12.2.612 · Energy and Respiration

Aerobic Respiration- Role of NAD and FAD — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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12.2.6 Aerobic Respiration: Role of NAD and FAD — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how NAD and FAD carry hydrogen in respiration and how many reduced coenzymes are made.Dehydrogenases — many respiratory enzymes remove hydrogen atoms from substra…

Regeneration
Cytoplasmic NAD
Entry points
Without oxygen
Energy in reduced forms
Coenzyme A

Part 1 of 8Coenzymes in respiration

Dehydrogenases

Dehydrogenases many respiratory enzymes remove hydrogen atoms from substrates and need coenzymes to accept them.

NAD

NAD nicotinamide adenine dinucleotide, derived from the vitamin niacin.

FAD

FAD flavin adenine dinucleotide, derived from riboflavin.

Nature

Nature both are non-protein organic molecules temporarily bound to the enzyme.

Hydrogen atom

Hydrogen atom it is a hydrogen ion plus an electron.

Part 2 of 8Reduction and oxidation

OIL RIG

Oxidation Is Loss, Reduction Is Gain.

Substrate

Substrate it is oxidised when a dehydrogenase removes two hydrogen atoms.

NAD

NAD it accepts a hydride and a proton is released: NAD + 2H → reduced NAD (NADH + H⁺).
- (one H and two electrons)

FAD

FAD it accepts both hydrogen atoms: FAD + 2H → reduced FAD (FADH₂).

Regeneration

Regeneration at the electron transport chain reduced coenzymes donate electrons and hydrogen and are oxidised, so NAD and FAD can be reused.

Part 3 of 8Reduced coenzymes made per glucose

Stage	Reduced NAD	Reduced FAD
Glycolysis	2	0
Link reaction	2	0
Krebs cycle	6	2
Total	10	2

Doubling

Doubling every stage is doubled because one glucose gives two triose phosphates and two pyruvates.

Cytoplasmic NAD

Cytoplasmic NAD reduced NAD from glycolysis passes its electrons into the mitochondrion by shuttle systems, with a small energy cost in some cells.

Part 4 of 8Why they are needed

Carriers

Carriers they carry hydrogen and electrons from glycolysis, the link reaction and Krebs cycle to the inner membrane where ATP is made.

Entry points

Entry points reduced NAD enters the electron transport chain at the first complex and reduced FAD at a later point.
Reduced NAD pumps more protons and gives about 2.5 ATP.
Reduced FAD gives about 1.5 ATP.

Recycling

Recycling only a small pool of NAD and FAD is needed because re-oxidation lets them accept more hydrogen.

Without oxygen

Without oxygen they cannot be re-oxidised, and cells re-oxidise NAD by fermentation to keep glycolysis going.

Energy in reduced forms

Energy in reduced forms one reduced NAD holds enough energy to pump about 10 protons.

Part 5 of 8Related coenzymes

1Coenzyme A

Coenzyme A it carries acetyl groups.

2NADP

NADP it carries hydrogen in photosynthesis, where reduced NADP is used to make triose phosphate.

3Pathways

NAD is mostly used in catabolic (breakdown) pathways and NADP in anabolic (building) pathways.

Part 6 of 8Redox in more detail

Redox reaction

Redox reaction oxidation and reduction always happen together.

Carriers in the chain

Carriers in the chain each is reduced when it gains electrons from the previous carrier and oxidised when it passes them on.

Redox potential

Redox potential each carrier is a slightly better electron acceptor than the one before,
- so electrons flow in one direction and release energy in small steps.

Oxygen

Oxygen it is the best acceptor and so is last.

Fat

Fat it gives lots of reduced NAD and FAD per carbon, so more energy per gram.

Part 7 of 8Redox indicators

Dyes

Dyes methylene blue and DCPIP change colour when reduced.

Method

Method mix yeast or mitochondria with a substrate and the dye; the dye takes the hydrogen instead of NAD.

Rate

Rate time to decolourise is a measure of respiration rate.

Part 8 of 8More from the specification

Coenzyme electron transport function

Coenzyme electron transport function reduced NAD and FAD transfer high-energy electrons to the mitochondrial respiratory chain, fulfilling vital transport function.

Roles of NAD and FAD in the Mitochondrial Matrix

In the mitochondrial matrix, dehydrogenation reactions in the link reaction and Krebs cycle reduce coenzymes NAD and FAD,
which transport high-energy electrons and protons to the cristae membrane.

Oxidative Phosphorylation and Water Formation

During oxidative phosphorylation, electrons travel down the electron transport chain to the terminal electron acceptor oxygen,
which combines with protons to form water as ATP is produced via chemiosmosis.

Anaerobic Respiration Pathways in Mammals and Yeast

In the absence of oxygen, glycolysis continues only if NAD+ is regenerated: mammalian muscle cells reduce pyruvate to lactate,
whereas yeast cells undergo alcoholic fermentation, decarboxylating pyruvate to produce ethanol and CO₂.

Effects of Temperature and Substrate Concentration on Respiration — The rate of aerobic respiration increases with temperature up to an optimum as respiratory enzyme collision frequency rises,

and increases with substrate concentration until mitochondrial transport proteins and enzymes become saturated.

Substrate concentration

The amount of substrate per unit volume.

Intermembrane space

The gap between the two membranes where protons accumulate.

Anaerobic

Without oxygen.

Phosphorylation

Adding a phosphate group.

Ethanol

Alcohol produced by yeast in anaerobic respiration.

Matrix

The solution inside the inner membrane containing Krebs cycle enzymes.

Lactate

The product of anaerobic respiration in human cells.

Anaerobic respiration

Respiration without oxygen, releasing much less energy per glucose molecule.

Oxidative phosphorylation

ATP production driven by the electron transport chain.

Key terms

Term	Meaning
NAD	A coenzyme that accepts hydrogen in respiration.
FAD	A coenzyme that accepts hydrogen in the Krebs cycle.
Reduction	Gain of electrons or hydrogen.
Oxidation	Loss of electrons or hydrogen.
Dehydrogenase	An enzyme that removes hydrogen from a substrate.
Redox reaction	A reaction in which oxidation and reduction occur together.

Avoid these mistakes

Saying FAD is used in glycolysis — it is used in only one Krebs cycle step.

Saying NAD is a protein enzyme — it is a non-protein coenzyme.

Saying reduced NAD is made only in the Krebs cycle — it is also made in glycolysis and the link reaction.

Forgetting to double the numbers for one glucose.

Last updated: 15 October 2026