By Microbiologist Doctor-Dr
Anaerobic Respiration: Definition, Types, Steps, Equation, Products, and Uses
Respiration is a metabolic process that releases cellular energy in the form of ATP (Adenosine Triphosphate) inside the living cell by dissolving basic organic substances (food material). An electron is converted from an electron donor to an electron acceptor through a sequence of oxidation-reduction reactions. The ATP molecule serves as a last type of energy release and storage. It is distinct from breathing, which entails the intake of ambient air (O2) and the expulsion of carbon dioxide (CO2). A common name for it is "cellular respiration".
Aerobic and anaerobic respiration may be distinguished based on the kind of terminal electron acceptor used in the respiration cycle. Oxygen (O2) is employed as a terminal electron acceptor during aerobic respiration. Other molecules than oxygen are utilised as terminal electron acceptors in anaerobic respiration.
What is Anaerobic Respiration?
Anaerobic respiration is simply the process of cellular respiration that takes place in an anoxic environment, or one in which oxygen is not present. Anaerobic respiration is a kind of respiration in which the energy molecule ATP is produced by transferring the terminal electrons generated during the oxidation-reduction of nutrients to a variety of organic and inorganic electron acceptors instead of oxygen molecules. Sulfur (S), sulphate (SO4-2), ferric ion (Fe3+), nitrate (NO3-), DMSO, and other sulfur-containing compounds are frequently utilised as terminal electron acceptors in anaerobic respiration. Therefore, it is the process of creating ATP molecules without oxygen.
Anaerobic respiration solely takes place in the cytoplasm, in contrast to aerobic respiration, which happens in both the cytoplasm and mitochondria. Anaerobic glycolysis takes place first, followed by fermentation. Either organic acids (lactic acid) or molecules of ethanol and ATP are the final products (energy). The following expressions can be used to describe the total response of a glucose molecule's anaerobic respiration:
Glucose + electron acceptors (other than O2) → Lactic acid / ethanol + 2 ATP molecules
Types of Anaerobic Respiration
Anaerobic respiration can be divided into a number of categories depending on the type of end product produced, such as organic acid fermentation (lactic acid, butyric acid, propionic acid, mixed acid, etc.), methanogenesis, acetogenesis, denitrification, sulphur reduction, alcohol (ethanol), butanediol, etc. Anaerobic respiration can be broadly classified as follows:
A. Lactic Acid Fermentation
When glucose and other six-carbon carbohydrates are converted to lactate, a kind of anaerobic respiration, chemical energy in the form of ATP molecules is released. Pyruvate, which is created at the conclusion of glycolysis and is changed to lactate by the enzyme lactate dehydrogenase, is formed during the fermentation process. Another name for it is "Lacto-fermentation."
The general equation of lactic acid fermentation can be expressed as:
Glucose + ADP + NADH → Lactic acid + ATP + NAD+
It is one of the most popular fermentations and is used extensively in the manufacturing of fermented foods. Fruits, vegetables, grains, milk (which is used to make fermented dairy products), and meat have all been preserved for use in the future through the fermentation process since ancient times.
It mostly takes place in the cytoplasm of bacteria. Lactic Acid Bacteria (LAB) are a kind of bacteria that convert glucose into lactic acid and are frequently employed in biotechnology and the food industry. Lactobacillus, Lactococcus, Leuconostoc, Streptococcus, Pediococcus, Enterococcus, etc. are examples of common LAB.
In addition, it happens in the cytoplasm of higher animals, like humans. Because there is a lack of oxygen during intense muscular activity, lactic acid fermentation takes place in our body muscles. After strenuous muscular activity, this will cause cramps, soreness, and a feeling of weariness in our muscles. Due to the absence of mitochondria in RBCs, this kind of respiration occurs often in tissues other than muscles.
B. Ethanol Fermentation
It is a typical example of anaerobic respiration in which glucose and other six-carbon carbohydrates are converted to ethanol and the chemical energy is released as ATP molecules. When the enzyme alcohol dehydrogenase is present, the glycolytic cycle's pyruvate is changed into ethanol. Additionally known as "alcoholic fermentation."
