Answer:
The passive transport of a substance by its concentration gradient through a protein channel is called facilitated diffusion.
Explanation:
Facilitated diffusion is a type of passive transport through the membrane, which involves the use of integral proteins called protein channels or transport proteins, and moves substances unable to cross the membrane by simple diffusion.
An example of facilitated diffusion through protein channels is the ion channel. Ions, being electrically charged particles, cannot pass through the plasma membrane. A protein channel -which crosses the whole thickness of the membrane- constitutes a space where ions can move depending on their concentration gradient.
In facilitated diffusion, both protein channels and transport proteins do not require energy, so it is a transmembrane passive transport process.
