Anaerobic Respiration- Definition, Types, Steps, Equation, Products, Uses

Respiration is a vital process for all living cells, as it provides them with the energy they need to perform various functions. Respiration involves the breakdown of organic molecules, such as glucose, and the transfer of electrons to an electron acceptor. The most common type of respiration is aerobic respiration, which uses oxygen as the final electron acceptor and produces carbon dioxide and water as waste products. Aerobic respiration is very efficient and can generate up to 38 molecules of ATP (adenosine triphosphate), the universal energy currency of cells, from one molecule of glucose.

However, not all cells can perform aerobic respiration, either because they lack the necessary organelles (such as mitochondria) or because they live in environments where oxygen is scarce or absent. In such cases, cells can resort to anaerobic respiration, which is a type of respiration that does not require oxygen. Anaerobic respiration uses other substances, such as nitrate, sulfate, sulfur, or organic compounds, as the final electron acceptors and produces different waste products, such as lactic acid, ethanol, methane, hydrogen sulfide, or nitrogen gas. Anaerobic respiration is less efficient than aerobic respiration and can generate only 2 molecules of ATP from one molecule of glucose.

Anaerobic respiration can be divided into two main types: fermentation and anaerobic cellular respiration. Fermentation is a simple process that occurs in the cytoplasm of cells and does not involve an electron transport chain. Fermentation converts pyruvate, the end product of glycolysis (the first stage of both aerobic and anaerobic respiration), into either lactic acid or ethanol, depending on the type of organism and the enzymes involved. Fermentation regenerates NAD+, which is needed for glycolysis to continue. Fermentation is common in bacteria, fungi (such as yeast), and some animal cells (such as muscle cells).

Anaerobic cellular respiration is a more complex process that occurs in some prokaryotic cells and involves an electron transport chain. Anaerobic cellular respiration transfers electrons from NADH and FADH2 (the reduced coenzymes produced during glycolysis and other metabolic pathways) to various electron acceptors other than oxygen. The electron transport chain creates a proton gradient across a membrane, which drives the synthesis of ATP by ATP synthase. Anaerobic cellular respiration is important for many ecological processes, such as biogeochemical cycles, sewage treatment, bioremediation, and biogas production.

In this article, we will explore the definition, types, steps, equation, products, and uses of anaerobic respiration in more detail. We will also compare and contrast anaerobic respiration with aerobic respiration and fermentation. By the end of this article, you will have a better understanding of how different organisms obtain energy in different ways depending on their environment and evolutionary history.

Anaerobic respiration is a type of cellular respiration that occurs in the absence of oxygen. It is a process of breaking down glucose and other organic molecules to produce energy in the form of adenosine triphosphate (ATP). Unlike aerobic respiration, which uses oxygen as the final electron acceptor, anaerobic respiration uses other molecules, such as sulfate, nitrate, sulfur, or organic compounds, as the final electron acceptors.

Anaerobic respiration occurs in the cytoplasm of the cells, and does not involve the mitochondria. It consists of two main steps: glycolysis and fermentation. Glycolysis is the same as in aerobic respiration, where glucose is split into two molecules of pyruvate, releasing two molecules of ATP and two molecules of NADH. Fermentation is the process where pyruvate is converted into different end products, depending on the type of anaerobic respiration. For example, in lactic acid fermentation, pyruvate is reduced to lactate; in ethanol fermentation, pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol.

The overall equation for anaerobic respiration varies depending on the type of fermentation and the final electron acceptor. For example, the equation for lactic acid fermentation is:

Glucose + 2 ADP + 2 Pi → 2 Lactate + 2 ATP

The equation for ethanol fermentation is:

Glucose + 2 ADP + 2 Pi → 2 Ethanol + 2 ATP + 2 CO2

Anaerobic respiration is less efficient than aerobic respiration in terms of ATP production. It only yields two molecules of ATP per glucose molecule, whereas aerobic respiration can yield up to 36 molecules of ATP per glucose molecule. However, anaerobic respiration is faster and can provide energy when oxygen is scarce or unavailable.

Anaerobic respiration is common in many microorganisms, such as bacteria, fungi, and protists. Some examples are lactic acid bacteria that ferment milk into yogurt and cheese; yeast that ferment sugar into alcohol and bread; and methanogens that produce methane gas from organic matter. Anaerobic respiration also occurs in some animal cells, such as muscle cells during intense exercise, when oxygen demand exceeds oxygen supply. This results in the accumulation of lactic acid in the muscles, causing fatigue and soreness. Anaerobic respiration also occurs in some plant cells, such as rice roots that grow in waterlogged soils.

