Final answer:
Active transport is an energy-dependent process necessary for maintaining concentration gradients within cells, requiring ATP to power the transport of substances across cellular membranes against their natural diffusion.
Therefore, option a is correct.
Step-by-step explanation:
Active transport must function continuously because it requires energy from ATP to move substances against their concentration gradient. This energy-dependent process involves specific membrane proteins that create an uphill movement of molecules, such as ions or larger substances, ensuring the necessary concentration levels for cell function are maintained. For instance, an ion flowing into a nerve cell relies on active transport to create an electrical potential essential for nerve signal transmission. Another example is the movement of oxygen into cells after oxygen deprivation; active transport allows the cells to maintain vital functions by adjusting concentration levels of oxygen as needed.