The molecules in the membrane that limit its permeability are the

Options

A. carbohydrates
B. phospholipids
C. proteins
D. water.

Explanation

To answer the question regarding which molecules in the membrane limit its permeability, we can break down the solution step by step:

### Step-by-Step Solution:

1. **Understanding Cell Membrane Composition**:
- The cell membrane is primarily composed of phospholipids, which are molecules that contain both a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.

2. **Role of Phospholipids**:
- The phospholipids arrange themselves into a bilayer, where the hydrophilic heads face outward towards the water inside and outside the cell, while the hydrophobic tails face inward, away from the water.

3. **Semi-Permeable Nature**:
- This arrangement creates a semi-permeable membrane, meaning that it allows certain molecules to pass through while restricting others. The hydrophobic interior of the membrane acts as a barrier to polar and charged molecules.

4. **Limiting Permeability**:
- The hydrophobic tails of the phospholipids are particularly important in limiting the permeability of the membrane. They prevent the passage of water-soluble substances, thus controlling what enters and exits the cell.

5. **Conclusion**:
- Therefore, the molecules in the membrane that limit its permeability are primarily the phospholipids. This makes option number 2 the correct answer.

### Final Answer:
The molecules in the membrane that limit its permeability are the **phospholipids**.

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