Final answer:
Active transport of molecules across plasma membranes must function continuously to maintain concentrations of ions and other substances needed by living cells.
Step-by-step explanation:
Active transport of molecules across plasma membranes must function continuously to move substances against a concentration or electrochemical gradient. This process requires energy, usually in the form of ATP, which is generated through the cell's metabolism. Active transport mechanisms, also known as pumps, work against electrochemical gradients to maintain concentrations of ions and other substances needed by living cells. Without continuous active transport, these concentrations would be disrupted by passive movements, resulting in an imbalance and ultimately affecting cell function.