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Anaerobic respiration/Fermentation

Anaerobic respiration/Fermentation

Topics covered:

  • Salient features of anaerobic respiration
  • Alcoholic fermentation
  • Lactic acid fermentation

Salient features of anaerobic respiration:

1. Alcoholic fermentation
2. Lactic acid fermentation
3. Propionic acid fermentation
4. Acidogenesis

  • The term anaerobic respiration is more commonly used for the respiration in organisms in the absence of oxygen.
  • In microorganisms, fermentation is the term used to describe the respiration in the absence of oxygen.
  • Fermentation can be of different types such as:
  • Buchner was a scientist who identified that the yeast cells can carry fermentation even after they are dead.
  • It was due to the presence of an enzyme that carries fermentation. Later, he named this enzyme zymase.
  • Hence, fermentation is also known as zymosis.
  • Definition: Fermentation is the breakdown of the respiratory substrates in the absence of oxygen to form alcohols, organic acids /and/or gasses by the microorganisms or their enzymes.
  • Glycolysis is the common step for both aerobic and anaerobic respiration.
  • In both the cases, alcoholic and lactic acid fermentation, very less energy is released i.e., not more than 7% of the energy is released from glucose
  • Also, all of it is not trapped as high energy bonds of ATP.
  • Additionally, as either acid or alcohol is produced during oxidation, the process of fermentation is proved to be hazardous.
  • In the case of yeasts, It may poison itself to death if the concentration of alcohol goes beyond about 13%.
  • Drawback of this process is that complete oxidation cannot be carried out by organisms.
  • Also, the process is not able to extract the energy stored to synthesize a larger number of ATP molecules needed for cellular metabolism.

Alcoholic fermentation:

  • In alcoholic fermentation, the fungi (e.g., yeast), or a few higher plants, undergo incomplete oxidation of glucose in anaerobic condition to form alcohol.
  • This involves a series of reactions in which pyruvic acid is converted to CO2 and ethanol.
  • During fermentation, yeasts release both ethyl alcohol and carbon dioxide gas, which forms bubbles.
  • This property is useful to make fizzy bears or soft breads.
  • This is achieved by two steps:

         1. Pyruvic acid is decarboxylated to acetaldehyde in the presence of enzyme pyruvic acid decarboxylase.
         2. This acetaldehyde is then reduced to ethyl alcohol or ethanol in the presence of enzyme alcohol dehydrogenase


Alcoholic fermentation

Lactic acid fermentation:

  • This type of fermentation is performed by human muscle cells in the absence of oxygen, and by some bacteria.
  • Lactic acid fermentation is actually used by humans to make yogurt.
  • In this, pyruvic acid produced in glycolysis is reduced by NADH + H+ to form lactic acid.
  • CO2 is not produced and NADH + H+ is oxidized to NAD+.
  • The reaction is catalyzed by lactic acid dehydrogenase enzyme.
  • In the same way, in the case of animal cells also (such as muscles) during heavy exercise and there is inadequate amount of oxygen for cellular respiration, pyruvic acid is reduced to lactic acid by the enzyme lactic acid dehydrogenase.
Lactic acid fermentation

Frequently asked questions (FAQ’s)

Q1. Why do we get muscle cramps after vigorous exercise?
Ans :
On doing vigorous exercise, the oxygen demand of the body increases and due to the absence of adequate oxygen, the muscle cells perform anaerobic respiration which produces lactic acid. The accumulation of the lactic acid in the muscle cell causes muscle cramp.

Q2. Where does anaerobic respiration take place in the cell?
Ans :
Anaerobic respiration takes place in the cytoplasm of the cell as the Krebs cycle and electron transport chain do not take place. Glycolysis takes place in the cytoplasm to generate pyruvic acid and it gets converted to ethanol or lactic acid through anaerobic respiration.

Q3. Why is less energy produced in anaerobic respiration than in aerobic respiration?
Ans :
In anaerobic respiration, glucose is not completely broken down but undergoes partial break down. This causes the inefficient release of the energy and hence less energy is generated in anaerobic respiration.

Q4. Which step of cellular respiration is common in both aerobic and anaerobic respiration?
Ans :
Glycolysis is the common step for both aerobic and anaerobic respiration in which one molecule of glucose is broken into 2 molecules of pyruvic acid.

Q5. Why does anaerobic respiration take place in white muscle?
Ans :
White muscle has less myoglobin protein compared to red muscle. Myoglobin helps in the storage of oxygen in the muscle cell and hence red muscle can work continuously for a longer period of time compared to white muscle through aerobic respiration. Whereas, white muscle can work for a shorter period through aerobic respiration until all the oxygen is exhausted and switch to anaerobic respiration.

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