The second law of thermodynamics states that the entropy (S), a measure of disorder, increases during any spontaneous process in an isolated system.
We can view the entire universe as an isolated system, leading to the conclusion that the entropy of the universe is tending to a maximum. However, all living things maintain a highly ordered, low entropy structure. The emergence of life in our universe therefore seems to be at odds with this thermodynamic law. If all spontaneous chemical processes result in increased disorder, it may seem unclear how highly ordered structures could be produced, let alone maintained. Although this view is not uncommon, there is no real contradiction between life and the second law of thermodynamics. The above statement of the second law works only in an isolated system. The Earth and all life on it are not isolated systems, so an alternative statement of the second law must be developed. We can start with the axiom that spontaneous processes increase the entropy of the universe, beyond the entropy change due to the heat transfer (q) alone.
Where T is the temperature of the system. Now, recall the first law of thermodynamics (the first law is discussed in more detail in my article about the molecular difference between heat and work):
Where U is the internal energy, w is the work, P is the pressure, and V is the volume. If this equation is combined with the above inequality for the entropy change, it can be shown that
It is a reasonable approximation to assume that the chemical processes in biological systems occur at constant temperature and pressure. This makes the above result extremely convenient. Notice that if T and P are held constant
For simplicity, we define U – TS + PV as a new quantity called the Gibbs energy (G). The quantity U + PV is defined as the enthalpy (H). For our purposes this is a more useful statement of the second law. This result works in any system with constant temperature and pressure, regardless of whether or not it is isolated. A process is spontaneous when its change in Gibbs energy is negative.
Now, let’s return to the issue of life’s highly ordered structure. Consistent with the idea of the second law, the highly ordered structural components of lifeforms are constantly degrading. New structural components must be continually synthesized to replace them. In fact, life has even been defined through this property, such as Pier Luisi’s definition [1]:
A system can be said to be living if it is able to transform external matter/energy into an internal process of self-maintenance and production of its own components.
The issue is how highly ordered structural components can be created when only processes which increase the universe's disorder occur spontaneously. Certainly, building complicated biological structures and biomolecules is not by itself spontaneous process. However, such processes can be driven forward by the input of work ("useful" energy), in turn generated through other spontaneous processes. Therefore, the highly ordered nature of life is possible through this coupling. Note that a net gain in entropy is still attained.The conversion of energy into work is not 100% efficient; some energy is always dissipated as unusable heat which increases the entropy of the universe. Living things, therefore, may actually be thought of as heat-dissipating machines, maintaining their order through the production of even larger amounts of disorder. A practical example of this principle follows.
In order for heterotrophs (such as ourselves) to stay alive, we must metabolize food. This provides the required work required to maintain our highly ordered structure. For instance, consider the aerobic respiration of glucose. The reaction below, of course, skips over a very large number of intermediates. However, this is unimportant since G, H and S are all state functions (they depend only on the present state, not the path through which it was achieved).
A system can be said to be living if it is able to transform external matter/energy into an internal process of self-maintenance and production of its own components.
The issue is how highly ordered structural components can be created when only processes which increase the universe's disorder occur spontaneously. Certainly, building complicated biological structures and biomolecules is not by itself spontaneous process. However, such processes can be driven forward by the input of work ("useful" energy), in turn generated through other spontaneous processes. Therefore, the highly ordered nature of life is possible through this coupling. Note that a net gain in entropy is still attained.The conversion of energy into work is not 100% efficient; some energy is always dissipated as unusable heat which increases the entropy of the universe. Living things, therefore, may actually be thought of as heat-dissipating machines, maintaining their order through the production of even larger amounts of disorder. A practical example of this principle follows.
In order for heterotrophs (such as ourselves) to stay alive, we must metabolize food. This provides the required work required to maintain our highly ordered structure. For instance, consider the aerobic respiration of glucose. The reaction below, of course, skips over a very large number of intermediates. However, this is unimportant since G, H and S are all state functions (they depend only on the present state, not the path through which it was achieved).
Entropy increases in this reaction. This can be seen qualitatively by how the total number of molecules increases. Furthermore, this reaction has a negative enthalpy change (it releases energy to the environment). Therefore the Gibbs energy change must be negative. The Gibbs energy change associated with synthesizing life's structural components is surely positive. However, its magnitude could not be greater than that of glucose respiration (plus any other metabolic sources of energy) since this work drives the synthesis of these components. As previously discussed, a portion of the released energy cannot be used to do work. Instead it is dissipated as heat and increases the entropy. Thus the total entropy of the universe increases despite the highly ordered structure of the organism being maintained.
The question which follows from that example, however, is how sugars like glucose are formed on Earth given the second law. Of course these are formed through photosynthesis which is driven by work from the sun. Fusion reactions occur in the sun, light elements combine to make heavier ones and there is a slight decrease in the total rest mass of the atoms. This releases an incredible amount of energy, some of which can be used by autotrophs on Earth to do work, like synthesizing glucose. Yet much of the energy released by the sun is not available to do work, dissipation of this heat increases the entropy of the universe. When the entropy produced by the sun's fusion is considered, there is no contradiction between life and the second law of thermodynamics. The common error that leads to this mistake is to forget that both the Earth and individual organisms are not isolated systems. Although biological systems have highly ordered structure, they are maintained by work derived from processes that produces even more disorder. The chemistry underlying all biological systems exactly follows the second law of thermodynamics.
References:
[1] Luisi PL. 2006. The emergence of life: From chemical origins to synthetic Biology. New York: Cambridge University Press, 25 p.
The question which follows from that example, however, is how sugars like glucose are formed on Earth given the second law. Of course these are formed through photosynthesis which is driven by work from the sun. Fusion reactions occur in the sun, light elements combine to make heavier ones and there is a slight decrease in the total rest mass of the atoms. This releases an incredible amount of energy, some of which can be used by autotrophs on Earth to do work, like synthesizing glucose. Yet much of the energy released by the sun is not available to do work, dissipation of this heat increases the entropy of the universe. When the entropy produced by the sun's fusion is considered, there is no contradiction between life and the second law of thermodynamics. The common error that leads to this mistake is to forget that both the Earth and individual organisms are not isolated systems. Although biological systems have highly ordered structure, they are maintained by work derived from processes that produces even more disorder. The chemistry underlying all biological systems exactly follows the second law of thermodynamics.
References:
[1] Luisi PL. 2006. The emergence of life: From chemical origins to synthetic Biology. New York: Cambridge University Press, 25 p.
RSS Feed