Which types of molecules require a carrier protein to cross a cell membrane?

Carrier proteins play a crucial role in the transport of various substances across cell membranes. Charged molecules and ions, due to their polar nature and inability to diffuse freely through the lipid bilayer, specifically require carrier proteins or channels to facilitate their movement into and out of cells. The hydrophobic interior of the cell membrane repels charged entities, making it essential for these molecules to utilize specialized transport proteins that can create a pathway through the membrane.

In contrast, small uncharged molecules can pass through the lipid bilayer via simple diffusion, as they do not face the same restrictions imposed by the membrane's hydrophobic nature. Simple sugars, while larger than small uncharged molecules, can often be transported by specific transport proteins that facilitate their movement without needing to pass through the lipid bilayer in significant numbers directly. Gases like oxygen and carbon dioxide are small and nonpolar; they can easily diffuse through the membrane without the need for a carrier protein. Therefore, charged molecules and ions are the correct answer as they are dependent on carrier proteins or channels to traverse the cell membrane effectively.