Difference Between Homolactic and Heterolactic Fermentation

The key difference between homolactic and heterolactic fermentation is that in homolactic fermentation, one glucose molecule converts into two lactic acid molecules, whereas in heterolactic fermentation, one glucose molecule forms lactic acid, carbon dioxide, and ethanol.

Fermentation is a metabolic process where organic molecules convert into acids, gases or alcohols. This process occurs in the absence of oxygen or any other electron transport chain. The major function of a fermentation process is to regenerate NAD+ from NADH so that it can be used again in the glycolysis process. There are two major types of fermentation as lactic acid fermentation and ethanol fermentation.

CONTENTS

1. Overview and Key Difference
2. What is Lactic Acid Fermentation
3. What is Homolactic Fermentation
4. What is Heterolactic Fermentation
5. Homolactic vs Heterolactic Fermentation in Tabular Form
6. Summary – Homolactic vs Heterolactic Fermentation

What is Lactic Acid Fermentation?

Lactic acid fermentation is a biological process in which glucose or a similar sugar molecule is converted into cellular energy and metabolite lactate. Here, the sugar molecule can be either glucose or another six-carbon sugar molecule. Disaccharides such as sucrose can also be used. Lactate is lactic acid in a solution. Lactic acid fermentation is an anaerobic process that takes place in some bacteria and animal cells, including muscle cells.

What is Homolactic Fermentation?

Homolactic fermentation is the conversion of one glucose molecule into two lactic acid molecules. It is the opposite of heterolactic fermentation. The process of homolactic fermentation involves homofermentative bacteria, which can convert glucose into two molecules of lactate, and this chemical reaction is used for the performance of substrate-level phosphorylation in order to make two ATP molecules. The reaction is as follows:

Glucose + 2 ADP + 2Pi → 2 lactate + 2 ATP

During the homolactic fermentation process, pyruvate undergoes reduction to lactate or lactic acid in the presence of lactate dehydrogenase enzyme. This process is named “homo-“ because it produces a single acid as the end product.

Figure 01: Lactic Acid Fermentation

Generally, the lactic acid bacteria species that can carry out homolactic fermentation are named homofermenters. These bacteria can mainly produce lactic acid through the glycolytic pathway. The most common forms of bacteria that can perform this process include Lactococcus lactis, Streptococcus species, and thermobacteria species.

What is Heterolactic Fermentation?

Heterolactic fermentation is the conversion of one glucose molecule into the lactic acid molecule, carbon dioxide, and ethanol. It is the opposite process of homolactic fermentation. This process involves heterofermentative bacteria that are able to produce less lactate and less amount ATP comparatively, but they can produce several other products, including ethanol and carbon dioxide. The chemical reaction for this process is as follows:

Glucose +  ADP + 2Pi →  lactate + ethanol + CO2 + ATP

Some examples of heterofermentative bacteria include Leuconostoc mesenteroides, Lactobacillus bifermentous, and Leconostoc lactis.

What is the Difference Between Homolactic and Heterolactic Fermentation?

Fermentation is an important biological process. There are two types as ethanol fermentation and lactic acid fermentation. Furthermore, lactic acid fermentation can be categorized into two groups as homolactic and heterolactic fermentation. The key difference between homolactic and heterolactic fermentation is that in homolactic fermentation, one glucose molecule converts into two lactic acid molecules whereas, in heterolactic fermentation, one glucose molecule forms lactic acid, carbon dioxide and ethanol.

Moreover, homolactic fermentation has a high ATP production compared to heterolactic fermentation.  Besides, homolactic fermentation involves homofermenters including Lactococcus lactis, Streptococcus species and thermobacteria species, whereas heterolactic fermentation involves heterofermenters including Leuconostoc mesenteroides, Lactobacillus bifermentous, and Leconostoc lactis.

The below infographic compiles the differences between homolactic and heterolactic fermentation in tabular form.

Summary – Homolactic vs Heterolactic Fermentation

Fermentation is an important biological process. There are two types as ethanol fermentation and lactic acid fermentation. Further, lactic acid fermentation can be categorized into two groups as homolactic and heterolactic fermentation. The key difference between homolactic and heterolactic fermentation is that in homolactic fermentation, one glucose molecule converts into two lactic acid molecules, whereas in heterolactic fermentation, one glucose molecule forms lactic acid, carbon dioxide and ethanol.

Reference:

1. “Anaerobic Respiration: Homolactic Fermentation.” SparkNotes.

Image Courtesy:

1. “Lactic acid fermentation” By Sjantoni – Own work (CC BY-SA 3.0) via Commons Wikimedia

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