Figure 1. Formula of coenzyme B ₁₂. This is one of the biologically active forms of B ₁₂. In vitamin B ₁₂ the deoxyadenosine-residue (bottom part of the molecule) is replaced by a CN- or an OH-group. B ₁₂-compounds can only be synthesized by a number of bacterial and archaeal species. (H.P.C. Hogenkamp, Vitamin B ₁₂ Coenzymes. Ann. Acad. Sci 112, 552–564 [1969].)

Figure 2. The bioelements in the periodic table. The macrobioelements are shown in green, the micro-bioelements, in yellow. Of the remaining elements, a few such as silicon play a role in certain organisms. (L.P. Wackett et al., Appl. Environ. Microbiol. 70, 647–655 [2004], modified.)