How does diffusion move molecules across a cell membrane?

Diffusion is a passive transport process where molecules move spontaneously from a region of higher concentration to a region of lower concentration, down a concentration gradient, without requiring external energy.12

How molecules cross the cell membrane depends on their chemical properties:

• Direct Permeation: Small, nonpolar, or lipid-soluble molecules can dissolve directly into the hydrophobic interior of the lipid bilayer and pass through the membrane. While the membrane is generally a barrier to polar substances, very small molecules like water can also permeate it.132
• Protein Channels: Electrically charged ions and certain small polar molecules that cannot pass through the lipid bilayer often move through specialized, water-filled protein channels that span the membrane.132
• Facilitated Diffusion: Some molecules require "facilitators"—specific carrier proteins—to move across the membrane. In this process, the molecule binds to a carrier protein, which then helps it diffuse through the membrane. Like simple diffusion, this is a passive process that does not require energy and cannot move substances against a concentration gradient.12

Diffusion continues until a state of equilibrium is reached, where concentrations are equal on both sides and there is no net flow.12