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 (beta oxidation), which are able to be converted into acetyl CoA and used in the Krebs cycle (aerobic respiration only)
  • Proteins can be broken down into amino acids, which are also capable of being converted into intermediates of the Krebs cycle and 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