Although silicon is one of the most common elements in the earth's crust, its role in the biosphere is of secodary importance.
This is likely due to the low solubility of Si(OH)4, which is lowered by the presence of metal ions such as Ca2+, Al3+ or Fe3+.
Silicates are thus used as matrix material in only a few organisms. These include the diatomes and sponges. Especially the single-celled diatomes form bizarre structures.
A passive support function is achieved by the incorporation of amorphous silica in the cell walls of certain grasses.