Cell respiration — IB Diploma Biology HL
IB Biology C1.2: ATP, aerobic and anaerobic respiration, measuring respiration rate, and HL glycolysis, Krebs cycle, electron transport and chemiosmosis.
Revision notes
8 short notes, in the order of the specification. Each one in short:
ATP, adenosine triphosphate, is a nucleotide that distributes energy within cells. Hydrolysis of ATP to ADP and phosphate releases enough energy for many tasks, such as active transport, synthesis of macromolecules and movement of cells or chromosomes. Synthesising ATP from ADP and phosphate requires energy, which comes from cell respiration.
Cell respiration is a system for producing ATP within the cell using energy released from carbon compounds, mainly glucose and fatty acids. Aerobic respiration needs oxygen and mitochondria, gives a large ATP yield and produces carbon dioxide and water. Anaerobic respiration in humans happens in the cytoplasm without oxygen, uses only glucose, gives a small yield and produces lactate.
The rate of cell respiration can be measured as oxygen used or carbon dioxide produced per unit time, often per gram of organism. Variables that affect it include temperature, oxygen concentration and the availability of respiratory substrate. A simple respirometer with an alkali to absorb carbon dioxide measures oxygen uptake from the movement of a liquid drop.
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.
In anaerobic respiration, reduced NAD from glycolysis cannot pass hydrogen to the electron transport chain. In humans, pyruvate accepts the hydrogen and becomes lactate, regenerating NAD so glycolysis continues, with a net yield of 2 ATP per glucose. Yeast uses the same pathway except that pyruvate becomes ethanol and carbon dioxide, which are used in brewing and baking.
In the link reaction, pyruvate in the mitochondrial matrix is oxidised and decarboxylated, and the acetyl group is transferred by coenzyme A to the Krebs cycle. In the cycle, the acetyl group joins oxaloacetate (4C) to form citrate (6C). Two decarboxylations and four oxidations regenerate oxaloacetate, releasing carbon dioxide and producing reduced NAD and ATP.
Reduced NAD from glycolysis, the link reaction and the Krebs cycle passes a pair of electrons to the first carrier of the electron transport chain on the inner mitochondrial membrane. Electron flow releases energy that pumps protons into the intermembrane space. Protons diffuse back through ATP synthase, which phosphorylates ADP: chemiosmosis. Oxygen is the terminal electron acceptor, forming water.
Lipids give a higher yield of energy per gram than carbohydrates because they contain less oxygen and more oxidisable hydrogen and carbon. Fatty acids are broken into 2C acetyl groups that enter the Krebs cycle as acetyl CoA. Glycolysis and anaerobic respiration occur only if carbohydrate is the substrate, so fatty acids can be respired only aerobically.
Exam questions
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Quick check questions
What is the full name of ATP and what type of molecule is it?
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Adenosine triphosphate; a nucleotide.
Write the word equation for anaerobic respiration in humans.
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glucose → lactate
Why is soda lime used in a respirometer?
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It absorbs carbon dioxide, so the movement of the drop measures oxygen uptake.
HL only Name the two Krebs cycle intermediates you need to know and their number of carbons.
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Citrate (6C) and oxaloacetate (4C).
HL only What is the role of oxygen in aerobic respiration?
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It is the terminal electron acceptor: it accepts electrons and protons, forming water, so electrons keep flowing along the chain.
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.
What is chemiosmosis in cell respiration?
HL only Chemiosmosis is the diffusion of protons down their gradient through ATP synthase, which uses the energy released to phosphorylate ADP into ATP. In mitochondria, the electron transport chain pumps protons into the intermembrane space, and they flow back into the matrix through ATP synthase. Most ATP in aerobic respiration is made this way.
Why do lipids release more energy than carbohydrates?
HL only Lipids release more energy per gram because their molecules contain less oxygen and more oxidisable hydrogen and carbon than carbohydrates. Oxidising this extra hydrogen produces more reduced NAD and so more ATP. Fatty acids enter respiration as acetyl CoA, so they bypass glycolysis and can only be respired aerobically.
Written and checked against the IB Biology HL specification · Updated October 2026