Anaerobic respiration has various applications in biotechnology, food industry, waste management, and energy production. For example, anaerobic respiration is used to produce biogas from organic waste; to make vinegar from ethanol; to preserve food by fermentation; and to generate electricity from microbial fuel cells. Anaerobic respiration also plays a role in biogeochemical cycles, such as nitrogen cycle, sulfur cycle, and carbon cycle.

Based on the type of end product of respiration, anaerobic respiration can be classified into several types. The major types of anaerobic respiration are:

• Lactic acid fermentation: It is a type of anaerobic respiration where sugar molecules (glucose and other six-carbon sugars) are metabolized to lactate, releasing the chemical energy in the form of ATP molecules. It is the fermentation process where pyruvate is produced at the end of glycolysis and is converted to lactate by the lactate dehydrogenase enzyme. It is also called ‘Lacto-fermentation’. It mainly occurs in the bacterial cytoplasm and in some animal cells, such as muscle cells and red blood cells .

• Ethanol fermentation: It is another common type of anaerobic respiration process where the sugar molecules (glucose and other six-carbon sugars) are metabolized to ethanol, releasing the chemical energy in the form of ATP molecules. The pyruvate produced by the glycolytic cycle is converted to ethanol in the presence of the alcohol dehydrogenase enzyme. It is also called ‘alcoholic fermentation’. It is performed by yeasts and some bacteria and fungi .

• Anaerobic cellular respiration: It is a type of anaerobic respiration where an inorganic molecule other than oxygen (such as sulfate, nitrate, sulfur, etc.) is used as a final electron acceptor for an electron transport chain. This process is similar to aerobic cellular respiration in that electrons extracted from a fuel molecule are passed through an electron transport chain, driving ATP synthesis. However, it differs from aerobic cellular respiration in that it does not use oxygen as a final electron acceptor. This process is performed by some bacteria and archaea that live in low-oxygen environments .

Some examples of anaerobic cellular respiration are:

• Methanogenesis: It is a type of anaerobic cellular respiration where carbon dioxide (CO2) is used as a final electron acceptor, producing methane (CH4) as a by-product. It is performed by some archaea called methanogens that are found in soil and in the digestive systems of some animals .

• Sulfate reduction: It is a type of anaerobic cellular respiration where sulfate (SO4^-2) is used as a final electron acceptor, producing hydrogen sulfide (H2S) as a by-product. It is performed by some bacteria and archaea called sulfate-reducing microorganisms that are found in marine and freshwater sediments, soil, and biofilms .

• Denitrification: It is a type of anaerobic cellular respiration where nitrate (NO3^-) is used as a final electron acceptor, producing nitrogen gas (N2) as a by-product. It is performed by some bacteria such as Pseudomonas, Clostridium, Geobacter, etc. that are involved in the nitrogen cycle .

  Lactic Acid Fermentation

Lactic acid fermentation is a type of anaerobic respiration where sugar molecules (glucose and other six-carbon sugars) are metabolized to lactate, releasing the chemical energy in the form of ATP molecules. It is the fermentation process where pyruvate is produced at the end of glycolysis and is converted to lactate by the lactate dehydrogenase enzyme. It is also called `Lacto-fermentation`.

The general equation of lactic acid fermentation can be expressed as:

$$\text{Glucose} + \text{ADP} + \text{NADH} \rightarrow \text{Lactic acid} + \text{ATP} + \text{NAD}^+$$

It is one of the most common fermentation widely used in food preservation and fermented food production. Since ancient times, it has been in practice to ferment fruits, vegetables, cereals, milk (production of fermented dairy products), and meat and preserve them for later use.

It mainly occurs in the bacterial cytoplasm. Bacteria fermenting glucose to lactic acid are called "Lactic Acid Bacteria" (LAB), and they are widely used in the food industry and biotechnology. Common LAB are; Lactobacillus, Lactococcus, Leuconostoc, Streptococcus, Pediococcus, Enterococcus, etc. Besides, it also occurs in the cytoplasm of higher animals, including humans. During strenuous muscular activities, lactic acid fermentation occurs in our body muscles due to the shortage of oxygen supply. This will result in muscle cramps or pain and fatigue sensation in our muscles after laborious muscular activities. Apart from muscles, this type of respiration is common in RBCs also as they lack mitochondria.

