Organisms require a continuous supply of energy, primarily in the form of adenosine triphosphate (ATP), often referred to as the “energy currency” of the cell. ATP production is a fundamental process for sustaining life.
Glycolysis The Initial Energy Pathway
The initial step in extracting energy from glucose is glycolysis. This metabolic pathway occurs in the cytoplasm of nearly all living cells, breaking down one six-carbon glucose molecule into two three-carbon pyruvate molecules. During this breakdown, a small amount of ATP is directly produced, along with NADH. Glycolysis yields a net gain of two ATP and two NADH molecules per glucose molecule.
Glycolysis depends on a continuous supply of NAD+. During one specific step, NAD+ acts as an electron acceptor, becoming reduced to NADH. If NAD+ is not regenerated, it would be depleted, causing glycolysis to halt. The accumulation of NADH would effectively stop the energy production pathway.
The Crucial Role of Fermentation
When oxygen is unavailable, cells cannot use the electron transport chain, which is the primary method for regenerating NAD+ from NADH in aerobic conditions. Fermentation becomes essential, as its main purpose is to regenerate NAD+ from NADH. This allows glycolysis to continue producing a limited amount of ATP, ensuring cells can still generate energy without oxygen.
During fermentation, NADH donates its electrons to an organic molecule, oxidizing NADH back to NAD+. This regeneration of NAD+ sustains the electron-accepting capacity for glycolysis. While fermentation itself does not produce additional ATP, it is crucial for keeping the ATP-generating glycolysis pathway operational under anaerobic conditions.
Common Fermentation Pathways
Two widely recognized types of fermentation are lactic acid fermentation and alcoholic fermentation, each distinguished by their end products and the organisms that perform them. Lactic acid fermentation is carried out by certain bacteria (e.g., Lactobacillus, Streptococcus) and animal muscle cells during intense activity when oxygen is insufficient. In this one-step process, pyruvate directly accepts electrons from NADH, reducing it to lactate and regenerating NAD+. This pathway allows muscles to sustain short bursts of energy and is used in producing foods like yogurt and sauerkraut.
Alcoholic fermentation is primarily performed by yeasts, such as Saccharomyces cerevisiae, and some bacteria. This process involves two steps: first, pyruvate is converted into acetaldehyde and carbon dioxide, and then acetaldehyde is reduced to ethanol by accepting electrons from NADH, regenerating NAD+. The carbon dioxide produced is responsible for the rise in bread dough, while the ethanol is the alcohol found in alcoholic beverages. Both lactic acid and alcoholic fermentation pathways ultimately serve the same function: to regenerate NAD+ for glycolysis to continue, enabling cells to produce ATP without oxygen.