2. THERMODYNAMICS OF LIVING SYSTEMS
25
the reaction conditions. A reversible reaction is one which proceeds in
such a way that it is always essentially at equilibrium. The concept of
reaction rate or time to reach equilibrium is not involved. As a matter
of fact, we usually must postulate that our ideal reactions will take
place infinitely slowly in order that they be reversible. As has been
emphasized by several writers, the field of classical thermodynamics
might have better been termed thermostatics. Thermodynamically reversible reactions or processes do not occur to any extent in our physical universe. The reversible process may be regarded as the limiting
condition to which actual processes approach. Such limiting conditions
can be defined precisely and calculations of work, heat, energy, etc.
made which can then be applied with reasonable reliability to actual
processes.
With respect to biological systems none of the over-all reactions are
readily reversible. For example a typical living cell is maintained in a
high-energy, nonequilibrium condition by a continuous inflow of energy
in the form of light or in the form of oxidizable organic compounds
synthesized using light energy. However, if one considers a specific enzymatic reaction within a cell, he may find the reversible reaction
E n ^±E d
occurring. It is only in death that the cell comes to equilibrium, and,
moreover, death is an irreversible process.
We must recognize then that thermodynamics gives us only an approximation, although in many cases a very good approximation, when
applied to biological systems. In an attempt to provide a closer approach, a well-known field termed thermodynamics of irreversible processes has been developed. This branch of thermodynamics owes much
to Onsager's attempts to treat irreversible processes on a microscopic
level analogous to classical theory. In a later section we shall consider
this field in more detail.
G. THE FIRST LAW OF THERMODYNAMICS: CONSERVATION OF ENERGY
The sum of mass and energy in any kind of change is always a constant. This is the First Law of Thermodynamics. It takes the familiar
mathematical form of
dE = dQ + dW
(19)
where dE is the change in energy accompanying a thermodynamic
change of state of a system. Such a change may be due to the performance of work on or by the system, e.g., dW, and also to the transfer
of energy to or from the system in the form of heat, e.g., dQ. The work
25
the reaction conditions. A reversible reaction is one which proceeds in
such a way that it is always essentially at equilibrium. The concept of
reaction rate or time to reach equilibrium is not involved. As a matter
of fact, we usually must postulate that our ideal reactions will take
place infinitely slowly in order that they be reversible. As has been
emphasized by several writers, the field of classical thermodynamics
might have better been termed thermostatics. Thermodynamically reversible reactions or processes do not occur to any extent in our physical universe. The reversible process may be regarded as the limiting
condition to which actual processes approach. Such limiting conditions
can be defined precisely and calculations of work, heat, energy, etc.
made which can then be applied with reasonable reliability to actual
processes.
With respect to biological systems none of the over-all reactions are
readily reversible. For example a typical living cell is maintained in a
high-energy, nonequilibrium condition by a continuous inflow of energy
in the form of light or in the form of oxidizable organic compounds
synthesized using light energy. However, if one considers a specific enzymatic reaction within a cell, he may find the reversible reaction
E n ^±E d
occurring. It is only in death that the cell comes to equilibrium, and,
moreover, death is an irreversible process.
We must recognize then that thermodynamics gives us only an approximation, although in many cases a very good approximation, when
applied to biological systems. In an attempt to provide a closer approach, a well-known field termed thermodynamics of irreversible processes has been developed. This branch of thermodynamics owes much
to Onsager's attempts to treat irreversible processes on a microscopic
level analogous to classical theory. In a later section we shall consider
this field in more detail.
G. THE FIRST LAW OF THERMODYNAMICS: CONSERVATION OF ENERGY
The sum of mass and energy in any kind of change is always a constant. This is the First Law of Thermodynamics. It takes the familiar
mathematical form of
dE = dQ + dW
(19)
where dE is the change in energy accompanying a thermodynamic
change of state of a system. Such a change may be due to the performance of work on or by the system, e.g., dW, and also to the transfer
of energy to or from the system in the form of heat, e.g., dQ. The work