Lactic acid fermentation can be carried out under three basic types of conditions: dry salted, brined, or non-salted. Salting provides a suitable environment for lactic acid bacteria to grow, imparting an acid flavor to the vegetable. Brining involves submerging the vegetable in a salt solution that inhibits spoilage microbes and allows lactic acid bacteria to thrive. Non-salted fermentation relies on the natural sugars and microbes present in the vegetable to produce lactic acid.

Some examples of foods produced by lactic acid fermentation are:

• Pickles: Fermented cucumbers or other vegetables with salt and spices.
• Kimchi: Fermented cabbage or radish with chili peppers, garlic, ginger, and other seasonings.
• Sauerkraut: Fermented shredded cabbage with salt.
• Yogurt: Fermented milk with live cultures of LAB.

• Cheese: Fermented milk with rennet and starter cultures of LAB.

  Ethanol Fermentation

Ethanol fermentation is a type of anaerobic respiration where glucose and other six-carbon sugars are converted to ethanol and carbon dioxide by yeasts and some bacteria. It is also called alcoholic fermentation and is used for the production of alcoholic beverages, ethanol fuel and bread dough rising.

The process of ethanol fermentation involves two main steps: glycolysis and fermentation. In glycolysis, glucose is broken down into two molecules of pyruvate, releasing two molecules of ATP and two molecules of NADH. In fermentation, pyruvate is decarboxylated into acetaldehyde by the enzyme pyruvate decarboxylase, and then reduced to ethanol by the enzyme alcohol dehydrogenase, regenerating NAD+ for glycolysis.

The overall equation of ethanol fermentation can be written as:

Glucose + 2 ADP + 2 P_i + 2 NAD+ → 2 Ethanol + 2 CO_2 + 2 ATP + 2 NAD+

Ethanol fermentation occurs in the cytoplasm of yeast cells and some bacteria, such as Zymomonas mobilis. It also occurs in some fish, such as goldfish and carp, where it provides energy when oxygen is scarce.

Ethanol fermentation has many applications in various industries and biotechnology. Some examples are:

• Alcoholic beverage production: The fermentation of sugars from fruits, grains or tubers by yeasts produces different types of alcoholic drinks, such as wine, beer and distilled spirits.
• Ethanol fuel production: The fermentation of sugars from crops, such as corn or sugarcane, or from cellulosic biomass, such as wood or straw, produces ethanol that can be used as a renewable biofuel for vehicles or power generation.

• Bread dough rising: The fermentation of sugars from flour by yeasts produces carbon dioxide gas that makes the dough rise and gives bread its texture and flavor.

  Steps of Anaerobic Respiration

Anaerobic respiration is the process of breaking down glucose in the absence of oxygen to produce ATP, the energy currency of the cell. Unlike aerobic respiration, which uses oxygen as the final electron acceptor, anaerobic respiration uses other molecules, such as sulfate, nitrate, or organic compounds, to accept electrons at the end of the electron transport chain. Anaerobic respiration occurs in some bacteria and archaea that live in low-oxygen environments, such as soil, water, and the digestive tract of animals.

Anaerobic respiration consists of two main steps: glycolysis and fermentation.

Glycolysis

Glycolysis is the first step of both aerobic and anaerobic respiration. It takes place in the cytoplasm of the cell and does not require oxygen. In glycolysis, a glucose molecule (a six-carbon sugar) is split into two molecules of pyruvate (a three-carbon compound). This process releases two molecules of ATP (the energy currency of the cell) and two molecules of NADH (an electron carrier).

Glycolysis can be divided into three phases: energy investment, splitting, and energy generation.

Energy investment

In this phase, two molecules of ATP are used to phosphorylate (add a phosphate group to) glucose, forming fructose-1,6-bisphosphate. This makes glucose more unstable and ready to be split into two smaller molecules.

Splitting

In this phase, fructose-1,6-bisphosphate is cleaved by an enzyme called aldolase into two molecules of glyceraldehyde-3-phosphate (G3P). Another enzyme, triosephosphate isomerase, converts one of the G3P molecules into dihydroxyacetone phosphate (DHAP), which can be interconverted with G3P. Thus, there are two molecules of G3P available for the next phase.

