Active transport is the process that requires energy input to occur in cells. This is because, during active transport, substances move against their concentration gradient, meaning they are transported from an area of lower concentration to an area of higher concentration. This movement cannot happen spontaneously, as it opposes the natural flow of diffusion.
Energy, typically in the form of ATP (adenosine triphosphate), is necessary to power the transport proteins that carry out this process. This is crucial for various cellular functions, such as the uptake of essential nutrients and the maintenance of ion gradients that are vital for processes like nerve impulse transmission and muscle contractions.
In contrast, the other processes listed, including passive transport, diffusion, and facilitated diffusion, do not require energy input. These processes leverage natural concentration gradients or channel proteins to allow substances to move more freely without the need for additional energy, focusing instead on maintaining homeostasis in the cell.