Lower organisms like yeast respire by __________

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Respiration in Yeast:** Lower organisms like yeast respire through a process known as diffusion, which involves the exchange of gases.

2. **Types of Respiration:** Yeast can perform two types of respiration:
- **Aerobic Respiration:** This occurs when yeast has sufficient oxygen. In this process, oxygen is used to break down glucose, producing carbon dioxide and water as by-products. Aerobic respiration takes place in the mitochondria of the yeast cells.
- **Anaerobic Respiration:** This occurs when there is a lack of oxygen. In this case, yeast breaks down glucose without oxygen, resulting in the production of ethanol and carbon dioxide. This process occurs in the cytoplasm of the yeast cells.

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