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  • Last Updated : 27 Sep, 2022
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Every living organism on earth breathes, including plants, animals, and microbes. The breaking of C-C bonds of the complex compounds through oxidation in the cell, to release energy, is called respiration. Cellular respiration in plants is the process of the breakdown of bond materials within the cell to produce energy, and the use of this energy for the synthesis of ATP. The breakdown of these complexes occurs in the cytoplasm and mitochondria. The compounds that are oxidized during the process are known as respiratory substrates. Usually, carbohydrates are oxidized, but protein, fats, and organic acids are also used as respiratory substrates in some plants under certain conditions. The energy contained in the respiratory substrate is released in a slow stepwise reaction controlled by the enzymes and is stored in the form of ATP. The energy released by oxidation in respiration cannot be directly used but is used to synthesize ATP, which is broken down whenever there is a need for energy. Hence, ATP is used as the energy currency of cells. This energy stored in ATP is used in various functions of cells.

Plants, require O2 for respiration, and they give out CO2. Unlike animals, they do not have special organs for the exchange of gases. Plants have stomata and lenticels that ensure the availability of O2. Each plant part takes care of its own gas need, there is very little gas exchange between the parts of plants. Plants do not possess much need for gas exchange, roots and stems respire at much lower rates than animals. Gases exchange occurs only during photosynthesis, and each leaf is adapted to take care of its own needs during this period. O2 is released within the cell. The distance that gases must diffuse in plants is less. Each living cell in plants is located close to the surface of the plant. The living cell is arranged inside and beneath the bark. They have openings called lenticels. Thus, most plant cells have at least a part of their surface in contact with the air.

Fermentation – Anaerobic Respiration

This term is often used in connection with higher organisms where it occurs in the roots of some water-logged plants, and muscles of animals and as a supplementary mode of respiration in massive tissues. It is the exclusive mode of respiration in some parasitic worms and microorganisms. In microorganisms, the term fermentation is more commonly used where anaerobic respiration is known after the name of products like alcoholic fermentation, and lactic acid fermentation. In some cases, carbon dioxide is evolved. It gives a frothy appearance(L.fermentum – to boil). Buchner found that fermentation could be caused without the living yeast cells by grinding them under pressure and mixing the extract with a sugary solution. The enzyme present in the extract is named zymase. Because of the latter, fermentation is also called zymosis. 

Fermentation is the anaerobic breakdown of carbohydrates and other organic compounds into alcohols, organic acids, gases, etc. with the help of micro-organisms or their enzymes.

The mechanism of anaerobic respiration is similar to the common pathway of aerobic respiration up to glycolysis. 

Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. In this process, under anaerobic conditions (absence of oxygen) the pyruvic acid which is obtained from incomplete oxidation of glucose is converted into CO2 and ethanol. Fermentation results in a partial breakdown of glucose. Lactic acid is also produced from pyruvic acid due to some bacteria. The enzymes, pyruvic acid decarboxylase, and alcohol dehydrogenase are used as catalysts in these reactions. The pyruvic acid is reduced to lactic acid by lactate dehydrogenase. In this reaction, the reducing agent NADH+H+ is reoxidized to NAD+. The energy released during the lactic acid and alcohol fermentation is low (2 ATP molecules), which is very less compared to the reaction of glucose in glycolysis. These processes are hazardous, either acid or alcohol is produced as end products.



Types of Fermentation

  • Lactic Acid Fermentation: It is a metabolic process by which starch or sugar is converted into metabolite lactate, which is lactic acid by yeast strains and bacteria. It is an anaerobic fermentation process that occurs in some bacteria and animal cells when energy expenditure is more than the supplied oxygen. In this process, one glucose molecule is converted into pyruvate and then into lactic acid.


  • Alcohol Fermentation: Ethanol Fermentation or Alcohol Fermentation is a biological process where pyruvate, the end product of glycolysis, is broken down into Ethanol (alcohol) and carbon dioxide. Yeast performs this process in the absence of oxygen, alcoholic fermentation is an anaerobic process. Ethanol Fermentation is mainly used for the preparation of Alcoholic beverages like beer and the production of ethanol fuel.



  • Acetic acid Fermentation: Acetic acid is also known as vinegar. The production of acetic acid generally involves two processes, the first is using yeast alcohol produced from sugar and the second is utilizing acetic acid bacteria to oxidize ethyl alcohol to acetic acid through acetaldehyde. Fermentation of acetic acid is done by two methods, surface fermentation and submerged fermentation.


Importance of Fermentation

  • Fermentation has many uses in various fields ranging from the food industry to the manufacturing industry. 
  • Food products can be preserved and stored using the fermentation process, which increases the shelf life of food.
  • Various flavors and tastes can be added to food items through this process.
  • Fermentation is used to produce ethanol which is used as biofuel, it is generated from feedstock like grains, sugar cane, sugar beet, and cassava which is a renewable resource.
  • A number of products are made with this technique like dyes, inks, coagulants and binding agents, etc. 
  • Fermentation helps in digestion by degrading nutrients into digestible form. 

FAQs on Fermentation

Question 1: What is the net ATP that is synthesized when one molecule of glucose is fermented to alcohol or lactic acid?


When Glucose is fermented, there is a net gain of 2 ATP during the glycolysis process, in this process, glucose is first converted into pyruvates and then to lactic acid or alcohol. 2-pyruvates, 2-ATP, and 2-NADH are produced during glycolysis. It is the same for alcoholic, lactic acid, and acetic acid fermentation.

Question 2: Explain the Significance of Fermentation.


Fermentation is mainly and widely used in industries. Microorganisms like yeast and bacteria are used in large numbers. In the food industry fermentation plays a major role, preparation of bread, biscuits, and cakes requires fermentation. 

Question 3: How does Glycolysis differ from Fermentation?


  • Glycolysis—The starting material in glycolysis is glucose, and the end product is pyruvate. NADH molecules are formed in glycolysis.
  • Fermentation—The starting material in fermentation is pyruvate, and the end products are ethyl alcohol and CO2. In this process, NADH is consumed. 

Question 4: Why is fermenting of food required?


Fermentation helps in improving the shelf life of any food or vegetable and allows it to stand for long period of time. Fermented food is more beneficial to our body and nutritionally dense as compared to normal or unfermented food. 

Question 5: How is pickling different from Fermentation?


Pickles are generally obtained by using a pickling liquid made with vinegar, salt, sugar, oil, herbs, and spices, and heating it with vegetables, later it is placed in jars and left for a certain amount of time. In this process, there is no involvement of fermentation. 

Fermenting vegetables or (maybe fruits) is a process where a suitable environment is created for good bacteria to grow. After that, they are transmuted into the living food full of probiotics.

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