The electron transport system (ETS) has five protein complexes, all located on the inner mitochondrial membrane. Complex I is NADH dehydrogenase, Complex II is succinate dehydrogenase, Complex III is cytochrome bc1, Complex IV is cytochrome c oxidase, and Complex V is ATP synthase. Memory hook: "Never Skip Cyto-Cyto ATP" = NADH dehydrogenase, Succinate dehydrogenase, Cytochrome bc1, Cytochrome c oxidase, ATP synthase.
The five ETS complexes sit on the inner mitochondrial membrane. Complexes I and II feed electrons to mobile carrier ubiquinone (Q); Complex III passes them via cytochrome c to Complex IV, which reduces O2 to water. Complex V (ATP synthase, F0 + F1) uses the proton gradient to make ATP.
Your doubts, answered
What are the five complexes of the electron transport system?
Complex I = NADH dehydrogenase (oxidises NADH). Complex II = succinate dehydrogenase (feeds electrons from FADH2). Complex III = cytochrome bc1 complex. Complex IV = cytochrome c oxidase complex. Complex V = ATP synthase (makes ATP). NEET asks their names and order directly, so learn all five.
Where exactly are the ETS complexes located?
All five complexes are embedded in the inner mitochondrial membrane. NCERT states clearly that the ETS 'is present in the inner mitochondrial membrane.' The Krebs cycle and pyruvate oxidation happen in the matrix, so the matrix produces the NADH and FADH2, and the inner membrane then uses them.
Is Complex II part of the Krebs cycle or the ETS?
Both. Complex II is succinate dehydrogenase. In the Krebs cycle it oxidises succinate to fumarate, and it sits in the inner membrane as an ETS complex that passes electrons (via FADH2) to ubiquinone. This overlap is why NCERT calls it the link between the citric acid cycle and the chain, and it is a common NEET trap.
What is the role of Complex V (ATP synthase)?
Complex V does not carry electrons. It uses the proton gradient built by complexes I, III and IV to make ATP. It has two parts: F0, an integral membrane channel through which protons cross, and F1, the headpiece that joins ADP and inorganic phosphate into ATP. For each ATP, 4 protons pass through F0 from the intermembrane space to the matrix.
What carriers move electrons between the complexes?
Two mobile carriers connect the fixed complexes. Ubiquinone (coenzyme Q) is inside the membrane and receives electrons from both Complex I and Complex II, then passes them to Complex III. Cytochrome c is a small protein on the outer surface of the inner membrane that carries electrons from Complex III to Complex IV. They are carriers, not numbered complexes.
⚠️ The NEET trap ✗ Complex II (succinate dehydrogenase) is only part of the Krebs cycle, and oxidative phosphorylation happens on the outer mitochondrial membrane. ✓ Complex II belongs to BOTH the Krebs cycle and the ETS, and all ETS complexes plus oxidative phosphorylation are on the INNER mitochondrial membrane, not the outer. 🧠 If a NEET option says 'oxidative phosphorylation on outer membrane', it is wrong every time. ETS = inner membrane. Complex II is the shared bridge, not Krebs-only.
Real NEET questions
NEET 2019
Where is respiratory electron transport system (ETS) located in plants?
A · Mitochondrial matrix
B · Outer mitochondrial membrane
C · Inner mitochondrial membrane ✓
D · Intermembrane space
Solution: The ETS, the chain of carriers (complexes I to V) that passes electrons from NADH/FADH2 to O2, is embedded in the inner mitochondrial membrane. The matrix runs the TCA cycle, while the outer membrane and intermembrane space do not house the ETS complexes.
NEET 2025
The complex II of mitochondrial electron transport chain is also known as
A · Cytochrome c oxidase
B · NADH dehydrogenase
C · Cytochrome bc1
D · Succinate dehydrogenase ✓
Solution: Complex II is succinate dehydrogenase. It oxidises succinate to fumarate in the TCA cycle and feeds reducing equivalents via FADH2 to ubiquinone. The distractors are the other complexes: NADH dehydrogenase = I, cytochrome bc1 = III, cytochrome c oxidase = IV.
NEET 2026
Match List I (Process) with List II (Location): A. Glycolysis B. ETS C. Accumulation of protons D. Krebs' cycle | I. Inner mitochondrial membrane II. Mitochondrial matrix III. Cytoplasm IV. Intermembrane space
Five. Complexes I to IV pass electrons, and Complex V (ATP synthase) makes ATP. All are on the inner mitochondrial membrane.
Which complex receives electrons from FADH2?
Complex II (succinate dehydrogenase). FADH2 is generated during oxidation of succinate, and Complex II passes those electrons to ubiquinone.
Are ubiquinone and cytochrome c complexes?
No. They are mobile electron carriers, not numbered complexes. Ubiquinone (coenzyme Q) shuttles electrons to Complex III, and cytochrome c shuttles from Complex III to Complex IV.
What are F0 and F1 in Complex V?
F0 is the integral membrane channel that lets protons cross the inner membrane. F1 is the headpiece that synthesises ATP from ADP and inorganic phosphate. Together they form ATP synthase, Complex V.
Why does NEET keep asking about ETS location?
Because matching-type questions test whether you separate cytoplasm (glycolysis), matrix (Krebs), inner membrane (ETS) and intermembrane space (proton accumulation). One location error loses the whole match mark.
