= Silicic acid
<Silicic acid> names a family of <silicon> oxoacids; the simple dissolved monomer is <orthosilicic acid>, $\mathrm{Si(OH)_4}$. It contains one <silicon> atom bonded to four <hydroxyl groups>. Distinguishing a soluble monomer from a solid network of <silica> matters in both <chemistry> and <biology>: organisms that build silica structures must first acquire a dissolved source of <silicon>.
A simplified <condensation reaction> illustrates how monomers can begin to join:
$$
\mathrm{2Si(OH)_4\longrightarrow (HO)_3Si{-}O{-}Si(OH)_3+H_2O}.
$$
Two hydroxyl-bearing sites form a <silicon>–<oxygen>–<silicon> bridge and release <water>. Further joining creates more connected structures, so a nominal concentration of total <silicon> does not alone specify which molecular forms are present.
A 2016 experiment reconstructed <diatom> <silicon transporters> in artificial <liposomes>. The investigators trapped a <zinc>-based indicator inside and applied a <sodium> gradient; entry of <silicic acid> increased <fluorescence>. Controls separated transport from background signals. The near-neutral conditions supported transport of undissociated <orthosilicic acid>, illustrating how a soluble mineral precursor can be studied with a purified membrane system.<Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others>[Zinc silicate fluorescence; Proteoliposome transport assays; Discussion]{ref}
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