Which type of molecules can readily pass through the plasma membrane?

• A. Hydrophilic
• B. Electrolytes
• C. Nonpolar molecules
• D. Polar molecules

Answer: (C)

Nonpolar molecules can easily pass through the plasma membrane due to the structure and properties of the membrane itself. The plasma membrane is primarily composed of a phospholipid bilayer, which consists of hydrophobic (water-repelling) tails that face inward and hydrophilic (water-attracting) heads that face outward. This arrangement creates a selectively permeable barrier.

Nonpolar molecules, such as oxygen and carbon dioxide, do not interact favorably with water and are thus compatible with the hydrophobic interior of the phospholipid bilayer. As a result, they can diffuse freely across the membrane without the need for transport proteins or energy expenditure, which makes their passage both efficient and straightforward.

In contrast, hydrophilic and polar molecules tend to be repelled by the hydrophobic core of the membrane, making it more challenging for them to cross without assistance from specific transport proteins. Similarly, electrolytes, which are charged particles, require channels or carriers to move across the membrane, as they cannot easily penetrate the lipid bilayer. This understanding of membrane permeability is crucial in physiology, as it influences how substances enter and exit cells.