In the picture the carrier channel on the right is how diffusion would usually go directly across the phospholipid bilayer, but in facilitated diffusion, some molecules cannot easily cross a membrane. They may cross more easily through a carrier or transport protein, as in the image on the right. Facilitated diffusion enables molecules hat cannot directly cross the phospholipid bilayer through transport protein to get in the membrane. There are many types of transport proteins. Most allow only certain ions or molecules to pass. Some transport proteins are simple channels, or tunnels, through which particles such as ions can pass. Others act more like enzymes. When bound the protein changes shape, allowing the molecule to travel the rest of the way into the cell.
Some molecules cannot easily diffuse across a membrane. They may cross more easily through transport proteins-openings found by proteins that pierce the cell membrane. Facilitated diffusion is the diffusion of molecules across a membrane through transport proteins. The word facilitate means “to make easier.” That’s exactly what transport proteins do for molecules to enter or exit a cell. The process is still a form of passive transport. The molecules move down a concentration gradient requiring no energy expenditure by the cell. A concentration gradient exists for these molecules, so they have the potential to diffuse into, or out of, the cell by moving down it. Two major classes of facilitated transport proteins are channels and carrier or transport proteins. Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion. Channels are very selective and will accept only one type of molecule, or a few closely related molecules, for transport. Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise sloe or block their entry into the cell. Carrier or transport proteins can change their shape to move a target molecule from one side of the membrane to the other. Like channel proteins, carrier proteins are typically selective for one or a few substances. Often they will change shape in response to binding of their target molecule, with shape change moving the molecule to the opposite side of the membrane. The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps).