What is a good mnemonic for the electron transport chain?

Use "I-Q, 3-C, 4-O": complex I passes electrons to coenzyme Q, complex III passes them to cytochrome c and complex IV passes them to oxygen, which becomes water; complex II feeds electrons from succinate into Q alongside complex I. Add "4-0-4-2" for the protons complexes I to IV pump per electron pair, "Rotten Apples Cut Off ATP" for the inhibitors (rotenone, antimycin A, cyanide and CO, oligomycin) and "hot DATE" for the uncouplers DNP, aspirin and thermogenin.

What are the steps of the electron transport chain in order?

NADH gives two electrons to complex I, which passes them through FMN and iron-sulfur clusters to coenzyme Q and pumps 4 protons; succinate enters separately at complex II, which pumps none. Coenzyme Q carries the electrons to complex III, which releases 4 protons through the Q cycle and reduces cytochrome c. Cytochrome c carries single electrons to complex IV, which reduces oxygen to water and pumps 2 protons per pair. ATP synthase then uses the gradient to make ATP.

What is a mnemonic for electron transport chain inhibitors?

"Rotten Apples Cut Off ATP" lists the classic inhibitors in chain order: rotenone (and amytal) at complex I, antimycin A at complex III, cyanide and carbon monoxide at complex IV, and oligomycin at the F0 proton channel of ATP synthase. For complex IV, remember "4 for 4": cyanide, carbon monoxide, azide and hydrogen sulfide. All of these lower oxygen consumption and ATP synthesis; uncouplers, by contrast, raise oxygen consumption.

Why does FADH2 produce less ATP than NADH?

Electrons from FADH2 enter at coenzyme Q and skip complex I, one of the three proton pumps. Per electron pair, NADH moves about 10 protons (4 at complex I, 4 at complex III and 2 at complex IV), while FADH2 moves about 6. At roughly 4 protons per ATP delivered to the cytoplasm, that is about 2.5 ATP per NADH and 1.5 per FADH2.

What is the difference between an electron transport inhibitor and an uncoupler?

Inhibitors such as rotenone, antimycin A, cyanide and carbon monoxide stop electron flow, so the proton gradient, oxygen consumption and ATP synthesis all fall. Uncouplers such as 2,4-dinitrophenol, high-dose salicylates and thermogenin let protons leak back into the matrix, so electron flow and oxygen consumption rise while ATP synthesis falls and the energy is released as heat. Oligomycin is a third pattern: it blocks ATP synthase, so the gradient rises.

How does cyanide poisoning differ from carbon monoxide poisoning?

Cyanide inactivates cytochrome c oxidase, so tissues cannot use the oxygen delivered: severe lactic acidosis with oxygen-rich venous blood, treated with hydroxocobalamin or sodium nitrite plus sodium thiosulfate. Carbon monoxide displaces oxygen from hemoglobin, shifts the dissociation curve to the left and inhibits mitochondrial respiration; pulse oximetry reads falsely normal, so carboxyhemoglobin is measured by CO-oximetry, and treatment is 100% or hyperbaric oxygen. Smoke inhalation can cause both.

Electron Transport Chain Mnemonic: Order & Inhibitors

The core electron transport chain mnemonic is "I-Q, 3-C, 4-O": complex I hands electrons to coenzyme Q, complex III hands them to cytochrome c and complex IV hands them to oxygen, so the order is NADH → I → Q → III → cyt c → IV → O₂, with succinate (FADH₂) entering through complex II and joining at Q. Three more cover the rest: "Rotten Apples Cut Off ATP" for the classic inhibitors in chain order (rotenone at I, antimycin A at III, cyanide and carbon monoxide at IV, oligomycin at ATP synthase), "hot DATE" for the uncouplers (DNP, aspirin, thermogenin, with the energy lost as heat) and "4-0-4-2" for the protons complexes I to IV pump per electron pair (10 per NADH, about 2.5 ATP; 6 per FADH₂, about 1.5 ATP). Inhibitors stop electron flow, so oxygen use and ATP both fall; uncouplers let protons leak back into the matrix, so oxygen use rises, ATP falls and the energy is released as heat.