Energy generation

In this phase, each G3P molecule is oxidized by an enzyme called glyceraldehyde-3-phosphate dehydrogenase, producing 1,3-bisphosphoglycerate (BPG) and NADH. The BPG molecule then transfers a phosphate group to ADP, forming ATP and 3-phosphoglycerate (3PG). This process is repeated for the second G3P molecule, resulting in two molecules of ATP and two molecules of 3PG. The 3PG molecules are then converted into 2-phosphoglycerate (2PG) by an enzyme called phosphoglycerate mutase. The 2PG molecules are then dehydrated by an enzyme called enolase, forming phosphoenolpyruvate (PEP). The PEP molecules then transfer a phosphate group to ADP, forming ATP and pyruvate. This process is repeated for the second PEP molecule, resulting in two more molecules of ATP and two molecules of pyruvate.

The overall equation for glycolysis is:

Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Fermentation

Fermentation is the process where pyruvate produced in glycolysis is reduced to an end product of anaerobic respiration (mainly ethanol or lactic acid). In the process, NADH is oxidized to NAD+, and the released proton is used to reduce the pyruvate in the presence of a different enzyme. Fermentation may be homo-fermentation, i.e., producing only one type of product by reducing pyruvate, or maybe hetero-fermentation, i.e., producing two or more end products while reducing the pyruvate. The end product of pyruvate reduction depends on the enzyme involved and the types of fermentation. In lactic acid fermentation, the end product is lactic acid, and in alcohol fermentation, the end product is ethanol.

Fermentation does not involve an electron transport system and does not directly produce any additional ATP beyond that produced during glycolysis by substrate-level phosphorylation. Organisms carrying out fermentation, called fermenters, produce a maximum of two ATP molecules per glucose during glycolysis. Fermentation does not require oxygen and can occur in both aerobic and anaerobic environments. The study of fermentation is called zymology.

Fermentation can be classified into several types based on the type of end product of respiration, such as organic acid fermentation (lactic acid fermentation, butyric acid fermentation, propionic acid fermentation, mixed acid fermentation, etc.), methanogenesis, acetogenesis, denitrification, sulfur reduction, alcohol (ethanol) fermentation, butanediol fermentation, etc. The major types of anaerobic respiration are:

• Lactic Acid Fermentation: It is a type of anaerobic respiration where sugar molecules (glucose and other six-carbon sugars) are metabolized to lactate, releasing the chemical energy in the form of ATP molecules. It is the fermentation process where pyruvate is produced at the end of glycolysis and is converted to lactate by the lactate dehydrogenase enzyme. It is also called ‘Lacto-fermentation’. The general equation of lactic acid fermentation can be expressed as: Glucose + ADP + NADH → Lactic acid + ATP + NAD+
• Ethanol Fermentation: It is another common type of anaerobic respiration process where the sugar molecules (glucose and other six-carbon sugars) are metabolized to ethanol, releasing the chemical energy in the form of ATP molecules. The pyruvate produced by the glycolytic cycle is converted to ethanol in the presence of the alcohol dehydrogenase enzyme. It is also called ‘alcoholic fermentation’. The general equation of ethanol fermentation can be expressed as: Glucose + ADP + NADH → Ethanol + ATP + NAD+

Anaerobic Respiration Equation

Anaerobic respiration is the process of breaking down glucose in the absence of oxygen to produce energy. Unlike aerobic respiration, which uses oxygen as the final electron acceptor, anaerobic respiration uses other molecules, such as sulfate, nitrate, sulfur, or organic compounds. The end products of anaerobic respiration vary depending on the type of electron acceptor and the organism involved. Some common examples of anaerobic respiration are lactic acid fermentation and ethanol fermentation.

Products of Anaerobic Respiration

Anaerobic respiration is the process of breaking down glucose in the absence of oxygen to produce ATP, the energy currency of the cell. Unlike aerobic respiration, which uses oxygen as the final electron acceptor, anaerobic respiration uses other molecules, such as sulfate, nitrate, or organic compounds, to accept electrons at the end of the electron transport chain. Anaerobic respiration also produces different byproducts depending on the type of electron acceptor and the organism performing it.

Lactic Acid and Ethanol

The most common types of anaerobic respiration are lactic acid fermentation and ethanol fermentation. These processes occur in some bacteria, yeasts, and animal cells, such as muscle cells. In both cases, glycolysis (the first stage of glucose breakdown) produces two molecules of pyruvate and two molecules of ATP. However, since there is no oxygen available, pyruvate cannot enter the Krebs cycle or the electron transport chain. Instead, it is converted into another molecule that regenerates NAD+, which is needed for glycolysis to continue.

In lactic acid fermentation, pyruvate is reduced to lactate (the conjugate base of lactic acid) by the enzyme lactate dehydrogenase. This process occurs in some bacteria, such as Lactobacillus, which are used to make yogurt and cheese. It also occurs in animal cells when oxygen is scarce, such as during intense exercise. Lactic acid accumulation in the muscles causes fatigue and soreness.

