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 "The molecules in the membrane that limit its permeability are the", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Cell Membrane Structure**: The cell membrane is primarily composed of a phospholipid bilayer. This bilayer is crucial for maintaining the integrity of the cell and regulating what enters and exits.

2. **Role of Phospholipids**: Phospholipids are the main components of the cell membrane. Each phospholipid molecule has a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails". This unique structure allows phospholipids to form a bilayer.

3. **Amphiphilic Nature**: The amphiphilic nature of phospholipids means that they can interact with both water and lipid environments. This property is essential for creating a semi-permeable membrane.

4. **Limiting Permeability**: The hydrophobic tails of the phospholipids create a barrier to most water-soluble substances, thereby limiting the permeability of the membrane. Only certain small, non-polar molecules can easily pass through the membrane.

5. **Conclusion**: Therefore, the molecules in the membrane that limit its permeability are primarily the phospholipids.

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

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