The ETC mnemonics at a glance

What you need to recall	Mnemonic	What it stands for
Order of the carriers	I-Q, 3-C, 4-O	NADH → I → Q → III → cyt c → IV → O₂; succinate → II → Q
Inhibitors, in chain order	Rotten Apples Cut Off ATP	Rotenone (I), Antimycin A (III), Cyanide and CO (IV), Oligomycin (ATP synthase)
Complex IV poisons	4 for 4	Cyanide, carbon monoxide, azide, hydrogen sulfide
Uncouplers	hot DATE	DNP, Aspirin, Thermogenin; Energy lost as heat
Protons pumped by complexes I, II, III, IV	4-0-4-2	10 H⁺ per NADH (about 2.5 ATP); 6 per FADH₂ (about 1.5 ATP)

Mnemonic 1: the order, "I-Q, 3-C, 4-O"

Each complex hands its electrons to the letter written after it:

I → Q: complex I passes electrons from NADH to coenzyme Q. Complex II does the same for succinate, so II → Q as well.
3 → C: complex III passes them from reduced coenzyme Q (ubiquinol) to cytochrome c.
4 → O: complex IV passes them to O₂, the final electron acceptor, forming water.

NADH → I → Q → III → cyt c → IV → O₂

Succinate (FADH₂) → II → Q → III → cyt c → IV → O₂

Complexes I and II are parallel entrances to coenzyme Q; electrons never flow from complex I into complex II.

Complex	Name	Electrons move	H⁺ released into intermembrane space per electron pair
I	NADH-Q oxidoreductase (NADH dehydrogenase)	NADH → Q	4
II	Succinate dehydrogenase, also a TCA cycle enzyme	Succinate (via FADH₂) → Q	0
Q	Coenzyme Q (ubiquinone), a hydrophobic carrier in the inner membrane	I or II → III	None
III	Q-cytochrome c oxidoreductase (cytochrome bc₁ complex)	Ubiquinol → cytochrome c	4 (Q cycle)
cyt c	Cytochrome c, a small soluble protein	III (cytochrome c₁) → IV	None
IV	Cytochrome c oxidase	Cytochrome c → O₂ → H₂O	2 (4 per O₂, which takes 4 electrons)
V	ATP synthase (F₀ proton channel + F₁)	Protons flow back into the matrix, driving ATP synthesis	None

Coenzyme Q10, named for the 10 five-carbon isoprene units in its tail, is built partly from farnesyl pyrophosphate, the same precursor that feeds sterol synthesis in the cholesterol synthesis pathway.

Mnemonic 2: the inhibitors, "Rotten Apples Cut Off ATP"

The classic inhibitors spell R-A-C-O in chain order (I, III, IV, then ATP synthase), and all of them cut ATP:

R: rotenONE blocks complex ONE, stopping electrons passing from its iron-sulfur clusters to ubiquinone. Amytal, a barbiturate sedative, blocks the same complex.
A: Antimycin A blocks the b in bc₁. It binds complex III at the Qi site and stops electron flow out of cytochrome bH, so the Q cycle stalls and electron flow on to cytochrome c stops.
C: Cyanide and CO choke Cytochrome c oxidase. Complex IV has "4 for 4" poisons: cyanide, carbon monoxide, azide and hydrogen sulfide.
O: Oligomycin plugs "F-O", the F₀ proton channel of ATP synthase.
Uncouplers are a "hot DATE": DNP, Aspirin (salicylates), Thermogenin (UCP1 in brown fat), with Energy lost as heat.

