Entropy of living systems

Living systems are systems characterized by the ability to maintain or even reduce their degree of disorder, or entropy (S), i.e. they can dynamically ensure the stability of their internal environment in terms of structure, chemistry and energy. Living systems achieve this through a complex set of processes called metabolism.

A prerequisite for the existence of living systems is the constant exchange of energy and matter with the surrounding environment. Therefore, living systems are open systems from the thermodynamic point of view, so they do not contradict the wording of the second theorem of thermodynamics, which allows a zero or negative change in entropy for open systems.

The internal environment of a living system is characterized by a high degree of constancy, which is fundamentally different from thermodynamic equilibrium, as the entropy of a living system is lower than the maximum possible. A living system reaches true thermodynamic equilibrium only after its death, when the regulatory functions cease and the entropy of the now non-living system increases until it reaches a maximum due to the action of irreversible processes.

Entropy change mechanisms
It is not realistically possible to determine the entropy value of a living system, therefore in practice we always determine only the total or partial change in entropy, while in living systems we can briefly describe two types of mechanisms causing entropy change:


 * Irreversible processes described by the second theorem of thermodynamics always causing positive or zero changes in entropy, i.e. increase the disorder of the system:

$$ d_{in}S \geq 0 $$


 * Controlled regulatory processes of a living system conditioned by communication with the surrounding environment, i.e. metabolism. Partial changes in system entropy induced in this way can have either a positive value (gain of energy from the surroundings and therefore an increase in system entropy) or a zero or negative value (use of energy to perform cellular work or release of energy to the surroundings and therefore a decrease in system entropy). The overall change in entropy caused by these regulatory mechanisms is always negative, as the living system maintains or increases the orderliness of its structure through them:

$$ d_{ex}S=\frac{-\delta Q}{T} $$ The total change in entropy of a living system is then described by the relation: $$ dS= d_{in}S - \frac{\delta Q}{T} $$.

Entropy and metabolism
The stability of the environment at the cellular level is ensured by the constant activity of cellular metabolism, conditioned by obtaining energy from sunlight directly (phototrophic organisms) or indirectly (chemotrophic organisms). This energy is used to synthesize more complex substances from simpler, energy-poor precursors (anabolism), which reduces the entropy of the system.

In parallel, the opposite process takes place, i.e. the breakdown of energy-rich complex substances and their subsequent processing (e.g. by cellular respiration) with the simultaneous release of energy (catabolism). Part of this energy is again used by anabolic processes, but part is released into the environment in the form of heat or other forms of energy. The positive change in entropy is thus distributed between the cell itself and its surroundings.

The cell thus increases its degree of order, i.e. its total entropy change is negative. However, the absolute value of this change, in accordance with the second theorem of thermodynamics, does not exceed the positive change in entropy of the isolated system of the cell and its surroundings:

$$ \Delta S_{bu\check nky}+ \Delta S_{okol\acute i}> 0 $$

From a thermodynamic point of view, however, it is not possible to examine the change in entropy ∆S in isolation, because for metabolic processes we also describe the change in enthalpy ∆H, i.e. the change in chemical energy for a given metabolic reaction. Since metabolic processes take place at constant pressure and temperature, a more appropriate quantity used in practice to describe metabolic reactions is the change in Gibbs energy, for which the relation applies:

$$ \Delta G= \Delta H-T\Delta S $$

Related articles

 * Entropy
 * Entropie (česká wikipedie)
 * Entropy (anglická wikipedie)