Facilitated Diffusion: How Cells Move Molecules Across Membranes Without Energy Expenditure

Facilitated diffusion is a type of passive transport that allows molecules or ions to move across cell membranes from an area of higher concentration to an area of lower concentration, with the help of specific membrane proteins called transporters or channels. Unlike active transport, facilitated diffusion does not require energy from the cell.

Facilitated diffusion occurs when molecules or ions are too large or too polar to diffuse freely across the lipid bilayer of the cell membrane. The transporters or channels provide a pathway for these molecules or ions to move across the membrane. The transporters or channels are specific for a particular molecule or ion, and they allow them to pass through the membrane by changing shape.

There are two types of facilitated diffusion:

Carrier-mediated diffusion

In carrier-mediated diffusion, transporters bind to the molecule or ion to be transported, and then undergo a conformational change that moves the molecule or ion across the membrane. Carrier-mediated diffusion is specific for a particular molecule or ion, and it can be saturated when all the transporters are in use.

Channel-mediated diffusion

In channel-mediated diffusion, channels provide a pathway for molecules or ions to pass through the membrane. Channels are usually selective for a particular molecule or ion based on size and charge. Some channels are gated, which means they can open or close in response to a specific stimulus, such as a change in membrane potential or the binding of a ligand.

Facilitated diffusion is an important process for the movement of molecules and ions across cell membranes, and it is involved in many physiological processes, such as the uptake of nutrients by cells and the removal of waste products. It is also a target for drug development, as many drugs work by binding to specific transporters or channels to modify their activity.