The Krebs cycle and electron transport chain (ETC) are where aerobic respiration generates the bulk of its ATP — yet NEET questions on this topic frequently punish students who mix up the locations, carrier molecules, and net yields across these two stages.
Krebs cycle (citric acid cycle / TCA cycle) occurs in the mitochondrial matrix. Pyruvate from glycolysis first undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex, producing acetyl-CoA + CO₂ + NADH. Acetyl-CoA (2C) then condenses with oxaloacetate (OAA, 4C) to form citrate (6C). Through a cyclic series of reactions, citrate is progressively oxidised back to OAA, releasing 2 CO₂, 3 NADH, 1 FADH₂, and 1 GTP (equivalent to 1 ATP) per turn. Since each glucose yields two acetyl-CoA molecules, the cycle turns twice per glucose (NCERT Class 11 Biology Chapter 12, page 158).
A common confusion: students forget that the CO₂ released during respiration comes from the Krebs cycle (and the pyruvate decarboxylation step), not from the ETC. The ETC consumes no carbon substrate — it transfers electrons.
Electron transport chain is located on the inner mitochondrial membrane. NADH and FADH₂ donate electrons to a series of complexes (Complex I → ubiquinone → Complex III → cytochrome c → Complex IV). The terminal electron acceptor is molecular oxygen (O₂), which combines with H⁺ to form water. Proton pumping across the inner membrane creates a gradient; ATP synthase (Complex V) harnesses this gradient to synthesise ATP via chemiosmosis — Peter Mitchell's chemiosmotic hypothesis.
The amphibolic pathway. Glucose is respiration's favoured substrate, but other substrates can also be respired — they just enter the pathway at different points rather than at the first step. Fats are split into glycerol and fatty acids; fatty acids are degraded to acetyl CoA before entering, while glycerol enters after conversion to PGAL. Proteins are broken down by proteases into amino acids, which (after deamination) enter within the Krebs' cycle or as pyruvate or acetyl CoA. Because these same intermediates are withdrawn from the pathway when the cell needs to synthesise fatty acids or proteins, the respiratory pathway serves both breakdown (catabolism) and synthesis (anabolism) — so it is better described as an amphibolic pathway, not a purely catabolic one (NCERT Class 11 Biology, Chapter 12, page 162).
Watch out: NEET distractors often swap NADH and FADH₂ entry points. NADH donates electrons at Complex I; FADH₂ donates at Complex II (succinate dehydrogenase). FADH₂ bypasses Complex I, yielding fewer ATP per molecule.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The Krebs cycle occurs in which compartment of the mitochondria?
Answer: C. The Krebs cycle (TCA cycle) takes place in the mitochondrial matrix, as described in NCERT Class 11 Biology Chapter 12, page 158.
Why A is wrong: A is wrong because the outer membrane is permeable to small molecules via porins but hosts no TCA enzymes.
Why B is wrong: B is wrong because the intermembrane space is the region where protons accumulate during the ETC, not where the Krebs cycle runs.
Why D is wrong: D is wrong because the inner membrane is the site of the electron transport chain and ATP synthase, not the Krebs cycle enzymes.
What is the terminal electron acceptor in the mitochondrial electron transport chain?
Answer: C. Molecular oxygen is the terminal (final) electron acceptor in aerobic respiration. It combines with electrons and H⁺ to form water at Complex IV (NCERT Class 11 Biology Chapter 12).
Why A is wrong: A is wrong because NAD⁺ is an electron carrier that accepts electrons during glycolysis and the Krebs cycle to form NADH — it is not the terminal acceptor in the ETC.
Why B is wrong: B is wrong because FAD accepts electrons during the Krebs cycle to form FADH₂ — it donates to the ETC at Complex II, not the terminal acceptor.
Why D is wrong: D is wrong because cytochrome c is a mobile electron carrier between Complex III and Complex IV — it is an intermediate, not the terminal acceptor.
How many molecules of CO₂ are released per single turn of the Krebs cycle?
Answer: B. Each turn of the Krebs cycle releases 2 CO₂ molecules — one during isocitrate → α-ketoglutarate and one during α-ketoglutarate → succinyl-CoA (NCERT Class 11 Biology Chapter 12, pages 158–159).