In ethanol fermentation, pyruvate is first decarboxylated to acetaldehyde by the enzyme pyruvate decarboxylase, releasing a molecule of carbon dioxide. Then, acetaldehyde is reduced to ethanol by the enzyme alcohol dehydrogenase. This process occurs in some yeasts, such as Saccharomyces cerevisiae, which are used to make bread, beer, and wine.

The general equations for lactic acid fermentation and ethanol fermentation are:

Glucose + 2 ADP + 2 Pi → 2 Lactate + 2 ATP

Glucose + 2 ADP + 2 Pi → 2 Ethanol + 2 ATP + 2 CO2

Other Products

Some prokaryotes (bacteria and archaea) can use other inorganic molecules as electron acceptors in anaerobic respiration. For example, some bacteria can use sulfate (SO4^2-) as the final electron acceptor, producing hydrogen sulfide (H2S) as a byproduct. This process is called sulfate reduction and occurs in some marine sediments and wetlands.

Some bacteria can use nitrate (NO3^-) as the final electron acceptor, producing nitrite (NO2^-), nitric oxide (NO), nitrous oxide (N2O), or nitrogen gas (N2) as byproducts. This process is called denitrification and occurs in some soils and aquatic environments.

Some archaea can use carbon dioxide (CO2) as the final electron acceptor, producing methane (CH4) as a byproduct. This process is called methanogenesis and occurs in some anaerobic habitats, such as swamps and animal guts.

The general equations for these processes are:

Glucose + SO4^2- → H2S + CO2 + ATP

Glucose + NO3^- → NO2^-/NO/N2O/N2 + CO2 + ATP

Glucose + CO2 → CH4 + CO2 + ATP

These products of anaerobic respiration have various ecological and economic impacts. For instance, hydrogen sulfide is toxic and corrosive, but it can also be used as a source of energy by some chemosynthetic bacteria. Nitrous oxide and methane are potent greenhouse gases that contribute to global warming. Methane can also be used as a fuel or a feedstock for organic synthesis.

Application of Anaerobic Respiration

Anaerobic respiration is a process that has many applications in different fields, such as biotechnology, environmental engineering, food production, and medicine. Some of the applications are:

• Biogas production: Anaerobic respiration is used to produce biogas, which is a mixture of methane and carbon dioxide, from organic wastes such as sewage, animal manure, crop residues, and food waste . Biogas can be used as a renewable source of energy for cooking, heating, electricity generation, and transportation. Anaerobic respiration also reduces the emission of greenhouse gases and the pollution of water and soil by decomposing the organic wastes.
• Microbial fuel cells: Anaerobic respiration is useful in generating electricity in microbial fuel cells, which employ bacteria that respire solid electron acceptors (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity. Microbial fuel cells can be used for wastewater treatment, biosensors, bioremediation, and biohydrogen production.
• Food production: Anaerobic respiration is involved in the production of many fermented foods and beverages, such as yogurt, cheese, bread, beer, wine, vinegar, soy sauce, sauerkraut, kimchi, and pickles . Anaerobic respiration enhances the flavor, texture, shelf-life, and nutritional value of these products by producing organic acids, alcohols, carbon dioxide, and other compounds . Anaerobic respiration also prevents the growth of spoilage and pathogenic microorganisms by lowering the pH and creating anaerobic conditions .

• Medicine: Anaerobic respiration plays a role in some physiological processes and pathological conditions in humans and animals. For example, anaerobic respiration occurs in muscle cells during intense exercise when oxygen supply is insufficient, producing lactic acid that causes muscle fatigue and pain . Anaerobic respiration also occurs in red blood cells, brain cells, retina cells, and some cancer cells that lack mitochondria or have impaired oxygen utilization . Moreover, anaerobic respiration is performed by some pathogenic bacteria that cause infections such as tetanus, botulism, gangrene, dental caries, and periodontal disease .

  Examples of Anaerobic Respiration

Anaerobic respiration is the process of breaking down glucose or other organic molecules in the absence of oxygen, using different electron acceptors and producing different end products. Some examples of anaerobic respiration are:

• Lactic acid production in muscles: During intense exercise, the muscles in our body cannot get enough oxygen and thus perform more glycolysis than the body can transfer oxygen to the electron transport chain. This results in anaerobic respiration due to insufficient oxygen in our muscles. The pyruvate produced by glycolysis is converted to lactic acid by the enzyme lactate dehydrogenase, regenerating NAD+ for glycolysis to continue. Lactic acid accumulates in the muscles, causing fatigue and soreness .

