What is respiration?
Respiration is the chemical reaction that releases energy from glucose in every living cell. The energy released is used for muscle contraction, active transport, growth, cell division and keeping warm. Respiration is not the same as breathing; breathing (ventilation) simply moves air in and out.
Key idea
Aerobic respiration (with oxygen):
glucose + oxygen → carbon dioxide + water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
This releases a large amount of energy.
Aerobic respiration
Aerobic respiration uses oxygen to break down glucose completely, releasing a large amount of energy and producing carbon dioxide and water as waste. Most of the time our cells respire aerobically because it provides the most energy per glucose molecule.
Anaerobic respiration
Anaerobic respiration releases energy from glucose without oxygen, but releases much less energy. In human muscle during hard exercise:
glucose → lactic acid. In yeast (used in brewing and baking):
glucose → alcohol (ethanol) + carbon dioxide.
Worked example
A sprinter's muscles cannot get oxygen fast enough, so they respire anaerobically, producing lactic acid. This builds up an oxygen debt. After the race the runner keeps breathing deeply to supply extra oxygen that breaks down the lactic acid.
Remember
- Aerobic = with oxygen, more energy, products CO₂ + water.
- Anaerobic in muscle → lactic acid; in yeast → ethanol + CO₂.
- Anaerobic respiration releases much less energy than aerobic.