Life and Death
Looking at various features of a city can help us to understand the problems that cells face, and how to solve them. Like all comparisons, however, they are not perfect: cells in fact are more complex and versatile than even the largest city.
If we count protein molecules, for example, as inhabitants, the average cell would have a population of about 10 thousand million - that's a thousand times the population of London. And yet cells can duplicate their entire contents and divide into two daughter cells in only a few hours. A fertilised egg becomes two cells, each of these divides in turn and before long there are millions of cells as they form the young embryo.
In making a new living being, not all of the cells are used. Very large numbers of cells die, carefully committing suicide at the right place and at the right time, to ensure the correct patterning and organisation of the remainder.
It is hard to make parallels between this sort of activity and events within a city, although cities do of course die: for example Pompeii in Italy and the ghost mining towns in the Rocky Mountains.
Cells constantly produce molecules that ensure the survival of their neighbours. Most cells only die when they begin to lose this support. This concept of programmed cell death - or apoptosis as it is called - is one of the most exciting new concepts in modern biological research.