A cell membrane has

Options

A. Middle electron dense layer
B. Middle electron transparent layer
C. Outer electron transparent layer
D. Inner electron transparent layer

Answer

### Step-by-Step Solution:

1. **Understanding the Cell Membrane**: The cell membrane, also known as the plasma membrane, is a crucial structure found in all cells. It serves as a barrier that separates the interior of the cell from the external environment.

2. **Composition of the Cell Membrane**: The cell membrane is primarily composed of a lipid bilayer. This bilayer is semi-permeable, meaning it allows certain substances to pass through while blocking others.

3. **Function of the Cell Membrane**: One of the main functions of the cell membrane is to regulate the transport of materials into and out of the cell. This regulation is vital for maintaining the cell's internal environment.

4. **Identifying the Layers of the Cell Membrane**: The cell membrane has different layers that can be described based on their electron density. The options provided include:
- Middle electron dense layer
- Middle electron transparent layer
- Outer electron transparent layer
- Inner electron transparent layer

5. **Analyzing the Options**:
- The **middle electron dense layer** is incorrect because the cell membrane is not characterized by a dense layer.
- The **middle electron transparent layer** is correct as it refers to the semi-permeable nature of the membrane, allowing certain substances to pass through.
- The **outer electron transparent layer** and **inner electron transparent layer** do not accurately describe the structure of the cell membrane.

6. **Conclusion**: Based on the analysis, the correct answer is the **middle electron transparent layer**. This layer allows the passage of certain substances, particularly solvents, while restricting others.