Thermodynamics and Statistical Mechanics

Thermodynamics started as the study of heat engines, but applies to all processes in the universe, including those of chemistry and, crucially, the workings of life at every level. So, though this theme concerns the thermodynamics of living processes, we illustrate it with, perhaps the quintessence of thermodynamics, the formation of and working of stars, where the chemical elements used by all known life are created and the source of energy for very nearly all life on earth (we mean our nearest star, of course). The statistical mechanics interpretation of thermodynamics has both inspired and confused the use of information theory in biology (and other fields beyond physics), most notably regarding entropy. Here we try to convey a clear understanding of both thermodynamics and statistical mechanics interpretations of living processes in a way that supports the integrated description of life as an information-process, at all levels from atoms to the global ecosystem.


Some fundamental principles


All of living involves the transformation of energy from one form to another: in this respect life as a whole and all individual organisms themselves are engines. Since energy transformations can never be 100% efficient, all living things radiate heat energy (they glow in the infra-red and indeed this has been proposes as one of the methods for detecting life beyond our planet). The essential reason why living must trasnform energy is that this is the way to 'pump' entropy* from with a system (the organism in this case) to the outside world. This is essential because to live, a system must maintain a high level of organisation and to do that requires work to be done. Thermodynamics teaches us that work can only be obtained by 'pumping' out  entropy, usually into energy. As it gains the excess entropy, energy degrades from high value (such as light) to low value (usually heat). This is what engines do - they pump entropy from hot things into cold things: it was the study of steam engines gave us these insights in the first place and from this developed the science of thermodynamics. 

* Note that since entropy is a property of a system (describing its 'spread'), it is not a material, nor energy, it cannot really be 'pumped' - this is just an analogy using rather figurative language. Read more about entropy here.

This idea does not only apply to organisms. In fact the thermodynamic interpretation is well established and very useful in chemistry and therefore biochemistry, where it explains the necessary conditions and direction of all of life’s chemical reactions, including the transcription of DNA. Further, a simple organism, such as a bacterial cell can be explained this way and when we put organisms together, again thermodynamics explains the flows of energy and materials that relate them in ecological systems: these flows all obey the laws of thermodynamics, enabling us to quantify the processes that make up ecological functions. Understanding this is critical to understanding how life originated: it must have been able to maintain a state of thermodynamic non-equilibrium, meaning that thermodynamic gradients (differences) were constantly maintained. They still are, of course. Living cells are little engines in the following sense: they transform energy from more to less organised forms (increasing the spread of energy density, e.g. from chemical bonds to heat) and do so in a way that performs work. In thermodynamics, work is the organised (coherent) movement, or combination (in the sense of making chemical bonds) of material, as opposed to random movement and bond-making and breaking. This coherence is a specific consequence of the organisational structure of the little engine: it is the inevitable result of the information embodied in the cell's chemical and physical structure. The extra problem for life, as opposed to any human-made machine, is that life is solely responsible for making, maintaining and reproducing itself: the process of autopoiesis.

The Theme is led by Dr Keith Farnsworth