Final answer:
Facilitated diffusion is a type of passive transport where molecules move down a concentration gradient through specific protein carriers in the cell membrane without expending energy.
Step-by-step explanation:
The student is referring to facilitated diffusion which is a process where solute molecules bind specifically to a membrane protein carrier and move from an area of high concentration to an area of low concentration. In facilitated diffusion, substances move across the plasma membrane with the help of transmembrane proteins like channel proteins and carrier proteins. Carrier proteins, in particular, are very selective and typically only allow one particular type of molecule to pass through. They work by binding to the molecule on one side of the membrane, undergoing a change in shape or position, and then releasing the molecule on the other side.
Passive transport mechanisms like facilitated diffusion do not require cellular energy as they rely on the concentration gradient, meaning that the movement of molecules takes place from an area where they are more concentrated to an area where they are less concentrated, releasing free energy in the process. This passive transport includes not only facilitated diffusion but also simple diffusion and osmosis.