= Silicon transporter
A <silicon transporter> is a <membrane protein> that moves a dissolved silicon-containing <solute> across a <cell membrane>. In the diatom SIT family, the substrate is <silicic acid>, the soluble precursor used in building a <silica>-rich <cell wall>. The functional category need not imply that transporters in every lineage have the same structure or coupling mechanism.<Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others>[Introduction and Discussion, diatom silicic-acid transport]{ref}
A purified-system experiment reconstituted SIT <proteins> into <liposomes>. An inward-directed <sodium> gradient drove uptake, detected by <fluorescence> from zinc-dependent complexes formed inside the <liposomes>. Protein-free and unrelated-transporter controls helped distinguish transport from mixing background. Selected transporters had apparent <Michaelis constants> near $20\,\mu\mathrm M$.<Direct evidence of the molecular basis for biological silicon transport by Michael J. Knight and others>[Figs.3–5, Zinc silicate fluorescence and Proteoliposome transport assays]{ref}
Using that approximate scale in a simple <Michaelis-Menten equation>, $v/V_{\max}=C/(K_M+C)$ gives $1/2$ at $C=20\,\mu\mathrm M$ and $4/5$ at $C=80\,\mu\mathrm M$. This calculation illustrates saturation, not an additional experimental fit. A substrate-binding ratio measured at equilibrium does not by itself establish the number of <sodium> ions coupled to each transport event.
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