Final answer:
Small, uncharged substances like oxygen and carbon dioxide, as well as hydrophobic molecules, can simply diffuse across the plasma membrane. Polar substances and charged ions require transport proteins for diffusion.
Step-by-step explanation:
Small, uncharged substances such as oxygen and carbon dioxide, as well as hydrophobic molecules such as lipids, can simply diffuse across the plasma membrane (PM) of a cell. This is because the structure of the lipid bilayer allows these molecules to pass through the membrane down their concentration gradient by simple diffusion.
On the other hand, polar substances and charged ions cannot easily pass through the PM via simple diffusion. Polar molecules, such as water, diffuse across the membrane through transport proteins, which completely span the membrane and allow certain molecules or ions to diffuse across. Examples of transport proteins involved in facilitated diffusion are channel proteins, gated channel proteins, and carrier proteins.