Site	Inhibitor	What still works
Complex I	Rotenone (a fish and insect poison); amytal	Succinate oxidation through complex II
Complex II	No classic clinical poison (the fungicide carboxin blocks it)	NADH oxidation through complex I
Complex III	Antimycin A	Nothing: NADH and succinate electrons both reach III through Q
Complex IV	Cyanide, carbon monoxide, azide, hydrogen sulfide	Nothing: it is the last stop before O₂
ATP synthase (F₀)	Oligomycin	Electron flow slows as the gradient builds

Inhibitors vs uncouplers

Class	Examples	Proton gradient	O₂ consumption	ATP made	Heat
Electron transport inhibitors	Rotenone, amytal (I); antimycin A (III); cyanide, CO, azide, H₂S (IV)	↓	↓	↓	—
ATP synthase inhibitor	Oligomycin (F₀)	↑	↓	↓	—
Uncouplers	2,4-DNP, high-dose salicylates, thermogenin	↓	↑	↓	↑

The trap is oligomycin: with F₀ plugged, protons cannot flow back, so the gradient rises, ATP synthesis stops and oxygen use falls, because electrons pass to O₂ only while ADP is being phosphorylated (respiratory control).

Mnemonic 3: the protons, "4-0-4-2"

Read the pumps in order, complexes I, II, III, IV: 4, 0, 4, 2 protons released into the intermembrane space for each pair of electrons. Think of complex II's pump as a "404 error: not found." Complex III's 4 come from the Q cycle, and complex IV's 2 are half of the 4 it pumps per O₂. (Berg's Biochemistry tallies the same 10 differently, as 4, 2 and 4 for complexes I, III and IV, by counting protons removed from the matrix: complex III's net 2, and complex IV's 2 pumped plus 2 used to make water.)

NADH: $4 + 4 + 2 = 10$ H⁺. At about 4 H⁺ per ATP delivered to the cytoplasm, $10 \div 4 = 2.5$ ATP per NADH.
FADH₂: $0 + 4 + 2 = 6$ H⁺, so $6 \div 4 = 1.5$ ATP per FADH₂.

FADH₂ yields less ATP because its electrons skip complex I, one of the three pumps. For where the NADH and FADH₂ come from, see the TCA cycle Q-bank breakdown and the β-oxidation overview.

Cytosolic NADH depends on its shuttle. The inner membrane is impermeable to NADH, so the electrons of glycolytic NADH, not NADH itself, are shuttled in. The glycerol 3-phosphate shuttle, prominent in muscle, hands the electrons to an FAD enzyme that reduces Q directly, so each cytosolic NADH is worth about 1.5 ATP; the malate–aspartate shuttle of heart and liver regenerates NADH inside the matrix, worth about 2.5. One glucose therefore yields about 30 to 32 ATP; the old figure of 36 is outdated.

Electron transport chain steps in order

NADH unloads at complex I. Two electrons pass from NADH to FMN, then through iron-sulfur clusters to ubiquinone, which becomes ubiquinol (QH₂); 4 H⁺ are pumped out of the matrix.
Succinate unloads at complex II. Succinate dehydrogenase oxidizes succinate in the TCA cycle; the FADH₂ never leaves the enzyme, and its electrons pass through iron-sulfur centers to Q. No protons are pumped.
Ubiquinol unloads at complex III. Cytochrome c accepts only one electron, so complex III splits each pair (the Q cycle): one electron goes via the Rieske iron-sulfur center and cytochrome c₁ to cytochrome c, the other via cytochrome b's two hemes to a second quinone. Per pair, 4 H⁺ go out to the intermembrane space and 2 are taken from the matrix.
Cytochrome c ferries single electrons from complex III to complex IV.
Complex IV reduces oxygen. Electrons flow from CuA to heme a, then to the heme a₃–CuB center, where 4 electrons and 4 matrix protons turn one O₂ into 2 H₂O; 4 more H⁺ are pumped per O₂.
ATP synthase spends the gradient. Protons flow back through F₀ and drive ATP synthesis in F₁: about 3 H⁺ per ATP, plus 1 more to bring ADP and phosphate in and send the ATP out to the cytoplasm, hence about 4 H⁺ per usable ATP.

