What characterizes hydrophilic molecules in relation to the plasma membrane?

• A. They can pass easily through membranes
• B. They are nonpolar
• C. They cannot pass without assistance
• D. They are typically large molecules

Answer: (C)

Hydrophilic molecules have a tendency to interact favorably with water and typically have polar or charged characteristics. The plasma membrane, which consists of a phospholipid bilayer, acts as a barrier that separates the interior of the cell from its external environment. This bilayer is primarily made up of nonpolar lipid tails that create a hydrophobic core.

Because hydrophilic molecules are polar, they do not easily pass through this hydrophobic core without help. Instead, they require assistance from transport proteins such as channels or carriers to facilitate their passage across the membrane. This characteristic distinguishes them from hydrophobic (nonpolar) molecules, which can more easily diffuse through the membrane.

Contextually, while hydrophilic molecules can be large, size alone does not dictate their ability to pass through the membrane, and being nonpolar is a description of hydrophobic molecules, not hydrophilic ones. Therefore, the correct understanding of hydrophilic molecules is that they cannot pass through the plasma membrane without the aid of specialized mechanisms.