• 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
• Carbon dioxide and water are produced as waste products as well as releasing useful cellular energy

• This equation can also be shown as a balanced chemical equation
  • One molecule of glucose combines with six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water

   

Exam Tip

• 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

   

• It involves the incomplete breakdown of glucose and so releases a relatively small amount of energy for use in cell processes
• Different breakdown products are formed depending on the type of organism that the anaerobic respiration is taking place in
• You need to know the equations for anaerobic respiration in animals and plants (or fungi)

Anaerobic respiration in animals

• Anaerobic respiration mainly takes place in muscle cells during vigorous exercise
• When we exercise at high intensities, our muscles have a higher demand for energy
• Our bodies can only deliver so much oxygen to our muscle cells for aerobic respiration
• When oxygen runs out, glucose is broken down without it, producing lactic acid instead
• Glucose has not been fully broken down meaning there is still energy stored within the bonds of lactic acid molecules
• Anaerobic respiration releases less energy than aerobic respiration

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

   

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 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 carbon dioxide produced helps the dough to rise) and in brewing (where the ethanol produced makes beer)

• 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

   

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