Define with example:
Selectively permeable membrane

Explanation

### Step-by-Step Text Solution

1. **Definition of Selectively Permeable Membrane**:
A selectively permeable membrane is a type of biological membrane that allows certain substances to pass through while restricting others. This means that the membrane can control what enters and exits the cell, ensuring that essential molecules can enter while waste products can be removed.

2. **Mechanism of Selective Permeability**:
The selectively permeable membrane operates through two main processes:
- **Passive Transport**: This process does not require energy and allows substances to move across the membrane along their concentration gradient (from high to low concentration).
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