Facilitated Diffusion

The phospholipid bilayer is picky. Small nonpolar molecules like carbon dioxide and oxygen slide straight through the fatty tails. But glucose is bulky and polar, and ions like potassium carry charge, so the hydrophobic core of the membrane blocks them. They need a protein doorway. That doorway comes in two styles. Channel proteins form a pore that is simply open, like a tunnel with the right shape and charge lining to let a specific ion pass. Carrier proteins work more like a revolving door: the molecule binds on one side, the protein flexes into a new shape, and that motion releases the molecule on the other side. Both are passive. No ATP is spent moving the solute, because the solute is already pushed by its own concentration gradient, or in the case of ions, by an electrochemical gradient that combines concentration and charge.

That electrochemical gradient matters beyond single molecules. When channels open and ions rush through, they shift the charge balance across the membrane, changing its voltage. This is exactly how neurons fire: sodium and potassium channels open in sequence and the resulting charge shifts generate the signal. The gradient itself is often built earlier by active transport, like the sodium-potassium pump spending ATP to set up an imbalance; the channel just lets that stored energy discharge.

Because only a fixed number of transport proteins sit in the membrane, facilitated diffusion has a ceiling. At low solute concentration, adding more molecules speeds up transport, but once every carrier and channel is working flat out, the rate plateaus even as concentration keeps rising. Simple diffusion through the lipid bilayer has no such protein bottleneck, so its rate keeps climbing linearly instead of leveling off.