INTRODUCTION - Nanoindentation with Biological Applications - page 12

this regular structure, there are large numbers of hydrogen bonds
between parallel cellulose chains, forming a regular crystalline structure
with outstanding mechanical properties for an organic biopolymer. (The
other molecule “building block” of most importance in the plant
kingdom is lignin, which is a poorly-understood ring-based complex
macromolecule formed from alcohols, and which forms an integral part
of the secondary cell walls of plants.)
Figure 1-8. Cellulose molecule in short-hand, where each hexagon represents a carbon
ring and in which only the oxygen atoms are identified.
The saccharides in the vertebrate ECM are mainly
glycosaminoglycans, which are long, unbranched, highly charged
molecules formed of disaccharide repeats. There are two important
categories of protein-sugar complexes, glycoproteins and proteoglycans.
Glycoproteins are proteins with covalently linked oligosaccharide chains,
while proteoglycans are a subset of glycoproteins that are heavily
glycosylated, in which many glycosaminoglycans are linked to a single
protein core. These glycoproteins and proteoglycans assist in protein
folding and improve the protein stability; as well as frequently binding
water and controlling the local water concentration.
2.1.4. Biominerals
Biominerals are the ceramics of the biological world. There are a small
number of bioceramics with dominantly structural functions; the
biominerals of primary interest here are calcite, aragonite, hydroxyapatite
and bio-silica. It is these biominerals that give stiffness, hardness and
strength to many natural materials, including bone, teeth and shells.
Figure 1-9 shows the formulae and shape of the unit cell that describes
the structures of calcite, aragonite and hydroxyapatite; bio-silica is an