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Aerobic respiration: the overall reaction

Respiration, Muscles and the Internal Environment · Stages of respiration · note 1 of 5

Spec 7.1
A Level only (what this means)A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean
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Aerobic respiration: the overall reactionSpec 7.1

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.

Respiration transfers the chemical energy in a respiratory substrate (usually glucose) to ATP, the energy currency of the cell. Aerobic respiration needs oxygen. The overall reaction is shown below.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP and heat)

Two things happen to the substrate. Its carbon atoms are removed as carbon dioxide, which is a waste product (decarboxylation). Its hydrogen atoms are removed and picked up by the hydrogen carriers NAD and FAD, which become reduced NAD and reduced FAD (dehydrogenation). At the end of the process the hydrogen is reunited with atmospheric oxygen to make water, and this releases a large amount of energy that is used to make ATP.

Respiratory substrate
An organic molecule, such as glucose, that is oxidised in respiration to release energy.
Decarboxylation
Removal of carbon dioxide from a substrate molecule.
Dehydrogenation
Removal of hydrogen atoms from a substrate molecule, usually by an enzyme with NAD or FAD as the hydrogen acceptor.

Why respiration is a stepped process

Respiration is not one reaction but a series of small steps. Each step is controlled and catalysed by a specific intracellular enzyme, so the names of the individual enzymes are not required.

  • Energy is released in small, manageable amounts rather than one large burst.
  • More of the energy can be captured in ATP, and less is lost as heat that could damage the cell.
  • Each step can be controlled separately, so the cell can speed up or slow down respiration to match its needs.
  • Intermediate compounds can be diverted into other metabolic pathways.
Common mistake:

The carbon dioxide does not come from the oxygen you breathe in. It comes from the carbon in the substrate, removed by decarboxylation. Oxygen only accepts hydrogen at the end, forming water.

Flow diagram of aerobic respiration: glucose enters glycolysis in the cytoplasm (net 2 ATP), 2 pyruvate go to the link reaction (2 CO₂), 2 acetyl CoA to the Krebs cycle (4 CO₂, 2 ATP), and reduced NAD and reduced FAD carry hydrogen to oxidative phosphorylation on the inner mitochondrial membrane, which uses O₂, makes water and makes most of the ATP. (opens full size in a new tab)
The four stages of aerobic respiration for one glucose molecule. Reduced NAD and reduced FAD carry hydrogen to the electron transport chain.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

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.

All 4 questions on Stages of respiration