Cell respiration can be either anaerobic (does not require oxygen) or aerobic (requires oxygen)

• The two forms differ in the products that are formed, where the reactions occur and the overall ATP yield
  

• Not all respiratory substrates can undertake both forms or respiration (fatty acids are only digested aerobically)
  

Anaerobic Respiration

Anaerobic respiration begins with the process of glycolysis, whereby simple sugars (glucose) are partially broken down into two pyruvate molecules with a small yield of ATP

• Certain amino acids and glycerol (from triglycerides) may also feed into the glycolytic pathway and produce pyruvate anaerobically

In the absence of oxygen, the pyruvate molecules are fermented to form lactic acid (in animals) or ethanol and carbon dioxide (in plants and yeasts)

• The anaerobic processes of glycolysis and fermentation both occur within the cytosol of the cell

Aerobic Respiration

Aerobic respiration also begins with the process of glycolysis, but oxygen is then used to completely break down the pyruvate for a much larger ATP yield

• The pyruvate is transported to the mitochondria and is broken down into carbon dioxide and water

• The complete breakdown of pyruvate involves three key stages: the link reaction, the Krebs cycle and the electron transport chain

Anaerobic versus Aerobic

Both anaerobic and aerobic respiration use the digestion and oxidation of organic molecules to synthesise ATP

• The process of glycolysis is common to both pathways, even though it is technically an anaerobic reaction

• While sugars are the main respiratory substrate, lipids and proteins can both be converted into usable intermediaries