Chemistry Reference
In-Depth Information
magnesium ammonium phosphate). Birds make
eggshells
from calcite (a
mineral form of calcium carbonate) together with aragonite, another mineral
form of calcium carbonate. Some microorganisms, for example, the diatoms
(microscopic algae) produce hard symmetric exoskeletons, mainly from
silica, and many plants synthesize extremely small biomineral structures,
known as
phytoliths
, made up of silica or calcium oxalate. Grasses, too,
contain large numbers of phytoliths, which are one of the main causes of the
erosion and wear of the teeth of animals and humans who feed on grass
(Piperno 1988). Altogether, over 60 different bioinorganic substances are
known to be synthesized by living organisms, including metal carbonates
(aragonite and calcite), citrates, oxalates, phosphates (dahlite and apatite),
silicates (opal), sulfates (gypsum), sulfides (pyrite and gelena), and many
others (Wilkins and Wilkins 1997; Lippard and Berg 1994).
15.1.
HARD ANIMAL TISSUES
Bone
Bone, the compact and firm material that makes up the bulk of the skeleton
of vertebrate animals, combines two extremely useful properties, light
weight and strength; bone is, for example, relatively stronger, although a
great deal lighter, than concrete. Regardless of differences in the composi-
tion of bone according to species and sex, significant quantitative facts are
general and well known. The typical, approximate composition of fresh bone
is given in Table 97 (Price 1989; Chaplin 1971).
The bones and teeth of humans and other vertebrate animals, for example,
consist mainly of a composite material made up of an organic substance,
col-
lagen
, and a biomineral,
calcium carbonate phosphate
(see Textboxes 32 and 61).
The latter, which makes up about two-thirds of the total dry weight of bone,
is composed of calcium phosphate containing between 4-6% calcite (com-
posed of calcium carbonate) as well as small amounts of sodium, magnesium,
fluorine, and other trace elements. The formula Ca
4
(HPO
4
)(PO
4
)
5
(CO
3
)(OH)
approximately represents its composition; its crystal structure is akin to that
TABLE 97
Fresh Bone Composition
Bone component
Relative amount (%)
Inorganic component
Carbonated hydroxyapatite
70
Organic components
30
Collagen
28
Fats and carbohydrates
2
