1. Cell Analogies
“A Cell is like a city …..”

2. In a far away city called Eukaryote City, the main export and production product is the bolt. Everyone in the town has something to do with bolt making and the entire town is designed to build and export bolts.
cell
protein

3. The town hall has the instructions for bolt making
The town hall has the instructions for bolt making. Bolts come in all shapes and sizes and any citizen of Eukaryote can get the instructions and begin making their own bolts.
ribosome
DNA
nucleus

4. Bolts are generally produced in small shops around the city.
ribosomes

5. These small shops can be built by the carpenter's union (whose headquarters are in town hall).
nucleolus
nucleus

6. Endoplasmic reticulum
After the bolt is constructed, they are placed on special carts which can deliver the bolt anywhere in the city by traveling on special roads.
vesicles
Endoplasmic reticulum

7. In order for a bolt to be exported, the carts take the bolt to the postal office, where the bolts are packaged and labeled for export.
Golgi apparatus

8. Sometimes bolts don't turn out right, and the "rejects" are sent to the scrap yard where they are broken down for parts or destroyed altogether.
lysosome

9. mitochondria
The town powers the bolt shops and carts from a power plant that is in the city.

10. Vesicles Cell membrane
The entire city is enclosed by a large wooden fence. Only the postal trucks are allowed outside the city.
Cell membrane

15. The cell is like a city.
Viewed from outer space, it is hard to see how human life is organized. It is only when we get closer that we start to see the structure of individual communities.
Closer still, and we see that these communities are highly organized.
Although it may seem rather a large jump from a working city to a living cell
the organization of different activities into different areas
the transport of raw materials
the building of new structures
the production of energy
the removal of waste
effective communication systems

16. Towns and cities vary greatly in size
Towns and cities vary greatly in size. Nevertheless, whatever country they are in, they all have many similar features. Likewise, despite the great variety of life forms and cell types, all cells have many features in common. The same basic structures and organizing principles apply equally well to cells in plants, for example, as they do to cells in humans.

17. The Outer Boundary
The city has a clearly defined edge. In medieval times this was a hard defensive wall; today the boundary is less distinct and more likely to be a ring road, but the principle is the same. Healthy and productive life can carry on within the city, in the shelter offered by the boundary. In the cell, the cell membrane performs a similar function to the city wall. It completely surrounds the cell and provides the boundary between one cell and the next. It is constructed, not from bricks and mortar, but from molecules called lipids. The outer boundary of the city is not, of course, continuous. For the city to survive materials, and people, have both to enter and to leave. The cell membrane too must allow materials into and out of the cell. It is important that fuel, raw materials and some signaling molecules can cross the membrane, and that waste products and some manufactured goods can leave. Other materials must be prevented from crossing the membrane at all.

18. 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 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.

19. The Factory
New goods and products are continually being manufactured from raw materials. In cities this takes place in workshops and factories. Raw materials are transformed, usually in a sequence of steps on a production line, into finished products. The process is governed by a clear set of instructions or specifications. In some cases the final products are for immediate or local use, in others they are packaged for export. In the cell too there are production lines, in this case manufacturing new proteins of many different sorts. The messages from the cell's DNA act as a blueprint. This blueprint is interpreted by machines on a production line, which are called ribosomes, to assemble the correct sequence of amino acids. Amino acids are the building blocks used to make proteins. As in cities where products made in factories are used within the city, so the completed proteins are often for home consumption, but others may be exported outside the cell for use elsewhere.

20. The Framework
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. Within a cell there is a set of structures that define the center, distinguish one end of the cell from the other, and provide routes for transportation. These structures are collectively called the cytoskeleton and consist of a series of fibrous proteins that help organize and structure the cell.

21. The Nucleus
The nucleus is the command center, the planning department and the central library - roughly equivalent to City Hall. Inside the nucleus lie the chromosomes that carry the genetic instructions. They are a sort of master plan that specifies how the cell should develop. It would many millions of such files to write down all the instructions for even one cell, so a special code is used, based on very long chain molecules of DNA - deoxyribonucleic acid.