Which of the following requires energy in the form of ATP?

Concept

Substances cross the plasma membrane by two broad routes. In a passive route the solute travels along its own electrochemical gradient, so the drive comes from the random thermal motion of the particles themselves and no metabolic fuel is consumed. In an active route a carrier protein drives the solute against its electrochemical gradient, and that uphill work has to be paid for with cellular energy released by hydrolysing ATP to ADP and inorganic phosphate.

Applying it here

The stem asks which route is coupled to ATP hydrolysis, so we look for the route that does uphill work. That route is active transport: a pump protein binds ATP, hydrolyses it, changes its shape and carries the solute to the side where the solute is already more concentrated. The classic example is the sodium-potassium pump of the nerve cell membrane, which drives three sodium ions out and two potassium ions in for every ATP molecule split.

Contrasting the four routes

• Passive transport - the umbrella name for downhill movement across the membrane; the existing gradient itself supplies the drive, so no ATP is spent.

• Facilitated transport - downhill movement guided by a channel or carrier protein such as GLUT1, which ferries glucose into a red blood cell; the protein supplies a path, not fuel, so no ATP is spent.

• Simple diffusion - small non-polar molecules such as oxygen and carbon dioxide slip straight through the lipid bilayer down their gradient, with no protein and no ATP involved.

• Active transport - a pump protein moves the solute uphill against its gradient, and the hydrolysis of ATP supplies the energy for that work.

Hence, among the four routes listed, active transport is the one that requires energy in the form of ATP.