Fermentation: Keeping the Cell Running Without Oxygen

Cellular respiration is the sequence of reactions and processes that cells use to produce ATP, but the processes inside of the mitochondria depend on oxygen. When oxygen is unavailable, anaerobic conditions, cells face a critical problem. Glycolysis, the first stage of respiration, requires a steady supply of NAD⁺ to keep running. NAD⁺ can be regenerated through the electron transport chain, but this requires oxygen. When oxygen is not present, the cell cannot regenerate NAD⁺ using the electron transport chain, glycolysis stalls and ATP production stops entirely. Fermentation solves this molecular problem.

Fermentation produces no additional ATP beyond the 2 net ATP already generated by glycolysis. Its function is to regenerate NAD⁺ by transferring electrons from NADH to another molecule, converting it back to its oxidized form NAD⁺. This keeps glycolysis running so the cell can continue producing ATP under anaerobic conditions.

Two Types of Fermentation

There are two main fermentation pathways, and which one a cell uses depends on the organism.

Lactic acid fermentation occurs in animal muscle cells and certain bacteria. Pyruvate, the end product of glycolysis, directly accepts electrons from NADH and is converted into lactate. NAD⁺ is regenerated in the process and returned to glycolysis. This is the pathway responsible for the burning sensation in muscles during intense exercise, when oxygen delivery cannot keep up with demand, muscle cells switch to lactic acid fermentation to maintain ATP production, and the accumulating lactate lowers the pH of the muscle tissue.

Alcoholic fermentation occurs primarily in yeast and some plant cells. The pathway runs in two steps: pyruvate is first converted to acetaldehyde, releasing CO₂ in the process, and acetaldehyde then accepts electrons from NADH to become ethanol. As in lactic acid fermentation, NAD⁺ is regenerated and recycled back into glycolysis. The CO₂ released during this process is responsible for the bubbles in bread and carbonated beverages produced through fermentation.

Why Fermentation Matters

Despite producing no ATP directly, fermentation is essential for the survival of cells in low-oxygen, or anaerobic environments. By regenerating NAD⁺, it ensures glycolysis can continue running.