The Organic Model
The line between inorganic and organic chemistry is an important one in the evolution of life on Earth, and possibly on other planets. However, it is a division that is not really understood – see `The Beginnings of Life`. However, the 2nd Law of Thermodynamics states that energy tends not to remain in high energy levels within an atom and must eventually disperse to as many lower energy levels as possible. As a consequence, the 2nd law is closely associated with the concept of entropy, sometimes referred to as the `law of disorder`, which would seem to prohibit the formation of more complex structures by pure chance. As a result, one might naturally assume that complex molecules cannot spontaneously form from simple ones, which some have characterized by the saying:
`A watch must have a watchmaker`
While this may seem to be a requirement of most chemical compounds that exist in the cosmos, we might need to take a closer look at the dynamics of the energy equation associated with the formation of molecules to verify whether this is actually the case. The chemical potential energy, i.e. the enthalpy of formation, which is bound up in most of the 20,000,000 known kinds of molecules, is less than that of their elements. Thus, energetically, the second law says that the majority of compounds known could spontaneously form from the corresponding elements. So, with reference to the watchmaker analogy above, it would appear that watches are not lower in thermodynamic energy than the total energy of their individual components. Therefore, the second law is suggesting that it is simply inappropriate to compare a watch with the behaviour of chemical compounds and elements.
Typically, atoms in a gas can be moving at a few hundred to several thousand miles an hour at ordinary temperatures. Therefore, hydrogen and many other atoms behave in a way that is impossible for watch parts. With the increase of kinetic energy of the atoms, due to temperature, atoms can spontaneously `bond` when they collide and, in so doing, can form new arrangements. These new arrangements can create molecules that are so stable even temperatures of thousands of degrees cannot tear them apart again. However, molecules are not random arrangements, so when three or more atoms join to form a molecule, they are arranged in a precise order, normally unchanging over time, and with a relatively fixed geometric relationship. By the same token, different types of molecules can collide so violently that they produce totally new types of molecules, i.e. other more complex formations of ordered structures.
Amino acids when simply melted with other amino acids, i.e. when energy is added to make them move more rapidly, form huge new compounds. Although these compounds are not true proteins, they still utilize hundreds to thousands of amino acids using the same kind of bonds that hold proteins together. Another example of the spontaneous behaviour of elements is the reaction of hydrogen gas with oxygen. Hydrogen atoms have such an inherent tendency to form strong bonds with oxygen that even a small amount of activation energy causes the two substances to react. This process still obeys the 2nd law, which requires the energy in hydrogen and oxygen to spread out, when lower energy formations are possible, i.e. water. Yet, water is a far more complex formation than its constituent elements.
As such, there are millions of compounds that have less energy in them than the elements of which they are composed.
This sentence is highlighted because is suggests a mechanism in which complexity and order can increase without violating the 2nd Law of thermodynamics. This would appear to be critical to any incremental process that ultimately leads to the building blocks of life. In fact, the 2nd law actually favours the formation of the majority of complex chemical compounds directly from their simpler elements. Thus, contrary to a wide spread belief, the 2nd law does not dictate a decrease of ordered structure; it only demands that energy diffuse when lower energy formations are possible.
In the context of the discussion of Life, organisms produce some of the most complex compounds known. For example, by photosynthesis, plants make sugars, cellulose and chlorophyll plus hundreds of other compounds. They use energy from sunlight to synthesize the complex higher-energy substances from lower-energy compounds like carbon dioxide and water. However, the 2nd law states that these higher-energy compounds cannot spontaneously form from lower-energy elements, which they do not, as they derive the additional energy to `swim against entropy’s tide` from the sun in the form of high-energy photons. The 2nd law does not prevent the entropy of a local system being reversed through the input of external energy, as in the case of heating a gas, as long as the system as a whole doesn't disobey the 2nd law.
So, to recap at this point, there are both spontaneous and non-spontaneous processes, which can lead to the formation of more complex compounds. Spontaneous processes can be directly attributed to the 2nd law, where molecules are formed because they are in a lower-energy state. A non-spontaneous process requires external energy to reverse entropy, but can then form stable molecules. Again, in the context of our wider discussion, there appears to be no reason why these processes do not take place throughout the universe.
Hydrogen and carbon are both prevalent in the universe along with oxygen and nitrogen, albeit in much smaller quantities. Organic compounds are substances that contain carbon and the simplest organic compounds are called alkanes. Alkanes are composed of one carbon atom bonded to two or three hydrogen atoms. All alkanes have less energy than their elements and the 2nd law would suggest they would form spontaneously whenever carbon and hydrogen were present. Therefore, it is not so surprising that there is spectroscopic evidence showing that these alkane compounds do exist naturally in the wider universe.
However, far more complicated types of compounds called `Polycyclic Aromatic Hydrocarbons (PAH)` have also been detected in space. If oxygen is present in the PAH it would have less energy than its elements and could therefore form spontaneously in accordance to the 2nd law. If PAH is composed only of carbon and hydrogen it will contain more energy than its elements and, as a consequence could only form via a non-spontaneous process. However, this type of PAH is also detected in space, but could also have been formed, without intervention, given sufficient energy being provided from stellar sources, i.e. radiation. So while accepting that we still have much to learn in this area, there does appear to be evidence of processes, based on the laws of chemistry and thermodynamics, which support the automatic formation of complex compounds.
To conclude this aspect of our discussion, we know that biological life on Earth is based on the element carbon. We also know that very early in the formation of the Earth, carbon was made available as volcanoes spewed out many key elements into the Earth's atmosphere. As a consequence, various chemical reactions began to take place, which would have resulted in the creation of new and more complex compounds. Our discussion has suggested that one of these compounds could have been the amino acids. In 1953, Stanley Miller and Harold Urey showed that basic amino acids could be formed, when an electrical field was applied to a certain mixture of gases, similar in proportion to Earth’s early atmosphere.
However, in-line with
Clifford’s warning about the limits of inference, it should be noted
that some have likened the Miller-Urey experiment to creating a few words that, by chance,
appear in a Shakespeare play.