Stages of respiration — Edexcel International A Level Biology
Glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, plus anaerobic respiration and what happens to lactate.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Aerobic respiration splits a respiratory substrate such as glucose, releasing carbon dioxide as a waste product and reuniting the hydrogen removed with atmospheric oxygen to form water, releasing large amounts of energy. It is a stepped process: each step is controlled and catalysed by a specific intracellular enzyme.
Glycolysis is the first stage of both aerobic and anaerobic respiration and takes place in the cytoplasm. A hexose is phosphorylated using ATP, split into two triose phosphates and oxidised to two pyruvate, making a net gain of two ATP by substrate-level phosphorylation and two reduced NAD.
The link reaction and the Krebs cycle take place in the mitochondrial matrix and completely oxidise glucose. Pyruvate is decarboxylated and dehydrogenated, then acetyl groups enter the Krebs cycle, which releases carbon dioxide, makes ATP by substrate-level phosphorylation and produces reduced NAD and reduced FAD by dehydrogenation.
Oxidative phosphorylation makes ATP using energy released as electrons pass along the electron transport chain on the inner mitochondrial membrane. This energy pumps protons into the intermembrane space; protons flow back through ATP synthase by chemiosmosis, driving ADP and phosphate to ATP. Oxygen is the final electron acceptor.
In animals, anaerobic respiration is glycolysis followed by the conversion of pyruvate to lactate, which regenerates the NAD that glycolysis needs. After the period of anaerobic respiration, lactate is carried in the blood to the liver, where it is converted back to pyruvate and oxidised or converted to glycogen.
Exam questions
8 exam-style questions (28 marks), each with its mark scheme.
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14 cards: flip them, mark what you knew, and practise the rest.
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The whole of respiration, muscles and the internal environment on one page, so you can see where this subtopic fits.
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Quick check questions
In which part of the cell does glycolysis take place?
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In the cytoplasm.
How many ATP molecules are made by glycolysis, net, per glucose molecule?
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Two (four are made, two are used).
What are the products of the link reaction?
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Acetyl CoA, carbon dioxide and reduced NAD.
What is the role of oxygen in aerobic respiration?
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It is the final electron acceptor, combining with electrons and protons to form water.
Why does glycolysis stop if pyruvate cannot be converted to lactate in anaerobic conditions?
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Reduced NAD would not be oxidised, so NAD would not be regenerated for glycolysis.
Frequently asked questions
What are the stages of aerobic respiration in order?
The stages are glycolysis in the cytoplasm, the link reaction and the Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane. Glucose is split, carbon is lost as carbon dioxide, hydrogen is carried by reduced NAD and reduced FAD, and ATP is made.
Where does the Krebs cycle take place?
The Krebs cycle takes place in the matrix of the mitochondrion. The link reaction also happens in the matrix, whereas glycolysis happens in the cytoplasm. Oxidative phosphorylation takes place on the inner mitochondrial membrane, which is folded into cristae to give a large surface area.
What is chemiosmosis in respiration?
Chemiosmosis is the movement of protons down their concentration gradient through ATP synthase. The electron transport chain pumps protons from the matrix into the intermembrane space. When they flow back through ATP synthase, the energy released is used to join ADP and inorganic phosphate to make ATP.
Why is lactate produced during anaerobic respiration?
Lactate is made so that NAD is regenerated. Without oxygen the electron transport chain stops, so reduced NAD cannot be oxidised. Pyruvate accepts the hydrogen from reduced NAD and becomes lactate, which allows glycolysis to continue making a small amount of ATP.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026