Are all (known) organisms carbon-based? For example, are there also living beings with silicon as a base, for example?

Answer

Since silicon is very similar to carbon, especially in its valence value, some people have indeed suggested the possibility of Si-based life. A big and important difference (for building biomolecules) between C and Si, however, is that Si does not form double and triple bonds so easily.

Life as we know it could not have evolved by Si-based biochemistry. The main reason for this is that (almost) all life on earth is dependent on the carbon cycle: autotrophic organisms (eg plants) use carbon dioxide to make C-based biomolecules. These autotrophs are then used as food by heterotrophs (e.g. a cow) as an energy source, releasing CO₂which completes the circle.

If we were to replace carbon with Si, then a Si cycle would be necessary. SiO₂ however, precipitates in aqueous environments (so it settles). SiO₂ can therefore NOT be transported between living organisms via the general biological pathways (after all, water is necessary for life). So another solvent would be needed to sustain Si-based life. However, it is very difficult (if not impossible) to find another so common component with the same properties as water that would allow Si life.

Although there are no life forms based entirely on Si chemistry, some do use Si for other functions. For example, diatoms (single-celled diatoms) have skeletons made of opaline (silicon dioxide) and some sponges have spines (spicules) made of the same material.