Anaerobic respiration in yeast, commonly known as alcoholic fermentation, is a metabolic pathway that allows the single-celled fungus Saccharomyces cerevisiae to generate energy when oxygen is scarce or absent. This method of metabolism enables the yeast to break down sugars and sustain its life functions. The process is distinct from aerobic respiration, which occurs in the presence of oxygen and produces a significantly higher amount of energy. Fermentation begins with the uptake of simple sugars, such as glucose.
The Key Chemical Products
The direct result of this anaerobic metabolic process is the creation of two primary chemical compounds: ethanol and carbon dioxide (\(\text{CO}_2\)). Ethanol, or ethyl alcohol, is the main organic product and represents the end of the yeast’s sugar-processing chain. For the yeast cell, ethanol production is a necessary step to continue generating energy, but this compound is toxic in high concentrations. Most strains of Saccharomyces cerevisiae can tolerate ethanol levels up to about 15% before their growth is inhibited.
Carbon dioxide, the other principal product, is a gaseous byproduct released during the final stages of the sugar conversion process. For every molecule of glucose consumed, two molecules of ethanol and two molecules of \(\text{CO}_2\) are generated. This gas is expelled from the yeast cell and is often observed as bubbling or foaming in liquid environments. Humans have found significant utility in both the alcohol and the gas they produce.
The Energy Generated
Although the chemical end products are the most noticeable output, the yeast’s primary goal in fermentation is the production of usable energy in the form of adenosine triphosphate (ATP). This process occurs entirely in the cytoplasm. For each molecule of glucose metabolized through anaerobic respiration, the net energy yield for the yeast is two molecules of ATP.
This low yield is inefficient compared to aerobic respiration. When oxygen is available, the yeast can fully break down one glucose molecule to generate a higher amount of energy, yielding between 36 and 38 molecules of ATP. The cell resorts to fermentation only when oxygen is unavailable, prioritizing rapid, low-yield energy production for survival.
How Yeast Converts Sugar
The conversion of sugar begins with a universal pathway called glycolysis, where a six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvate. This initial step directly generates the two net ATP molecules used by the yeast for energy. Glycolysis also produces NADH, an electron carrier that must be recycled back into \(\text{NAD}^+\) for the process to continue.
The second stage is fermentation, which serves the function of regenerating \(\text{NAD}^+\). The two pyruvate molecules are first converted into two molecules of acetaldehyde, with two molecules of \(\text{CO}_2\) released as a byproduct. Next, the enzyme alcohol dehydrogenase catalyzes the conversion of acetaldehyde into ethanol, simultaneously oxidizing NADH back to \(\text{NAD}^+\). This regeneration allows glycolysis to cycle continuously, ensuring the yeast’s ongoing ability to produce its minimal ATP requirement.
Practical Applications of Fermentation
The products of yeast fermentation have been utilized by humans for millennia, forming the basis of countless food and beverage industries. The ethanol produced is the desired compound in the brewing of beer, the making of wine, and the distillation of spirits, determining the alcoholic content. Different strains of Saccharomyces cerevisiae are selected to produce certain flavor compounds alongside the ethanol.
The carbon dioxide byproduct is important, particularly in the baking industry. When yeast is mixed into bread dough, the \(\text{CO}_2\) gas becomes trapped within the elastic gluten network, causing the dough to rise or “leaven.” During the baking process, the heat causes most of the ethanol to evaporate, leaving behind a light, porous texture and contributing to the characteristic flavor of the bread. This dual utility highlights the impact of this specific anaerobic metabolic pathway.