What type of transport requires energy to move substances across a membrane?

Active transport is the process that requires energy to move substances across a cell membrane against their concentration gradient. This means that when cells need to transport molecules from an area of lower concentration to an area of higher concentration, energy in the form of ATP is utilized. This is essential for maintaining Homeostasis within the cell and facilitating various cell functions, such as nutrient uptake and waste removal.

In contrast, passive transport mechanisms, such as facilitated diffusion and simple diffusion, do not require energy. These processes rely on the natural movement of molecules along their concentration gradient, meaning substances move from areas of higher concentration to areas of lower concentration without the need for cellular energy input. This distinction underscores the significance of active transport in processes that defy natural diffusion trends, which is vital for cellular functioning and survival.