Fermentation is a fundamental metabolic process used by various organisms to extract energy from carbohydrates. It is an anaerobic process, meaning it occurs without the presence of oxygen. This biological pathway allows single-celled organisms, and even some cells in the human body, to sustain energy production when oxygen is scarce or completely absent. The ability to switch between this method and aerobic respiration is a major survival advantage for many microbes.
Energy Production Pathways
Organisms generally have two distinct strategies for creating adenosine triphosphate (ATP), the primary energy currency of the cell. The first and most common method is aerobic respiration, which requires oxygen to act as the final electron acceptor in the electron transport chain. This process completely breaks down a glucose molecule, resulting in a high energy yield of up to 38 molecules of ATP from a single sugar molecule. Aerobic respiration is the preferred method for cells when oxygen is readily available because of its high efficiency.
Fermentation is the alternative, anaerobic strategy. It is a less efficient pathway that allows the cell to continue producing energy when oxygen is depleted. This method only partially breaks down the glucose molecule, resulting in a much lower energy output. The total ATP yield from fermentation is a net gain of only two ATP molecules per molecule of glucose processed, which is sufficient to sustain life for many single-celled organisms.
How Cells Generate Energy Without Oxygen
Fermentation is linked to the first stage of all cellular energy production, a process called glycolysis. Glycolysis occurs in the cytoplasm and breaks down a six-carbon glucose molecule into two three-carbon molecules of pyruvate. This initial breakdown produces the net gain of two ATP molecules and also generates a molecule called NADH.
The continuation of glycolysis depends on a steady supply of its coenzyme, NAD+, which is converted into NADH during the process. In the absence of oxygen, the cell cannot recycle NADH back to NAD+ using the electron transport chain, causing NAD+ stores to become depleted. Fermentation’s primary purpose is to regenerate NAD+ from NADH so that glycolysis can keep running. It achieves this by transferring electrons from NADH to the pyruvate, which produces the characteristic waste products of fermentation, such as lactic acid or ethanol.
Real-World Applications of Fermentation
Fermentation has two main types with widespread use in the food industry and biological systems. One common type is lactic acid fermentation, where bacteria convert sugars into lactic acid. This process is harnessed in the production of dairy products like yogurt and cheese, as well as fermented vegetables such as sauerkraut and kimchi.
Lactic acid fermentation also occurs in muscle cells during strenuous exercise when the oxygen supply cannot meet the high energy demand. The rapid energy production under these anaerobic conditions results in the accumulation of lactate, which is then transported to the liver for further metabolism.
The second major type is alcoholic fermentation, primarily carried out by yeast. In this pathway, sugars are converted into ethanol and carbon dioxide gas. This process is foundational to the brewing industry, where yeast ferments sugars in malt to produce alcohol and carbonation in beer. Alcoholic fermentation is also used to create wine and is responsible for the carbon dioxide gas that causes bread dough to rise.