2. THERMODYNAMICS OF LIVING SYSTEMS
45
In discussing the energetics of living systems, the first consideration
to keep in mind is that they obey the First Law of Thermodynamics. It
is convenient to categorize the multitude of reactions taking place
within cells, e.g., metabolism, into two parts: (a) anabolism, which
consists of the formation of complex molecules from simpler reactants, a
process which requires energy; and (b) catabolism, a process which
breaks larger molecules into smaller ones with a subsequent release of
energy. An important corollary to the fact that living systems obey the
First Law is that energy required in anabolic activities must be supplied by catabolic processes. This leads to the idea of coupling whereby
the catabolic energy may be utilized for synthesis and other activities.
In order to pursue this idea, let us first discuss the concept of reversibility in terms of biological reactions.
All chemical reactions tend to equilibrium if left to themselves, e.g.,
A+B+C+'-'—L+M+N+···
Thermodynamics has nothing to say about the rate of attainment of
equilibrium; however, in the case in which a suitable catalyst is present
equilibrium is rapidly established. In biological systems these catalysts
are generally present in the form of enzymes. We have seen that the
composition of the equilibrium mixture is determined by the over-all
free energy change of the system and by the activities of the reactants
and products. When we say a reaction is irreversible, e.g.,
what we mean is that the free energy of the products is much smaller
than the free energy of the reactants. Thus, the forward reaction greatly
predominates over the backward reaction. The over-all reaction is
therefore considered as irreversible in the forward direction. For example, in a biological system consider a catabolic process characterized
by
A-*L+M+N-\-'·-;
AF = -Q kcal.
Assuming the presence of an appropriate enzyme, Q kcal, per mole of
free energy become available for anabolic activities or for other endergonic processes. In an in vitro preparation, where there is no suitable
transformer to convert the free energy into an appropriate form for
synthesis or biological work, the free energy is dissipated as heat in
the amount given by
AH = AF + TAS
In principle, to drive the reaction backward, e.g., to anabolize the
products, Q kcal, of energy must be supplied. The Second Law assures
45
In discussing the energetics of living systems, the first consideration
to keep in mind is that they obey the First Law of Thermodynamics. It
is convenient to categorize the multitude of reactions taking place
within cells, e.g., metabolism, into two parts: (a) anabolism, which
consists of the formation of complex molecules from simpler reactants, a
process which requires energy; and (b) catabolism, a process which
breaks larger molecules into smaller ones with a subsequent release of
energy. An important corollary to the fact that living systems obey the
First Law is that energy required in anabolic activities must be supplied by catabolic processes. This leads to the idea of coupling whereby
the catabolic energy may be utilized for synthesis and other activities.
In order to pursue this idea, let us first discuss the concept of reversibility in terms of biological reactions.
All chemical reactions tend to equilibrium if left to themselves, e.g.,
A+B+C+'-'—L+M+N+···
Thermodynamics has nothing to say about the rate of attainment of
equilibrium; however, in the case in which a suitable catalyst is present
equilibrium is rapidly established. In biological systems these catalysts
are generally present in the form of enzymes. We have seen that the
composition of the equilibrium mixture is determined by the over-all
free energy change of the system and by the activities of the reactants
and products. When we say a reaction is irreversible, e.g.,
what we mean is that the free energy of the products is much smaller
than the free energy of the reactants. Thus, the forward reaction greatly
predominates over the backward reaction. The over-all reaction is
therefore considered as irreversible in the forward direction. For example, in a biological system consider a catabolic process characterized
by
A-*L+M+N-\-'·-;
AF = -Q kcal.
Assuming the presence of an appropriate enzyme, Q kcal, per mole of
free energy become available for anabolic activities or for other endergonic processes. In an in vitro preparation, where there is no suitable
transformer to convert the free energy into an appropriate form for
synthesis or biological work, the free energy is dissipated as heat in
the amount given by
AH = AF + TAS
In principle, to drive the reaction backward, e.g., to anabolize the
products, Q kcal, of energy must be supplied. The Second Law assures