8. Life, Metabolism and Energy

The question of why Lotka views have scarcely been considered by scientists (except in a few instances as for example ref. [22]) involved in origin of life studies during the twentieth century is open to debate by historians of science.

It is worthy to note that considering the emergence of life as an event in a continuous process means that locating the emergence of life is arbitrary, as that of any transition in history, so that the mere definition of life cannot be purely objective but the result of a convention established and adopted by the scientific community.

Interestingly, the height of barriers inhibiting spontaneous nuclear reactions and related to the strong repulsion existing between nuclei could be depicted in the same way as responsible for the possibilities of reaction cycles presenting all the attributes of catalytic or autocatalytic cycles in chemistry. For instance, the direct formation of ⁴He helium nucleus from four protons is hardly possible by nuclear fusion of four protons but takes place through the C N O cycle in the core of massive stars (with ¹² C, ¹³ N, ¹³ C, ¹⁴ N, ¹⁵O, and ¹⁵ N as components of the reacting loop in which four protons are captured stepwise and an α particle is produced). Eigen and Schuster already pointed out this analogy with chemical catalytic cycles [24]. In the same way, nuclear fission reactions may also be described as autocatalytic replications of neutrons starting from unstable heavy nuclei. However, the main difference in the self-organisation processes that take place through nuclear reactions compared to chemistry probably lies in the limited set of atoms presenting a sufficient stability that can be obtained, whereas there is no limit to the number of structures accessible to organic chemistry.

In this chapter we use the term proto-metabolism for specifying networks of reactions capable of performing chemical transformations and inducing self-organisation features in a chemical system. From this definition, there is fundamentally no difference between metabolic and proto-metabolic pathways except that enzyme catalysis makes metabolism much more efficient in achieving its function and generates multiple possibilities of feedback control almost without any limitations.

Efficient catalysis requires that the interaction of a catalyst with a transition state is preceded by an interaction with reactants that has been analysed to impose constraints on enzymatic catalysis [52].

This limitation does not apply to folded genetically encoded biocatalysts in which a well-defined three-dimensional structure gives rise to multiple non-covalent interactions specific of the transition state of the reaction and allows the binding energy with non-reacting portions of the substrate to substantially contribute to catalysis [53].

It is important to notice that the conversion of heat into other forms of energy by other processes requires heat engines that use a cold sink (entropy sink) to get energy with higher thermodynamic potentials. Natural processes such as storms are capable of doing so by giving rise to lightning with local temperatures transiently exceeding 10,000 K, but considering lightning as an alternative source of energy will be preferred here in order to avoid any misleading statement about the potential of heat sources.

Carbon at the carbonyl state of oxidation (sugars) represents the most efficient source of organic matter capable of feeding early heterotrophic organisms in energy in the absence of a respiratory metabolism [14, 78, 79].

The difference between photosynthesis and the photochemical generation of activated molecules must be emphasised. The latter corresponds to the chemistry occurring after bringing a chemical system into a highly activated state that can generate intermediates with a chemical potential in a determined environment. Oxygenic photosynthesis is a complex process coupling three different actions: the collection of energy, the oxidation of water, and the generation of reducing power exploitable for the synthesis of organic matter.