All cells need energy to perform their functions. For this, cells use the chemical energy of external molecules such as glucose and transform this chemical energy into ATP which is the form of energy they use to function.
This glucose will pass through a glycolysis to be converted into pyruvate, ATP and NADH, then go into Krebs cycle to manage more NADH, as well as FADH2 and ATP and eventually oxidative phosphorylation to transform NADH and FADH2 into ATP (molecule supplying energy during its consumption).