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

Respiration is a vital metabolic process occurring within living cells, involving the breakdown of organic nutrients to liberate cellular energy in the form of ATP (Adenosine Triphosphate). It encompasses a series of oxidation-reduction reactions wherein electrons are transferred from a donor to an acceptor molecule. This ultimately leads to the release and conservation of energy in the form of ATP. It is crucial to note that respiration is distinct from the act of breathing, which pertains to the inhalation of atmospheric oxygen (O2) and the exhalation of carbon dioxide (CO2). This biological process is often referred to as “cellular respiration.”

Respiration is categorized into two main types based on the nature of the terminal electron acceptor involved: aerobic respiration and anaerobic respiration. Aerobic respiration relies on oxygen (O2) as the final electron acceptor, while anaerobic respiration utilizes alternative molecules as terminal electron acceptors, distinct from oxygen.

Anaerobic Respiration

Anaerobic respiration refers to the cellular respiration process that occurs in an environment devoid of oxygen. In this type of respiration, electrons resulting from the oxidation-reduction of nutrients are transferred to various organic and inorganic electron acceptors, excluding oxygen molecules. This leads to the production of ATP, the energy currency of the cell. Common electron acceptors in anaerobic respiration include sulfur (S), sulfate (SO4-2), ferric ion (Fe3+), nitrate (NO3–), and DMSO.

Unlike aerobic respiration, which takes place in both the cytoplasm and mitochondria, anaerobic respiration is confined to the cytoplasm. It typically occurs in two stages: anaerobic glycolysis followed by fermentation. The end products of anaerobic respiration are either organic acids like lactic acid or ethanol, along with ATP molecules, representing stored energy. The overall reaction for anaerobic respiration of a glucose molecule can be summarized as:

Glucose + Electron Acceptors (other than O2) → Lactic acid / Ethanol + 2 ATP molecules

Types of Anaerobic Respiration

Anaerobic respiration encompasses various types, each characterized by the specific electron acceptor involved in the process. Here are some types of anaerobic respiration:

1. Lactic Acid Fermentation:

Lactic acid fermentation is a form of anaerobic respiration where six-carbon sugars, including glucose, are metabolized to produce lactate, along with the release of chemical energy in the form of ATP molecules. This process occurs in the absence of oxygen and takes place in the cytoplasm.

At the end of glycolysis, pyruvate is produced, which is then converted to lactate by the enzyme lactate dehydrogenase. The chemical equation for lactic acid fermentation can be summarized as:

Glucose + ADP + NADH → Lactic acid + ATP + NAD+

This type of fermentation, also known as ‘Lacto-fermentation,’ is widely used in food preservation and the production of fermented food items. Since ancient times, it has been employed to ferment various food groups like fruits, vegetables, cereals, milk (for producing fermented dairy products), and meat, ensuring their preservation for future consumption.

Lactic Acid Bacteria (LAB), including genera like Lactobacillus, Lactococcus, Leuconostoc, Streptococcus, Pediococcus, and Enterococcus, are instrumental in carrying out this fermentation process. LAB play a crucial role in the food industry and biotechnology.

Additionally, lactic acid fermentation occurs in the cytoplasm of higher animals, including humans. In situations of intense physical exertion, such as strenuous muscular activities, this type of respiration takes place in muscle tissues due to a limited oxygen supply. This can lead to muscle cramps, pain, and a sensation of fatigue after intense physical labor. Red Blood Cells (RBCs) also utilize lactic acid fermentation as they lack mitochondria, which are essential for aerobic respiration.

• Organisms: Certain bacteria (e.g., Lactobacillus), fungi, and animal muscle cells.
• End Product: Lactic acid.
• Example: Conversion of pyruvate to lactic acid during strenuous exercise in muscles.

2. Alcoholic Fermentation:

Ethanol fermentation is a prevalent form of anaerobic respiration where six-carbon sugars, including glucose, undergo metabolism to produce ethanol, along with the release of chemical energy in the form of ATP molecules. This process occurs in the absence of oxygen and takes place in the cytoplasm.

