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. This is the cytoskeleton. It consists of many networks of fibrous proteins - micro-filaments, intermediate filaments and microtubules that help organise, structure and orient the cell.

Each individual molecule can behave like a tiny motor, propelling itself by all sorts of methods within a cell.

The cytoskeleton provides both a network of tracks down which cell components move but they also help the whole cell move if required. This includes proteins.

Proteins are the building blocks of the cell. So if DNA is the master plan as to how a cell develops, then proteins are the means by which the plan is implemented. Proteins in cells have something resembling a postcode which ensures that they all end up in the right place and the cytoskeleton plays a role in their delivery.

TRANSPORT SYSTEM / INFRASTRUCTURE SYSTEM

A city has a framework, or structure, which enables movement and organisation. Everything knows where it belongs and where it is going.

The sites of the various activities in a city are not randomly distributed, but tend to be organised: houses in residential areas, factories in industrial estates, shops in the City Centre etc.

Roads and railways form an effective network of communications between these areas and give an overall shape and structure to the city. They are the most important routes for moving people and materials around.

In the city, letters, for example, carry a postcode or "zip code" to help the postman ensure they reach their destination.