2. THERMODYNAMICS OF LIVING SYSTEMS
17
are in a sense a permanent part of the interior of the cell, except as
they too enter into reactions and disappear.
If one considers the cell as a system enclosed within another system,
e.g., the environment in which the cell is embedded, it is obvious that
there is a continual change of matter across the phase boundary or
membrane. Materials enter, react, and waste products are extruded.
Elaborate pathways have evolved by which energy is accumulated,
transferred, stored, and released as the requirements of the cell demand. Comparative biochemistry reveals the universality of certain of
these pathways and one sees variations of a central theme throughout
all nature. Besides the movement of matter necessary to sustain life,
there is a need for an energy flux as well. Heat produced by chemical
reactions must be utilized within the cell, dissipated, or directed to
energize reactions which are energy-deficient.
In addition to these properties most cells also possess the ability to
duplicate themselves. Furthermore, they house an elaborate information
center, capable of transmitting information from daughter cell to
daughter cell with a remarkable degree of accuracy.
All of the above processes are carried out under very mild conditions, e.g., at room temperature and at a pH of about 7. All of this
points to the marked difference of such systems compared with the
usual models employed to explain classical thermodynamics. Classical
theory concerns itself with equilibrium states and reversible reactions.
While it is true that it takes cognizance of nonequilibrium states, it
leads only to inequalities not sufficiently restricted to yield useful results. The steady state provides a fruitful alternative restriction.
Thermodynamics of irreversible processes is largely concerned with
entropy production and nonequilibrium states. As we shall later see,
such an analysis leads to a method of determining which processes become possible by means of an entropy coupling, e.g., the use of entropy
production of another process. Since entropy does play the central role
in classical as well as in irreversible thermodynamics, a great deal of
attention will be devoted to it.
B. EQUILIBRIUM AND OPEN SYSTEMS DEFINED; STEADY STATES
It is more or less intuitive that a system left to itself will eventually
come to equilibrium. After all backward reactions balance forward reactions, after all thermal gradients have disappeared, and after all
pressure gradients have vanished, the system no longer changes with
time. This time-independent state of an isolated system is called equilibrium. It is also possible to have another type of time-independent
state which is quite different from the one above. Suppose that energy
17
are in a sense a permanent part of the interior of the cell, except as
they too enter into reactions and disappear.
If one considers the cell as a system enclosed within another system,
e.g., the environment in which the cell is embedded, it is obvious that
there is a continual change of matter across the phase boundary or
membrane. Materials enter, react, and waste products are extruded.
Elaborate pathways have evolved by which energy is accumulated,
transferred, stored, and released as the requirements of the cell demand. Comparative biochemistry reveals the universality of certain of
these pathways and one sees variations of a central theme throughout
all nature. Besides the movement of matter necessary to sustain life,
there is a need for an energy flux as well. Heat produced by chemical
reactions must be utilized within the cell, dissipated, or directed to
energize reactions which are energy-deficient.
In addition to these properties most cells also possess the ability to
duplicate themselves. Furthermore, they house an elaborate information
center, capable of transmitting information from daughter cell to
daughter cell with a remarkable degree of accuracy.
All of the above processes are carried out under very mild conditions, e.g., at room temperature and at a pH of about 7. All of this
points to the marked difference of such systems compared with the
usual models employed to explain classical thermodynamics. Classical
theory concerns itself with equilibrium states and reversible reactions.
While it is true that it takes cognizance of nonequilibrium states, it
leads only to inequalities not sufficiently restricted to yield useful results. The steady state provides a fruitful alternative restriction.
Thermodynamics of irreversible processes is largely concerned with
entropy production and nonequilibrium states. As we shall later see,
such an analysis leads to a method of determining which processes become possible by means of an entropy coupling, e.g., the use of entropy
production of another process. Since entropy does play the central role
in classical as well as in irreversible thermodynamics, a great deal of
attention will be devoted to it.
B. EQUILIBRIUM AND OPEN SYSTEMS DEFINED; STEADY STATES
It is more or less intuitive that a system left to itself will eventually
come to equilibrium. After all backward reactions balance forward reactions, after all thermal gradients have disappeared, and after all
pressure gradients have vanished, the system no longer changes with
time. This time-independent state of an isolated system is called equilibrium. It is also possible to have another type of time-independent
state which is quite different from the one above. Suppose that energy