Biological oxidation and the electron transport chain
In mitochondria, electrons from NADH and FADH2 pass along four protein complexes to oxygen, which becomes water. The energy released is used to make ATP.
Biological oxidation in simple words
Oxidation is loss of electrons (or hydrogen); reduction is gain of electrons. The two always happen together. Biological oxidation is the step-by-step oxidation of food inside cells. Enzymes called dehydrogenases remove hydrogen from food molecules and load it onto the coenzymes NAD+ and FAD, giving NADH and FADH2 (their reduced, hydrogen-carrying forms). The electron transport chain (ETC), also called the respiratory chain, then passes these electrons in small steps to oxygen. Releasing energy in small packets lets the cell trap it as ATP (adenosine triphosphate) instead of losing it all as heat.
Flow of electrons along the chain
Electrons move from carriers with a more negative redox potential (NAD+/NADH, −0.32 V) to more positive ones (½O2/H2O, +0.82 V). Starred complexes pump protons (H+) out of the matrix.
- 1★NADH → FMN → Fe-S → CoQComplex I (NADH dehydrogenase) · pumps protons
- 2Succinate → FAD → Fe-S → CoQComplex II (succinate dehydrogenase) · FADH2 enters here · no proton pumping
- 3CoQH2 carries electrons to Complex IIICoenzyme Q (ubiquinone) · mobile, fat-soluble carrier in the membrane
- 4★CoQH2 → cyt b → Fe-S → cyt c1 → cyt cComplex III (cytochrome bc1 complex) · pumps protons
- 5Cytochrome c carries one electron at a time to Complex IVCytochrome c · mobile, water-soluble protein on the outer face of the inner membrane
- 6★cyt c → cyt a → cyt a3 (+ Cu) → ½O2 + 2H+ → H2OComplex IV (cytochrome c oxidase) · pumps protons · blocked by cyanide
★ one-way, controls the pace
The four complexes at a glance
| Complex | Name | Carriers inside | Pumps protons? |
|---|---|---|---|
| I | NADH dehydrogenase (NADH–CoQ reductase) | FMN, iron–sulphur (Fe-S) centres | Yes |
| II | Succinate dehydrogenase (succinate–CoQ reductase) | FAD, Fe-S centres | No |
| III | Cytochrome bc1 complex (CoQ–cytochrome c reductase) | Cytochrome b, Fe-S, cytochrome c1 | Yes |
| IV | Cytochrome c oxidase | Cytochromes a and a3, copper centres | Yes |
The chain is built from nutrients: niacin (B3) makes NAD+, riboflavin (B2) makes FMN and FAD, iron sits in the cytochromes and Fe-S centres, and copper is part of Complex IV. Coenzyme Q is made in the body. Iron deficiency causes tiredness partly because cytochromes, not just haemoglobin, fall.
10 marks: “Describe the electron transport chain with a neat diagram.” Draw the chain on the inner mitochondrial membrane, name Complexes I–IV with their carriers, show CoQ and cytochrome c, end at O2 → H2O and mark the proton-pumping complexes.
5 marks: “Write a note on cytochromes.” Haem proteins of the ETC (b, c1, c, a, a3) whose iron switches between Fe3+ and Fe2+ as it passes electrons. Cytochrome oxidase (a + a3) also contains copper.
2 marks: “What is the final electron acceptor of the respiratory chain?” Molecular oxygen. Complex IV passes electrons to O2, which joins with H+ to form water.
I, III and IV pump; II does not. That is why FADH2, which enters at Complex II, gives less ATP than NADH.