Chemistry Reference
In-Depth Information
matters more complicated, experiments in which there is not complete lack of biomineralisation after particular
gene knock-outs of single bone proteins suggest that there is likely to be a redundancy of function, and that many
of the actors in this particularly complicated process still need to be identified.
Finally, it is intriguing that in terms of biomineralisation, invertebrates have based their reliance on calcium
carbonates, while vertebrates appear to have used almost exclusively calcium phosphate. We say almost, because,
while the use of calcium phosphates for biomineralisation is an invention of some vertebrates, they still use
calcium carbonate for the formation of otoliths
4
of the inner ear. It remains to be established if the equivalent of the
gene starmaker required for otolith formation in zebrafish has homologues among invertebrates.
Silica-Based Biominerals
We saw in Chapter 1 that silicon is an essential element, and that it is particularly found in diatoms, in plants, and
in sponges. Diatoms are unicellular algae which produce intrinsically structured cell walls made of nanopatterned
silica (Si0
2
). There are tens of thousands of diatom species, each forming differently sculpted silica cell walls
(
Figure 19.19
)
. Diatom biosilica is an inorganic
e
organic hybrid material composed of inorganic silica attached
FIGURE 19.19
Scanning electron microscopy (SEM) images of the cell walls of four different diatom species. (From
Kro¨ger & Poulson,
to specific organic macromolecules, including proteins, polysaccharides, and long chain polyamines (LCPA).
The monomeric precursor for diatom silica orthosilicic acid Si(OH)
4
is transported into the cell by the SIT family
of silicic acid transporter proteins (see Chapter 1). Silica morphogenesis takes place within specialised
membrane-bound compartments termed silica deposition vesicles (SDVs). After completion of morphogenesis,
the silica is deposited on the cell surface through SDV exocytosis. Each diatom species appears to contain
specific silaffin
5
and LCPA molecules. Both silaffins and LCPA accelerate and control silica morphogenesis
4. Otolith
e
a tiny bony structure in the inner ear of lower vertebrates.
5. Silaffins (proteins with silica affinity) are a family of phosphoproteins, which are loosely associated with diatom silica; two other families
of cell proteins, frustulins, and pleuralins are found bound to the cell walls of diatoms.
