Which process uses energy directly from ATP to move substances across a membrane?

Active transport is the process that directly utilizes energy from ATP to move substances across a membrane, typically against their concentration gradient. This means that it can transport molecules from areas of lower concentration to areas of higher concentration. This mechanism is crucial for maintaining cellular homeostasis and ensuring that essential nutrients and ions are available within the cell, even if they are in lower concentrations outside.

In contrast, osmosis, facilitated diffusion, and simple diffusion do not require ATP. Osmosis involves the passive movement of water molecules across a semi-permeable membrane from a region of lower solute concentration to one of higher concentration. Facilitated diffusion also functions passively, using specific transport proteins to help molecules cross a membrane without the need for energy. Simple diffusion is the movement of molecules from an area of higher concentration to one of lower concentration, driven solely by the concentration gradient. These processes rely on passive mechanisms rather than the energy investment that active transport entails.