The processes studied in the theories of mineralization and phase transformation are described by a large number of equations as previously seen. Phenomena like Wetting, Diffusion, Nucleation and Growth, Surface Tension and Capilarity are modeled after mathematical formulas, always based on parameter selection and boundary conditions implied in the occurrences of these phenomena. Within several parameters contained in the mathematical deduction, temperature (T) certainly occupies an important role: the sintering of ceramics and fusion of metals, for example, happen through the supply of energy, generally in the form of heat for liquefaction to occur in large quantities of solids. This situation is absolutely impossible for biological systems, not only because of the fact that this doesn’t provide energy to add to the phase transformation process (many times they extract energy from the process, like in the case of Gallionella ferruginea, a bacteria that oxidizes Fe+2 a Fe+3) and by the fact that oxides, carbonates and phosphates in biomineralization have phase transformation temperatures hundreds of degrees higher (400 oC, 1000 oC, etc) than environment temperature.
How can the organisms produce metals (like magnetite teeth in chitons, or the gold aggregates and nucleated heavy metals by fungus and bacteria), ceramics (composite eye lenses of organisms from trilobites to an incredible variety of carapaces, spicules, shells, bones and teeth) and glass (like glass sponge skeletons and SiO2 particles in diverse vegetable examples)? No organism is capable of surviving the temperatures necessary for the fusion or sintering of these materials.
The answer for the occurrence isothermic processes of phase transformation could be in the scale and in the hierarchy.
The majority of the mineralized biomaterials observed is, in the hierarchical in which they organize, composed by primary nanometric units. The spicules of sea urchins, the bones and teeth of humans, the skeletons of glass sponges and the teeth of chitons all possess mineral nanospheres, initially amorphous. Through assembly and auto-organizing processes which still need to be clarified, the nanospheres are the basic units, composed of, at most, millions of atoms.
After the nucleation of the amorphous nanospheres, their lumping into larger bodies results into the first step of phase transformation.
Nanospheres are porous indicating that the shaping process happened in subunits that stuck together with van der Waals forces. The nanospheres have diameters from 3 to 20 nanometers and can have complex compositions, like the case of the ferridrite nanospheres of the bacteria that oxidize iron, and CaCO3 nanospheres used in the creation of shells. Organic systems live in relatively constant bands of temperature, and depend on the physiological maintenance of the bands, for biochemical reasons: proteins work best at specific at specific points, in function of temperature – lower than the optimal point, the processes lose their efficiency; above, the proteins loses its three dimensional form (denaturing) and as a consequence its capacity to perform its specific function (an irreversible process). Because of this, the nucleation of nanospheres occurs in isothermic processes.
Heterogeneous nucleation takes place on the cell surface of single cell organisms, extrapallial spaces and intra and extracellular vesicles, vacuoles and specific regions in different tissues, along the organic matrix of different proteic compositions. If the first unicellular organisms that biomineralized did so to obtain energy (in an O2 poor atmosphere), then the nanospheres appeared as a result metabolic processes.
Even though the specific site details of the enzymatic oxidation occurs aren’t completely understood, the alteration in local pH alters the zeta potential of the particles and the viscosity of the micro-environment exercising direct influence on the aggregation of nano-particles, modulating its critical radius (r*) and making breaking the activation barrier easier in intracellular nano-environments or interstitial micro-environments isolated and controlled. Instead of heterogeneous sites, the organisms create heterogeneous micro-environments, many times organized with help of the curves and holes in its own membranes (as the case of unicellular organisms) and or by the existing delimited spaces where the pH can be altered and, as a consequence, change the zeta potential of the particles in the aggregation. In contrast to an analyses that strengthens the extraordinary character of biomineralization, like the challenges of thermodynamics, what reveals itself in a more realistic form is natural selection of organisms that manage to obtain important advantages in controlling, even microlocally, the pH.
At the atomic scale the quantity of energy is defined by the availability of electrons in the different energetic layers, and can be translated into the agitation of the atoms (like when we compare the molecule H2O in ice, in liquid water and vapor). Macroscopically, this agitation can be measured as temperature; but if we stay at the atomic scale, the arrival of protons and electrons can locally alter the vibration of the atoms, without perceivable alterations in the temperature. The electrons and/or protons obtained from a variety of metabolic processes could act as a source of energy for the nucleation of nanometric structures, by influencing the distribution of surface charges of atoms and their groupings.
