Alcoholic fermentation, also known as ethanol fermentation, is a metabolic process used by certain organisms, primarily yeasts, to extract energy from sugars in an environment lacking oxygen. This process transforms glucose and other simple sugars into two main byproducts: ethanol (a type of alcohol) and carbon dioxide gas. As an anaerobic pathway, alcoholic fermentation allows these microbes to sustain themselves and is the biological reaction at the heart of many human industries.
Why Cells Rely on Alcoholic Fermentation
Cells utilize alcoholic fermentation when they cannot perform aerobic respiration, which is the far more efficient energy-generating process that requires oxygen. In conditions where oxygen is absent or scarce, such as deep within a batch of dough or a fermentation tank, a cell must turn to alternative methods to produce the energy required for survival. The first stage of energy extraction, known as glycolysis, does not require oxygen and breaks down one glucose molecule into two molecules of pyruvate, generating a small net gain of two molecules of adenosine triphosphate (ATP) for the cell.
The challenge for the cell lies in sustaining glycolysis, which requires a specific molecule called NAD+ (nicotinamide adenine dinucleotide) to function. Glycolysis consumes NAD+ by reducing it to NADH, and if the cell cannot convert NADH back into NAD+ for reuse, the entire energy pathway halts. In oxygen-rich conditions, aerobic respiration handles this recycling, but without oxygen, fermentation must take over this task.
Alcoholic fermentation functions primarily as a survival mechanism designed to regenerate the necessary NAD+ so that glycolysis can continue to produce its minimal output of ATP. Although two ATP molecules per glucose is a low energy yield compared to the nearly thirty-two molecules produced by aerobic respiration, this small amount is sufficient to keep the organism alive in temporary low-oxygen environments. The regeneration of NAD+ is accomplished by using pyruvate, the product of glycolysis, as an electron acceptor in the final steps of the fermentation process.
The Two-Step Chemical Conversion
The conversion of pyruvate into ethanol and carbon dioxide occurs in two distinct enzymatic steps within the cell’s cytoplasm. Pyruvate, a three-carbon molecule resulting from glycolysis, must first be modified to accept electrons and regenerate NAD+. This chemical sequence characterizes alcoholic fermentation and differentiates it from other anaerobic metabolism forms, such as lactic acid fermentation.
In the initial step, the enzyme pyruvate decarboxylase removes a carboxyl group from the three-carbon pyruvate molecule. This reaction releases carbon dioxide (CO2) and leaves behind a two-carbon compound called acetaldehyde. The release of CO2 is an irreversible step characteristic of this specific fermentation pathway.
In the second step, the two-carbon acetaldehyde acts as the electron acceptor needed to complete the process. The enzyme alcohol dehydrogenase catalyzes a reaction where acetaldehyde accepts two electrons from NADH, converting it into ethanol. This transfer oxidizes NADH back into the NAD+ required to keep glycolysis operational.
The yeast species Saccharomyces cerevisiae is the primary organism responsible for this process in nature and industry. This microbe is well-suited for ethanol fermentation because it possesses both the pyruvate decarboxylase and alcohol dehydrogenase enzymes necessary to carry out the entire sequence. The complete conversion of a single glucose molecule yields two molecules of ethanol, two molecules of carbon dioxide, and two molecules of ATP.
Practical Uses of Fermentation Byproducts
Ethanol and carbon dioxide, the two byproducts of alcoholic fermentation, have been utilized by humans for millennia, forming the basis of several major industries. Ethanol is the desired product in the brewing and distilling of alcoholic beverages like wine, beer, and spirits. Different strains of Saccharomyces cerevisiae are selected to produce the varying flavors and alcohol concentrations required for these products.
Ethanol is not solely used for consumption; it is also used in industrial and renewable fuel sources. Bioethanol, typically produced through the fermentation of starchy crops like corn or sugarcane, is blended with gasoline to reduce emissions. This application leverages the same microbial process on an industrial scale to produce a cleaner-burning fuel.
Carbon dioxide is the agent behind the leavening of bread. In bread dough, the gas released during fermentation becomes trapped in the gluten network, causing the dough to rise and giving the final baked product its light, airy texture. While the ethanol produced largely evaporates during baking, the CO2 remains responsible for the volume increase.
Carbon dioxide is also used for the carbonation of beverages. In the beverage industry, the gas is captured and either remains dissolved in the liquid to provide natural carbonation, as in some beers, or is used to carbonate other drinks.