Cramps

The idea

When muscle cells lack oxygen, pyruvate is converted into lactic acid, also a three-carbon molecule; its build-up in the muscles causes cramps.

A sudden sprint

You run as fast as you can for a short time.

What do you think can happen in your muscle cells?

• They may run short of oxygen
• They have too much oxygen
• They stop working for ever

Inside the muscle

Press Next step.

The muscle's other path

When a muscle cell does not get enough oxygen, it cannot break pyruvate down with oxygen. Instead the pyruvate is converted into lactic acid. Lactic acid is also a three-carbon molecule. If it builds up in the muscles during sudden activity, it causes cramps.

When muscle cells lack oxygen, pyruvate is converted into lactic acid, also a three-carbon molecule; its build-up in the muscles causes cramps.

Notes

When muscle cells lack oxygen, pyruvate becomes lactic acid (also three carbons); its build-up in muscles causes cramps.

Check yourself

When muscle cells lack oxygen, pyruvate is converted into ...

What does the build-up of lactic acid in the muscles cause?

How many carbon atoms does a molecule of lactic acid have?

Answer: 3

The book says lactic acid is also a three-carbon molecule.

When do muscle cells make lactic acid?

• ethanol and carbon dioxide. This happens in yeast during fermentation.
• lactic acid — correct. Yes!
• carbon dioxide and water. This happens in the mitochondria when oxygen is used.
• A lot more energy than aerobic respiration. The release of energy in the aerobic process is a lot greater than in the anaerobic process.
• Ethanol in the muscle. Ethanol is formed from pyruvate in yeast, not in muscle cells.
• Cramps — correct. Yes!
• When they lack oxygen, as in sudden activity — correct. Yes!
• When plenty of oxygen reaches them. With oxygen, pyruvate is broken down in the mitochondria into carbon dioxide and water.
• Only when glucose is not broken down at all. Glucose is first broken down into pyruvate in all cases; lactic acid is made from pyruvate.