Which type of transport requires the use of ATP to move molecules against their concentration gradient?

• A. Facilitated diffusion
• B. Primary active transport
• C. Passive transport
• D. Simple diffusion

Answer: (B)

The correct answer is primary active transport, as this type of transport specifically utilizes ATP to move molecules from an area of lower concentration to an area of higher concentration, which is known as moving against the concentration gradient. This process involves specific transport proteins, often referred to as pumps, that harness energy derived from ATP hydrolysis to change shape and transport substances across the cell membrane.

In contrast, facilitated diffusion, passive transport, and simple diffusion do not require energy input in the form of ATP. Facilitated diffusion and simple diffusion occur along the concentration gradient, allowing molecules to move from areas of higher concentration to areas of lower concentration without any energy expenditure by the cell. Passive transport encompasses both facilitated diffusion and simple diffusion, relying solely on the inherent kinetic energy of molecules and the permeability of the cell membrane to different substances. Therefore, it is the primary active transport mechanism that distinguishes itself by requiring energy to transport molecules in opposition to their natural tendency to move towards equilibrium.