Modern Thermodynamics
- Chapter 1
1.3 Biological and Other Open Systems
Open systems are particularly interesting because in them we see spontaneous self-organization. The
most spectacular example of self-organization in open systems is life. Every living cell is an open system
that exchanges matter and energy with its exterior. The cells of a leaf absorb energy from the sun and
exchange matter by absorbing CO2, H2O and other nutrients and releasing O2 into the atmosphere. A
biological open system can be defined more generally: it could be a single cell, an organ, an organism or
an ecosystem. Other examples of open systems can be found in the industry; in chemical reactors, for
example, raw material and energy are the input and the desired and waste products are the output.
As noted in the previous section, when a system is not in equilibrium, processes such as chemical
reactions, conduction of heat and transport of matter take place so as to drive the system towards
equilibrium. And all of these processes generate entropy in accordance with the second law (see Fig.
1.2). This does not mean however that the entropy of the system must always increase; the exchange of
energy and matter may also result in the net output of entropy in such a way that the entropy of a system
is maintained at a low value. One of the most remarkable aspects of nonequilibrium systems that came to
light in the twentieth century is the phenomenon of self-organization. Under certain nonequilibrium
conditions, systems can spontaneously undergo transitions to organized states, which in general are states
with lower entropy. For example, nonequilibrium chemical systems can make a transition to a state in
which the concentrations of reacting compounds vary periodically, thus becoming a "chemical clock".
The reacting chemicals can also spatially organize into patterns with great symmetry. If fact, it can be
argued that most of the "organized" behavior we see in nature is created by irreversible processes that
dissipate energy and generate entropy. For this reasons, these structures are called dissipative
structures1 and we shall study more about them in chapter 11. In an open system, these organized states
could be maintained indefinitely, but only at the expense of exchange of energy and matter and increase
of entropy outside the system.
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