3.12 : 协助扩散

The plasma membrane consists of several transporters or carrier proteins interspersed between the lipid bilayer that facilitate solute transport.

Facilitated diffusion, a type of passive diffusion, is a transporter-mediated pathway wherein substrates such as organic compounds and ions interact with a transporter on one side. This induces conformational changes in proteins, allowing them to translocate the substrate without ATP expenditure.

Substrates can move into or out of the cell, depending on the electrochemical gradient.

Once the concentration and charge inside and outside of the cell become balanced, a steady state is reached.

 Facilitated diffusion helps solutes to move faster than passive diffusion without a transporter.

These carrier proteins are selective in structure, enabling only specific endogenous compounds or drugs to bind.

For example, the organic cation transporter, or OCT1, facilitates the absorption of vitamins such as thiamine and the hepatic uptake of certain drugs, such as metformin and levodopa.