Unit two. The Living Cell
4.11. Facilitated diffusion
The selective permeability of biological membranes is perhaps their most important property. Ions and polar molecules can only cross the lipid core of membranes by passing through protein channels that bridge the bilayer.
The simplest of these channels are so-called open channels, shaped like tubes and functioning as open doors. As long as a molecule fits the channel, it is free to pass through in either direction, like a marble through a donut. Diffusion tends to equalize the concentration of such molecules on both sides of the membrane, with the molecules moving toward the side where they are scarcest. Many of the cell’s water and ion channels are open channels, simple open pores that span the membrane. The open channels are selective, as only ions and molecules that precisely fit the pore can diffuse through it, in either direction. Often the pore has a “gate,” a door that must be opened before an ion can pass through. Open ion channels with gates that swing open or shut in response to electrical charge play an essential role in signaling by the nervous system.
There are limits to the specificity of open channels, as many different polar molecules are roughly the same size, shape, and charge. To increase the selectivity of membrane transport, cells employ a more complex channel that requires the diffusing molecule to bind to the surface of a “carrier” protein. Such hand-in-glove binding can be very specific. Once having bound their cargo, a carrier protein then physically carries the diffusing molecule across the membrane.
Each carrier protein binds with only a certain molecule, such as a particular sugar, amino acid, or ion, physically binding them on one side of the membrane and releasing them on the other. The direction of the molecule’s net movement depends only on its concentration gradient across the membrane. If the concentration is greater outside the cell, the molecule is more likely to bind to the carrier on the extracellular side of the membrane, as in panel 1 of the Key Biological Process illustration below, and be released on the cytoplasmic side, as in panel 3. The net movement always occurs from high concentration to low, just as it does in simple diffusion, but the process is facilitated by the carriers. For this reason, this mechanism of transport is given a special name, facilitated diffusion.
A characteristic feature of transport by carrier proteins is that its rate can be saturated. If the concentration of a substance is progressively increased, the rate of transport of the substance increases up to a certain point and then levels off. There are a limited number of carrier proteins in the membrane, and when the concentration of the transported substance is raised high enough, all the carriers will be in use. The transport system is then said to be “saturated.”
Key Learning Outcome 4.11. Facilitated diffusion is the selective transport of substances across a membrane using a protein channel or carrier in the direction of lower concentration.