This is a bit of a trick question! All cells need ATP to function. Even the simplest processes, like maintaining a concentration gradient across a membrane, require energy.

Here's why:

* Active transport: Cells need to move molecules against their concentration gradient (from low to high concentration). This requires energy, provided by ATP.

* Protein synthesis: Building proteins requires energy to create the peptide bonds and fold the protein properly.

* Cellular signaling: Sending and receiving signals, a fundamental part of cell communication, often involves energy-dependent processes.

* Maintaining homeostasis: Cells need to maintain a stable internal environment, which requires energy to regulate temperature, pH, and other factors.

So, there's no cell that can operate without ATP. It's the fundamental energy currency of life.

Perhaps the question is asking about a specific process that can occur passively, without ATP. In that case, you might be thinking about:

* Simple diffusion: Movement of molecules down their concentration gradient (from high to low concentration), which doesn't require ATP.

* Osmosis: The movement of water across a semipermeable membrane, following the concentration gradient of water, also doesn't require ATP.

However, even these passive processes are indirectly influenced by the cell's overall energy state and the activity of ATP-powered pumps that maintain the gradients.