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What you need to know

Reflections and Exam tips




All living things need energy for life processes (remember MRS GREN), this energy comes from respiration.

Respiration is the process which releases energy from glucose. It happens in the cytoplasm, inside mitochondria.

Mitochondria diagram

If oxygen is present, it is called aerobic respiration (with oxygen). Equation for aerobic respiration:

Glucose + Oxygen —› Carbon Dioxide + Water + Energy

Unlike what this equation shows, respiration happens in many small steps, each controlled by an enzyme. So respiration will be affected by:

  • temperature

  • pH

  • substrate concentration

Anaerobic respiration

In the absence of oxygen, or during vigorous excercise when oxygen needs to be delivered fast enough, anerobic respiration occurs. This is the incomplete burning of glucose in the absence of oxygen.

Equation for anaerobic respiration in animal cells (muscles):

Glucose —› Lactic acid + small amounts of energy

Anaerobic respiration produces:

  • Lactic acid

  • Very little energy

Oxygen Debt

Lactic acid is toxic and causes signs of fatigue and cramps during exercise. After exercise, you need to take in more oxygen to get rid of the lactic acid. This amount of oxygen required to get rid of lactic acid is called the Oxygen Debt.

Equation for anaerobic respiration in fungi (yeast) and in plants:

Glucose —› Ethanol (alcohol)+ Carbon dioxide + small amounts of energy

This is sometimes called fermantation. It is the reaction used to make beer, wine, spirits and ethanol for biofuels.

The carbon dioxide is used in baking to make dough rise.