Lesson 3 of 5

Running out of oxygen

Goal: Describe anaerobic respiration in humans.

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Children sprinting in a race on a school field

In a sprint, your muscles need more oxygen than the blood can bring. They release extra energy without oxygen. This is anaerobic respiration.

glucose → lactic acidA little energy is released.

  • Anaerobic means without oxygen. Muscle cells respire this way during hard exercise, on top of their aerobic respiration.
  • Lactic acid is the product. It builds up in the muscles, which ache and tire.
  • Less energy is released from each glucose molecule than in aerobic respiration.
A girl bent over with her hands on her knees, out of breath after a race
The oxygen debt. The race is over, but she is still breathing hard. The extra oxygen breaks down the lactic acid.

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Key words in this lesson

Respiration A chemical reaction in cells that releases energy from glucose

Breathing Moving air into and out of the lungs

Aerobic respiration Respiration that uses oxygen

Anaerobic respiration Respiration without oxygen

Glucose The sugar that cells break down in respiration

Oxygen The gas that aerobic respiration uses up

Lactic acid The product of anaerobic respiration in muscles

Oxygen debt The extra oxygen needed after hard exercise to break down lactic acid

Product A substance that is made in a chemical reaction

For teachers: AQA 3.9.3 · Pearson Edexcel 1.5