• Alcohol fermentation in yeast: Yeast cells are able to ferment sugars into ethanol and carbon dioxide in anaerobic conditions, such as in bread dough or wine production. The pyruvate produced by glycolysis is first decarboxylated to acetaldehyde by the enzyme pyruvate decarboxylase, releasing CO2. Then, acetaldehyde is reduced to ethanol by the enzyme alcohol dehydrogenase, regenerating NAD+ for glycolysis to continue. Ethanol gives alcoholic beverages their intoxicating effects, while CO2 makes bread rise .

• Methanogenesis in methanogens: Methanogens are a group of archaea that live in anaerobic environments, such as swamps, marshes, or the guts of ruminants. They use carbon dioxide as the final electron acceptor in their electron transport chain, producing methane as a by-product. Methane is a potent greenhouse gas that contributes to global warming. Methanogens also play a role in biogas production, which can be used as a renewable source of energy .

• Propionic acid fermentation in bacteria: Some bacteria, such as Propionibacterium and Clostridium, can ferment sugars or lactate into propionic acid and other organic acids in anaerobic conditions. Propionic acid fermentation is responsible for the flavor and texture of Swiss cheese, as the bacteria produce CO2 that forms holes in the cheese. Propionic acid fermentation also occurs in the rumen of ruminants, where it helps digest cellulose .

These are some of the examples of anaerobic respiration that show how diverse and important this process is for different organisms and industries.

Glycolysis is the first step of both aerobic and anaerobic respiration. It takes place in the cytoplasm of the cell and does not require oxygen. In glycolysis, a glucose molecule (a six-carbon sugar) is split into two molecules of pyruvate (a three-carbon compound). This process releases two molecules of ATP (the energy currency of the cell) and two molecules of NADH (an electron carrier).

Glycolysis can be divided into three phases: energy investment, splitting, and energy generation.

Energy investment

In this phase, two molecules of ATP are used to phosphorylate (add a phosphate group to) glucose, forming fructose-1,6-bisphosphate. This makes glucose more unstable and ready to be split into two smaller molecules.

Splitting

In this phase, fructose-1,6-bisphosphate is cleaved by an enzyme called aldolase into two molecules of glyceraldehyde-3-phosphate (G3P). Another enzyme, triosephosphate isomerase, converts one of the G3P molecules into dihydroxyacetone phosphate (DHAP), which can be interconverted with G3P. Thus, there are two molecules of G3P available for the next phase.

Energy generation

In this phase, each G3P molecule is oxidized by an enzyme called glyceraldehyde-3-phosphate dehydrogenase, producing 1,3-bisphosphoglycerate (BPG) and NADH. The BPG molecule then transfers a phosphate group to ADP, forming ATP and 3-phosphoglycerate (3PG). This process is repeated for the second G3P molecule, resulting in two molecules of ATP and two molecules of 3PG. The 3PG molecules are then converted into 2-phosphoglycerate (2PG) by an enzyme called phosphoglycerate mutase. The 2PG molecules are then dehydrated by an enzyme called enolase, forming phosphoenolpyruvate (PEP). The PEP molecules then transfer a phosphate group to ADP, forming ATP and pyruvate. This process is repeated for the second PEP molecule, resulting in two more molecules of ATP and two molecules of pyruvate.

The overall equation for glycolysis is:

Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Fermentation is the process where pyruvate produced in glycolysis is reduced to an end product of anaerobic respiration (mainly ethanol or lactic acid). In the process, NADH is oxidized to NAD+, and the released proton is used to reduce the pyruvate in the presence of a different enzyme. Fermentation may be homo-fermentation, i.e., producing only one type of product by reducing pyruvate, or maybe hetero-fermentation, i.e., producing two or more end products while reducing the pyruvate. The end product of pyruvate reduction depends on the enzyme involved and the types of fermentation. In lactic acid fermentation, the end product is lactic acid, and in alcohol fermentation, the end product is ethanol.

Fermentation does not involve an electron transport system and does not directly produce any additional ATP beyond that produced during glycolysis by substrate-level phosphorylation. Organisms carrying out fermentation, called fermenters, produce a maximum of two ATP molecules per glucose during glycolysis. Fermentation does not require oxygen and can occur in both aerobic and anaerobic environments. The study of fermentation is called zymology.

