Which is not bounded by a membrane

Options

A. Plastid
B. Mitochondrion
C. Nucleus
D. Centrioles

Answer

To determine which structure is not bounded by a membrane, we can analyze the options provided: plastid, mitochondrion, nucleus, and centriole.

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a structure that is not surrounded by a membrane. This means we need to identify which of the given options lacks a membrane.

2. **Analyze Each Option**:
- **Plastid**: Plastids are membrane-bound organelles found in plant cells. They are involved in processes like photosynthesis and storage of products. Therefore, plastids are not the answer.
- **Mitochondrion**: Mitochondria are also membrane-bound organelles known as the powerhouse of the cell. They have an inner and outer membrane. Hence, mitochondria are not the answer.
- **Nucleus**: The nucleus is a membrane-bound organelle that contains the cell's genetic material. It is surrounded by a nuclear envelope. Thus, the nucleus is not the answer.
- **Centriole**: Centrioles are cylindrical structures found in the cells of animals and some lower plants. They are not surrounded by a membrane and are considered membrane-less organelles.

3. **Conclusion**: After analyzing all the options, the only structure that is not bounded by a membrane is the **centriole**. Therefore, the correct answer is option D: centriole.

### Final Answer:
The structure that is not bounded by a membrane is the **centriole**.

---