Why is plasma membrane said to be semipermeable ?

Explanation

### Step-by-Step Solution

1. **Definition of Plasma Membrane**: The plasma membrane, also known as the cell membrane, is a biological membrane that separates and protects the interior of the cell from the external environment.

2. **Structure of Plasma Membrane**: The plasma membrane is primarily composed of a lipid bilayer, which consists of two layers of phospholipids. This bilayer is embedded with various proteins, including integral proteins and peripheral proteins.

3. **Understanding Semipermeability**: The term "semipermeable" refers to the ability of the plasma membrane to selectively allow certain substances to pass through while restricting others. This means that not all molecules can freely enter or exit the cell.

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