Carrier	Prosthetic groups	Nutrient link
Complex I	FMN and 8 iron-sulfur clusters (2Fe-2S and 4Fe-4S)	Riboflavin (B₂) for FMN; iron
Complex II	FAD and iron-sulfur clusters	Riboflavin (B₂) for FAD; iron
Complex III	Hemes bL, bH and c₁; Rieske 2Fe-2S center	Iron
Cytochrome c	Heme	Iron
Complex IV	Hemes a and a₃; copper centers CuA and CuB	Iron, copper

More pathway reviews are on the biochemistry hub.

How the poisons show up in USMLE stems

Cyanide. The usual source is smoke from burning synthetic materials in house fires; others are prolonged sodium nitroprusside infusions, improperly processed cassava and apricot pits. Cyanide inactivates cytochrome oxidase, so tissues cannot use the oxygen delivered: gasping, tachycardia and hypertension, then loss of consciousness and convulsions within as little as 30 seconds. Expect lactic acidosis (a lactate of 8 mmol/L or more is a sensitive marker of toxic cyanide levels; after smoke inhalation, where carbon monoxide also raises lactate, the antidote labels use 10 mmol/L or more) and a narrow arteriovenous oxygen difference: venous blood stays oxygen-rich and may look bright red, and a central venous saturation above 90% is suggestive. Many people cannot smell its bitter-almond odor, so it is an unreliable clue.

Hydroxocobalamin, 5 g IV over 15 minutes in adults, binds cyanide as cyanocobalamin, which is excreted in the urine; it discolors skin and urine. It does not lower oxygen-carrying capacity, which matters when carbon monoxide poisoning coexists after smoke inhalation.
Sodium nitrite plus sodium thiosulfate: nitrite turns hemoglobin into methemoglobin, which binds cyanide in preference to cytochrome a₃, forming nontoxic cyanomethemoglobin; thiosulfate is the sulfur donor rhodanese uses to turn cyanide into thiocyanate, excreted in the urine. Nitrite can cause severe hypotension, and its label urges caution after smoke inhalation or CO poisoning.

Carbon monoxide. Sources are house fires, poorly vented vehicles and fuel-burning heaters, furnaces and stoves. CO displaces O₂ from hemoglobin, shifts the oxygen-hemoglobin dissociation curve to the left and inhibits mitochondrial respiration, binding complex IV too. Pulse oximetry reads falsely normal, so measure carboxyhemoglobin with CO-oximetry. Give 100% oxygen: the CO half-life falls from about 4.5 hours on room air to about 1.5 hours, and to about 20 minutes with hyperbaric oxygen at 3 atmospheres. Hyperbaric oxygen is considered for any loss of consciousness, altered mental status, life-threatening cardiopulmonary complications, ongoing chest pain or COHb above 25%. Neuropsychiatric problems can appear days to weeks later.

Hydrogen sulfide. Sewer, manure-pit and oil and gas workers are exposed. Like cyanide, it blocks cytochrome c oxidase. It smells of rotten eggs at low concentrations, but around 100 ppm it paralyzes the sense of smell, and it can cause sudden unconsciousness ("knockdown"). There is no antidote: remove the patient from the source and give oxygen and supportive care.

Salicylates. Aspirin uncouples oxidative phosphorylation and causes fever. It also stimulates the medullary respiratory center (a primary respiratory alkalosis) and independently causes a metabolic acidosis, a classic mixed acid-base disorder; early symptoms are nausea, vomiting, tinnitus and hyperventilation. Management includes activated charcoal, urine alkalinization with sodium bicarbonate and, for severe toxicity, hemodialysis.

2,4-Dinitrophenol (DNP). It is still taken for weight loss despite an FDA ban dating to 1938. It carries protons back across the inner membrane, so the gradient's energy is released as heat instead of ATP while oxygen consumption rises. Toxicity is hyperthermia, tachycardia, sweating and tachypnea, sometimes with yellow skin, sclerae and urine, and it can be fatal. There is no specific antidote; treatment is aggressive cooling and supportive care.

