OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What process occurs when yeast cells respire anaerobically?

1. Photosynthesis

2. Fermentation

3. Cell division

4. Aerobic respiration

The correct answer is: Fermentation

When yeast cells respire anaerobically, the process that takes place is fermentation. In this process, glucose is converted into ethanol and carbon dioxide in the absence of oxygen. This is particularly relevant in the production of alcoholic beverages and in baking, where the carbon dioxide produced causes dough to rise. During fermentation, yeast cells break down sugars to extract energy without the need for oxygen. This is essential for their survival in environments where oxygen is limited. The overall equation for alcoholic fermentation can be summarized as: Glucose → Ethanol + Carbon Dioxide + Energy This process is a key characteristic of yeast and is distinct from aerobic respiration, which occurs in the presence of oxygen and produces carbon dioxide and water, along with a much higher yield of energy.