The general equation of ethanol fermentation can be expressed as:
Glucose + ADP + NADH → Ethanol + ATP + NAD+
It is always employed in the brewing of alcoholic drinks. Saccharomyces cerevisiae, Schizosaccharomyces, Zygosaccharomyces, Candida, Pichia, and other yeasts carry out this function. In addition, bacteria like Zymomonas and fungus like Aspergillus oryzae are also employed. Additionally, it affects higher species like goldfish.
Steps of Anaerobic Respiration
Glycolysis and fermentation are the first and second processes in anaerobic respiration.
A. Glycolysis
The process of turning a glucose (or glycogen) molecule into pyruvic acid via several enzymes found in each cell's cytoplasm is known as glycolysis. Two molecules of pyruvic acids, two molecules of NADH, and two molecules of ATP are produced throughout the process from one glucose molecule by catalysis. Every live cell's cytoplasm contains it during both anaerobic and aerobic respiration. The "Embden-Meyerhof-Parnas (EMP) Pathway" is another name for it.
The breakdown of the six-carbon sugar glucose into two molecules of three carbon pyruvate occurs in a complicated 10-step catabolic process. The whole response is broken down into three phases, including;
1. Energy Investment Phase
It is the first stage of a three-step process when glucose is transformed into fructose-1, 6-bisphosphate utilising two ATP molecules (therefore named the energy investment phase).
The enzyme "hexokinase" first phosphorylates glucose to glucose - 6 - phosphate (G-6-P). The enzyme "glucose - 6 - phosphate isomerase" converts the G - 6 - P to fructose - 6 - phosphate (F-6-P) in the next step in the presence of Mg++ ion.
Finally, the "phosphofructokinase" enzyme phosphorylates the F - 6 - P to fructose - 1,6 - bisphosphate.
2. Splitting Phase
In the second phase, two molecules of glyceraldehyde-3-phosphate (G - 3 - P), each with three carbons, are formed from the 6-carbon fructose-1,6-bisphosphate molecule.
First, an enzyme known as "aldolase (fructose bisphosphate aldolase)" breaks down the fructose - 1,6 - bisphosphate enzymatically to produce G - 3 - P and dihydroxyacetone phosphate. The enzyme "triosephosphate isomerase" converts dihydroxyacetone phosphate to G - 3 - P in the following step.
3. Energy Generation Phase
The last stage of glycolysis is when G-3-P is ultimately converted to pyruvate, releasing two ATP molecules.
The enzyme "glyceraldehyde - 3 - phosphate dehydrogenase" converts the G - 3 - P into 1,3 - bisphosphoglycerate in the first step, resulting in a molecule of NADH. An ATP molecule is produced when the enzyme "phosphoglycerate kinase" catalyses the conversion of 1,3-bisphosphate to 3 phosphoglycerate, which is subsequently catalysed by another enzyme, "phosphoglycerate mutase," to produce 2 phosphoglycerate.
Phosphoglycerate is converted to phosphoenolpyruvate in a reversible process by "enolase" 2. In turn, "pyruvate kinase" will dephosphorylate phosphoenolpyruvate, yielding "pyruvate" and an ATP molecule.
The following equation provides a summary of the entire chemical process;
Glucose + 2 ATP + 2 ADP + 2Pi + 2 NAD+ → 2 Pyruvate + 2 NADH + 2 ATP
B. Fermentation
It is the process through which pyruvate generated during glycolysis is converted into an anaerobic respiration byproduct (mainly ethanol or lactic acid). As a result, pyruvate is reduced in the presence of a separate enzyme using the proton that was released when NADH was converted to NAD+.
When pyruvate is reduced, fermentation can either be homo-fermentation, which results in the production of only one kind of product, or it can be hetero-fermentation, which results in the production of two or more end products.
The type of enzyme used and the method of fermentation determine the final result of pyruvate reduction. The final result of lactic acid fermentation is lactic acid, whereas the final product of alcohol fermentation is ethanol.
1. Lactic Acid Fermentation
This kind reduces the pyruvate to L-lactate (the conjugate base of lactic acid). The "lactate dehydrogenase" enzyme catalyses the reduction, which causes NADH to oxidise into NAD.
