One way to find out what a part does is to remove it and see what fails: a knock-out experiment. Biologists do it with drugs and gene changes, and nature does it too: mature human red blood cells have no mitochondria and no nucleus. A fair test needs a control, the same cell under the same conditions with nothing removed. Mitochondria run aerobic respiration and make about 30 ATP from each glucose; without them, glycolysis in the cytoplasm still makes 2. Many other jobs, such as building proteins, making lipids and digesting waste, use ATP. So removing one part can have a direct effect and knock-on effects. Predict first, then run.