Respiration in Plants · Lesson 5 of 9
The Mitochondrial Electron Transport System
“Complexes I–IV, NADH, FAD-linked complex II, ubiquinone, cytochrome c, proton pumping and oxygen.”
• Trace electron flow from NADH and FAD-linked reactions through mitochondrial complexes. • Identify the locations and roles of complexes I, II, III and IV. • Explain how ubiquinone and cytochrome c carry electrons. • Distinguish proton-pumping complexes from complex II. • Explain why oxygen is indispensable as the final electron acceptor.
Where the energy in NADH and FADH₂ goes
After glucose is split and its pyruvate is oxidised, a large portion of the conserved chemical energy resides in reduced electron carriers rather than directly in ATP. NADH and FAD-linked reducing equivalents can release electrons through a series of carriers in the inner mitochondrial membrane. As electrons move to progressively more favourable acceptors, the respiratory system harnesses some of their energy to move protons out of the matrix. The resulting separation of charge and concentration stores potential energy.
The electron transport system (ETS) is not a single molecule that converts NADH directly into ATP. It is an ordered network of protein complexes, smaller mobile carriers and a final oxygen-reduction step. ATP synthase is a separate membrane machine that uses the proton gradient produced by electron transport. Understanding the separation of these two jobs explains oxidative phosphorylation.
A chain of inner-mitochondrial-membrane electron carriers that transfers electrons from reduced cofactors ultimately to oxygen.
An electrochemical gradient across the inner mitochondrial membrane created by proton movement and membrane potential.
Inner mitochondrial membrane and two compartments
The mitochondrion has an outer membrane and a highly folded inner membrane surrounding the matrix. The intermembrane space lies between the membranes. Electron transport complexes are embedded in the inner membrane, while many reactions making NADH occur in the matrix. When complexes pump protons, they move from matrix to intermembrane-space side, establishing a gradient. A useful diagram should show these two compartments clearly; otherwise proton direction can be misread.
The outer membrane is comparatively permeable to many small molecules, but the inner membrane restricts free proton movement. This resistance to unrestricted H⁺ flow permits the system to accumulate a gradient that can later drive ATP synthesis.
Complex I: electrons enter from NADH
A membrane complex that transfers electrons from NADH to ubiquinone and contributes to pumping protons across the inner membrane.
Complex I accepts electrons from NADH at its matrix-facing part. NADH becomes oxidised to NAD⁺, replenishing the electron-acceptor pool used in the TCA cycle and other metabolism. Electrons travel through internal cofactors and ultimately reduce ubiquinone, a mobile lipid-soluble carrier. Coupled to this transfer, complex I pumps protons to the intermembrane space. For a standard mammalian-like schematic, about four H⁺ are translocated per pair of electrons entering from NADH, though energy coupling in different organisms can vary.
Complex II: a second entry route
A membrane-associated enzyme linking the TCA succinate-to-fumarate reaction with electron transfer to ubiquinone.
Complex II oxidises succinate to fumarate as part of the TCA cycle and transfers its electrons through a bound FAD-containing system to ubiquinone. This is the source's FADH₂-linked entry route. Unlike complexes I, III and IV, complex II does not pump protons across the inner membrane. Therefore electrons arriving through complex II contribute less to the proton gradient and generally produce fewer ATP molecules than electrons from NADH that enter complex I.
The phrase 'FADH₂ enters complex II' is often used in simplified teaching. More precisely, FAD is bound to succinate dehydrogenase; it is reduced and reoxidised locally as electrons are passed to other carriers. Keeping this nuance in mind avoids thinking free FADH₂ always travels independently through the cytosol to complex II.
Ubiquinone: mobile carrier within membrane
A lipid-soluble electron carrier that collects electrons from different respiratory entry routes and transfers them to complex III.
Ubiquinone can accept electrons and protons to become reduced ubiquinol. It diffuses within the lipid-rich inner membrane and brings reducing power from complex I, complex II and certain other dehydrogenases toward complex III. Its mobility and location are important: it connects several entry pathways without requiring every metabolic enzyme to contact the next large membrane complex directly.
Complex III and cytochrome c
Complex III, often called the cytochrome bc₁ complex, transfers electrons from reduced coenzyme Q to cytochrome c. Through a mechanism called the Q cycle, electron flow contributes to movement of protons toward the intermembrane space. A common simplified stoichiometry attributes four net H⁺ translocated per electron pair at complex III under standard conditions. This is one of the gradient-forming steps.
A small mobile haem-containing protein on the outer (intermembrane-space-facing) side of the inner mitochondrial membrane that transports single electrons to complex IV.
