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 14, page 270).
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.
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?
What is the terminal electron acceptor in the mitochondrial electron transport chain?
How many molecules of CO₂ are released per single turn of the Krebs cycle?
FADH₂ produced in the Krebs cycle donates its electrons to the ETC at which complex?
Per molecule of glucose, the Krebs cycle produces a net of:
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:
Starting from one molecule of glucose, calculate the total number of NADH molecules produced across glycolysis (2 NADH), pyruvate decarboxylation, and the Krebs cycle.
If the inner mitochondrial membrane were made freely permeable to protons (H⁺), what would be the immediate effect on ATP production via the ETC?
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.) ---
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, Ch. 14, p. 270These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
C3: 3-PGA (3-carbon) is first stable product. C4: oxaloacetate (4-carbon) is first stable product.
Question asks first stable product of CO2 fixation in C3/C4 plant.
Pathway name = carbons in first product. C3 → 3C; C4 → 4C.
Category: Negative Marking
Light reactions: water splits at PS II (P680) FIRST, electrons flow PS II → PS I (P700). PS II numbered AFTER PS I in discovery, but reaction order is PS II → PS I.
Question on Z-scheme order, water photolysis, electron source.
Discovery order ≠ reaction order. Photolysis at PS II; PS II reduces PS I.
Root cause: concept gap
Mnemonic: C3 → 3-PGA (3-carbon); C4 → OAA (4-carbon). Hatch-Slack pathway is C4.
Root cause: concept gap
Glycolysis is in CYTOPLASM (cytosol). Krebs cycle in mitochondrial matrix; ETC in inner membrane.
Root cause: term confusion
RuBisCO = ribulose-1,5-bisphosphate CARBOXYLASE/OXYGENASE. Carboxylase activity drives Calvin; oxygenase causes photorespiration.
38 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Which of the following statements about RuBisCO is true?
Auxin is used by gardeners to prepare weed-free lawns. But no damage is caused to grass as auxin
Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
The reaction centre in PS II has an absorption maxima at
Which micronutrient is required for splitting of water molecule during photosynthesis?
Which hormone promotes internode/petiole elongation in deep water rice?
Which of the following combinations is required for chemiosmosis?
Which one of the following plants does not show plasticity?
What amount of energy is released from glucose during lactic acid fermentation?
The first stable product of CO fixation in sorghum is 2
Which of the following statements is correct ? cambial ring
flÎÁh ∑§Ë ¬˝Á∑˝§ÿÊ •Áœ∑§Ã◊ Á∑§‚ ŒÊÒ⁄UÊŸ „Ê ÃË „Ò? 114. The process of growth is maximum during :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
photosynthesis pathway product confusion
C3 and C4 pathways share the Calvin cycle; students who cannot anchor the first stable product in each pathway invert the carbon counts: 3-PGA (3-carbon) is the C3 first stable product, OAA (4-carbon) is the C4 first stable product. RuBisCO has dual carboxylase and oxygenase activity; students who recall only one role select incorrect statements about its function, including denying that it causes photorespiration.
ps i ps ii assignment reversal
PS I and PS II are numbered in order of discovery, not order of action in the Z-scheme. PS II acts first (water photolysis, O2 release, P680 reaction centre at 680 nm); PS I acts second (P700 reaction centre at 700 nm, NADPH production). Students who use Roman numeral order assign water photolysis to PS I and attribute the 700 nm absorption maximum to PS II.
respiration compartment swap
Glycolysis feeds the mitochondrial Krebs cycle, creating a mental link: students place glycolysis in the mitochondrial matrix. The anchor -- glycolysis occurs in the cytosol because it predates mitochondria and operates in anaerobes -- is lost under time pressure.
rubisco function confusion
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) has dual activity: carboxylase drives CO2 fixation in the Calvin cycle; oxygenase drives photorespiration. Questions give multiple statements about RuBisCO and ask which set is correct; students who have not anchored the dual-function name omit or deny the oxygenase role, selecting option sets that describe it as carboxylase only.
assertion reason unchecked reason
A/R questions in plant physiology present a true Assertion (e.g., ATP is used at two steps in glycolysis) alongside a Reason that sounds mechanistically plausible but may be factually imprecise or not the correct explanation. Students accept the Reason without independently checking whether its specific biochemical claim is accurate.
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