Editing Second law of thermodynamics (section)

== Living organisms ==
There are two principal ways of formulating thermodynamics, (a) through passages from one state of thermodynamic equilibrium to another, and (b) through cyclic processes, by which the system is left unchanged, while the total entropy of the surroundings is increased. These two ways help to understand the processes of life. The thermodynamics of living organisms has been considered by many authors, including [[Erwin Schrödinger]] (in his book ''[[What is Life?]]'') and [[Léon Brillouin]].<ref name="Brillouin 2013 p. ">{{cite book | last=Brillouin | first=L. | title=Science and Information Theory | publisher=Dover Publications, Incorporated | series=Dover Books on Physics | year=2013 | isbn=978-0-486-49755-6 | url=https://books.google.com/books?id=tPXVbiw_1P0C | access-date=26 March 2021 | page=}}</ref>

To a fair approximation, living organisms may be considered as examples of (b). Approximately, an animal's physical state cycles by the day, leaving the animal nearly unchanged. Animals take in food, water, and oxygen, and, as a result of [[metabolism]], give out breakdown products and heat. Plants [[Photosynthesis|take in radiative energy]] from the sun, which may be regarded as heat, and carbon dioxide and water. They give out oxygen. In this way they grow. Eventually they die, and their remains rot away, turning mostly back into carbon dioxide and water. This can be regarded as a cyclic process. Overall, the sunlight is from a high temperature source, the sun, and its energy is passed to a lower temperature sink, i.e. radiated into space. This is an increase of entropy of the surroundings of the plant. Thus animals and plants obey the second law of thermodynamics, considered in terms of cyclic processes.

Furthermore, the ability of living organisms to grow and increase in complexity, as well as to form correlations with their environment in the form of adaption and memory, is not opposed to the second law – rather, it is akin to general results following from it: Under some definitions, an increase in entropy also results in an increase in complexity,<ref name="Ladyman Lambert Wiesner pp. 33–67">{{cite journal | last1=Ladyman | first1=James | last2=Lambert | first2=James | last3=Wiesner | first3=Karoline | title=What is a complex system? | journal=European Journal for Philosophy of Science | publisher=Springer Science and Business Media LLC | volume=3 | issue=1 | date=19 June 2012 | issn=1879-4912 | doi=10.1007/s13194-012-0056-8 | pages=33–67| s2cid=18787276 }}</ref> and for a finite system interacting with finite reservoirs, an increase in entropy is equivalent to an increase in correlations between the system and the reservoirs.<ref>{{cite journal | last1=Esposito | first1=Massimiliano | last2=Lindenberg | first2=Katja |author-link2=Katja Lindenberg | last3=Van den Broeck | first3=Christian | title=Entropy production as correlation between system and reservoir | journal=New Journal of Physics | volume=12 | issue=1 | date=15 January 2010 | issn=1367-2630 | doi=10.1088/1367-2630/12/1/013013 | page=013013| arxiv=0908.1125 | bibcode=2010NJPh...12a3013E | doi-access=free }}</ref>

Living organisms may be considered as open systems, because matter passes into and out from them. Thermodynamics of open systems is currently often considered in terms of passages from one state of thermodynamic equilibrium to another, or in terms of flows in the approximation of local thermodynamic equilibrium. The problem for living organisms may be further simplified by the approximation of assuming a steady state with unchanging flows. General principles of entropy production for such approximations are a subject of [[Non-equilibrium thermodynamics|ongoing research]].