Aerobic & Anaerobic Respiration (OCR Gateway GCSE Biology: Combined Science)

Aerobic Respiration

• Aerobic respiration requires oxygen
  • It is defined as the chemical reaction in cells that uses oxygen to break down nutrient molecules to release energy
• Aerobic respiration is the complete breakdown of glucose to release a relatively large amount of energy for use in cell processes and reactions
  • Large amounts of energy in the form of ATP can be generated
  • Aerobic respiration can produce approximately 32 molecules of ATP per molecule of glucose
• Carbon dioxide and water are produced as waste products as well as releasing useful cellular energy

Anaerobic Respiration

• Anaerobic respiration does not require oxygen
  • It is defined as the chemical reaction in cells that breaks down nutrient molecules to release energy without using oxygen
• Anaerobic respiration releases much less energy than aerobic respiration as it involves the incomplete breakdown of glucose
  • Only 2 molecules of ATP are produced for every molecule of glucose
• The process of anaerobic respiration is slightly different in different organisms
  • Different breakdown products are formed depending if the process is happening in animals or plants/fungi
• You need to know the equations for anaerobic respiration in animals and plants/fungi

Anaerobic respiration in animals

• Anaerobic respiration in animals mainly takes place in muscle cells during vigorous exercise
• When individuals exercise at high intensities, their muscles have a higher demand for energy
• Bodies can only deliver so much oxygen to muscle cells for aerobic respiration
• When oxygen becomes a limiting factor, glucose is broken down without it, producing lactic acid instead
• The glucose has not been fully broken down meaning there is still energy stored within the bonds of lactic acid molecules
• Anaerobic respiration releases much less energy than aerobic respiration
  • Only 2 molecules of ATP are produced for every molecule of glucose

Word equation for anaerobic respiration in animals. Glucose is converted to lactic acid and energy is released

• This equation can also be shown as a balanced chemical equation
  • One molecule of glucose is broken down into two molecules of lactic acid

The balanced chemical equation for anaerobic respiration in animals

Lactic acid and oxygen debt

• Lactic acid builds up in muscle cells and lowers the pH of the muscle tissue (making the conditions more acidic)
  • Acidic conditions can denature the enzymes in cells

• Lactic acid will eventually be broken down using oxygen to produce carbon dioxide and water as waste products
• The amount of oxygen required to break down the lactic acid that has built up is referred to as the 'oxygen debt'
• The process of breaking down the lactic acid is known as ‘repaying the oxygen debt’

Anaerobic respiration in plants and fungi

• Plants and fungi can respire without oxygen as well, breaking down glucose in the absence of oxygen to produce ethanol and carbon dioxide
• Under certain conditions plants need to utilise anaerobic respiration
  • If plants are in waterlogged soil, there is little or no oxygen available
  • The plant root cells will respire anaerobically
• Anaerobic respiration in yeast (a unicellular fungi) is called fermentation
• Fermentation is economically important in the manufacture of bread (where the carbon dioxide produced helps the dough to rise) and in brewing (where the ethanol produced makes beer)

Word equation for anaerobic respiration in plants and fungi

• This equation can also be shown as a balanced chemical equation
  • One molecule of glucose is broken down into two molecules of alcohol and two molecules of carbon dioxide

Balanced equation for anaerobic respiration in plants and yeast

Comparing Aerobic & Anaerobic Respiration Table