Final answer:
Facilitated diffusion is a type of passive transport that requires carrier proteins and allows molecules to move along their concentration gradient without the use of ATP.
Step-by-step explanation:
The correct answer is D. Facilitated diffusion constitutes a type of passive transport, meaning one where energy is not directly consumed like in ATP hydrolysis, requiring trans-membrane or carrier proteins. This process involves the movement of substances across a cell membrane along their concentration gradient, and unlike active transport, it does not require cellular energy in the form of ATP. Facilitated diffusion relies on specific proteins, such as carrier proteins or channel proteins, embedded in the membrane to assist the transport of molecules that otherwise cannot pass freely through the lipid bilayer, such as glucose, amino acids, and ions.
Carrier proteins for active transport differ in their function as they do expend energy (ATP) to move substances against their concentration gradient, usually with the assistance of protein carriers known as 'pumps'. In contrast, facilitated diffusion allows molecules to move down their concentration gradient without the use of ATP, facilitated by transport proteins that provide a pathway for the molecules to pass through the hydrophobic interior of the cell membrane.