The sites of the various activities are not just randomly distributed, but tend to be organized: houses in residential areas, factories in industrial estates, shops in the City Centre, and so on. Roads form an effective network of communications between these areas and give an overall shape and structure to the city. They are the most important route for moving people and materials around.
Within a cell there is a complex set of structures that define the centre, distinguish one end of the cell from the other, and provide routes for transportation. These structures are collectively called the cell skeleton - or cytoskeleton - and consist of a series of fibrous proteins that help organize, structure and orient the cell.
Everything knows where it belongs and where it is going. In the city, letters, for example, carry a postcode to help the postman ensure that they reach their destination.
Proteins in cells have their own postcodes too. 10 thousand million proteins, 10 thousand different kinds, and yet they all end up in the right place.