Lactic acid fermentation is described by the equation:
2. Ethanol Fermentation
In this kind, the pyruvate is converted in two steps to ethanol. The enzyme "pyruvate decarboxylase" first converts pyruvate into acetaldehyde in the presence of TPP (Thymine pyrophosphate). This procedure results in the release of a CO2 molecule. The "alcohol dehydrogenase" enzyme reduces acetaldehyde to ethanol in the following process. This process results in the oxidation of one NADH molecule to NAD+.
Anerobic Respiration Equation
Lactic Acid Fermentation
Step I: Glucose + 2 ATP + 2 ADP + 2 Pi + 2 NAD → 2 Pyruvate + 4 ATP + 2 NADH
Step II: 2 Pyruvate + 2 NADH → 2 Lactate (lactic acid) + 2 NAD
∴ Glucose + 2ADP + 2Pi → 2 Lactate + 2 ATP
Ethanol Fermentation
Step I: Glucose + 2 ATP + 2 ADP + 2 Pi + 2 NAD → 2 Pyruvate + 4 ATP + 2 NADH
Step II: 2 Pyruvate → 2 Acetaldehyde + 2 CO2
Step III: 2 Acetaldehyde + 2 NADH → 2 Ethanol + 2 NAD
∴ Glucose + 2 ADP + 2 Pi → 2 Ethanol + 2 ATP + 2 CO2 ↑
Products of Anaerobic Respiration
Products of anaerobic respiration differ depending on the kinds of electron acceptors utilised in the respiration process. Anaerobic respiration often results in the production of lactic acid, ethanol, CO2, and ATP. Acetic acid, butyric acid, methane, Fe(II), H2S, halide ions, succinate, NO2- and N2, U (IV), and other substances are also generated.
Application of Anaerobic Respiration
It is essential to biogeochemical cycles such as the carbon, nitrogen, iron, and sulphur cycles. These cycles include a number of phases that call for anaerobic respiration. Anaerobic respiration processes include, for instance, methanogenesis, sulphur reduction, and denitrification.
Waste management systems and sewage treatment facilities both utilise it. In the sewage treatment process, the denitrification stage is utilised to remove nitrate. In bioremediation techniques, pollutants such as petroleum, arsenic, radioactive, and chlorinated compounds are reduced anaerobically.
Anaerobic respiration is the technique used to generate electricity using microbial fuel cells.
production of organic acids for industrial and domestic usage, such as lactic acid, propionic acid, acetic acid, and butyric acid.
The area of anaerobic respiration with the largest applications is alcohol fermentation. Beer, wine, distilled alcohols, and other alcoholic drinks are still produced today, and despite their lengthy history, they still represent a sizable portion of the global economy.
Fermented foods are created using lactic acid fermentation, acetic acid fermentation, and other organic acid fermentation techniques. The organic acid fermentation process results in the production of fermented dairy products, soy sauce, sourdough bread, Indian Idli and Dosa, fermented mustard greens, pickled vegetables, sauerkraut, etc.
Ammonia is transformed into ammonium ions in our intestines by anaerobic lactic acid fermentation. This shields our bodies from the damaging effects of ammonia.
Examples of Anaerobic Respiration
Alcoholic Beverage Production
Alcohol fermentation results in the creation of wine, beer, and other distilled ethanol products. For the generation of ethanol, yeast such as Saccharomyces cerevisiae, Schizosaccharomyces, Zygosaccharomyces, etc. are frequently utilised.
Biogas Production
Anaerobic fermentation is a method for breaking down organic waste that results in the creation of methane gas. A popular alternative energy source is methane gas.
Production of Swiss Cheese
The fermentation of propionic acid produces the distinctive flavour and porous texture of Swiss cheese. Acid imparts taste, and the CO2 gas that is released causes the surface of the cheese to grow the recognisable holes.
Vinegar Production
A combination of 5–8% acetic acid makes up vinegar used as a flavour and food preservation. The procedure of twofold fermentation is used in its manufacturing. Ethanol is the first product of alcohol production. The acetic acid bacteria (Acetobacter) subsequently convert the ethanol to acetic acid.
Nitrate Reduction in Nitrogen Cycle
Denitrification, an anaerobic respiration process, is how nitrate (NO3 -) is reduced to nitrite (NO2 -) in the nitrogen cycle. Denitrification is carried out by bacteria like Pseudomonas, Clostridium, Geobacter, etc.
