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Thermodynamics/Entropy of living organisms?


Hello Kevin,

I've been thinking about entropy and living organisms.  Everything I read seems to suggest that living organisms have lower entropy that their environment (because they are open systems).  For example, the Wikipedia article on "Entropy and Life" mentions that Schrödinger thought living organisms lowered their internal entropy (again, does not violate Second Law because they are open systems).

On first thought this made sense to me; living organisms are more 'organized' than non-living matter.  However, on closer inspection, I would think that living organisms have actually much higher entropy than non-living matter.  

I say this because a more precise definition of entropy is a measure of how dispersed is the energy between the various possible energy modes of the molecules and atoms.  In a living organism, the energy contained in its matter is highly distributed between many energy modes (movement, vibration, electricity, chemical), while non-living matter this energy is more concentrated in less energy modes (mostly chemical).  

As an example, my cat eats only dry cat food and water.  Therefore the matter that composes my cat comes completely from the matter contained in the dry food, water and air.  The energy in the food is concentrated mostly in chemical energy.  However, the energy in my cat is more widely distributed between heat energy (which is really molecular kinetic energy), kinetic and potential energy as it runs around, electrical energy in its nervous system, and chemical energy in its fat and tissues.  Clearly the energy is more dispersed between different energy modes, meaning much higher entropy.  Another way to see it, there is more 'unknown information' about the microstates of the energy, given the macrostate of the energy contained in the cat, as opposed to the energy in the food.

What do you think?

Thank you in advance!

Hi Juan,

I think you have it more or less right, especially with respect to your cat.
We know that animals use enthalpy in the form of calorific value of food and that this is quite straight forward. Where our knowledge is lacking is, as you have said, in the entropy.
A self reproducing system such as cats or bacteria must be using ethalpy to overcome the entropy loss on organising matter (water, food, air) into a new organism.

Well done. You think like a thermochemist.

Best wishes


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Thermodynamics, chemistry, chemical reactions, kinetics, chemical reaction safety, dust explosion technology, static electricity. General science.


Over 40 years experience as a practicing thermochemist in industry. Head of the fire and explosion laboratory of a major European chemical company (Ciba-Geigy). Now retired.

Institute of Chemical Engineers. Royal Society of Chemistry.

Chartered Chemist, Member of the Royal Society of Chemistry (C.Chem MRSC). Msc Sheffield.

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