Orthosilicic acid
= Orthosilicic acid
{title2=$\mathrm{Si(OH)_4}$}
<Orthosilicic acid> is the monomeric <silicic acid> $\mathrm{Si(OH)_4}$, with four <hydroxyl groups> attached to a <silicon> center in approximately <tetrahedral molecular geometry>. The formula can also be written $\mathrm{H_4SiO_4}$. These are two ways of counting the same atoms, not two different compounds. Dissolved <orthosilicic acid> is a molecular precursor from which <organisms> can construct <silica>.<Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others>[Discussion, undissociated orthosilicic acid and biological silicon transport]{ref}
Its distinction from a silica network is visible in a <condensation reaction>: two hydroxyl-bearing sites can form a <silicon>–<oxygen>–<silicon> bridge while releasing <water>. Continuing such reactions produces connected species, so total dissolved <silicon> is not automatically the concentration of free monomer.
For a numerical example, one millimole of molecular <orthosilicic acid> contains one millimole of <silicon> atoms and four millimoles of <oxygen> atoms. In a transport experiment, however, counting atoms alone is insufficient: a <silicon transporter> must recognize the actual dissolved species. Purified <diatom> transporters tested near neutral <pH> transported <silicic acid> in a <sodium>-dependent assay, linking this molecular form to biological uptake.<Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others>[Proteoliposome transport assays and Discussion]{ref}