Which type of transport allows substances to move down their concentration gradient without energy?

The type of transport that allows substances to move down their concentration gradient without the expenditure of energy is passive transport. This process relies on the natural tendency of molecules to move from areas of higher concentration to areas of lower concentration, striving for equilibrium.

In passive transport, the movement occurs spontaneously, driven solely by the concentration gradient. This encompasses several modes of transport, including osmosis, which is the movement of water across a selectively permeable membrane, and facilitated diffusion, which is the movement of specific molecules across a biological membrane via protein channels.

The term "passive transport" serves as an umbrella concept that includes these specific processes, emphasizing that no energy input is required. The lack of energy expenditure distinguishes passive transport from active transport, where substances move against their concentration gradient necessitating energy consumption, usually in the form of ATP.

Understanding the differences between these transport mechanisms is crucial for grasping how substances move in and out of cells, which is fundamental to many biological processes.