Advanced Placement (AP) Biology Practice Exam

Which of the following processes requires energy?

1. Facilitated diffusion

2. Passive transport

3. Active transport

4. Osmosis

The correct answer is: Active transport

Active transport is the process that requires energy because it involves the movement of substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This movement necessitates the use of energy, typically supplied in the form of adenosine triphosphate (ATP). In contrast, facilitated diffusion, passive transport, and osmosis do not require energy. Instead, these processes rely on the natural tendency of molecules to move from areas of higher concentration to areas of lower concentration until equilibrium is reached. In facilitated diffusion, specific transport proteins assist in moving molecules across the membrane without energy input, while osmosis specifically refers to the movement of water across a selectively permeable membrane. Therefore, only active transport is distinctively characterized by its energy requirement for moving substances against their natural flow.