2. THERMODYNAMICS OF LIVING SYSTEMS
49
The ATP system is involved in numerous syntheses and metabolic
transformations. It is believed to be involved in transmethylation, sulfur
assimilation in autotrophic bacteria, and in photosynthesis. This is evidence of its diverse nature. Its importance to biological energetics can
hardly be overestimated.
L. SUMMARY OF CLASSICAL THERMODYNAMICS
We should like to summarize some of the more important ideas
pertinent to biological systems as found in classical thermodynamics.
1. The sum of mass and energy in any kind of change is always a constant.
2. There exists a state function, the entropy, which has the following
properties:
a. Entropy is an extensive property. The total entropy of a system is
the sum of the entropy of its parts.
b. The change in entropy may be expressed as
dS = d e S + diS
(69)
where d e S is the flow of entropy arising from interactions with
the exterior and diS is entropy arising from changes within the
system itself. Furthermore,
dJS Ϊ 0
(70)
where the equal sign has reference to a reversible process, and
the inequality sign pertains to an irreversible process.
c. The only general criterion of irreversibility is Eq. 70 above; however, state functions like the Gibbs free energy, for example, exist
under certain conditions. AF is a very useful concept. It represents
the maximum amount of work obtainable from any particular
process; it determines the direction of reactions; and it is directly
related to the equilibrium constant and to the standard oxidation
potential. Although the evaluation of AF is often based on equilibrium theory, its significance remains the same, whether or not
a system is in equilibrium. Furthermore, general expressions may
be developed which can be used to evaluate AF providing the activities of the products and reactants can be determined. Many
times activities cannot be ascertained, and it becomes necessary
to make simplifying assumptions; however, the theoretical significance of AF remains the same. We belabor this point because
there is some confusion about what is meant by the statement that
classical thermodynamics does not apply to living systems since
they are not equilibrium systems. The problem of accumulation,
49
The ATP system is involved in numerous syntheses and metabolic
transformations. It is believed to be involved in transmethylation, sulfur
assimilation in autotrophic bacteria, and in photosynthesis. This is evidence of its diverse nature. Its importance to biological energetics can
hardly be overestimated.
L. SUMMARY OF CLASSICAL THERMODYNAMICS
We should like to summarize some of the more important ideas
pertinent to biological systems as found in classical thermodynamics.
1. The sum of mass and energy in any kind of change is always a constant.
2. There exists a state function, the entropy, which has the following
properties:
a. Entropy is an extensive property. The total entropy of a system is
the sum of the entropy of its parts.
b. The change in entropy may be expressed as
dS = d e S + diS
(69)
where d e S is the flow of entropy arising from interactions with
the exterior and diS is entropy arising from changes within the
system itself. Furthermore,
dJS Ϊ 0
(70)
where the equal sign has reference to a reversible process, and
the inequality sign pertains to an irreversible process.
c. The only general criterion of irreversibility is Eq. 70 above; however, state functions like the Gibbs free energy, for example, exist
under certain conditions. AF is a very useful concept. It represents
the maximum amount of work obtainable from any particular
process; it determines the direction of reactions; and it is directly
related to the equilibrium constant and to the standard oxidation
potential. Although the evaluation of AF is often based on equilibrium theory, its significance remains the same, whether or not
a system is in equilibrium. Furthermore, general expressions may
be developed which can be used to evaluate AF providing the activities of the products and reactants can be determined. Many
times activities cannot be ascertained, and it becomes necessary
to make simplifying assumptions; however, the theoretical significance of AF remains the same. We belabor this point because
there is some confusion about what is meant by the statement that
classical thermodynamics does not apply to living systems since
they are not equilibrium systems. The problem of accumulation,