Fermentation can be classified into several types based on the type of end product of respiration, such as organic acid fermentation (lactic acid fermentation, butyric acid fermentation, propionic acid fermentation, mixed acid fermentation, etc.), methanogenesis, acetogenesis, denitrification, sulfur reduction, alcohol (ethanol) fermentation, butanediol fermentation, etc. The major types of anaerobic respiration are:

• Lactic Acid Fermentation: It is a type of anaerobic respiration where sugar molecules (glucose and other six-carbon sugars) are metabolized to lactate, releasing the chemical energy in the form of ATP molecules. It is the fermentation process where pyruvate is produced at the end of glycolysis and is converted to lactate by the lactate dehydrogenase enzyme. It is also called ‘Lacto-fermentation’. The general equation of lactic acid fermentation can be expressed as: Glucose + ADP + NADH → Lactic acid + ATP + NAD+
• Ethanol Fermentation: It is another common type of anaerobic respiration process where the sugar molecules (glucose and other six-carbon sugars) are metabolized to ethanol, releasing the chemical energy in the form of ATP molecules. The pyruvate produced by the glycolytic cycle is converted to ethanol in the presence of the alcohol dehydrogenase enzyme. It is also called ‘alcoholic fermentation’. The general equation of ethanol fermentation can be expressed as: Glucose + ADP + NADH → Ethanol + ATP + NAD+

Anaerobic respiration is the process of breaking down glucose in the absence of oxygen to produce energy. Unlike aerobic respiration, which uses oxygen as the final electron acceptor, anaerobic respiration uses other molecules, such as sulfate, nitrate, sulfur, or organic compounds. The end products of anaerobic respiration vary depending on the type of electron acceptor and the organism involved. Some common examples of anaerobic respiration are lactic acid fermentation and ethanol fermentation.

Anaerobic respiration is the process of breaking down glucose in the absence of oxygen to produce ATP, the energy currency of the cell. Unlike aerobic respiration, which uses oxygen as the final electron acceptor, anaerobic respiration uses other molecules, such as sulfate, nitrate, or organic compounds, to accept electrons at the end of the electron transport chain. Anaerobic respiration also produces different byproducts depending on the type of electron acceptor and the organism performing it.

Lactic Acid and Ethanol

The most common types of anaerobic respiration are lactic acid fermentation and ethanol fermentation. These processes occur in some bacteria, yeasts, and animal cells, such as muscle cells. In both cases, glycolysis (the first stage of glucose breakdown) produces two molecules of pyruvate and two molecules of ATP. However, since there is no oxygen available, pyruvate cannot enter the Krebs cycle or the electron transport chain. Instead, it is converted into another molecule that regenerates NAD+, which is needed for glycolysis to continue.

In lactic acid fermentation, pyruvate is reduced to lactate (the conjugate base of lactic acid) by the enzyme lactate dehydrogenase. This process occurs in some bacteria, such as Lactobacillus, which are used to make yogurt and cheese. It also occurs in animal cells when oxygen is scarce, such as during intense exercise. Lactic acid accumulation in the muscles causes fatigue and soreness.

In ethanol fermentation, pyruvate is first decarboxylated to acetaldehyde by the enzyme pyruvate decarboxylase, releasing a molecule of carbon dioxide. Then, acetaldehyde is reduced to ethanol by the enzyme alcohol dehydrogenase. This process occurs in some yeasts, such as Saccharomyces cerevisiae, which are used to make bread, beer, and wine.

The general equations for lactic acid fermentation and ethanol fermentation are:

Glucose + 2 ADP + 2 Pi → 2 Lactate + 2 ATP

Glucose + 2 ADP + 2 Pi → 2 Ethanol + 2 ATP + 2 CO2

Other Products

Some prokaryotes (bacteria and archaea) can use other inorganic molecules as electron acceptors in anaerobic respiration. For example, some bacteria can use sulfate (SO4^2-) as the final electron acceptor, producing hydrogen sulfide (H2S) as a byproduct. This process is called sulfate reduction and occurs in some marine sediments and wetlands.

Some bacteria can use nitrate (NO3^-) as the final electron acceptor, producing nitrite (NO2^-), nitric oxide (NO), nitrous oxide (N2O), or nitrogen gas (N2) as byproducts. This process is called denitrification and occurs in some soils and aquatic environments.

Some archaea can use carbon dioxide (CO2) as the final electron acceptor, producing methane (CH4) as a byproduct. This process is called methanogenesis and occurs in some anaerobic habitats, such as swamps and animal guts.

