Yeast is

Options

A. Purely aerobic
B. Anaerobic
C. Rarely anaerobic
D. Both aerobic and anaerobic

Answer

**Step-by-Step Solution:**

1. **Understanding Yeast**: Yeast is a type of fungus, specifically a unicellular organism. The most common yeast used in baking and brewing is Saccharomyces cerevisiae.

2. **Respiration Types**: Yeast can perform two types of respiration:
- **Aerobic Respiration**: This process occurs in the presence of oxygen. Yeast uses oxygen to convert glucose into energy, producing carbon dioxide and water as byproducts.
- **Anaerobic Respiration**: This process occurs in the absence of oxygen. Yeast can ferment sugars to produce energy, resulting in ethanol (alcohol) and carbon dioxide as byproducts. This is known as alcoholic fermentation.

3. **Environmental Conditions**: The type of respiration that yeast undergoes depends on the environmental conditions:
- In oxygen-rich environments, yeast will primarily use aerobic respiration.
- In low or no oxygen environments, yeast will switch to anaerobic respiration.

4. **Conclusion**: Given that yeast can perform both types of respiration depending on the availability of oxygen, the correct answer to the question "Yeast is" is that it is **both aerobic and anaerobic**.

**Final Answer**: Both aerobic and anaerobic.

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