Y10 Common Test 3





Topics assessed:

1. Respiration

2. Metabolism

3. The Brain and the Eyes

4. The Human Nervous System

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  • Energy needed for cellular processes is supplied by respiration.
  • Respiration is the process of transferring energy from glucose, which goes on in every cell.


  • It uses oxygen 
  • ATP (adenosine triophosphate) is produced by mitochondria.

Glucose + oxygen ------> carbon dioxide + water

Energy is also used:

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Aerobic Respiration

How do organisms use energy from respiration?

  • Organisms use energy to build up larger molecules from smaller ones (like proteins from amino acids)
  • Keep body temperature steady
  • Use energy to allow muscles to contract OR in plants for the transport of substances in phloem.


Respiration using oxygen to break down food molecules is called aerobic respiration. Glucose is the molecule normally used for respiration – it is the main respiratory substrate. Glucose is oxidised to release its energy.

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Anaerobic Respiration - describe the process

Most organisms cannot respire without oxygen but some organisms and tissues can continue to respire if the oxygen runs out. These organisms and tissues use the process of anaerobic respiration.

Human muscle can respire anaerobically for short periods of time – even though the process is relatively inefficient, it's better to continue respiring and be able to run away from danger – or run a race.

The glucose in muscle is converted to lactic acid:

glucose → lactic acid + energy released

Some plants, and some fungi such as yeast can respire anaerobically – it's preferable to release less energy but remain alive.

Glucose in yeast cells is converted to carbon dioxide and ethanol, which we refer to simply as 'alcohol':

glucose → ethanol + carbon dioxide + energy released

Anaerobic respiration occurs only in the cytoplasm of cells.

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Anaerobic Respiration LIGHTNING SUMMARY


- incomplete breakdown of glucose to lactic acid

- glucose ----> lactic acid

- used in fermentation of alcohol from yeast

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Summary and Comparison

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Response to Exercise

Muscles need energy to contract. While exercising, the muscles need additional energy as:

  • - the breathing rate and volume of each breath increases to bring more oxygen into the body and remove the carbon dioxide produced
  • - the heart rate increases, to supply the muscles with extra oxygen and remove the carbon dioxide produced

During long periods of vigorous activity:

  • lactic acid levels build up
  • glycogen reserves in the muscles become low as more glucose is used for respiration, and additional glucose is transported from the liver.

This build-up of lactic acid produces an oxygen debt.

As body stores of glycogen become low, the person suffers from muscle fatigue.

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Describe oxygen debt.

Liver converts lactic acid back to glucose which needs oxygen.

Oxygen debt

When a period of exercise is over, lactic acid must be removed. The body's tolerance of lactic acid is limited.

Lactic acid is taken to the liver by the blood, and either:

  • oxidised to carbon dioxide and water, or
  • converted to glucose, then glycogen - glycogen levels in the liver and muscles can then be restored

These processes require oxygen. This is why, when the period of activity is over, a person’s breathing rate and heart rate do not return to normal straightaway.

The amount of oxygen required to remove the lactic acid, and replace the body's reserves of oxygen, is called the oxygen debt.

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