Cytochrome c accepts one electron at a time and carries it between complexes III and IV. It is not ATP synthase and does not independently create the entire proton gradient. Its iron-containing haem group participates in reversible oxidation and reduction, permitting repeated electron transfer.
Complex IV transfers electrons to oxygen
The terminal respiratory complex that accepts electrons from cytochrome c and reduces molecular oxygen to water.
Complex IV contains cytochrome haem groups and copper centres. It collects electrons and transfers them to molecular oxygen. Oxygen combines with electrons and protons to form water. This is why O₂ is described as the ultimate electron acceptor of aerobic respiration. Complex IV also helps build the proton gradient by pumping protons while oxygen reduction consumes matrix protons in chemical water formation.
Without oxygen as terminal acceptor, electrons cannot continue flowing through the usual chain. Upstream electron carriers remain reduced, and reoxidation of NADH slows. Glycolysis may continue through fermentation if NAD⁺ is regenerated, but typical oxygen-supported mitochondrial ATP production falls severely.
Read the entire transport route
Why NADH and FAD-linked electron entry give different yields
| Entry route | Complex sequence | Proton pumping encountered | Approximate ATP equivalent |
|---|---|---|---|
| NADH | I → CoQ → III → cytochrome c → IV | I, III, IV | About 2.5 ATP / NADH in modern simplified estimates |
| FAD-linked complex II | II → CoQ → III → cytochrome c → IV | III, IV; not II | About 1.5 ATP / FADH₂-equivalent in modern simplified estimates |
The source uses an older accounting convention of 3 ATP per NADH and 2 ATP per FADH₂. Modern estimates commonly use about 2.5 and 1.5 ATP respectively when proton stoichiometry and phosphate/nucleotide transport costs are considered. These should not be mixed midway through a single balance-sheet calculation. The next lesson explains why the source's 38 ATP is a theoretical idealisation, whereas real ATP yields vary.
Oxygen is at the end, but it controls the whole system
Oxygen does not have to take part directly in glycolysis or each TCA chemical step. Yet its ability to accept terminal electrons is essential for reoxidising carriers and maintaining sustained oxidative metabolism. This dependency links cell-level oxygen supply with pathway-level redox chemistry. Under oxygen limitation, the decline in electron transport affects carrier availability and cellular ATP production long before the leaf or root completely stops using glucose.
The ETS also differs from photosynthetic electron transport. In respiration, energy released by oxidation of NADH and other reduced substrates drives proton-gradient formation. In chloroplast photophosphorylation, absorbed light supplies much of the energy needed to build a proton gradient. The organelles, electron donors and energy sources differ even though both exploit chemiosmosis.
Problem
A diagram labels complexes I–IV and shows three H⁺ pumping arrows. A student places one over complex II instead of complex IV. Correct the picture and explain the ATP consequence.
- 1.Complexes I, III and IV pump protons in the standard respiratory chain; complex II does not.
- 2.Electrons entering via complex II bypass the proton-pumping contribution of complex I.
- 3.Thus FAD-linked electrons generally support less ATP production than NADH electrons entering complex I.
Quick Quiz
Quiz
Where is the respiratory ETS located in a typical plant mitochondrion?
Which complex directly accepts NADH electrons?
Which complex is linked to succinate oxidation?
Which of the following does not directly pump protons?
Which mobile carrier moves in the inner-membrane lipid phase?
What transfers electrons from complex III to IV?
What is oxygen's direct role at the terminal respiratory complex?
Why do FAD-linked electrons yield less ATP than NADH electrons in typical estimates?
Practice Problems
- Trace the main electron-transfer route from NADH to oxygen.
- Explain the route from succinate-derived electrons and how it differs.
- State the functions and locations of ubiquinone and cytochrome c.
- Explain why oxygen deprivation indirectly slows the TCA cycle.
- Compare oxidative respiratory electron transport with photosynthetic electron transport.
NADH → complex I → CoQ/ubiquinol → complex III → cytochrome c → complex IV → oxygen, producing water. Complexes I, III and IV pump H⁺.
Key Takeaways
• The mitochondrial respiratory ETS is embedded in the inner membrane. • NADH enters at complex I; succinate/FAD-linked electrons enter at complex II. • Ubiquinone joins electron streams from I and II and carries electrons to complex III. • Cytochrome c carries electrons between complexes III and IV. • Complexes I, III and IV pump protons; complex II does not. • O₂ is reduced to water at complex IV and enables sustained carrier reoxidation. • NADH-linked electrons usually support more proton pumping and ATP synthesis than FAD-linked electrons.