The second principle of thermodynamics had been discovered even on Mars, and greenmen knew that an organism must be in a perpetual state of activity in order to be alive. Not only must a body be capable of repairing itself when something breaks down, but it must be repairing itself all the time, it must always be demolishing and rebuilding its own structures, i.e. it must be capable of permanent self-production, or autopoiesis.
Varela and Maturana add that autopoiesis must be a property of every living system, including its smallest units, which means that any cell can be represented by a scheme that illustrates its continuous transformation of external matter into cellular components (Figure 1-3). When production is equal to demolition, a cell is in a stationary state (self-maintenance); when production is greater than demolition, a cell grows and eventually divides itself into two (self-reproduction).
Varela and Maturana arrive in this way at a definition of the living system in general and of the cell in particular: a physical system is alive if it is capable of transforming a flux of external matter and energy into an internal flux of self-maintenance and self-reproduction.
This definition, as we have seen, is an automatic consequence of the second principle of thermodynamics, and the autopoietic machine represents therefore an updated version of the 19th century concept of thermodynamic machine.
But is this really the most general definition of life?

The epigenetic cell

One of the most important biological achievements of the 20th century was the discovery that the information of a gene is determined by the order of its nucleotides, pretty much as the information of a word is due to the order of its letters, and in both cases information corresponds to the positional order of elementary units along a line.
Genetic information is therefore a linear quantity, but the function of proteins is determined by the arrangement of their amino acids in space, i.e. by their three-dimensional information. Clearly genes are not transporting all the information that is going to appear in proteins. Where then does the missing information come from?