Anaerobic respiration is a metabolic process cells use to generate chemical energy in the form of adenosine triphosphate (ATP) when the supply of oxygen is limited or entirely absent. This energy pathway allows organisms, from single-celled bacteria to human muscle tissue, to sustain temporary function under anoxic conditions. While oxygen-dependent respiration yields a large energy return, the anaerobic mechanism provides a rapid, though much smaller, burst of ATP production. This process is distinct from aerobic respiration because it does not utilize oxygen as the final electron acceptor. It ensures that immediate energy demands can be met even when the environment cannot support the primary power source.
The Initial Step: Generating Energy Without Oxygen
All anaerobic pathways begin with glycolysis, which occurs within the cell’s cytoplasm. During glycolysis, a six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvate. This initial conversion releases a net yield of two ATP molecules per glucose molecule.
Glycolysis also generates molecules of the reduced electron carrier NADH from the oxidized form, NAD+. The continued operation of glycolysis depends on a constant supply of NAD+ to accept electrons released during the glucose breakdown. In the absence of oxygen, the cell cannot recycle NADH back to NAD+ through the usual aerobic pathways.
Pyruvate must therefore be converted into a different compound to perform this recycling function. This conversion step, known as fermentation, is where NADH transfers its electrons to pyruvate or a derivative. This action regenerates the necessary supply of NAD+, allowing glycolysis to continue and providing the cell with ATP.
Byproduct Pathway 1: Lactic Acid Fermentation
The byproduct of lactic acid fermentation is lactate. This pathway is most relevant to the human body, occurring in muscle cells during periods of intense exercise when oxygen demand exceeds supply. It is also the mechanism used by certain bacteria, such as those belonging to the Lactobacillus genus, to ferment milk into dairy products.
In this single-step conversion, the enzyme lactate dehydrogenase directly reduces pyruvate by accepting electrons from NADH. The resulting chemical products are two molecules of lactate and the regenerated NAD+. This regeneration of NAD+ ensures the short-term continuation of energy production.
Lactate accumulation in muscle cells has been associated with the burning sensation and fatigue experienced during strenuous activity. However, lactate is readily transported to the liver where it can be converted back into glucose through the Cori cycle. In the food industry, the production of lactate by bacteria lowers the pH of milk, causing the proteins to coagulate and creating the characteristic flavor and texture of yogurt and cheese.
Byproduct Pathway 2: Alcoholic Fermentation
Alcoholic fermentation is a two-step anaerobic process carried out primarily by yeast, such as Saccharomyces cerevisiae, and some plant cells. This pathway produces two byproducts: ethanol and carbon dioxide. The first step involves the enzyme pyruvate decarboxylase removing a carboxyl group from pyruvate, releasing carbon dioxide (CO2) and forming the intermediate molecule acetaldehyde.
In the second step, the enzyme alcohol dehydrogenase uses the electrons carried by NADH to reduce acetaldehyde, resulting in the final byproduct, ethanol. This reaction simultaneously regenerates the NAD+ needed to maintain the flow of glycolysis. The overall process converts one glucose molecule into two molecules of ethanol, two molecules of carbon dioxide, and two net molecules of ATP.
The byproducts of this pathway have commercial applications in the baking and brewing industries. In bread making, the released CO2 forms bubbles, causing the dough to rise before the ethanol evaporates during baking. For alcoholic beverages, ethanol is the desired product, and the CO2 contributes to the carbonation.