Cells and how to run them

THE CHEMICAL reactions on which life depends need a place to happen. That place is the cell. All the things which biology recognises as indisputably alive are either cells or conglomerations of cells (viruses fall into disputable territory). Since the middle of the 19th century the cell has been seen as the basic unit of life.

A cell requires something to keep its insides in and the outside out. That is the role of the cell membrane, a flexible film made largely of lipids. These are smallish tadpole-shaped molecules with heads that are comfortable in water and twin tails that shun it. When put into a watery solution they naturally form double layers in which the water-tolerant heads are on the outside and the water-wary bits on the inside. Some plant, fungal and bacterial cells employ more rigid structures, called cell walls, as further fortifications beyond their membranes. But it is the membrane which defines the cell.