The fuel of ecosystems is organic matter, detritus. It is therefore relevant to calculate the free energy of dead organic matter, consisting of poly-organic molecules. The chemical potential of dead organic matter, indexed i = 1, can be expressed from classical thermodynamics (e.g., Russel and Adebiyi, 1993) as:
where mi is the chemical potential. The difference mi - is known for detritus organic matter, which is a mixture of carbohydrates, fats and proteins. Approximately 18.7 kJ/g may be applied for the free energy content of average detritus. Obviously, the value is higher (22-24 kJ/g) for detritus originated from birds, as they in average contain more fat. Coal has a free energy content of about 30 kJ/g and mineral oil of 42 kJ/g. Both coal and mineral oil are a concentrated form of detritus from previous periods of the Earth. c1 is the concentration of the detritus in the considered ecosystem and c1o is the concentration of detritus in the same ecosystem but at thermodynamic equilibrium.
Generally, c1o can be calculated from the definition of the probability, Pi o, of finding component i at thermodynamic equilibrium, which is
If this probability can be determined, then in effect the ratio of cieq to the total concentration is also determined. As the inorganic component, c0, is very dominant at thermodynamic equilibrium, Equation 6.2 can be approximated as
By a combination of Equations 6.1 and 6.2, we get
The equilibrium constant for the process describing the aerobic (presence of oxygen) decomposition of detritus at 300 K can be found based upon the above-mentioned values. We could presume a molecular weight of about 100,000 (more accurate 104,400) (Morowitz, 1968) for the typical poly-organic molecules that make up dead organic matter, and with a typical composition of 3500 carbon, 6000 hydrogen, 3000 oxygen and 600 nitrogen:
C3500H6000O3000N600 + 4250 O2 ! 3500 CO2 + 2700 H2O + 600 NO-+ 600H+
K = [CO2]3500[NO-]600 [H+]600/[C3500H6000O3000N600][O2]4250 since water is omitted from the expression of K. We have that
— AG = —18.7kJ/g 104,400 g/mole = 1952MJ/mole = 8.2J/mole 300 lnK, which implies that ln K = 793,496 or K is about 10344998.
The equilibrium constant is, in other words, enormous. The spontaneous formation of detritus in the form of a compound with the molecular weight of about 100,000 has therefore a very low probability.
Figure 6.2 illustrates the free energy of various ecologically important nitrogen compounds. By a steady input of energy, it is possible to convert nitrate or ammonium stepwise to proteins—to go up the free-energy staircase in Figure 6.2 step by step. Proteins with the highest free energy are in the present ecosystems, the result of the photosynthesis or of the biochemical synthesis in heterotroph organisms.
Detritus has an eco-exergy density corresponding to 18.7kJ/g. The distance to thermodynamic equilibrium is therefore 18.7 kJ/g, which is a high value as also indicated by the high K-value. While the eco-exergy flow density is higher on the Earth than on the Sun, the eco-exergy density on the Sun is of course higher (see Section 5.1) as the Sun contains a total eco-exergy amount corresponding to the nuclear processes for the entire life-time of the Sun. Even if we consider detritus with a low molecular weight
Figure 6.2 The level of free energy for some ecologically important nitrogen compounds.
corresponding to detritus partially decomposed, the K-value is still very high. If we presume a 100 times smaller molecular weight the exponent is 100 times smaller or about 3500—still a very high K-value. It is therefore understandable that detritus is decomposed spontaneously and thereby can yield energy to the heterotroph organisms. The opposite process corresponds to what may be the result of the photosynthesis, the conversion of solar radiation (energy) into chemical energy.
The formation of living cells in one step from non-living chemicals is impossible, and even the formation of the poly-organic molecules that are characteristic for life is impossible in one step according to the above shown calculations. The formation of poly-organic molecules must therefore have taken place stepwise, either by chemical non-biological processes or as a result of living organisms. The various components formed by an energy input to the primeval atmosphere can readily be adsorbed on clay particles or other fine particles. Amino acids have lower eco-exergy than detritus, for instance glycine 5kJ/g, lysine 10.5 kJ/g and tryptophan 11.9 kJ/g. It means that the formation of the poly-molecules in living matter (detritus when it is non-living) requires an inflow of free energy. Further inflow of energy implies that the amino acids react to form polypeptides, first with a few amino acids, later with many more amino acids. Autocatalysis may also have played a role, when we are considering the formation of higher molecular proteins from polypeptides. It is not possible to determine whether the formation of poly-molecules took place before the first primitive life forms emerged or they were a consequent of the first primitive life forms (see also next chapter).
It is, however, interesting that concentrated solutions of proteinoids after heating to between 120 and 200 °C followed by a very slow cooling can form spontaneously vesicles, which Fox and Dose (1972) called microspheres. They have a regular form and with a diameter of 1-2 mm. They are very stable and retain the weak catalytic activity of individual proteinoids. They are also able to absorb proteinoids from the surrounding solution, whereby they can grow and divide into two by fission or budding. They also present a rudimentary type of metabolism. Still, however, the evolutionary potential of the microspheres remains a mystery. They might have appeared on the primitive Earth, but it is not sure if they had a future.
Another interesting theory about the first poly-molecules is the so-called surface metabolism. It is based on a solid thermodynamic argument. The formation of peptide bond is not favoured in solution, but on a surface. This is true for many polymerization processes. A great number of enzymatic reactions require collisions of three molecules, an event which is highly unlikely in space but much more probable on a surface. Generally, it is a thermodynamic principle that spontaneous reactions are more likely to occur on surfaces than in space or solutions. It is therefore reasonable to conclude that the first metabolic structures started out as two-dimensional systems, for instance on the surface of clay crystals.
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