Why do life phenomena not violate the second law of thermodynamics?

For many people, the reconciliation of Life with the Second Law of Thermodynamics remains a mystery. This law says that nothing in the universe will happen spontaneously without the generation of entropy. This function measures how many microstates are accessible to the system and may thus be interpreted in a simplified way as disorganization, or “messiness” of the system. The spontaneous increase of messiness may be easy to understand when considering avalanches thundering down mountains, or buildings collapsing under the effect of an earthquake. But it is much more difficult to see how living organisms can spontaneously synthesize highly ordered functional systems during growth, and how they can defend themselves against the tendency of constant decay that the second law implies. We decided to offer a very simple explanation of this mystery as an introduction to this Special Issue, meant especially for colleagues who are interested in thermodynamics and convinced by its importance, but still see this reconciliation as a mystery. We first describe the historical genesis of the concept of entropy and of the Second Law. Using simple analogs to everyday processes we then explain the generation of entropy and Gibbs energy changes as a driving force for any process. In living organisms, there are two types of metabolisms. Anabolism, i.e. the biosynthetic reactions, would by itself reduce the entropy of the system and thus have a driving force of their own in the wrong direction, i.e. towards decay of biomass. But anabolism is coupled to catabolism, i.e. to energy yielding reactions, that produce far more entropy than anabolism would consume. Therefore, catabolism pushes anabolism to proceed against its own driving force towards growth.