Cellular respiration is the biological process by which living cells convert nutrients into usable energy, primarily adenosine triphosphate (ATP). This metabolic pathway powers all necessary cellular activities, such as movement, growth, and repair. When oxygen is plentiful, cells use aerobic respiration, a highly efficient method for energy extraction.
Anaerobic respiration is an alternative metabolic strategy employed when oxygen is absent or insufficient. This pathway allows for the continuous, less efficient production of ATP under oxygen-limited conditions. Many microorganisms rely on this process exclusively, while complex organisms, such as humans, use it temporarily during intense energy demand.
Glucose as the Primary Fuel Source
The primary reactant molecule for anaerobic respiration is the six-carbon sugar, glucose. Glucose is the most important organic fuel source for energy metabolism across nearly all forms of life. It is readily available from the breakdown of larger carbohydrates, such as starches or glycogen.
While glucose is the default starting material, other organic molecules, like certain sugars, can also feed into this process. Fats and proteins can enter the broader cellular respiration pathway, but glucose provides the most direct starting point for anaerobic metabolism. Its structure allows it to be broken down efficiently without the immediate requirement for oxygen.
The Universal First Step: Glycolysis
Anaerobic respiration begins with glycolysis, a metabolic pathway shared universally by both aerobic and anaerobic energy production. This sequence of ten enzyme-catalyzed reactions takes place entirely in the cytosol. Oxygen is not required for glycolysis to proceed, making it the initial step for anaerobic pathways.
The process starts with an energy investment phase, consuming two molecules of ATP to destabilize the glucose molecule. The six-carbon glucose is then cleaved into two separate three-carbon molecules called pyruvate. This splitting marks the beginning of the payoff phase.
During the payoff phase, the chemical energy stored in the three-carbon molecules is harvested. This phase generates four ATP molecules, resulting in a net gain of two ATP per glucose molecule. Additionally, two molecules of the electron carrier NADH are produced, capturing high-energy electrons. The resulting pyruvate holds chemical energy that the cell must process further.
Shifting to Fermentation and Final Products
The formation of pyruvate concludes glycolysis, but the anaerobic stage begins immediately after, focusing on regenerating the electron carrier NAD+. The continuation of glycolysis relies on a steady supply of NAD+, which is consumed to produce NADH during the splitting of glucose. Without oxygen to process the NADH, the cell must find an alternative way to convert it back to NAD+.
Two distinct processes, known collectively as fermentation, accomplish this goal. The two most common types are lactic acid fermentation and alcoholic fermentation.
Lactic acid fermentation occurs in certain bacteria and in animal muscle cells during intense exercise when oxygen delivery cannot keep pace with energy demand. The enzyme lactate dehydrogenase transfers electrons from NADH directly to the pyruvate molecule. This action converts NADH back into NAD+, allowing glycolysis to continue, and simultaneously converts pyruvate into the three-carbon compound lactate.
Lactic acid fermentation is used commercially in the production of foods like yogurt, cheese, and sauerkraut. Alcoholic fermentation is characteristic of yeast and some plant cells. This process is a two-step conversion: first, pyruvate is converted into acetaldehyde, releasing carbon dioxide (CO2) as a byproduct.
In the second step, NADH transfers its electrons to acetaldehyde, regenerating NAD+ and forming the final end product, ethanol. The CO2 released during this pathway causes bread dough to rise and creates bubbles in fermented beverages. Regardless of the final product, the net energy yield of anaerobic respiration from a single glucose molecule is only two molecules of ATP.