The Vital Question: Why is life the way it is?

by Nick Lane

The more deeply rooted a mechanism, the more it can become the basis of quite unrelated traits. So proton gradients are widely used to drive the uptake of nutrients and excretion of waste; they are used to turn the screw that is the bacterial flagellum, a rotating propeller that motors cells about; and they are deliberately dissipated to produce heat, as in brown fat cells. Most intriguingly, their collapse ushers in the abrupt programmed death of bacterial populations. In essence, when a bacterial cell becomes infected with a virus, it is most likely doomed. If it can kill itself quickly, before the virus copies itself, then its kin (nearby cells sharing related genes) might survive. The genes that orchestrate cell death will spread through the population. But these death genes have to act quickly, and few mechanisms are faster than perforating the cell membrane. Many cells do exactly this – when infected, they form pores in the membrane. These collapse the proton-motive force, which in turn trips the latent death machinery. Proton gradients have become the ultimate sensors of cellular health, the arbiters of life and death, a role that will loom large later in this chapter.