Topic: Plant Physiology
Subtopic: Respiration
Heading: Fermentation (anaerobic)
Title: Fermentation (anaerobic)
Fermentation is a metabolic process that occurs in the absence of oxygen, allowing
cells to generate energy in anaerobic conditions. It is characterized by several
properties that vary based on the type of organisms involved and the specific
fermentation pathway.
Fermentation takes place in the absence of oxygen. It provides an alternative way for
cells to generate ATP when oxygen is limited or unavailable.
One of the primary purposes of fermentation is to regenerate NAD + from NADH.
During glycolysis, NAD+ is used to oxidize glucose, forming NADH. Fermentation
ensures the recycling of NAD+ to sustain glycolysis.
Unlike aerobic respiration, which completely oxidizes glucose to carbon dioxide and
water, fermentation involves the partial oxidation of substrates. The end products
depend on the specific type of fermentation and the organism involved.
Different organisms engage in various fermentation pathways, leading to the
production of diverse end products. Examples include lactic acid, ethanol, carbon
dioxide, and various organic acids.
Fermentation is a way for cells to generate energy in the form of ATP. However, the
ATP yield in fermentation is lower compared to aerobic respiration.
In the absence of oxygen, cells use organic compounds (e.g., pyruvate) as electron
acceptors. This allows the continuation of glycolysis by regenerating NAD+.
Different organisms use specific enzymes and pathways for fermentation. For
example, lactic acid fermentation involves enzymes like lactate dehydrogenase, while
alcoholic fermentation involves alcohol dehydrogenase.
Fermentation often produces characteristic waste products that are excreted from the
cell. These waste products have various industrial applications and contribute to the
distinct flavors of fermented products.
Some fermentation processes, particularly lactic acid fermentation, result in the
production of organic acids, leading to a decrease in pH. Organisms involved in these
processes often exhibit tolerance to low pH.
Fermentation is extensively used in the production of various food and beverage
products. Examples include the fermentation of dough for bread, the production of
yogurt and cheese, and the brewing of beer.
Fermentation processes are economically significant in industries such as
biotechnology, pharmaceuticals, and biofuel production. Microorganisms are
employed to produce valuable compounds through fermentation.
Types of fermentation:
Lactic Acid Fermentation:
Lactic acid fermentation is a metabolic process that occurs in the absence of oxygen,
allowing cells to generate energy and regenerate NAD + from NADH. This type of
fermentation is common in various microorganisms, including certain bacteria, fungi,
and human muscle cells during strenuous exercise.
The process begins with the initial steps of glycolysis, where a molecule of glucose is
broken down into two molecules of pyruvate. Glycolysis occurs in the cytoplasm of
the cell and does not require oxygen.
In lactic acid fermentation, the pyruvate generated from glycolysis is reduced to lactic
acid. The reduction reaction is catalyzed by the enzyme lactate dehydrogenase.
NADH, which was produced during glycolysis, donates electrons to pyruvate,
converting it into lactic acid.
The reduction of pyruvate to lactic acid is essential for the regeneration of NAD+. By
accepting electrons from NADH, pyruvate is reduced, and NAD+ is produced.
Regenerating NAD+ is crucial to maintaining the flow of glycolysis. If NAD+ is not
regenerated, glycolysis would come to a halt. The net reaction for lactic acid
fermentation is represented by the conversion of glucose to lactic acid:
Glucose + NAD+ Lactic acid + NADH + H+
Lactic acid fermentation is commonly observed in certain bacteria (e.g.,
Lactobacillus) used in the fermentation of dairy products, as well as in human muscle
cells during intense physical activity when oxygen supply is insufficient.
Lactic acid produced through fermentation has industrial applications. It is used in the
food industry for the production of yogurt, cheese, and pickled vegetables.
Additionally, lactic acid is utilized in the production of biodegradable plastics.
Lactic acid fermentation has limitations in terms of ATP production compared to
aerobic respiration, as glycolysis is the only ATP-generating pathway involved. The
accumulation of lactic acid may lead to a decrease in pH, affecting cellular processes.
Lactic acid fermentation is reversible. Under aerobic conditions, lactic acid can be
converted back to pyruvate, and the resulting pyruvate can enter the citric acid cycle
for further energy production.
Alcoholic fermentation:
Alcoholic fermentation is an anaerobic metabolic process that occurs in certain
microorganisms, including yeast and some bacteria. It involves the conversion of
sugars, typically glucose, into ethanol (alcohol) and carbon dioxide. This process is
widely used in the production of alcoholic beverages and has industrial applications.
The process begins with the initial steps of glycolysis, where a molecule of glucose is
broken down into two molecules of pyruvate. Glycolysis occurs in the cytoplasm of
the cell and does not require oxygen.
In alcoholic fermentation, each pyruvate molecule undergoes decarboxylation, a
process where a carbon dioxide molecule is removed. This step is catalyzed by the
enzyme pyruvate decarboxylase, resulting in the formation of acetaldehyde.
Acetaldehyde, produced from the decarboxylation of pyruvate, is then reduced to
ethanol. The reduction reaction is catalyzed by alcohol dehydrogenase. NADH, which
was generated during glycolysis, donates electrons to acetaldehyde, converting it into
ethanol.
The overall reaction is:
Acetaldehyde + NADH Ethanol + NAD+ + CO2
Similar to lactic acid fermentation, one of the key functions of alcoholic fermentation
is the regeneration of NAD+. By accepting electrons from NADH, acetaldehyde is
reduced to ethanol, regenerating NAD+ for glycolysis to continue.
The net reaction for alcoholic fermentation is represented by the conversion of
glucose to ethanol and carbon dioxide:
Glucose + NAD+ Ethanol + CO2
Alcoholic fermentation is predominantly observed in yeast, such as Saccharomyces
cerevisiae. Yeast cells undergo alcoholic fermentation in the absence of oxygen or
under anaerobic conditions.
Pathways of fermentation
Reference- NCERT Textbook for class XI.
