Definition · Plain-language
Krebs cycle
The Krebs cycle is a stage of aerobic respiration, taking place in the mitochondria, that releases energy and gives off carbon dioxide.
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A stage of aerobic respiration
The Krebs cycle is one of the main stages of aerobic respiration — the oxygen-using process by which cells release energy from glucose. It happens inside the mitochondria, the organelles often called the powerhouse of the cell. The cycle takes products formed when glucose is broken down and processes them through a repeating series of reactions, releasing energy step by step. It is named after the scientist Hans Krebs, who worked out its reactions, and is also known as the citric acid cycle.
Where it fits in respiration
Aerobic respiration happens in stages. First, in glycolysis, glucose is split into smaller molecules in the cytoplasm. Those products then enter the mitochondria and feed into the Krebs cycle. The Krebs cycle releases carbon dioxide, which the body removes as waste, and produces energy-carrying molecules. These carriers pass to a final stage, where most of the cell’s ATP — its usable energy — is actually made. So the Krebs cycle sits in the middle of aerobic respiration, between glycolysis and the final energy-releasing step.
Why it is a cycle
The Krebs cycle is called a cycle because it ends with the same molecule it began with, ready to start again. Each turn takes in a fresh fuel molecule, runs it through the reactions, gives off carbon dioxide and energy carriers, and regenerates the starting compound to accept the next fuel. This looping design lets the same set of molecules be used over and over, processing fuel continuously. Because it depends on oxygen further down the line, the Krebs cycle is part of aerobic respiration only and does not run when oxygen is absent.
Key facts
At a glance
- Definition: a stage of aerobic respiration that releases energy in a cycle
- Also called: the citric acid cycle
- Location: inside the mitochondria
- Comes after: glycolysis
- Gives off: carbon dioxide and energy-carrying molecules
- Needs: oxygen (it is part of aerobic respiration)
Common misconceptions
What people often get wrong
Often heard: The Krebs cycle directly makes most of the cell’s ATP.
Actually: The Krebs cycle releases energy carriers, but most ATP is made in the final stage of respiration that follows it. The cycle prepares the energy carriers; the last stage uses them to make the bulk of the ATP.
Often heard: The Krebs cycle can run without oxygen.
Actually: It is part of aerobic respiration and depends on oxygen further along the pathway. When oxygen is absent, the cycle stops and cells switch to anaerobic respiration, which does not use the Krebs cycle.
Often heard: The Krebs cycle is where glucose is first broken down.
Actually: Glucose is first split in glycolysis, in the cytoplasm, before the Krebs cycle. The cycle processes products of that breakdown inside the mitochondria; it does not act on whole glucose directly.
Going deeper
