Which process do cells use to transport materials against a concentration gradient?

Cells utilize active transport to move materials against a concentration gradient, which means that substances are transported from an area of lower concentration to an area of higher concentration. This process is essential because it enables cells to uptake nutrients and ions that are present in lower concentrations outside the cell compared to the inside.

Active transport requires energy, usually in the form of ATP, because moving substances against a concentration gradient is not a spontaneous process; it goes against the natural flow of diffusion where materials typically move from high to low concentration areas. This energetic aspect distinguishes active transport from other transport mechanisms that do not require energy, such as diffusion, facilitated diffusion, and osmosis.

In summary, active transport is vital for maintaining the right concentrations of various substances within a cell and ensuring proper cellular function, making it the correct answer in this context.