Difference Between Aerobic and Anaerobic Fermentation

The main difference between aerobic and anaerobic fermentation is that aerobic fermentation regenerates NAD⁺ at electron transport chain whereas the regeneration of NAD⁺ in anaerobic respiration follows glycolysis.

Fermentation is a term used to describe the mechanisms of cellular respiration, which occurs in the absence of oxygen. However, in aerobic fermentation, the final electron acceptor in the electron transport chain is oxygen. Thereby, it is more precisely called aerobic respiration rather than aerobic fermentation. The two mechanisms of anaerobic fermentation are ethanol fermentation and lactic acid fermentation.

Key Areas Covered

1. What is Aerobic Fermentation
     – Definition, Process, Role
2. What is Anaerobic Fermentation
     – Definition, Process, Types, Role
3. What are the Similarities Between Aerobic and Anaerobic Fermentation
     – Outline of Common Features
4. What is the Difference Between Aerobic and Anaerobic Fermentation
     – Comparison of Key Differences

Key Terms: Aerobic Fermentation, Anaerobic Fermentation, ATP, Glucose, NAD⁺, Oxygen

What is Aerobic Fermentation

As mentioned above, aerobic respiration is the more precise and scientific term for aerobic fermentation.  Aerobic respiration refers to the set of chemical reactions involved in the production of energy by completely oxidizing food. It releases carbon dioxide and water as by-products. Aerobic respiration mainly occurs in higher animals and plants. It is the most efficient process among various processes of energy production. The three steps of aerobic respiration are glycolysis, Krebs cycle, and electron transport chain.

Glycolysis

Glycolysis is the first step of aerobic respiration, which occurs in the cytoplasm. This process breaks down glucose into two pyruvate molecules. The pyruvate molecules undergo oxidative decarboxylation to form acetyl-CoA. 2 ATP and 2 NADH are the yield of this process.

Krebs Cycle

Krebs cycle occurs inside the mitochondrial matrix. A complete breakdown of acetyl-CoA into carbon dioxide occurs in the Krebs cycle, regenerating the starting compound, oxaloacetate. During Krebs cycle, releasing the energy from acetyl-CoA produces 2 GTPs, 6 NADH, and 2 FADH₂.

Electron Transport Chain

The production of ATP during the oxidative phosphorylation uses the reducing power of NADH and FADH2. It occurs in the inner membrane of mitochondria. The below figure shows the overall chemical reaction of aerobic respiration.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 36ATP

Figure 1: Aerobic Respiration – Steps

What is Anaerobic Fermentation

Fermentation refers to the chemical breakdown of organic substrates by microorganisms into ethanol or lactic acid in the absence of oxygen. Typically, it gives off effervescence and heat. Fermentation occurs in the locality of the cytoplasm in microorganisms such as yeast, parasitic worms, and bacteria. The two steps of fermentation are glycolysis and partial oxidation of pyruvate. Based on the pathway of pyruvate oxidation, fermentation consists of two types; ethanol fermentation and lactic acid fermentation.  The net yield of fermentation is only 2 ATPs.

Figure 2: Aerobic and Anaerobic Fermentation

Ethanol Fermentation

Ethanol fermentation mainly occurs in yeast in the absence of oxygen. In this process, removing the carbon dioxide results in the decarboxylation of pyruvate into acetaldehyde. Acetaldehyde is then converted into ethanol by using the hydrogen atoms of the NADH. The effervescence occurs due to the release of carbon dioxide gas into the medium. The balanced chemical equation for ethanol fermentation is as follows:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

Lactic Acid Fermentation

Lactic acid fermentation mainly occurs in bacteria. During lactic acid fermentation, the pyruvate converts into lactic acid. The overall chemical reaction for ethanol fermentation and lactic acid fermentation are as follows:

C₆H₁₂O₆ → 2C₃H₆O₃ + 2ATP

Similarities Between Aerobic and Anaerobic Fermentation

• Aerobic and anaerobic fermentation are two mechanisms of cellular respiration that generates energy for the cellular processes.
• Both fermentations use glucose as the substrate and produce ATP during the processing.
• Carbon dioxide is a product in both processes.
• They both undergo glycolysis in the cytoplasm.

Difference Between Aerobic and Anaerobic Fermentation

Definition

Aerobic Fermentation: Set of chemical reactions involved in the production of energy by completely oxidizing food

Anaerobic Fermentation: Chemical breakdown of organic substrates into ethanol or lactic acid by microorganisms in the presence of oxygen

Occurrence

Aerobic Fermentation: Occurs in both cytoplasm and mitochondria

Anaerobic Fermentation: Occurs in the cytoplasm

Type of Organisms

Aerobic Fermentation: Occurs in higher animals and plants

Anaerobic Fermentation: Occurs in yeast, parasites, and bacteria

Oxygen

Aerobic Fermentation: Uses molecular oxygen as the final electron acceptor in the electron transport chain

Anaerobic Fermentation: Does not use oxygen

Water

Aerobic Fermentation: Produces six water molecules per glucose molecule

Anaerobic Fermentation: Does not produce water

Substrate Oxidization

Aerobic Fermentation: Glucose is completely broken down into carbon dioxide and oxygen 

Anaerobic Fermentation: Glucose is incompletely oxidized either into ethanol and lactic acid

NAD⁺ Regeneration

Aerobic Fermentation: NAD⁺ regeneration occurs in the electron transport chain

Anaerobic Fermentation: NAD⁺ regeneration occurs during the partial oxidation of pyruvate

ATP Production during NAD⁺ Regeneration

Aerobic Fermentation: ATP is a yield during the NAD⁺ regeneration 

Anaerobic Fermentation: ATP is not a yield during the NAD⁺ regeneration

Number of ATPs Produced

Aerobic Fermentation: Produces 36 ATP

Anaerobic Fermentation: Produces 2 ATP

Conclusion

Aerobic and anaerobic fermentation are two types of cellular respiration involved in the production of energy from glucose. Aerobic fermentation requires oxygen while anaerobic fermentation does not require oxygen. NAD⁺ regeneration occurs in the electron transport chain of the aerobic respiration while it occurs during the partial oxidation of pyruvate in anaerobic respiration. 

Reference:

1. “Fermentation and Anaerobic Respiration.” Khan Academy, Available here.

Image Courtesy:

1. “Cellular respiration flowchart” By Users Daycd, Pdefer, Bdesham on en.wikipedia – Created by bdesham with en:OmniGraffle; post-processed in en:GraphicConverter (Public Domain) via Commons Wikimedia
2. “Cellular respiration” By Darekk2 – Own work (CC BY-SA 3.0) via Commons Wikimedia