Brent Cornell

    
Both anaerobic and aerobic respiration pathways begin with the anaerobic breakdown of glucose in the cytosol by glycolysis

Glycolysis breaks down glucose (6-C) into two molecules of pyruvate (3C), and also produces:

• Hydrogen carriers (NADH) from an oxidised precursor (NAD⁺)
• A small yield of ATP (net gain of 2 molecules)
  

Overview of Glycolysis

Anaerobic Respiration

Anaerobic respiration proceeds in the absence of oxygen and does not result in the production of any further ATP molecules

• In animals, the pyruvate is converted into lactic acid (or lactate)
• In plants and yeasts, the pyruvate is converted into ethanol and carbon dioxide 

The purpose of anaerobic respiration is to restore stocks of NAD⁺ – as this molecule is needed for glycolysis

• By restoring stocks of NAD⁺ via anaerobic pathways, the organism can continue to produce ATP via glycolysis

The conversion of pyruvate into lactic acid (animals) or ethanol and CO₂ (plants / yeasts) is reversible

• Hence, pyruvate levels can be restored once oxygen is present and a greater yield of ATP may be produced aerobically

Summary of Anaerobic Respiration

    
Muscle contractions require the expenditure of high amounts of energy and thus require high levels of ATP

When exercising at high intensity, the cells’ energy demands will exceed what the available levels of O₂ can supply aerobically

• Hence the body will begin breaking down glucose anaerobically to maximise ATP production

This will result in an increase in the production of lactic acid, which leads to muscle fatigue

• When the individual stops exercising, oxygen levels will increase and lactate will be converted back to pyruvate

Although carbohydrates, lipids and proteins can all be consumed as energy sources, only carbohydrates will typically undergo anaerobic respiration

The Effect of Exercise Intensity on Carbohydrate Consumption (and Lactate Production)

The above graph demonstrates how the conditions of cell respiration change with increasing energy demand

• At high intensities, the aerobic consumption of fats is decreased while the anaerobic consumption of sugars increases
• Consequently, lactate levels will increase at higher levels of exercise intensity