Which of the following is not membrane bound?

Options

A. endoplasmic reticulum
B. Centriole
C. Chloroplast
D. Vacuoles

Answer

To determine which of the following organelles is not membrane-bound, we will analyze each option provided:

1. **Endoplasmic Reticulum (ER)**:
- The endoplasmic reticulum is a network of membranes that is continuous with the nuclear envelope. It is involved in the synthesis of proteins and lipids. Since it is surrounded by membranes, it is a membrane-bound organelle.

2. **Centrioles**:
- Centrioles are cylindrical structures found in animal cells, primarily involved in cell division. They are not surrounded by any membrane. Therefore, centrioles are not membrane-bound.

3. **Chloroplasts**:
- Chloroplasts are found in plant cells and are responsible for photosynthesis. They have a double membrane structure, which makes them membrane-bound organelles.

4. **Vacuoles**:
- Vacuoles are storage organelles found in plant cells (and some animal cells). They are surrounded by a membrane called the tonoplast, making them membrane-bound as well.

Based on the analysis of all the options, the correct answer is **Centrioles**, as they do not have a membrane surrounding them.

### Summary of Steps:
1. Identify and analyze each organelle.
2. Determine if each organelle is membrane-bound or not.
3. Conclude that centrioles are the only organelles listed that are not membrane-bound.