4
Thermodynamics of Bioreactions
Chapter 2 gives an overview of the many biochemical reactions occurring in a living cell, and as
discussed these biochemical reactions are exploited in the biotech industry to produce many
interesting compounds. The pathway activity is to a large extent determined by the kinetics of the
enzymes catalyzing the individual reactions as will be discussed in Chapter 6. However, of equal
importance is thermodynamic constraints, since all reactions, independent of the properties of the
enzymes, have to operate according to the laws of thermodynamics. Any treatment of bioreactions
is therefore not complete unless one considers the thermodynamic constraints imposed on
individual bioreactions. Since usually a bioreaction is accompanied by generation of heat the
thermodynamic principles are also of importance in connection with design of the heat exchangers
to be installed in bioreactors. In this chapter we will take a brief look at the thermodynamics
involved in bioreactions. Classical thermodynamics is a discipline, which is extensively described
in many textbooks, and it is not the purpose of this chapter to give an in-depth explanation of all the
fundamental
thermodynamic
aspects
involved
in
cellular processes.
Still,
to
give
some
understanding of the relation between growth of a microbial culture and the energy generation and
consumption in the process, certain topics of thermodynamics will be briefly reviewed here. As
mentioned above this will also provide a tool for an important part of bioreactor design.
4.1 Chemical Equilibrium and Thermodynamic State Functions
In thermodynamics, a
system
is defined as that part of the universe that is being studied, such as a
bioreactor or a cell, whereas the rest of the universe is referred to as its surroundings. A system is
said to be
open
or
closed
according to whether it can exchange matter and energy with its
surroundings. Because living cells take up nutrients, release metabolites, and generate work and
heat, they are open systems. The state of a system is defined by a set of state functions, which
include the enthalpy {//, equal to the heat absorbed at constant pressure when the only type of
work is due to volume change) and entropy (5, a measure of the degree of order in the system).
Biochemical reactions occurring within a cell have to satisfy the laws of thermodynamics, and
according to the second law of thermodynamics “spontaneous processes occur in a direction that
increases the overall disorder (or entropy) of the
universe
or, mathematically, A
S >
0” . Thus,
spontaneity of a process is determined from the
overall
change in entropy. In the study of cellular
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