Which statement best describes primary active transport?

• A. It moves substances down their gradient without energy input.
• B. Energy from ion gradients established by other primary transport to move solutes.
• C. It relies on diffusion through channels.
• D. Direct hydrolysis of ATP powers transport.

Answer: (D)

Primary active transport uses energy from ATP hydrolysis to push substances against their electrochemical gradient. The energy released when ATP is converted to ADP + Pi powers a transporter (pump) to change shape and physically move the solute across the membrane, allowing the cell to accumulate or maintain concentrations that oppose the gradient. Classic examples include the Na+/K+ pump and Ca2+-ATPases. This is different from passive transport, which moves substances down their gradient without energy input, and from secondary active transport, which uses the energy stored in an existing ion gradient established by a primary pump to drive transport. Moving through channels by diffusion is also passive. Therefore, the direct use of ATP hydrolysis to power transport is the defining feature.