2. Key approachers to classification of chemical elements
A body of any organism living on the Earth, plant or animal, consists in definite set of chemical elements strictly genetically regulated and transmitted from generation to generation in the same proportions. The question is only whether this transmission is absolutely stable over generations or it can change in the process of geochemical evolution of the Earth’s surface, but with the rate unperceivable by humans.
Mineral composition of contemporary organisms has been built under the action of two processes. On the one hand, it is evolution of hydro- and lithosphere composition characterized by permanent shift in relationship of elements due to leaching, volcanic activity. On the other hand, this "necessary" for organism genetic regulation of already existing in it relationships at this or that stage as according to outstanding scientist К. Bernar, "stability of inner environment is a necessary condition for free life of organism". The history of environment – organism relations is rather dramatic. Only having studied the history of organisms, their evolution and adaptation to changing geochemical habitat, extinction and flourishing of definite species, communities and families of plants and animals we could understand the present deeply, solve the urgent problems of medicine, ecology as well a problems related to them.
From this position the problem of development of natural classification of chemical elements has been stated. It can be solved at the interface of evolutionary geology, genetics, and medicine.
At present there are many various approaches to classification of chemical elements.
The Periodical System of D.I. Mendeleyev includes 105 elements, 15 of which (Fe, J, Cu, Zn, Co, Cr, Mo, Ni, V, Se, Mn, As, F, Si, Li) are referred to essential i.e. vital ones. Four (Cd, Pb, Sn, Rb) are «strong candidates for being essential» (Avtsyn, 1991, p.17). Most of specialists follow mainly this element classification (Fig.8).
In foreign literature the terms «trace elements» and «ultra trace» are accepted (Table3).
Detailed information about classifications is given in А.А. Kist’s work «Phenomenology of biochemistry ….» (1987), where the author expresses the opinion that most suitable for application are classifications used in geochemistry.
Fig. 8 The place of microelements (in italics) in the Periodic System
The letter М is used to designate the microelements, excess or lack of which has practical significance for human health. (Mertz W., 1987).
Table3
Macro, trace and ultra-trace essential elements for humans (Macro, trace and ultra trace elements which are essential for humans. H.R. Eschnauer, 1998)
-
Groups of elements
Elements
Macro elements
Trace elements
Ultra trace elements
Ca, Mg, P, Na, K, S, Cl
I, Fe, Cu, Zn, Se, Co, (Mn, Mo, Ni)
F, Cr, V, Li, Sn, Pb, Cd, B, Al, Br, Rb, Ti....
There are classification systems of Mason В. (1958) composed according to dependence on the role in plant nutrition; V.V. Kovalskiy (1974) based on abundance and degree of awareness in biological role in mammal organism; Bowen H.J.M. (1966) from the standpoint of biological functions (Kist, 1987).
One of the most convenient system for the purposes of biogeochemistry is considered the system suggested by А.I. Perelman (1979), interpreting the element behavior from the point of view of their concentration in living organisms and migration mechanisms.
A great number of various element classifications can be explained by the variety of their biological roles.
According to one of them all elements are conditionally divided into the following groups depending on their biological significance:
а) irreplaceable elements forming ferments, hormones, vitamins, – O, K, H, Ca, P, C, S, CI, Na, Mg, Zn, Fe, Cu, I, Mn, V, Mo, Co, Se;
b) constantly detectable in living organisms elements, the significance of which has not been studies enough, - Sr, Cd, F, Br, B, Si, Cr, Be, Li, Ni, Cs, Sn, Al, Ba, Rb, Ti, Ag, Ga, Ge, As, Hg, Pb, Ti, Bi, Sb, U, Th, Ra;
c) elements detectable in animal and human organisms the data about which with respect to their content in tissue, organs, and their biological role are absent – Tl, Nb, La, Pr, Sm, Tb, W, Re, Au.
There is an idea that if mass part of an element in organism exceeds 10-2%, it should be referred as macroelement. The fraction of microelements in organism amounts 10-3-10-5%. If content of an element is lower 10-5%, it is referred as ultramicroelement, such a gradation is conditional. According to it, for instance, magnesium belongs to intermediate group between macro-and microelements.
Some scientists believe that microelements are a group of elements which are found in human organism in low quantity in the range 10-3-10-12% (in this case microelements with content less 10-5% are sometimes called ultramicroelements). It is this fact that determines their names: trace elements in German and English, «oligoelements» – in French, «dissipated elements» – in V.I. Vernadskiy’s works. The chemical elements from the Periodic Table in biosphere was suggested to be classified into major, minor, trace and rare gases (Medical Geology, 2005).
According to the degree of their use and harm for organism macro- and microelements can be divided into the following groups (Underwood E., 1962) (Fig. 1):
Essential (vital) elements – all structural elements (H, O, N, C, Ca, Cl, F, K, Mg, Na, P, S) + 8 microelements (Cr, Cu, Fe, I, Mn, Mo, Se, Zn) –20 elements total.
Conditionally essential (vital, but harmful in high doses) microelements (Ag, Al, Au, B, Co, Ge, Li, Si, V) – 9 elements.
Conditionally-toxic microelements and ultra-microelements (Be, Sc, Ti, Ni, Ga, As, Br, Rb, Sr, Y, Zr, Nb, Ru, Rh, Pd, Cd, In, Sn, Sb, Te, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Hg, Tl, Pb, Bi, Th, U) – in total 52 elements. It is believed that mercury (Hg) is harmful for humans in any amount; therefore it can be referred to as an (unconditionally) toxic element.