Why A is wrong: A is wrong because only 1 CO₂ is released during oxidative decarboxylation of pyruvate (before the cycle), but within the cycle itself, 2 CO₂ are released per turn.
Why C is wrong: C is wrong — if you counted the CO₂ from pyruvate decarboxylation plus the 2 from the cycle, you get 3 per pyruvate, but per turn of the Krebs cycle alone, the count is 2.
Why D is wrong: D is wrong because 4 CO₂ would only arise if you counted both pyruvate decarboxylations plus both Krebs turns for one glucose molecule — the question asks per single turn.
FADH₂ produced in the Krebs cycle donates its electrons to the ETC at which complex?
Answer: B. FADH₂ donates electrons at Complex II (succinate dehydrogenase), bypassing Complex I entirely. This is why FADH₂ yields fewer ATP than NADH (NCERT Class 11 Biology Chapter 12).
Why A is wrong: A is wrong because Complex I is the entry point for NADH, not FADH₂. Confusing these two entry points is a frequent NEET distractor pattern.
Why C is wrong: C is wrong because Complex III receives electrons from ubiquinone (coenzyme Q) — it does not directly accept electrons from FADH₂.
Why D is wrong: D is wrong because Complex V (ATP synthase) uses the proton gradient to synthesise ATP — it does not accept electrons at all.
Per molecule of glucose, the Krebs cycle produces a net of:
Answer: B. Each turn of the Krebs cycle produces 3 NADH, 1 FADH₂, and 1 GTP. Two turns per glucose gives 6 NADH, 2 FADH₂, and 2 GTP. Note: pyruvate decarboxylation adds 2 more NADH but that step is not part of the cycle itself (NCERT Class 11 Biology Chapter 12).
Why A is wrong: A is wrong because it undercounts NADH — each turn of the Krebs cycle alone produces 3 NADH (not 1), so two turns give 6 NADH total.
Why C is wrong: C is wrong because 8 NADH would result from incorrectly adding the 2 NADH from pyruvate decarboxylation into the Krebs cycle count. Pyruvate decarboxylation is a separate linking step.
Why D is wrong: D is wrong because 3 NADH, 1 FADH₂ and 1 GTP is the yield of ONE turn of the cycle. One glucose gives two acetyl-CoA, so the cycle turns twice: 6 NADH, 2 FADH₂ and 2 GTP.
During the ETC, protons are pumped from the mitochondrial matrix into the intermembrane space. ATP synthesis driven by the return flow of these protons through ATP synthase is described by:
Answer: B. Peter Mitchell's chemiosmotic hypothesis explains that the proton-motive force (electrochemical gradient) across the inner mitochondrial membrane drives ATP synthesis through ATP synthase (NCERT Class 11 Biology Chapter 12).
Why A is wrong: A is wrong because substrate-level phosphorylation is ATP formation by direct transfer of a phosphate group from a substrate (as in glycolysis and one step of the Krebs cycle) — it does not involve a proton gradient.
Why C is wrong: C is wrong because oxidative decarboxylation refers to the removal of CO₂ with simultaneous oxidation (e.g., pyruvate → acetyl-CoA) — it describes a reaction type, not the ATP-synthesis mechanism.
Why D is wrong: D is wrong because photophosphorylation is the light-driven ATP synthesis in chloroplasts during photosynthesis, not in mitochondria during respiration.
Starting from one molecule of glucose, calculate the total number of NADH molecules produced across glycolysis (2 NADH), pyruvate decarboxylation, and the Krebs cycle.
Answer: B. Glycolysis: 2 NADH. Pyruvate decarboxylation (×2): 2 NADH. Krebs cycle (×2 turns, 3 NADH each): 6 NADH. Total: 2 + 2 + 6 = 10 NADH per glucose (NCERT Class 11 Biology Chapter 12).
Why A is wrong: A is wrong — 8 NADH results from omitting the 2 NADH produced during pyruvate decarboxylation (the linking step between glycolysis and the Krebs cycle).
Why C is wrong: C is wrong — 12 NADH overcounts, likely by adding an extra 2 NADH or by miscounting the Krebs cycle as producing 4 NADH per turn instead of 3.
