Cell wall of red algae protects the brown algae from drying or freezing on their exposure to air .

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question states that the cell wall of red algae protects brown algae from drying or freezing when exposed to air. We need to explain how this protection occurs.

2. **Composition of Red Algae Cell Wall**: The cell wall of red algae is primarily made up of polysulphate. This is a crucial point because the composition of the cell wall determines its protective capabilities.

3. **Role of Pectin**: In addition to polysulphate, the cell wall also contains pectin material. Pectin is a polysaccharide that contributes to the structural integrity and flexibility of the cell wall.

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