Which one of the following organelles is not surrounded by any membrane?

Options

A. Mitochondrion
B. Vacuole
C. Endoplasmic reticulum
D. Ribosome

Answer

To determine which organelle is not surrounded by any membrane, we can analyze the options provided:

1. **Mitochondria**:
- Mitochondria are known as the powerhouse of the cell and are surrounded by a double membrane. The outer membrane is selectively permeable, while the inner membrane is involved in ATP synthesis and the electron transport chain. Therefore, mitochondria are membrane-bound organelles.

2. **Vacuole**:
- Vacuoles are storage organelles that are surrounded by a single membrane called the tonoplast. They play a significant role in maintaining water balance and storing substances. Thus, vacuoles are also membrane-bound organelles.

3. **Endoplasmic Reticulum (ER)**:
- The endoplasmic reticulum is classified into two types: rough ER and smooth ER. Both types are surrounded by a single membrane. The rough ER is involved in protein synthesis due to the presence of ribosomes, while the smooth ER is involved in lipid synthesis. Therefore, the endoplasmic reticulum is also a membrane-bound organelle.

4. **Ribosome**:
- Ribosomes are unique organelles that are not surrounded by any membrane. They are the sites of protein synthesis and can be found freely floating in the cytoplasm or attached to the rough endoplasmic reticulum. Since ribosomes lack a surrounding membrane, they are the correct answer to the question.

**Conclusion**: The organelle that is not surrounded by any membrane is **Ribosome** (Option 4).

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