So the cell has energy. Three main activities Chemical energy - synthesis of complex biological molecules. In other words, to make more of themselves. Transport - Cells often live in dilute environments. They are sitting in a lake and they need some amino acid, the concentration of that nutrient is very low in the environment.
Cells have to expend energy to transport that nutrient into the cell.
Mechanical Energy - Cells may be able to change their physical location and all cells need to move structures within them. This requires energy. The oxidized form is on the left and the reduced form is on the right. After reduction, NAD can carry two protons and two electrons. ATP is a carrier of chemical energy in the form of high energy phosphate bonds.