Like cities, cells are active, energetic beings. They too need a constant supply of energy and they produce it by converting fuel into useable cellular energy.
The power stations of the cell are called mitochondria and the most common fuel that they consume is sugar (glucose). In this case the energy generated is passed on, not as electricity, but as small universal molecules called ATP (the energy source of the molecule).
Mitochondria are spherical or sausage shaped structures with folded internal sides which gives them a large internal surface area. On this surface sit the proteins which convert glucose into ATP. The ATP is then passed out of the mitochondria into the cell for use. ATP is the basic unit of energy required in the cell. It's rather like a specific type of battery which is needed to power all the chemical reactions which go on in the cell.
These can be considered the local power station in the cell: we have many in each cell. In specialised cells which require lots of energy (such as muscle) there are many thousands. They convert the storage material glucose into ATP using oxygen. A bit like burning coal to generate electricity, only the energy is passed out into the cell as small packets of chemical energy rather than pumped out onto power lines. Rather like producing a lot of small batteries.
The more power a process in the cell needs the more mitochondria are needed.
Constant production of energy is necessary to keep the city alive. Energy arrives stored in fuel (such as coal, gas, or atomic rods). It is converted in a power station, into a more useable form - electricity - that is then distributed to where it is needed throughout the city.