The carbon you breathe out

Section 4 of 4

Each breath you breathe out carries carbon atoms that were once part of your food.

Follow the six carbon atoms of one glucose molecule. Glycolysis splits them three and three. As each pyruvate moves into a mitochondrion, it gives off one carbon atom as carbon dioxide. Each turn of the citric acid cycle gives off two carbon dioxide molecules, as many carbon atoms as the acetyl group brought in. All six carbon atoms leave the cell as carbon dioxide.

Where the six carbon atoms of glucose go A flow chart of carbon atoms, drawn as grey balls. At the top, glucose: a row of six carbon atoms. An arrow labelled glycolysis leads down to two pyruvate molecules of three carbon atoms each. An arrow leads down into a shaded region labelled mitochondrion. On the way, a side arrow gives off 2 CO₂, and two acetyl groups of two carbon atoms each are left. They go into a loop arrow labelled citric acid cycle, which gives off 4 CO₂. At the bottom: 6 carbons in, 6 CO₂ out. mitochondrion glucose glycolysis 2 pyruvate 2 CO₂ 2 acetyl groups 4 CO₂ citric acid cycle 6 carbons in, 6 CO₂ out
The six carbon atoms of one glucose molecule. Two leave as carbon dioxide as the pyruvate goes into the mitochondrion, and four more in two turns of the citric acid cycle.

The carbon dioxide dissolves in your blood, which carries it to your lungs, and you breathe it out. The oxygen you breathe in goes a different way. At the end of the steps it takes the hydrogen atoms collected from the food and becomes water. The oxygen atoms in the carbon dioxide come from the food itself and from water inside the cell.

Oxygen, carbon dioxide and water vapour in the air you breathe in and out, as a share of its molecules, at rest
GasBreathed inBreathed out
oxygen, O₂21%about 16%
carbon dioxide, CO₂0.04%about 4%
water vapour, H₂Oabout 1%5 to 6%

Where fat goes

Fat is broken down in the same mitochondria. Its fatty acid chains are cut two carbon atoms at a time into acetyl groups, which join the same citric acid cycle. In 2014 Ruben Meerman and Andrew Brown worked out where the atoms of lost body fat go. Using up 10 kg of fat takes 29 kg of oxygen and makes 28 kg of carbon dioxide and 11 kg of water. Of the 10 kg of fat, 8.4 kg leaves through your lungs as carbon dioxide, and 1.6 kg leaves as water.

The carbon in your food was carbon dioxide in the air not long ago. Plants took it in and built it into glucose with the energy of sunlight, and your breath returns it to the air.

Quick check

Your cells use the oxygen you breathe in. Which molecule do its atoms end up in?

That's the end of lesson 37

In this lesson you found out:

  • Why respiration gives out the same energy as burning glucose
  • Why cells break glucose down in many small steps
  • How ATP carries energy around a cell
  • Where the carbon dioxide you breathe out comes from
Start Lesson 38: Carbon dating All lessons