Final answer:
Active transport mechanisms, such as pumps, work against electrochemical gradients using energy from ATP. These mechanisms maintain the concentrations of ions and other substances needed by living cells. They are sensitive to metabolic poisons that can disrupt the supply of ATP.
Step-by-step explanation:
To move substances against a concentration or electrochemical gradient, the cell must use energy, usually in the form of ATP. Active transport mechanisms, or pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Because active transport mechanisms depend on a cell's metabolism for energy, they are sensitive to many metabolic poisons that interfere with the ATP supply.