Final answer:
Facilitated diffusion is a passive transport mechanism where materials such as ions or polar molecules move down their concentration gradients across the plasma membrane with the help of carrier or channel proteins.
Step-by-step explanation:
In facilitated diffusion, materials such as ions or polar molecules move across the plasma membrane without the use of cellular energy but with the help of membrane proteins. These molecules cannot freely pass through the hydrophobic lipid bilayer due to their polarity or charge. Two main types of proteins assist in this process: carrier proteins and channel proteins.
Carrier proteins are selective; they bind to specific molecules and facilitate their passage across the membrane by changing shape. Channel proteins, on the other hand, act as pores for ions and small molecules to pass through. These proteins are crucial because they protect the molecules from the hydrophobic interior of the membrane, making it possible for them to move down their concentration gradients.
It's important to note that facilitated transport only allows molecules to move down their concentration gradient, which is a passive process and differs from active transport, where molecules are moved against their gradient requiring energy.