 2 NADH from glycolysis, giving 10 NADH overall.

Where does each direct (substrate-level) ATP come from?

Only 4 ATP are made directly, without the ETS. Glycolysis makes 2 net ATP by substrate-level phosphorylation. The Krebs cycle makes 1 GTP per turn (which becomes 1 ATP), and the cycle turns twice, so 2 more. That gives 4 direct ATP. The remaining 34 ATP come from oxidative phosphorylation in the ETS using the 10 NADH and 2 FADH2.

⚠️ The NEET trap
✗ NADH gives 2 ATP and FADH2 gives 3 ATP.
✓ NADH gives 3 ATP and FADH2 gives 2 ATP (10 NADH x 3 = 30, 2 FADH2 x 2 = 4).
🧠 NEET 2019 (and repeated) asked exactly this. Students swap the two numbers under pressure. NADH enters earlier at Complex I, so it earns MORE ATP: NADH = 3, FADH2 = 2.

Real NEET questions

2019

Which of the following statements is incorrect?

A · ATP is synthesized through complex V
B · Oxidation-reduction reactions produce proton gradient in respiration.
C · During aerobic respiration, role of oxygen is limited to the terminal stage
D · In ETC (Electron Transport Chain), one molecule of NADH + H+ gives rise to 2 ATP molecules, and one FADH2 gives rise to 3 ATP molecules ✓
Solution: The statement is incorrect because the values are reversed. One NADH gives 3 ATP and one FADH2 gives 2 ATP, not the other way round. NADH enters the ETS at Complex I and pumps more protons, so it makes more ATP. The other three options are correct: ATP synthase is Complex V, redox reactions build the proton gradient, and oxygen acts only at the terminal stage. (Repeated in NEET 2021 and ReNEET 2026.)
2022

What is the net gain of ATP when each molecule of glucose is converted to two molecules of pyruvic acid?

A · Four
B · Six
C · Two ✓
D · Eight
Solution: This is the glycolysis part of the balance sheet. Glycolysis makes 4 ATP by substrate-level phosphorylation but spends 2 ATP in the investment phase, so the net gain is 2 ATP. 'Four' is the gross figure and 'Six' ignores the ATP spent, so both are wrong.

Solved Respiration in Plants NEET PYQs

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Frequently asked

What is a respiratory balance sheet?

It is a stage-by-stage tally of every ATP, NADH and FADH2 formed when one glucose is fully oxidised in aerobic respiration, ending in the total net ATP gain of 38.

What is the net gain of ATP from one glucose?

38 ATP per glucose according to NCERT, in aerobic respiration with complete oxidation of glucose to CO2 and water.

How much ATP comes from glycolysis alone?

A net of 2 ATP directly, plus 2 NADH that later yield 6 more ATP in the ETS. So glycolysis contributes 2 direct + 6 from NADH = about 8 ATP to the total.

How much ATP does the Krebs cycle contribute?

Per glucose (two turns): 2 ATP from GTP, 6 NADH (18 ATP), and 2 FADH2 (4 ATP), giving 24 ATP linked to the Krebs cycle.

Why do some books say 36 ATP instead of 38?

They subtract 2 ATP for shuttling the cytoplasmic NADH of glycolysis into the mitochondria. NCERT does not make this subtraction, so for NEET the answer is 38.

Related concepts:
Assumptions Behind the 36/38 ATP CalculationWhat is Substrate-Level Phosphorylation?NADH and FADH2 Produced in RespirationWhat is Oxidative Phosphorylation?Net Gain of ATP and Pyruvate in Glycolysis
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Respiration in Plants PYQs