Aerobic & Anaerobic Respiration (AQA GCSE Biology: Combined Science)

• Cellular respiration is an exothermic reaction which is continuously occurring in living cells
• The chemical process of cellular respiration releases energy
• The energy transferred supplies all the energy needed for living processes to occur within cells and organisms as a whole
• Organisms need energy for:
  • Chemical reactions to build larger molecules from smaller molecules
  • Muscle contraction to allow movement
  • Keeping warm (to maintain a constant temperature suitable for enzyme activity)

Uses of the energy released from respiration

• Respiration in cells can take place aerobically (using oxygen) to transfer energy; glucose is reacted with oxygen in this process
• The equations that summarise the chemical reactions of respiration that release energy from glucose are:

Word equation for aerobic respiration

Balanced symbol equation for aerobic respiration

• Aerobic respiration uses oxygen and most of the reaction takes place in the mitochondria (these are shown above the arrow in the equations)

• Respiration in cells can take place anaerobically (without oxygen), to transfer energy; it simply involves the incomplete breakdown of glucose into lactic acid
• This occurs when the body can’t supply enough oxygen for aerobic respiration, such as during vigorous exercise
• Anaerobic respiration is represented by the equation:

Word equation for anaerobic respiration in animals – some bacterial cells respire in this way too

• As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration
• Anaerobic respiration takes place without the need of oxygen

• Plants and yeast can respire without oxygen as well, breaking down glucose in the absence of oxygen to produce ethanol and carbon dioxide
• Anaerobic respiration in yeast cells is called fermentation
• Fermentation is economically important in the manufacture of bread (where the production of carbon dioxide makes dough rise) and alcoholic drinks (as ethanol is a type of alcohol)

The process outlined above is the same in plants

• You need to be able to compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the differing products and the relative amounts of energy transferred: