Which of the following molecules is NOT typically allowed through the phospholipid bilayer?

The correct choice identifies large polar molecules as those not typically allowed through the phospholipid bilayer. The structure of the phospholipid bilayer consists of hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads, which effectively creates a selective barrier.

Small nonpolar molecules, such as oxygen and carbon dioxide, can readily diffuse through the bilayer due to their compatibility with the hydrophobic interior. Water molecules, while polar, are relatively small and can also pass through the bilayer, albeit more slowly than nonpolar molecules, often aided by special channels or proteins.

In contrast, large polar molecules—due to their size and hydrophilic nature—are unable to penetrate the hydrophobic core of the bilayer. They typically require specific transport mechanisms, such as facilitated diffusion or active transport, to cross the membrane effectively. This selective permeability is crucial for maintaining cellular homeostasis, allowing the cell to control its internal environment by regulating the passage of substances in and out.