What process involves the passive transport of polar molecules and ions with the assistance of transport proteins?

Facilitated diffusion is the process that allows polar molecules and ions to cross the cell membrane with the assistance of transport proteins. Unlike simple diffusion, which occurs when small, nonpolar molecules pass directly through the lipid bilayer, facilitated diffusion specifically involves helper proteins that provide pathways for these otherwise impermeable substances to move down their concentration gradient.

This process is termed "passive" because it does not require energy expenditure from the cell; the molecules move in response to their concentration gradient, much like how a ball rolls down a hill without needing an external force. Transport proteins can either be channel proteins, which create a pore through which ions and polar molecules can flow, or carrier proteins, which undergo a conformational change to move substances across the membrane.

Osmosis, while related to the movement of water, specifically refers to the diffusion of water molecules across a semipermeable membrane, rather than the transport of polar molecules and ions in general. Active transport, on the other hand, requires energy to move substances against their concentration gradients and is not passive. Simple diffusion applies only to nonpolar molecules that can easily cross the lipid bilayer without the aid of proteins.