1When oxygen can't keep up
In a sprint, your muscles need energy faster than your blood can bring oxygen. So they also respire anaerobically: without oxygen.
anaerobic
Lesson 22.3 notes
Anaerobic respiration
Respiration without oxygen in hard exercise, lactic acid, the oxygen debt, and how aerobic and anaerobic respiration compare.
2The word summary
Anaerobic respiration in muscles: glucose → lactic acid. The glucose is only partly broken down, so much less energy is released than in aerobic respiration.
lactic acid
3Much less energy
Breaking glucose down completely, with oxygen, releases a lot of energy. Without oxygen, each glucose releases only a small fraction of that. It's a short-term top-up, not a replacement.
4Lactic acid builds up
Lactic acid builds up in the muscles. It makes them ache and tire, and soon they can't keep going at full speed. That's why a sprint can't last long.
ache
5The oxygen debt
After a sprint you keep breathing hard. The extra oxygen is used to break down the lactic acid. The amount of extra oxygen needed is called the oxygen debt.
oxygen debt
6Aerobic or anaerobic?
Aerobic: uses oxygen, makes carbon dioxide and water, releases a lot of energy, happens all the time. Anaerobic in muscles: no oxygen, makes lactic acid, releases much less, only in hard exercise.
7Other organisms
Some bacteria respire anaerobically, making lactic acid: that's how milk is turned into yoghurt. Yeast respires anaerobically too, but makes different products (next lesson).
bacteria
Explore: Sprint lab
Set the pace and run. Watch the oxygen supply fall behind, lactic acid build up, and the breathing rate stay high while the oxygen debt is paid off.
Common mistake: Anaerobic means without oxygen, not without breathing
In a sprint you are still breathing hard. Anaerobic respiration happens because oxygen can't reach the muscles fast enough to meet the demand, so the cells respire without it.
Remember
- Anaerobic respiration in muscles: glucose → lactic acid. It releases much less energy than aerobic respiration.
- It happens in hard exercise, when oxygen can't be supplied fast enough. Lactic acid builds up, making muscles ache and tire.
- Afterwards you keep breathing hard: the extra oxygen breaks the lactic acid down. This is the oxygen debt.
Going further at GCSE