Final answer:
In facilitated diffusion, molecules are moved down their concentration gradient with the help of a protein carrier molecule without requiring energy. It is a type of passive transport as opposed to active transport which requires ATP energy to move substances against their concentration gradient.
Step-by-step explanation:
In facilitated diffusion, a molecule is moved down its concentration gradient with the assistance of a protein carrier molecule, and no energy is required. This is because facilitated diffusion is a form of passive transport where substances move along their concentration gradient through specific protein channels in the membrane, without the expenditure of cellular energy. Unlike in active transport, where molecules are pumped against their concentration gradient requiring energy in the form of ATP, facilitated diffusion does not require such energy input.
Facilitated transport proteins, such as channel proteins or carrier proteins, aid in this process by shielding polar molecules or ions from the hydrophobic parts of the cell membrane, which would otherwise repel them, allowing these substances to diffuse into the cell. Glucose transporters are a classic example of facilitated diffusion, where glucose is transferred into cells by proteins that exploit the concentration gradient without using energy.