What is the main characteristic of active transport in cells?

The main characteristic of active transport in cells is that it requires energy to move substances against their concentration gradient. This process is essential for maintaining the proper concentrations of various ions, nutrients, and other solutes within the cell. Active transport allows cells to accumulate necessary substances even when those substances are already present in higher concentrations inside the cell compared to the outside environment, which would otherwise make passive transport, like diffusion, insufficient.

By using energy, typically in the form of ATP, active transport enables cells to regulate their internal environment actively. This regulation is crucial for various cellular functions, such as maintaining osmotic balance and enabling cellular processes that depend on specific concentrations of ions and molecules.

The other options relate either to passive processes or specific types of transport not characteristic of active transport. For example, while diffusion is a fundamental transport process, it does not require energy and moves substances down their concentration gradient, which contrasts with the energy-dependent mechanism of active transport.