Which type of transport requires ATP to move substances against the concentration gradient?

Active transport is the correct answer because it specifically involves the movement of substances against their concentration gradient, which requires energy in the form of ATP. This process is essential for maintaining cellular homeostasis, allowing cells to uptake essential nutrients and ions even when they are in lower concentrations outside the cell than inside. An example of active transport is the sodium-potassium pump, which transports sodium ions out of the cell and potassium ions into the cell, crucial for various cellular functions, including maintaining osmotic balance and generating action potentials in nerve cells.

On the other hand, passive transport mechanisms, such as simple diffusion and facilitated diffusion, do not require ATP because they rely on the natural movement of particles along their concentration gradients. In passive transport, substances move from areas of higher concentration to areas of lower concentration until equilibrium is reached, which does not require energy expenditure.