A Biomolecular Survey of Calcification 



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The following survey is designed to compare, at a macromolecular level, 

 analogous structures and analogous calcium deposits in a variety of animals and 

 plants. Certain features have been selected to comment on the three principal prob- 

 lems of distribution, structure and genesis. 



Distribution 



Comprehensive reports and reviews of the relative amounts of mineral matter in 

 animals (Clarke and Wheeler, 1922) and in plants (Pobeguin, 1954) serve as a 

 useful guide to the distribution of calcium salts in nature, but there are gaps in our 

 knowledge caused by lack of information about clearly defined crystalline salts (as 

 opposed to calculations based on chemical assay of cations and anions) and by the 

 precise geography of "mixed" salts within a single tissue or cell. It is well known, 

 for example, that many crustaceans contain calcium phosphate as well as carbonate 

 in their hard parts, yet until recently (Pautard and Trautz, in press) no species has 

 been found with crystallographic verification of bone salts, although the usual well- 

 detined X-ray diffraction pattern of calcite found in carbonate-rich invertebrates 

 might well conceal a second, less crystalline, phase of phosphate. The absence of 

 precise information about the relationship of anions to calcium has led to the 

 erroneous generalization that calcium phosphate is confined to the vertebrates, 

 calcium carbonate to the invertebrates and calcium oxalate to the plants. In fact, all 

 three salts are to be found in all three situations without any clear demarcation 

 between them, as can be seen by the distributions set out in Table 1. 



Table 1. Distribution of calcium carbonate, oxalate and phosphate in normal biological 



structures 



The most widely reported calcium salt in plants and animals is the carbonate, 

 ranging from the shells and tests in the Protozoa, through the hard exoskeletons of 

 invertebrates to the eggs of birds and statoconia in man, from intracellular granules 

 in bacteria through spherulites in algae to a wide variety of inclusions in cystoliths, 

 tracheids and leaf parenchyma in angiosperms. The occurrence of calcium phosphate 

 has been less well recorded, partly because bone has dominated the phosphate scene 

 and partly because much of the salt present in tissues other than vertebrate is less 

 crystalline than the more abundant carbonate and may hence be undetected. Deposits 



