The process by which substances move across the plasma membrane against a concentration gradient (from an area of lower concentration to an area of higher concentration) is called active transport. This process requires energy, typically in the form of ATP, to transport molecules through specific transport proteins or pumps embedded in the plasma membrane.
Active transport is essential for various cellular functions, including the uptake of essential nutrients, the removal of waste products, and the maintenance of ion balance within the cell. Examples of active transport mechanisms include the sodium-potassium pump (which exchanges sodium ions out of the cell for potassium ions into the cell) and the proton pump (which moves protons across a membrane to create an electrochemical gradient).