OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which type of molecules can easily diffuse through the cell membrane?

1. Larger protein molecules

2. Very small molecules like glucose and oxygen

3. Complex carbohydrates

4. All types of molecules

The correct answer is: Very small molecules like glucose and oxygen

The ability of molecules to diffuse through the cell membrane primarily depends on their size and polarity. Very small molecules, such as glucose and oxygen, can easily pass through the lipid bilayer of the cell membrane due to their small size and relatively nonpolar nature, allowing them to interact favorably with the hydrophobic interior of the membrane. This allows for efficient transport into and out of the cell, which is vital for processes such as respiration and cellular metabolism. Larger protein molecules, complex carbohydrates, and other larger or polar substances face a significant barrier due to their size and the interaction of charged or polar functional groups with the hydrophobic core of the membrane. They often require specific transport mechanisms, such as facilitated diffusion or active transport, to cross the membrane effectively. Therefore, very small molecules are the ones that can diffuse easily through the cell membrane.