Which type of molecules primarily require facilitated diffusion to cross the cell membrane?

Facilitated diffusion is a specific type of passive transport that allows certain molecules to cross the cell membrane with the assistance of specific proteins, such as carrier proteins or channel proteins. The type of molecules that primarily require facilitated diffusion to cross the cell membrane are polar molecules or larger charged molecules.

This is because the lipid bilayer of the cell membrane is composed primarily of nonpolar phospholipid tails, which create a hydrophobic environment that repels polar substances and ions. Polar molecules, like glucose and water, as well as larger charged molecules (such as ions), cannot easily diffuse through this hydrophobic barrier due to their charge or polarity. Instead, they require specific transport proteins that facilitate their movement across the membrane.

By contrast, nonpolar molecules can easily cross the membrane without assistance, and small uncharged molecules can also pass through directly due to their size and lack of charge. Therefore, while facilitated diffusion plays a crucial role for polar and charged molecules, it is not necessary for nonpolar or small uncharged molecules. This is why the correct choice emphasizes the specific need for facilitated diffusion in the context of polar or larger charged molecules.