One of the fundamental characteristics of the cell membrane is

Options

A. Amino acid regulation
B. Fat regulation
C. Glucose regulation
D. Ion regulation

Explanation

### Step-by-Step Solution:

1. **Understanding Cell Membrane**: The cell membrane, also known as the plasma membrane, is a crucial structure that separates the interior of the cell from the external environment.

2. **Key Function**: One of the primary functions of the cell membrane is to regulate the movement of substances in and out of the cell.

3. **Selective Permeability**: A fundamental characteristic of the cell membrane is its selective permeability. This means that the membrane allows certain molecules or ions to pass through while restricting others.

4. **Mechanism of Selective Permeability**: The cell membrane contains protein channels that act as gates. These channels can open or close to allow specific substances to enter or exit the cell. For example, they can close to prevent pathogens or unwanted materials from entering.

5. **Importance of Regulation**: The selective permeability of the cell membrane is vital for maintaining homeostasis within the cell. It regulates essential substances such as amino acids, fats, glucose, and ions.

6. **Conclusion**: Therefore, the fundamental characteristic of the cell membrane is its selective permeability, which is crucial for the cell's interaction with its environment.