Physics - Molecular Theory of the Living Cell - page 5

Chapter 2
Physics

2.1 Thermodynamics of Living Systems

2.1.1 Three Thermodynamic Systems: Isolated, Closed, and Open

Thermodynamics is the scientific study of the production, transformation, and
storage of heat and other forms of energy in material systems, including organisms.
Thermodynamic systems can be divided into three classes - isolated, closed ,and open
(Prigogine 1980; Kondepudi and Prigogine 1998; Kurzynski 2006). Isolated thermo-
dynamic systems do not exchange any energy nor matter with their environment;
closed systems can exchange energy but not matter with their environment; and open
systems can exchange both energy and matter with their environment. Obviously
living systems are open thermodynamic systems. It should be pointed out that
physicists often use the term “closed systems” in the sense of “isolated systems” in
chemistry. Not distinguishing between closed and isolated systems can lead to
confusions in thermodynamic discourses. For example, the entropy of an isolated
system (e.g., the Universe) increases with time according to the Second Law of
thermodynamics, but the entropy of a closed system (e.g., a refrigerator) need not
increase with time.
Thermodynamic systems can be divided into two distinct groups based on
another criterion, namely, whether or not a given thermodynamic system involves
any dissipation of free energy. Free energy is different from energy in that it is a
function not only of energy, E, a measure of the capacity of a system to do work, but
also of entropy, S, a measure of the quality of energy. For example, the Gibbs free
energy , G, of a thermodynamic system is defined as

D G ¼ D E þ P D V T D S (2.1)

where P, V, and T are pressure, volume, and temperature, respectively, of the
thermodynamic system under consideration, and the symbol D X is defined as
the difference in variable X between the initial and the final states of the system,