The Power Station
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.
Like cities, cells are active, energetic beings. They too need a constant supply of energy and they produce it in exactly the same way: by converting fuel. The power stations of the cell are called mytochondria, 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. Carbon dioxide and water are the waste products.