Cellular respiration is the fundamental process by which cells convert the chemical energy stored in glucose and other nutrients into a usable form of energy called adenosine triphosphate (ATP). This complex pathway is divided into two phases based on the presence of oxygen: aerobic respiration (oxygen-dependent) and anaerobic respiration (oxygen-independent). The anaerobic reactions represent an ancient metabolic strategy that allows organisms to begin the energy extraction process even when oxygen is scarce or unavailable.
Pinpointing the Cellular Location
The anaerobic reactions of cellular respiration occur exclusively within the cytosol of the cell. The cytosol is the jelly-like substance that fills the cell and suspends the organelles. This location is consistent across both simple prokaryotic cells, which lack internal compartments, and complex eukaryotic cells, which contain membrane-bound organelles.
Anaerobic pathways do not require the specialized machinery found in the mitochondria, which are the powerhouses of the cell and the site of aerobic respiration. The necessary enzymes are freely dissolved and organized within the cytosol. This structural independence allows cells to begin breaking down glucose for energy without transporting starting materials into a specialized organelle.
The Initial Anaerobic Reaction
The initial stage of energy extraction is a process called glycolysis. Glycolysis begins with a single six-carbon molecule of glucose and ends with the formation of two three-carbon molecules of pyruvate. The pathway consists of ten sequential, enzyme-catalyzed steps that do not require molecular oxygen to proceed.
Glycolysis involves an energy-requiring phase where two ATP molecules are consumed to destabilize the glucose molecule. This is followed by an energy-releasing phase that generates four ATP molecules and two molecules of the electron carrier NADH. Therefore, the net yield of glycolysis for the cell is two ATP molecules and two NADH molecules per molecule of glucose. If oxygen is present, the pyruvate and NADH would move to the mitochondria to fuel the aerobic stages of respiration.
The Follow-Up Anaerobic Processes
When oxygen is absent, the pyruvate molecule cannot enter the mitochondria, and the cell must employ follow-up anaerobic processes to continue energy production. These processes are collectively known as fermentation. Fermentation occurs to address a specific chemical bottleneck: the regeneration of \(\text{NAD}^+\). Glycolysis consumes \(\text{NAD}^+\) by reducing it to NADH, and if the cell’s supply of \(\text{NAD}^+\) is exhausted, glycolysis would stop entirely, halting ATP production.
Fermentation solves this problem by using pyruvate, the product of glycolysis, as an electron acceptor to convert NADH back into \(\text{NAD}^+\). This recycling of the coenzyme allows the glycolytic pathway to continue running, providing a small but steady supply of two ATP per glucose molecule. The type of fermentation is defined by the end-product created from the pyruvate.
One common type is lactic acid fermentation, which occurs in human muscle cells during intense exercise when oxygen delivery is limited. In this reaction, pyruvate is directly converted into lactate, simultaneously oxidizing NADH back to \(\text{NAD}^+\). Another type is alcoholic fermentation, carried out by yeast and some bacteria, which converts pyruvate into ethanol and carbon dioxide.