The general equations for these processes are:

Glucose + SO4^2- → H2S + CO2 + ATP

Glucose + NO3^- → NO2^-/NO/N2O/N2 + CO2 + ATP

Glucose + CO2 → CH4 + CO2 + ATP

These products of anaerobic respiration have various ecological and economic impacts. For instance, hydrogen sulfide is toxic and corrosive, but it can also be used as a source of energy by some chemosynthetic bacteria. Nitrous oxide and methane are potent greenhouse gases that contribute to global warming. Methane can also be used as a fuel or a feedstock for organic synthesis.

Anaerobic respiration is a process that has many applications in different fields, such as biotechnology, environmental engineering, food production, and medicine. Some of the applications are:

• Biogas production: Anaerobic respiration is used to produce biogas, which is a mixture of methane and carbon dioxide, from organic wastes such as sewage, animal manure, crop residues, and food waste . Biogas can be used as a renewable source of energy for cooking, heating, electricity generation, and transportation. Anaerobic respiration also reduces the emission of greenhouse gases and the pollution of water and soil by decomposing the organic wastes.
• Microbial fuel cells: Anaerobic respiration is useful in generating electricity in microbial fuel cells, which employ bacteria that respire solid electron acceptors (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity. Microbial fuel cells can be used for wastewater treatment, biosensors, bioremediation, and biohydrogen production.
• Food production: Anaerobic respiration is involved in the production of many fermented foods and beverages, such as yogurt, cheese, bread, beer, wine, vinegar, soy sauce, sauerkraut, kimchi, and pickles . Anaerobic respiration enhances the flavor, texture, shelf-life, and nutritional value of these products by producing organic acids, alcohols, carbon dioxide, and other compounds . Anaerobic respiration also prevents the growth of spoilage and pathogenic microorganisms by lowering the pH and creating anaerobic conditions .

• Medicine: Anaerobic respiration plays a role in some physiological processes and pathological conditions in humans and animals. For example, anaerobic respiration occurs in muscle cells during intense exercise when oxygen supply is insufficient, producing lactic acid that causes muscle fatigue and pain . Anaerobic respiration also occurs in red blood cells, brain cells, retina cells, and some cancer cells that lack mitochondria or have impaired oxygen utilization . Moreover, anaerobic respiration is performed by some pathogenic bacteria that cause infections such as tetanus, botulism, gangrene, dental caries, and periodontal disease .

  Examples of Anaerobic Respiration

Anaerobic respiration is the process of breaking down glucose or other organic molecules in the absence of oxygen, using different electron acceptors and producing different end products. Some examples of anaerobic respiration are:

• Lactic acid production in muscles: During intense exercise, the muscles in our body cannot get enough oxygen and thus perform more glycolysis than the body can transfer oxygen to the electron transport chain. This results in anaerobic respiration due to insufficient oxygen in our muscles. The pyruvate produced by glycolysis is converted to lactic acid by the enzyme lactate dehydrogenase, regenerating NAD+ for glycolysis to continue. Lactic acid accumulates in the muscles, causing fatigue and soreness .

• Alcohol fermentation in yeast: Yeast cells are able to ferment sugars into ethanol and carbon dioxide in anaerobic conditions, such as in bread dough or wine production. The pyruvate produced by glycolysis is first decarboxylated to acetaldehyde by the enzyme pyruvate decarboxylase, releasing CO2. Then, acetaldehyde is reduced to ethanol by the enzyme alcohol dehydrogenase, regenerating NAD+ for glycolysis to continue. Ethanol gives alcoholic beverages their intoxicating effects, while CO2 makes bread rise .

• Methanogenesis in methanogens: Methanogens are a group of archaea that live in anaerobic environments, such as swamps, marshes, or the guts of ruminants. They use carbon dioxide as the final electron acceptor in their electron transport chain, producing methane as a by-product. Methane is a potent greenhouse gas that contributes to global warming. Methanogens also play a role in biogas production, which can be used as a renewable source of energy .

• Propionic acid fermentation in bacteria: Some bacteria, such as Propionibacterium and Clostridium, can ferment sugars or lactate into propionic acid and other organic acids in anaerobic conditions. Propionic acid fermentation is responsible for the flavor and texture of Swiss cheese, as the bacteria produce CO2 that forms holes in the cheese. Propionic acid fermentation also occurs in the rumen of ruminants, where it helps digest cellulose .

These are some of the examples of anaerobic respiration that show how diverse and important this process is for different organisms and industries.

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