An introduction to cells.

Abstract

Cells are the triumph of evolution.1 The rest of evolution can be thought of as an elaboration on this masterpiece. In some ways they are more complex than the organs to which they give rise, with the possible exception of the brain, in that their behaviour reflects the integrated activity of about 50000 genes, their products, and the complex biochemical and structural networks that result. In this biochemical network there are two different timescales. The first, which responds to changes in seconds or fractions of a second, is that concerned with metabolism. The enzymes in the cell cytoplasm, including the mitochondria, catalyse molecules along narrowly defined reaction pathways such as the Krebs cycle, the synthesis of purines, or the breakdown of carbohydrates. Many of these reactions require or generate energy in the form of adenosine triphosphate. The speed of response of these metabolic pathways can be contrasted with those in the second system, which entails the synthesis of macromolecules such as nucleic acids and proteins. Here the response times are minutes to hours. Understanding the integration of these two interdependent pathways is a major problem in cell biology.