In terminology, biological system is an expression, with the defunct prefix bio-, referring, historically, to a powered CHNOPS+ system; oft-used, but typically ill-defined, phrase referring to a "living system" or system containing biological entities, contained by a boundary, across which energy, matter, or work may pass. The typical example of a biological system, often used, is the cell, encased by a cell membrane. The term “biological system” is near-to synonymous with the term living system; although the latter is often used to convey a separate meaning in the sense of a system in possession of “life”. Belgian chemist Ilya Prigogine argues, in his 1972 article “Thermodynamics of Evolution”, that coherent behavior, similar to the act of Bénard cell formation, is “the characteristic feature of biological systems”. [5]

System stipulations
In biochemistry, a biological system is often defined as a cell, organism, or population. [1] In biological system situations, however, a caveat tends to stipulate that the system must be large relative to the molecular dimensions. [2] A simple definition would suffice that the system must contain a minimum of two molecules or two biological entities; thus allowing the definition of entropy, as the equivalence value of all uncompensated transformations, being loosely the measure of the internal work that the molecules of the system do on each other, to hold. [3]

Flows and affinities
The thermodynamic forces that drive most flows in biological systems, according to American physical chemist Dilip Kondepudi, are affinities. In particular, according to Kondepudi, when affinity is the difference in chemical potential between reactants and products, the corresponding flow is a chemical reaction; when it is the difference in chemical potential from one location to another, the flow is transport of matter. [4]

Gibbs free energy
The measure of affinity A, in biological systems is Gibbs free energy G, defined as follows:

A = -ΔG

Subsequently, because biological or living systems tend to be modeled as open systems, exchanging matter and energy with their surroundings, both energy and entropy changes will occur in coordination with reactions and movements internal to the system. To quantify these effects, the function of state used in biochemistry is the Gibbs free energy, defined as: [1]

G = H – TS

where H is the enthalpy, T the temperature, and S the entropy of the biological system.

1. Mathews, Christopher K., van Holde, K.E., and Ahern, Kevin G. (2000). Biochemistry, (pg. 63). Addison Wesley Longman, Inc.
2. Hammes, Gordon G. (2000). Thermodynamics and Kinetics for the Biological Sciences, (pg. 2, ch. 3: Applications of Thermodynamics to Biological Systems, pgs. 41-70). New York: John Wiley & Sons, Inc.
3. Clausius, Rudolf. (1879). The Mechanical Theory of Heat, (2nd ed). London: Macmillan & Co.
4. Kondepudi, Dilip. (2008). Introduction to Modern Thermodynamics, (section: Biological Systems, pg. 379). John Wiley and Sons.
5. (a) Prigogine, Ilya, Nicolis, Gregoire, and Babloyants, Agnes. (1972). "Thermodynamics of Evolution," (part I). Physics Today (pgs. 23-28), Vol. 25, November.
(b) Prigogine, Ilya, Nicolis, Gregoire, and Babloyants, Agnes. (1972). "Thermodynamics of Evolution," (part II). Physics Today (pgs. 38-44), Vol. 25, December.

Further reading
Reiner, J.M. and Spiegelman, S. (1945). “The Energetics of Transient and Steady States: with Special Reference to Biological Systems.” (abs) J Phys Chem 49: 81-92.
● Gladyshev, Georgi P. (1999). "On Thermodynamics, Entropy and Evolution of Biological Systems: What Is Life from a Physical Chemist's Viewpoint." Entropy 1, no. 2: 9-20.

External links
Decrease in Entropy in Biological Systems - HyperPhysics.

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