Abstract

Life can be defined as combining two entities that rest on completely different physico-chemical properties and on a particular way of handling information. The cell, first, is a « machine », that combines elements which are quite similar (although in a fairly fuzzy way) to those involved in a man-made factory. The machine combines two processes. First, it requires explicit compartmentalisation, including scaffolding structures similar to that of the châssis of engineered machines. In addition, cells define clearly an inside, the cytoplasm, and an outside. The cell envelope is more or less complicated in bacteria, and it is much more complicated in organisms made of cells with a nucleus (eukaryotes). Second, the machine also requires dynamic chemical processes, collectively named metabolism, that can be split into intermediary metabolism, managing chemical transformations and transport of small building blocks and management of energy (often with a rotating nanomachine, ATP synthase), and the macromolecule synthesis, salvage and turnover machinery which uses a variety of nanomachines, the ribosome being the most prominent one. The second entity which needs to be associated to life is the genetic program, in the form of the genome, composed of one or several chromosomes made of DNA. This is the entity which associates most clearly to information. Natural selection uses information in a particular way: it is a process that uses energy to prevent degradation of any functional entity. This process keeps building up novel information within organisms.