......... can not pass through the lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane

Options

A. Nonpolar molecules
B. Polar molecules
C. Hydrophobic molecules
D. Both (B) and (C)

Answer

To answer the question, we need to identify which type of molecules cannot pass through the lipid bilayer of the cell membrane and require a carrier protein for transport. Let's break down the options step by step:

1. **Understanding the Lipid Bilayer**: The lipid bilayer is a fundamental structure of cell membranes, composed of phospholipids. It is selectively permeable, meaning it allows certain substances to pass while blocking others.

2. **Analyzing the Options**:
- **Option 1: Non-polar molecules**: Non-polar molecules, such as oxygen (O2) and carbon dioxide (CO2), can easily pass through the lipid bilayer due to their solubility in lipids. Therefore, this option is incorrect.

- **Option 2: Polar molecules**: Polar molecules can be further divided into charged and uncharged polar molecules. While small uncharged polar molecules like water can diffuse through the membrane, larger uncharged polar molecules, such as glucose, cannot pass through easily and require a carrier protein to facilitate their transport. Thus, this option is correct.

- **Option 3: Hydrophobic molecules**: Hydrophobic molecules are water-repellent and can also pass through the lipid bilayer easily. Therefore, this option is incorrect.

- **Option 4: Both b and c**: Since option c is incorrect, this option is also incorrect.

3. **Conclusion**: Based on the analysis, the correct answer is that **polar molecules** (specifically larger uncharged polar molecules like glucose) cannot pass through the lipid bilayer without the assistance of a carrier protein.

### Final Answer:
**Polar molecules** cannot pass through the lipid bilayer; they require a carrier protein of the membrane to facilitate their transport across the membrane.