During this fermentation, pyruvate, generated in the glycolytic cycle, is converted to ethanol with the help of the enzyme alcohol dehydrogenase. The chemical equation for ethanol fermentation can be summarized as:

Glucose + ADP + NADH → Ethanol + ATP + NAD+

Commonly referred to as ‘alcoholic fermentation,’ this process finds universal application in the production of alcoholic beverages. It is carried out by a range of microorganisms, including yeasts like Saccharomyces cerevisiae, Schizosaccharomyces, Zygosaccharomyces, Candida, Pichia, and others. Additionally, certain fungi like Aspergillus oryzae, as well as bacteria like Zymomonas, are utilized for this purpose.

Apart from microorganisms, certain higher animals, such as Goldfish, are also capable of undergoing ethanol fermentation. This adaptation allows them to respire efficiently in oxygen-depleted environments.

• Organisms: Yeasts and some bacteria (e.g., Saccharomyces cerevisiae).
• End Product: Ethanol and carbon dioxide.
• Example: Used in the production of bread, beer, and wine.

3. Acetone-Butanol-Ethanol (ABE) Fermentation:
   • Organisms: Clostridium species.
   • End Products: Acetone, butanol, and ethanol.
   • Example: Historically used for industrial production of solvents.
4. Sulfate Respiration:
   • Organisms: Sulfate-reducing bacteria (e.g., Desulfovibrio).
   • End Product: Hydrogen sulfide (H2S).
   • Example: Common in anaerobic environments rich in sulfate, like sediments.
5. Methanogenesis:
   • Organisms: Methanogenic archaea (e.g., Methanobacterium).
   • End Product: Methane (CH4).
   • Example: Occurs in anaerobic environments like swamps, marshes, and the digestive tracts of certain animals.
6. Nitrate Respiration:
   • Organisms: Denitrifying bacteria (e.g., Pseudomonas).
   • End Product: Nitrogen gas (N2).
   • Example: Takes place in environments with limited oxygen, where nitrate (NO3-) serves as an alternative electron acceptor.
7. Fumarate Respiration:
   • Organisms: Bacteria like E. coli.
   • End Product: Succinate.
   • Example: Utilized when oxygen is scarce and fumarate is available.

Steps of Anaerobic Respiration

Glycolysis:

This is the first step of anaerobic respiration and it takes place in the cytoplasm. Glycolysis is a series of reactions that convert glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). During this process, two molecules of ATP and two molecules of NADH are generated.

Fermentation:

After glycolysis, in the absence of oxygen, pyruvate is converted into various end products depending on the organism and the type of fermentation:

1. Lactic Acid Fermentation: In this type, pyruvate is converted to lactic acid by the enzyme lactate dehydrogenase. This process regenerates NAD+ so that glycolysis can continue. Lactic acid fermentation is commonly utilized by certain bacteria and our muscles during strenuous exercise.
2. Alcohol (Ethanol) Fermentation: This type is carried out by organisms like yeast. Pyruvate is converted into acetaldehyde and carbon dioxide. Then, acetaldehyde is further reduced to ethanol by the enzyme alcohol dehydrogenase. This process also regenerates NAD+.

Anaerobic 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

The products of anaerobic respiration vary depending on the type of fermentation:

1. Lactic Acid Fermentation:
   • End Product: Lactic Acid
   • Regenerated Coenzyme: NAD+
2. Alcohol (Ethanol) Fermentation:
   • End Products: Ethanol, Carbon Dioxide
   • Regenerated Coenzyme: NAD+

These end products are essential for replenishing coenzymes (like NAD+) to sustain the glycolytic pathway, allowing cells to continue generating ATP in the absence of oxygen.

