When oxygen is not available, almost all organisms can harvest energy from organic matter from a process called fermentation. Instead of using inorganic matter as the final electron acceptor, it uses organic matter as the electron acceptor to capture electrons from NADH, and the resulting NAD⁺ re-enters glycolysis. It produces only a small amount of ATP, and most of the energy is still stored in organic matter.
Alcoholic fermentation
In certain organisms, such as yeast, plant and some bacteria, anaerobic respiration can produce alcohol as a byproduct. This process is often referred to as alcoholic fermentation where the acetaldehyde is served as final electron acceptor.
The process of anaerobic respiration that produces alcohol involves several steps:
Glycolysis: The first step is glycolysis, which occurs in the cytoplasm of the cell. During glycolysis, a molecule of glucose is broken down into two molecules of pyruvate. This process generates a small amount of ATP and NADH.
Conversion of pyruvate to acetaldehyde: In the presence of enzymes, one molecule of pyruvate is converted into acetaldehyde, releasing carbon dioxide as a byproduct. This step is catalyzed by the enzyme pyruvate decarboxylase.
Reduction of acetaldehyde to ethanol: Acetaldehyde, the intermediate product from the previous step, is then reduced to ethanol. This conversion involves the transfer of electrons from NADH to acetaldehyde, forming ethanol and regenerating NAD+.
The overall reaction for anaerobic respiration producing alcohol or ethanol can be summarized as:
Glucose → 2 pyruvate + 2 ATP + 2 NADH
2 pyruvate → 2 acetaldehyde + 2 CO₂
2 acetaldehyde + 2 NADH → 2 ethanol + 2 NAD⁺
Lactic acid fermentation
Anaerobic respiration can also produce lactic acid as a byproduct in certain organisms, including some bacteria and our own muscle cells in the case of oxygen deficiency. This process is known as lactic acid fermentation where the pyruvate is served as final electron acceptor.
The process of anaerobic respiration that produces lactic acid involves the following steps:
Glycolysis: The initial step of lactic acid fermentation is glycolysis, which occurs in the cytoplasm of the cell. During glycolysis, a molecule of glucose is broken down into two molecules of pyruvate. This process generates a small amount of ATP and NADH.
Conversion of pyruvate to lactic acid: In the absence of oxygen, pyruvate is converted into lactic acid through a process called reduction. This conversion involves the transfer of electrons from NADH to pyruvate, forming lactic acid and regenerating NAD+. The enzyme lactate dehydrogenase catalyzes this reaction.
The overall reaction for anaerobic respiration producing lactic acid can be summarized as:
Glucose → 2 pyruvate + 2 ATP + 2 NADH
2 pyruvate + 2 NADH → 2 lactic acid + 2 NAD⁺