Final answer:
Primary active transport requires ATP to function, moving substances against their concentration gradient with the help of protein carriers. Secondary active transport relies on the electrochemical gradient set by primary active transport and does not directly use ATP.
Step-by-step explanation:
Among the mechanisms mentioned for the transport of small molecules across cell membranes, primary active transport has a direct requirement for ATP to function properly. This process uses ATP to move substances against their concentration gradient, precisely with the help of protein carriers embedded in the cell membrane. Secondary active transport, on the other hand, does not directly use ATP but relies instead on the electrochemical gradient established by primary active transport to drive the movement of molecules.
Other methods like facilitated diffusion, simple diffusion, and osmosis do not require ATP for their operation. They are passive processes that rely on the concentration gradients and do not use cellular energy in the form of ATP. Therefore, out of the options provided, primary active transport is the one that directly entails the use of ATP.