= Frustule
A <frustule> is the intricately patterned, <silica>-containing <cell wall> of a <diatom>. Its two principal overlapping halves fit rather like a lid and a box, with connecting bands. Pores and ridges differ among <diatom> groups, making the mineral structure useful in microscopic identification.
How can a <biological cell> make a patterned mineral structure under mild conditions? Isolated <diatom> <peptides> called <silaffins> caused <silica> nanospheres to form rapidly when added to a solution of silicic acid. This demonstrated that cellular organic components can directly promote mineral formation.<Polycationic peptides from diatom biosilica that direct silica nanosphere formation by N. Kröger and others>[1999, abstract: cell-wall peptide isolation and silica precipitation]{ref} The experiment accounts for one chemical capacity; a complete <frustule> also requires control over where that capacity acts and how the resulting material is organized.
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