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Bamboo, horsetail and many grasses incorporate silicates into their tissues to protect themselves from grazers. Diatoms use it to build a glassy outer shell. Carbon is still the choice for metabolism, but I can see that life could have switched towards more silicon in cell walls and body structures. Silicones too (Si-O bonds) can be produced from silicates.

By what mechanisms marine as well as land organisms on Earth extract the silicates for building up their body? It is almost insoluble in water at standard temperature and pressure. Does it serve for metabolism or just physical structure?

EDIT: The question's scope is limited to silicon compounds the organism may absorb or ingest for the purpose on incorporating it into tissue or as a metabolic component.

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    $\begingroup$ Some silicates are readily soluble in water; for example, the widespread sodium silicate. Don't confuse silica (aka quartz) and silicates; they are very different substances. Biomineralization is a vast subject. Please consider taking the Tour to learn about this forum and then reformulating the question to focus on one problem relevant to your work; as it stands it risks being closed as too broad. $\endgroup$ – AlexP Sep 23 '17 at 22:49
  • $\begingroup$ It's dubious that there are silicon-based life forms. en.wikipedia.org/wiki/… "Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism. ... Silicon ... interacts with very few other types of atoms. ... silicon ... molecules (are) "monotonous compared with the combinatorial universe of organic macromolecules". ... silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds" $\endgroup$ – RonJohn Sep 24 '17 at 6:52
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This article gives some idea of the ways in which Silicates are used biologically. Bear in mind that silica (Silicon dioxide) is very distinct from silicates (such as Sodium silicate) The former is very insoluble but the latter is very soluble.

Silicon compounds are found in biological systems but their role is more supplementary and peripheral rather than central and metabolic. That’s not to say it is not extremely important to organisms that use it. So It has been suggested that Silicon might be capable of acting as the basic element for building life replacing carbon. Whilst Silicon can form a wide range of compounds, the scope for forming compounds is much less than that seen in carbon. So Silicon based life forms whilst not being entirely ruled out seem less likely than carbon based ones.

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Organisms that use silica for shells and tissues make opal out of sodium silicate; these include the horsetails and grasses you mention, sponges, radiolarians and diatoms.

There is another way organisms can use silicon, but as SiO2 or quartz (and other silicate minerals). Pick it up off the ground. caddisfly larva from http://bugguide.net/node/view/1302204

This caddisfly larva has used sand, probably (at least in part) from quartz and assembled a shell from it.

Here are some fine tests made of sand by the formidable and predatory Foraminiferans.

foraminiferan tests made of sand http://jfr.geoscienceworld.org/content/38/3/193

Sand is also useful for ad hoc temporary shells.

sandy seal from https://matthewmeierphoto.photoshelter.com/image/I00004RiJurIEhsQ

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