Energy Sources

Cell respiration is typically explained according to the oxidation of a carbohydrate (glucose), however lipids and proteins can also be used as energy sources

Carbohydrates are preferable as a ready-made energy source because: 

  • They are easier to transport and digest (unlike lipids) 
  • They don’t produce nitrogenous wastes (unlike proteins)


  • Carbohydrates can be broken down into monosaccharides (e.g. glucose), which are able to be used in glycolysis
  • Lipids (i.e. triglycerides) can be broken down into glycerol and fatty acids, which are able to be used in aerobic respiration
  • Proteins can be broken down into amino acids, which are also capable of being used in aerobic respiration

Energy Content

  • Lipids produce roughly twice as much energy (~4000 kJ per 100 g) as proteins and carbohydrates
  • Both proteins (~1,720 kJ per 100 g) and carbohydrates (~1,760 kJ per 100 g) produce similar levels of energy 

Role of Organic Molecules in Energy Production

energy sources