Component of the ubiquinol-cytochrome c reductase complex (complex III or cytochrome b-c1 complex) that is part of the mitochondrial respiratory chain. The b-c1 complex mediates electron transfer from ubiquinol to cytochrome c. Contributes to the generation of a proton gradient across the mitochondrial membrane that is then used for ATP synthesis. Binds 2 heme b groups non-covalently. The main subunits of complex b-c1 are: cytochrome b, cytochrome c1 and the Rieske protein. Heme 1 (or BL or b562) is low-potential and absorbs at about 562 nm, and heme 2 (or BH or b566) is high-potential and absorbs at about 566 nm. Belongs to the cytochrome b family. The full-length protein contains only eight transmembrane helices, not nine as predicted by bioinformatics tools.