7.3 - Fermentation

Fermentation is a metabolic process where glucose molecules are broken down anaerobically. As a result, the glucose molecules are not completely oxidized due to insufficient or complete absence of oxygen supply. Pyruvate produced will undergo alcoholic fermentation or lactic acid fermentation process , different from the normal process of glycolysis.

1. Alcoholic Fermentation

• Incomplete breakdown of glucose to ethanol, carbon dioxide and energy.
• Only 210 kJ of energy is produced.
• Ethanol produced is used in the making of wine and beer.
• Carbon dioxide produced also helps the dough to rise in the bread making process.
• Paddy plants carry out alcohol fermentation process due to the submerged paddy soil condition (lack of oxygen).
• Normally, the ethanol produced in tissues is toxic to plants. However, paddy plant cells are capable of producing alcohol dehydrogenase which can break down the ethanol molecules to non-toxic substances.

2. Lactic acid Fermentation

• Refers to the breakdown of glucose into lactic acid and energy.
• Milk fermentation in yoghurt production is one of the examples.
• The bacteria Lactobacillus turns lactose into lactic acid which then coagulates casein.
• Lactic acid is the source of the sour taste in yoghurt.

Figure 3 - Lactic acid fermentation can also happen in human muscle cells during vigorous exercise. When all the available oxygen has been used up, the breathing rate and heart rate will increase in order to supply more oxygen to the muscles. However, this is not enough to meet the muscles' requirements. The muscles are said to be in oxygen debt (oxygen deficiency state). Soon, in order to obtain extra energy, the muscles will start to perform anaerobic respiration. During anaerobic respiration, glucose molecules are only partially broken down into intermediate substances called lactic acid instead of carbon dioxide and water. Every glucose molecule broken down will produce only 2 molecules of ATP (150 kJ) because they are not completely oxidized and much of energy are still trapped within the lactic acid molecules. The accumulation of lactic acid can reach a higher level of concentration which can lead to fatigue and muscle cramps. The person needs to breathe in deeply to inhale more oxygen in order to oxidize the accumulated lactic acid to carbon dioxide and water. The oxygen debt is paid off when all the lactic acid is removed. Therefore, oxygen debt refers to the amount of oxygen required to remove lactic acid from the muscles.

Comparison between Aerobic Respiration and Fermentation

• Similarities
  • Breakdown process of glucose and chemical energy conversion
  • Both processes start in the cytoplasm
  • Both processes can occur in yeast, bacteria, animals and plants
  • Both processes begin with glycolysis
  • ATP is produced
• Differences
  • Aerobic Respiration
    • Oxygen supply: Continuous and sufficient
    • Breakdown of glucose: Complete
    • Site of action: Cytoplasm and mitochondria
    • Energy production: One molecule of glucose gives 2898 kJ of energy
    • End products: Carbon dioxide and water
  • Fermentation
    • Oxygen supply: Limited or completely absent
    • Breakdown of glucose: Incomplete
    • Site of action: Cytoplasm
    • Energy production: One molecule of glucose gives 210 kJ (alcoholic fermentation) or 150 kJ (lactic acid fermentation) of energy
    • End products: Ethanol and carbon dioxide or lactic acid