cell respiration
Meaning of cell respiration: The controlled release of energy from food inside a cell, captured as ATP rather than lost all at once as heat.
Definition of cell respiration
Cell respiration is the set of reactions by which a cell breaks down fuel molecules and captures the released energy in a usable form, ATP. Sugars, fats and amino acids can all serve as fuel. In the presence of oxygen, the process runs to completion and yields carbon dioxide, water and a large amount of ATP. Without oxygen, far less energy is recovered.
What are the stages?
| Stage | Where it happens | What it produces |
|---|---|---|
| Glycolysis | Cell fluid | Splits glucose, small amount of ATP |
| Citric acid cycle | Mitochondria | Carbon dioxide and electron carriers |
| Electron transport chain | Mitochondrial membrane | The great majority of the ATP, plus water |
Only the first stage runs without oxygen. The other two take place inside mitochondria, which is why those structures are described as the power plants of the cell.
What does oxygen actually do?
It waits at the end of the line. Fuel breakdown strips electrons from food molecules, and those electrons are passed down a chain of proteins, releasing energy in small steps that is used to pump protons across a membrane. The protons flow back through a turbine-like enzyme that manufactures ATP. Oxygen collects the spent electrons at the end and combines with hydrogen to form water. With no oxygen to accept them, the chain backs up and stops. This is why aerobic metabolism halts within seconds of a blocked blood supply, and why tissue starved of oxygen dies.
What happens without oxygen?
Glycolysis continues and produces a small amount of ATP, but the cell must dispose of the electrons somehow, and it does so by converting pyruvate into lactate. This sustains a muscle during a short sprint and keeps a red blood cell alive permanently, since red cells have no mitochondria at all. The yield is a small fraction of what oxygen allows, so it cannot support the body for long, and lactate accumulates in the blood. A related consequence of running the chain constantly is the leakage of reactive fragments of oxygen, the source of oxidative stress, which the cell neutralizes with its own defenses.
Also known as
Used in a sentence
Tissue injury followed the loss of oxygen required for cell respiration.
Good to know
Cell respiration uses fats and amino acids as well as sugars, and only the first stage proceeds without oxygen, which is why tissue deprived of blood supply fails within minutes.
Word origin
Where “cell respiration” comes from
Respiration comes from the Latin respirare, to breathe again, from spirare, to breathe.