Final answer:
Yeast carries out alcohol fermentation when oxygen is absent, converting glucose into ethyl alcohol and carbon dioxide. This process is crucial for the production of bread, wine, and beer.
Step-by-step explanation:
Yeast is a facultative anaerobe, which means that alcohol fermentation occurs when oxygen is absent. In this anaerobic condition, the yeast undergoes a process that converts sugars such as glucose into ethyl alcohol and carbon dioxide. This process is utilized in making various food and drinks, such as in the production of bread, where the carbon dioxide helps the bread to rise, and in the making of alcoholic beverages like beer and wine, where the alcohol is the desired product. Since yeast can perform this fermentation in the absence of oxygen, it differs from obligate aerobes, which require oxygen to survive, and from obligate anaerobes, for which oxygen can be harmful or lethal. Yeast's ability to switch between aerobic and anaerobic metabolic pathways allows it to grow in a variety of environments, making it versatile for various fermentation processes.