Application of Anaerobic Respiration

Anaerobic respiration has several applications in various fields:

1. Food and Beverage Industry:
   • Fermentation: Anaerobic respiration is utilized in the production of various food and beverage products. For example, in the production of bread, yeast undergoes alcoholic fermentation, producing carbon dioxide which causes the dough to rise.
2. Biotechnology:
   • Industrial Fermentation: Anaerobic respiration is used in large-scale fermentation processes to produce a wide range of products, including biofuels, pharmaceuticals, organic acids, enzymes, and more.
3. Waste Treatment:
   • Anaerobic Digestion: It is used in wastewater treatment plants to break down organic waste. Microorganisms convert organic matter into biogas (methane and carbon dioxide), which can be used as an energy source.
4. Bioenergy Production:
   • Biogas Production: Anaerobic digestion of organic materials like agricultural waste, manure, and sewage sludge generates biogas, which can be used as a renewable energy source.
5. Medical Research:
   • Study of Muscle Fatigue: Understanding lactic acid fermentation is important in sports medicine for studying muscle fatigue and recovery.
6. Pharmaceuticals:
   • Antibiotic Production: Some antibiotics, such as erythromycin, are produced through fermentation processes that involve anaerobic conditions.
7. Environmental Bioremediation:
   • Bioremediation of Contaminated Sites: Anaerobic microorganisms are used to degrade pollutants in contaminated soil and groundwater.
8. Animal Digestive Systems:
   • Gut Microbiota: Anaerobic microorganisms in the digestive tracts of animals help in the digestion of complex carbohydrates.

Examples of Anaerobic Respiration

1. Lactic Acid Fermentation:
   • Occurrence: Muscles of animals during intense exercise or when there’s a shortage of oxygen.
   • Microorganisms: Certain bacteria (e.g., Lactobacillus) and fungi (e.g., Rhizopus) carry out lactic acid fermentation.
2. Alcoholic Fermentation:
   • Occurrence: Yeasts, such as Saccharomyces cerevisiae, carry out alcoholic fermentation.
   • Applications: Used in the production of alcoholic beverages like beer, wine, and in bread-making.
3. Anaerobic Digestion:
   • Occurrence: In the digestive systems of animals (e.g., ruminants like cows) and in anaerobic environments like swamps and landfills.
   • Microorganisms: Methanogenic archaea and other anaerobic bacteria break down organic matter to produce biogas (methane and carbon dioxide).
4. Mixed Acid Fermentation:
   • Occurrence: Some bacteria, like Escherichia coli, perform mixed acid fermentation.
   • Products: Produces a mixture of organic acids, including formic, acetic, lactic, and succinic acids.
5. Propionic Acid Fermentation:
   • Occurrence: Certain bacteria, like Propionibacterium, carry out this type of fermentation.
   • Applications: Used in the production of Swiss cheese, where propionic acid contributes to flavor and texture.
6. Butyric Acid Fermentation:
   • Occurrence: Performed by bacteria like Clostridium.
   • Products: Produces butyric acid and other organic acids.

Alcoholic Beverage Production

• Alcoholic Beverage Production:

Alcoholic beverage production involves the fermentation of sugars by yeast to produce ethanol and other byproducts. The choice of raw material depends on the type of beverage. For example, grapes are used for wine, malted grains for beer, and fruits for spirits like brandy. The raw material is harvested and processed, with grapes being crushed for wine production. Grains, like barley, are malted for beer, converting starches into fermentable sugars. The sugary liquid (wort) is created through mashing malted grains with hot water. Yeast is added to the juice or wort, initiating the fermentation process. This process can take several days to weeks. In the case of spirits like whiskey or brandy, the fermented liquid may undergo distillation to increase the alcohol content. Some beverages benefit from aging in barrels.

• Biogas Production:

Biogas, which primarily consists of methane, is produced through anaerobic fermentation of organic waste. This methane gas serves as an alternative source of energy.

• Production of Swiss Cheese:

Swiss cheese owes its distinct flavor and porous texture to propionic acid fermentation. This acid contributes to the flavor, while the release of CO2 gas creates the characteristic holes in the cheese surface.

• Vinegar Production:

Vinegar, utilized as a flavoring agent and food preservative, is comprised of about 5 – 8% acetic acid. Its production involves a two-step fermentation process. Initially, alcohol fermentation produces ethanol. Subsequently, acetic acid bacteria (Acetobacter) oxidize ethanol to acetic acid.

• Nitrate Reduction in Nitrogen Cycle:

Denitrification is an anaerobic respiration process in the nitrogen cycle where nitrate (NO3 – ) is reduced to nitrite (NO2 – ). Bacteria such as Pseudomonas, Clostridium, Geobacter, among others, facilitate denitrification.

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