Advanced Placement (AP) Biology Practice Exam

What process do yeasts and microorganisms use to break down sugars in the absence of oxygen?

1. Photosynthesis

2. Cellular respiration

3. Alcoholic fermentation

4. Lactic fermentation

The correct answer is: Alcoholic fermentation

Yeasts and many microorganisms utilize alcoholic fermentation to break down sugars in the absence of oxygen. This anaerobic process allows them to convert glucose into ethanol and carbon dioxide, enabling them to generate energy when oxygen is not available. During alcoholic fermentation, enzymes present in the yeasts facilitate the conversion of sugars to ethanol, which can then be used in various applications, such as brewing and baking. In this process, the initial glycolysis pathway splits glucose into pyruvate, yielding energy in the form of ATP. Then, in the absence of oxygen, the pyruvate is further metabolized to ethanol and carbon dioxide rather than entering the aerobic pathway of cellular respiration. This method is crucial for survival in anaerobic environments and has important implications in various industries. The other processes mentioned, such as photosynthesis and cellular respiration, either require oxygen or involve the production of energy through different mechanisms not applicable in anaerobic conditions. Lactic fermentation, while also an anaerobic process, is primarily associated with certain bacteria and animal muscle cells and results in the production of lactic acid rather than ethanol. Thus, alcoholic fermentation is the most accurate and relevant process in this context.