Thermogenin (UCP1). The physiologic uncoupler of brown (and beige) fat, activated by norepinephrine and long-chain fatty acids: it lets protons back into the matrix so that fuel oxidation produces heat instead of ATP (non-shivering thermogenesis). Human newborns use it, and adults keep cold-activated brown fat in the neck and upper chest.

When the chain is broken genetically

Mitochondrial DNA encodes 13 subunits of the respiratory chain; nuclear genes encode the rest:

Complex	Subunits encoded by mtDNA
I	7 (ND1–ND6 and ND4L)
II	None
III	1 (cytochrome b)
IV	3 (CO I–III)
V (ATP synthase)	2 (ATPase 6 and 8)

Inheritance: mtDNA variants are transmitted maternally. Cells can hold a mix of mutant and normal mtDNA (heteroplasmy), and a biochemical defect appears only once the mutant share passes a threshold.
Clues: ptosis, external ophthalmoplegia, proximal myopathy with exercise intolerance, cardiomyopathy, sensorineural deafness, optic atrophy, diabetes and lactic acidosis (a fasting lactate above 3 mmol/L supports the diagnosis).
Complex I mutations are the most frequent cause of mitochondrial disease. They include Leber hereditary optic neuropathy: subacute, painless, bilateral visual failure, about four times as common in males, with a median onset at 24 years.
Leigh syndrome (psychomotor regression, usually in the first year of life, with basal ganglia and brainstem lesions on MRI and high lactate) can come from more than 110 genes; complex I defects cause nearly a third of cases, and others affect complex IV, ATP synthase (MT-ATP6), the pyruvate dehydrogenase complex or coenzyme Q10. Most cases are autosomal recessive; about 20% are mitochondrial.
MELAS (stroke-like episodes before age 40 with seizures or dementia, plus ragged-red fibers or lactic acidosis) and MERRF (myoclonus, seizures, cerebellar ataxia and myopathy) complete the classic list.

How USMLE tests it: traps and look-alikes

Four exposure look-alikes:

Look-alike	Where it acts	Oxygen clue	Distinguishing clue	Treatment
Cyanide	Complex IV	Oxygen-rich, bright red venous blood; central venous saturation above 90%	Lactate ≥8 mmol/L (≥10 after smoke inhalation); fire, nitroprusside, cassava	Hydroxocobalamin, or nitrite plus thiosulfate
Carbon monoxide	Hemoglobin (left shift) and complex IV	Pulse oximetry falsely normal; COHb on CO-oximetry	Headache and nausea; fire, heater or exhaust	100% oxygen; hyperbaric if criteria met
Hydrogen sulfide	Complex IV	—	Rotten eggs, then loss of smell; collapse in a sewer or manure pit	Removal, oxygen, supportive care
Methemoglobinemia	Ferric (Fe³⁺) hemoglobin cannot bind O₂	Pulse oximetry about 85% at high levels	Chocolate-brown blood; dapsone, benzocaine, nitrites	Methylene blue above 20–30% or with symptoms

"Complex I → complex II" is wrong. I and II both feed coenzyme Q in parallel.
Succinate rescues a complex I block, not a III or IV block, because it enters below complex I but above complex III.
Cytochrome c has a second job: when Bcl-2 family control fails and it leaks into the cytosol, it binds Apaf-1 and activates caspase-9, which drives apoptosis.

Worked vignettes

Vignette 1: the fat burner. A 22-year-old man is brought to the emergency department after taking a "fat-burning" powder bought online for several days. Temperature is 41.2°C, pulse 148/min and respirations 32/min; he is drenched in sweat, and his sclerae and urine are yellow. Which change in his mitochondria best explains these findings?

Answer: Protons leak back across the inner mitochondrial membrane without passing through ATP synthase: the gradient collapses, oxygen consumption rises, ATP synthesis falls and the energy is released as heat. This is uncoupling by 2,4-dinitrophenol.

Why the distractors are wrong:

Inhibition of cytochrome c oxidase (cyanide): electron flow stops, so oxygen consumption falls; the picture is abrupt collapse with lactic acidosis, not rising heat production.
Inhibition of the F₀ channel of ATP synthase (oligomycin): the gradient rises and oxygen consumption falls.
Inhibition of complex I or III (rotenone, antimycin A): blocks electron flow and lowers oxygen consumption.

