Final answer:
Facilitated diffusion is a type of passive transport that requires specific membrane receptors or channel proteins to move substances down their concentration gradient.
Step-by-step explanation:
Facilitated diffusion is a type of passive transport that moves substances down their concentration gradient with the help of specific membrane receptors or channel proteins. Unlike simple diffusion, it requires the presence of these proteins to facilitate the movement of molecules or ions across the membrane.
For facilitated diffusion to occur, there must be a concentration gradient, as substances move from an area of high concentration to an area of low concentration. The specific membrane receptors or channel proteins involved determine the selectivity of the process, allowing only certain substances to cross the membrane.
An example of facilitated diffusion is the movement of glucose or Na+ ions across the cell membrane. In certain cases, different substances can share the same channel protein port, and these mechanisms are known as symport and antiport.