NAD, oxidation and glycolysis

Molecules (Interaction and interdependence) · Cell respiration · note 4 of 8

Spec C1.2.7, C1.2.8

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NAD, oxidation and glycolysisSpec C1.2.7, C1.2.8

In short

Oxidation is loss of electrons, so removing hydrogen with its electron from a substrate (dehydrogenation) oxidises it. NAD accepts the hydrogen and is reduced. Glycolysis, in the cytoplasm, converts glucose to two pyruvate by stepwise reactions: phosphorylation, lysis, oxidation and ATP formation, each catalysed by a different enzyme. Net yield is 2 ATP and 2 reduced NAD.

Oxidation, reduction and NAD

Oxidation is a process of electron loss; reduction is electron gain. When hydrogen (with its electron) is removed from a substrate, a reaction called dehydrogenation, the substrate has been oxidised. Redox reactions involve both: whenever one molecule is oxidised, another is reduced.

NAD (nicotinamide adenine dinucleotide) is the main carrier of hydrogen in cell respiration. It accepts hydrogen removed from substrates and is reduced, forming reduced NAD. Reduced NAD carries the hydrogen (and its energy) to the electron transport chain.

NAD + 2H → reduced NAD

Glycolysis

Glycolysis is the conversion of glucose (6C) to two molecules of pyruvate (3C). It takes place in the cytoplasm, does not need oxygen and is a linear pathway in which each step is catalysed by a different enzyme. It has four key stages:

Phosphorylation: two phosphate groups are added to glucose, using 2 ATP. This makes the sugar less stable and ready to split.
Lysis: the 6C sugar bisphosphate splits into two 3C triose phosphate molecules.
Oxidation: hydrogen is removed from each triose phosphate and accepted by NAD, giving 2 reduced NAD per glucose.
ATP formation: phosphate groups are transferred to ADP, forming 4 ATP per glucose, and two pyruvate are produced.

Yield of glycolysis per molecule of glucose
Product	Number
ATP used	2
ATP produced	4
Net ATP	2
Reduced NAD	2
Pyruvate	2

Glycolysis flowchart: glucose (6C) is phosphorylated using 2 ATP to hexose bisphosphate (6C), which splits (lysis) into two triose phosphate (3C); each is oxidised (NAD to reduced NAD) and forms 2 ATP, producing two pyruvate (3C); net per glucose 2 ATP and 2 reduced NAD, in the cytoplasm. — Glycolysis: phosphorylation, lysis, oxidation and ATP formation give a net yield of 2 ATP and 2 reduced NAD per glucose.

Exam tip:

You do not need the names of the intermediates, but you must say that each step is catalysed by a different enzyme and give the net yield: 2 ATP and 2 reduced NAD.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is ATP called the energy currency of the cell?

ATP is called the energy currency because it distributes energy within cells. It is soluble, stable without enzymes and stays inside the cell, and hydrolysis to ADP and phosphate releases enough energy for tasks such as active transport, building macromolecules and movement. It is quickly regenerated from ADP using energy from respiration.

What is the difference between cell respiration and gas exchange?

Cell respiration is a set of enzyme-catalysed reactions inside cells that produce ATP using energy from carbon compounds. Gas exchange is the diffusion of oxygen and carbon dioxide between an organism and its environment, for example in the lungs. Gas exchange supplies oxygen for aerobic respiration and removes the carbon dioxide it produces.

What are the differences between aerobic and anaerobic respiration?

Aerobic respiration needs oxygen and mitochondria, can use carbohydrates, lipids and amino acids, gives a large yield of ATP and produces carbon dioxide and water. Anaerobic respiration in humans needs no oxygen, happens only in the cytoplasm, uses only glucose, gives just 2 ATP per glucose and produces lactate.

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