What does simple diffusion involve?

Simple diffusion involves the movement of small or nonpolar particles directly through the phospholipid bilayer of a cell membrane. This process occurs because these molecules can pass freely between the phospholipids, as they are small enough to slip through the spaces created by the lipid molecules. Simple diffusion is driven by the concentration gradient, with particles moving from an area of higher concentration to an area of lower concentration without the need for energy input or assistance from proteins.

In contrast, the other options describe different processes. The movement of large molecules through membrane proteins refers to facilitated diffusion, a distinct process that requires specific channel or carrier proteins. Active transport, as required in the third option, requires energy input to move ions against their concentration gradient, which is not a characteristic of simple diffusion. Lastly, the formation of ion channels pertains to facilitated transport, which involves specific pathways for charged particles and is distinct from the unaided movement seen in simple diffusion.