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.