Which statement best defines active transport?

• A. Endocytosis
• B. Osmosis
• C. Passive diffusion
• D. Movement of substances across a membrane against a concentration gradient that requires energy (usually ATP)

Answer: (D)

Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration, which requires energy input, typically in the form of ATP. This uphill movement and energy requirement are what distinguish active transport from other processes. Endocytosis is a specific method of active transport that brings material into the cell via vesicles, so it’s energy-dependent but not the general definition. Osmosis and passive diffusion are both passive processes: they move substances down their concentration gradient without energy. Therefore, the description that specifies moving against a concentration gradient and needing energy best defines active transport.