Define with example:
Semi-permeable membrane

Explanation

### Step-by-Step Solution:

1. **Definition of Semi-permeable Membrane**:
A semi-permeable membrane is a type of biological membrane that allows certain substances to pass through while restricting others. This selective permeability is based on factors such as the size, shape, and solubility of the substances.

2. **Characteristics**:
- **Selective Transport**: Only specific molecules can pass through the membrane.
- **Size and Shape**: Smaller molecules or those that fit the membrane's structure can pass, while larger or differently shaped molecules cannot.
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