The same ideas are tested from the ATP synthase side in this oxidative phosphorylation Q-bank breakdown.

Vignette 2: the apartment fire. A 45-year-old woman is carried out of a burning apartment. She is confused; blood pressure is 84/50 mm Hg and pulse oximetry reads 98% on a nonrebreather mask. Arterial pH is 7.08, lactate 11 mmol/L, central venous oxygen saturation 92% and carboxyhemoglobin 18%. Besides 100% oxygen, which antidote is most appropriate?

Answer: Hydroxocobalamin, 5 g IV over 15 minutes.

Reasoning: A lactate this high with oxygen-rich venous blood means the tissues are not using the oxygen they receive: cyanide from burning synthetic materials has blocked complex IV (after smoke inhalation, the antidote labels treat a lactate of 10 mmol/L or more as a sign of cyanide poisoning). She also has CO poisoning and is hypotensive, so nitrite is a poor choice: it deliberately makes methemoglobin, can cause severe hypotension, and its label urges caution after smoke inhalation. Hydroxocobalamin binds cyanide without lowering oxygen-carrying capacity. Her confusion also makes her a candidate for hyperbaric oxygen.

Vignette 3: the bench experiment. Isolated mitochondria oxidizing an NADH-linked substrate consume oxygen steadily once ADP is added. Drug X stops oxygen consumption, and adding succinate restores it. Drug Y stops it too, but succinate does not help. Drug Z stops both oxygen consumption and ATP synthesis; adding 2,4-dinitrophenol then restores oxygen consumption but not ATP synthesis. Using the inhibitors above, identify X, Y and Z.

Answer: X blocks complex I (rotenone or amytal); Y blocks complex III or IV (antimycin A or cyanide); Z is oligomycin.

Reasoning: Succinate feeds Q through complex II, below a complex I block, so electron flow resumes; a block at III or IV sits below both entrances. Oligomycin stops respiration indirectly: with F₀ closed, protons cannot return, so electron flow stops (respiratory control). DNP gives protons another route back into the matrix, so electrons flow to O₂ again, but ATP synthase stays blocked.

Vignette 4: the family history. A 24-year-old man has lost vision painlessly in both eyes over 2 months. His mother's brother lost his sight the same way in his twenties; no paternal relative is affected. A mutation in which part of the electron transport chain is most likely?

Answer: Complex I (NADH-Q oxidoreductase): Leber hereditary optic neuropathy.

Reasoning: Subacute, painless, bilateral visual failure in a young man, inherited through the maternal line, is LHON, caused by complex I mutations. Complex II is never the answer to an mtDNA question: none of its subunits is mtDNA-encoded. He will not pass the variant to his children, because mtDNA is transmitted maternally.

Key takeaways

Order: NADH → I → Q → III → cyt c → IV → O₂; succinate/FADH₂ enters at II and joins at Q ("I-Q, 3-C, 4-O").
Inhibitors: "Rotten Apples Cut Off ATP": rotenone and amytal (I), antimycin A (III), cyanide, CO, azide and H₂S (IV, "4 for 4"), oligomycin (F₀ of ATP synthase).
Uncouplers ("hot DATE": DNP, aspirin, thermogenin): gradient down, O₂ use up, ATP down, heat up.
Protons: "4-0-4-2" for complexes I to IV; 10 H⁺ per NADH gives about 2.5 ATP, 6 H⁺ per FADH₂ about 1.5 ATP, and one glucose about 30–32 ATP depending on the shuttle.
Cyanide vs CO: oxygen-rich venous blood and lactic acidosis (hydroxocobalamin, or nitrite plus thiosulfate) vs falsely normal pulse oximetry (100% or hyperbaric oxygen).
mtDNA encodes 13 subunits of complexes I, III, IV and V and none of complex II; inheritance is maternal, with heteroplasmy.