Orthosilicic acid is the monomeric silicic acid , with four hydroxyl groups attached to a silicon center in approximately tetrahedral molecular geometry. The formula can also be written . 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.[ref]
Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others. Discussion, undissociated orthosilicic acid and biological silicon transport
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.[ref]
Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others. Proteoliposome transport assays and Discussion
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