3.2.4Inquiry Process
Oxidation and Reduction — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 110 of 160
Section 1 of 4Redox Reactions
Electrons move from one substance to another in redox reactions, and this movement transfers energy.
Oxidation
- The loss of electrons from a substance;
- in respiration it is often seen as the removal of hydrogen atoms from a molecule.
Reduction
- The gain of electrons by a substance; in respiration it often involves the addition of hydrogen atoms.
Paired process
- Oxidation and reduction always occur together, because electrons removed from one substance must be accepted by another.
Energy release
- Oxidised molecules lose energy and reduced molecules gain it, so reduced molecules can act as energy carriers.
Section 2 of 4Coenzymes NAD and FAD
NAD
- A coenzyme that accepts hydrogen removed from substrates, taking two electrons and one proton to form reduced NAD (NADH),
- while a second proton stays in solution.
FAD
- A coenzyme in the Krebs cycle that accepts two hydrogen atoms to form reduced FAD, written FADH₂.
Dehydrogenase enzymes
- Enzymes that remove hydrogen from substrates in glycolysis, the link reaction and Krebs cycle, passing it to NAD or FAD.
Carriers
- Reduced NAD and reduced FAD carry electrons to the electron transport chain, where they are re-oxidised.
Recycling
- After releasing electrons, NAD⁺ and FAD return to the earlier stages, so the small pool of coenzymes can be reused.
| Coenzyme | Oxidised form | Reduced form | Where reduced |
|---|---|---|---|
| NAD | NAD⁺ | NADH + H⁺ | Glycolysis, link reaction, Krebs cycle |
| FAD | FAD | FADH₂ | Krebs cycle |
Section 3 of 4Redox in Respiration
1
Glucose is oxidised in stages, with hydrogen removed and passed to NAD and FAD, which are reduced.
2
Carbon dioxide is released in the link reaction and Krebs cycle as carbon-containing compounds are oxidised.
3
Reduced NAD and reduced FAD release electrons to the electron transport chain in the inner mitochondrial membrane and are oxidised.
4
Oxygen accepts the electrons at the end of the chain and is reduced to water.
5
The whole process transfers energy from glucose to ATP in small, controlled steps instead of releasing it all at once as heat.
Section 4 of 4Exam Checklist & Tips
3.2.4 – Key Points for the Exam
- Oxidation Loss of electrons or hydrogen from a substance.
- Reduction Gain of electrons or hydrogen by a substance.
- NAD A coenzyme that carries hydrogen in respiration.
- FAD A coenzyme that carries hydrogen from the Krebs cycle.
- Dehydrogenase An enzyme that removes hydrogen from a substrate.
Common Mistakes
- Writing that oxidation means gain of oxygen only, ignoring the loss of electrons or hydrogen.
- Saying NAD is used up; it is regenerated when reduced NAD is oxidised.
- Treating oxidation and reduction as separate events, when one cannot happen without the other.
- Forgetting that oxygen is itself reduced to water at the end of the chain.
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