Every cell in your body runs on energy from food. To get it, your cells burn sugar, slowly and without a flame.
The reaction is called respiration. Its main fuel is glucose, C₆H₁₂O₆, the sugar your blood carries to every cell. Glucose reacts with the oxygen you breathe in, and the products are carbon dioxide and water.
This is the equation for burning glucose. Sugar held in a hot flame burns to the same carbon dioxide and water. As with methane on a gas stove, the carbon atoms end up in carbon dioxide and the hydrogen atoms in water, and the strong C=O and O–H bonds that form give out the energy.
| Atom | C₆H₁₂O₆ + 6 O₂ | 6 CO₂ + 6 H₂O |
|---|---|---|
| carbon | 6 | 6 |
| hydrogen | 12 | 6 × 2 = 12 |
| oxygen | 6 + 6 × 2 = 18 | 6 × 2 + 6 = 18 |
The same energy, given out gently
The total energy given out depends only on the molecules at the start and the molecules at the end. A gram of glucose gives out about 16 kJ, in a flame and in your cells alike. The difference is in how the energy comes out. A flame gives it all out at once, as heat and light, hot enough to char the sugar. Your cells stay at about 37 °C and give the energy out a little at a time, as the next section shows.