Why D is wrong: D is wrong — 6 NADH accounts only for the Krebs cycle output (3 per turn × 2 turns) and omits glycolysis and pyruvate decarboxylation contributions entirely.
If the inner mitochondrial membrane were made freely permeable to protons (H⁺), what would be the immediate effect on ATP production via the ETC?
Answer: B. ATP synthase requires a proton-motive force (concentration gradient) to function. If the membrane becomes freely permeable, protons leak back without passing through ATP synthase, dissipating the gradient and halting oxidative phosphorylation (NCERT Class 11 Biology Chapter 12 — chemiosmotic hypothesis).
Why A is wrong: A is wrong because protons leaking freely across the membrane bypass ATP synthase entirely — they do not flow through it, so no ATP is synthesised from the gradient.
Why C is wrong: C is wrong because while substrate-level phosphorylation (glycolysis + Krebs) does continue independently, the question asks about ATP production via the ETC specifically, which would stop.
Why D is wrong: D is wrong because although electron transport may initially speed up (less back-pressure from the gradient), the uncoupling means this electron flow no longer drives ATP synthesis — energy is released as heat instead.
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Given
One molecule of glucose undergoes complete aerobic respiration. The following per-stage yields are provided:• Glycolysis: 2 ATP, 2 NADH• Pyruvate decarboxylation (×2): 2 NADH, 2 CO₂• Krebs cycle (×2 turns): 6 NADH, 2 FADH₂, 2 GTP, 4 CO₂
Required
Calculate the total CO₂ released and total reduced coenzymes (NADH + FADH₂) produced per glucose molecule.
Concept
Aerobic respiration releases CO₂ only during decarboxylation reactions — in pyruvate decarboxylation and two steps of the Krebs cycle. The ETC does not release CO₂. Reduced coenzymes accumulate across all three stages and feed into the ETC.
Formula
Total CO₂ = CO₂ from pyruvate decarboxylation + CO₂ from Krebs cycle
Total NADH = NADH(glycolysis) + NADH(pyruvate decarboxylation) + NADH(Krebs)
Total FADH₂ = FADH₂(Krebs)
Substitution
Total CO₂ = 2 + 4 = 6
Total NADH = 2 + 2 + 6 = 10
Total FADH₂ = 2
Calculation
CO₂: 2 (from 2 pyruvate → 2 acetyl-CoA) + 4 (from 2 turns of Krebs, 2 CO₂ per turn) = 6 CO₂
NADH: 2 + 2 + 6 = 10 NADH
FADH₂: 2 FADH₂
Note: the "2" in "2 turns" and "2 pyruvate" are exact counting integers derived from the stoichiometry of glucose (one glucose → two pyruvate) and do not involve approximation.
Final answer
Per glucose: 6 CO₂, 10 NADH, 2 FADH₂ produced in total across glycolysis, pyruvate decarboxylation, and the Krebs cycle.
Common trap
Conflating pyruvate decarboxylation with the Krebs cycle itself. Students who merge these two stages may report "8 NADH from the Krebs cycle" (incorrectly absorbing the 2 NADH from pyruvate decarboxylation) or report "only 4 CO₂" (omitting pyruvate decarboxylation CO₂). NEET distractors exploit this by offering 8 NADH or 4 CO₂ as tempting wrong options.
Similar NEET-style question
"How many molecules of FADH₂ are produced per glucose molecule during the Krebs cycle alone?" (Answer: 2 — one per turn, two turns per glucose.)
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Krebs (mitochondrial matrix): 1 acetyl-CoA → 3 NADH + 1 FADH2 + 1 GTP + 2 CO2. ETC (inner membrane): NADH/FADH2 → O2 via complexes I-IV; H+ pumped to intermembrane space; ATP synthase (complex V) makes ATP. Net per glucose: 36-38 ATP.
-- NCERT Class 11 Biology, Chapter 12, p. 158More in Plant Physiology: 5 exam traps and mistakes · 1 question pattern from its other lessons.
6 questions from NEET 2020, 2021, 2023, 2024, 2025. Answers verified against NTA official keys.
The complex II of mitochondrial electron transport chain is also known as
Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
Which of the following combinations is required for chemiosmosis?
Which of the following statements of incorrect?
The number of substrate level phosphorylations in one turn of citric acid cycle is :
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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