Which of the following structure is not bounded by a cell membrane?

Options

A. Ribosomes
B. Lysosomes
C. Mitochondria
D. Spherosomes

Explanation

To determine which structure is not bounded by a cell membrane, we will analyze each option provided.

### Step-by-Step Solution:

1. **Identify the Structures**: We have four options to consider: Ribosomes, Lysosomes, Mitochondria, and Spherozomes.

2. **Analyze Ribosomes**:
- Ribosomes are cellular structures involved in protein synthesis.
- They are not surrounded by a membrane and can be found freely floating in the cytoplasm.
- During protein synthesis, ribosomes may temporarily associate with membranes, but they do not have a permanent membrane structure.

3. **Analyze Lysosomes**:
- Lysosomes are organelles that contain digestive enzymes to break down waste materials and cellular debris.
- They are bounded by a single membrane known as the Tonoplast.

4. **Analyze Mitochondria**:
- Mitochondria are known as the powerhouse of the cell, responsible for energy production.
- They are bounded by a double membrane, consisting of an inner and outer membrane.

5. **Analyze Spherozomes**:
- Spherozomes are small organelles involved in the storage and synthesis of lipids.
- They are also bounded by a single membrane.

6. **Conclusion**:
- After analyzing all the options, we find that Ribosomes are the only structure that is not bounded by a cell membrane.
- Therefore, the answer to the question is **Ribosomes**.

### Final Answer:
**Ribosomes**