What type of transport mechanism uses ATP energy to move substances against a concentration gradient?

The correct answer is primary active transport, which specifically refers to the movement of substances against their concentration gradient using energy derived directly from ATP. In this transport mechanism, ATP is hydrolyzed to provide the necessary energy to pump ions or molecules from an area of lower concentration to an area of higher concentration.

An example of primary active transport is the sodium-potassium pump (Na+/K+ pump), which moves sodium ions out of the cell and potassium ions into the cell. This process is crucial for maintaining the electrochemical gradient across the cell membrane, which is essential for various cellular functions, including nerve impulse transmission and muscle contraction.

Other transport mechanisms mentioned differ in their energy requirements and the direction relative to the concentration gradient. Primary passive transport would involve movement down a concentration gradient without energy expenditure, while facilitated diffusion also requires no energy and allows substances to move through a membrane via specific transport proteins, again down their concentration gradient. Secondary active transport relies on the ion gradient established by primary active transport, but it does not directly use ATP to move the substance in question; instead, it uses the energy from the movement of another ion down its gradient to facilitate the transport of a different molecule against its gradient.