Component of the ubiquinol-cytochrome c oxidoreductase, a multisubunit transmembrane complex that is part of the mitochondrial electron transport chain which drives oxidative phosphorylation. The respiratory chain contains 3 multisubunit complexes succinate dehydrogenase (complex II, CII), ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII) and cytochrome c oxidase (complex IV, CIV), that cooperate to transfer electrons derived from NADH and succinate to molecular oxygen, creating an electrochemical gradient over the inner membrane that drives transmembrane transport and the ATP synthase. The cytochrome b-c1 complex catalyzes electron transfer from ubiquinol to cytochrome c, linking this redox reaction to translocation of protons across the mitochondrial inner membrane, with protons being carried across the membrane as hydrogens on the quinol. In the process called Q cycle, 2 protons are consumed from the matrix, 4 protons are released into the intermembrane space and 2 electrons are passed to cytochrome c. Cytochrome b is a catalytic core subunit containing 2 b-type hemes BL and BH topographically segregated in the quinone reduction (Qi) and quinol oxidation (Q0) sites on opposite sides of the membrane. Binds 2 heme b groups non-covalently. Component of the ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII), a multisubunit enzyme composed of 10 subunits. The complex is composed of 3 respiratory subunits cytochrome b (MT-CYB), cytochrome c1 (CYC1-1 or CYC1-2) and Rieske protein (UCR1-1 or UCR1-2), 2 core protein subunits MPPalpha1 (or MPPalpha2) and MPPB, and 5 low-molecular weight protein subunits QCR7-1 (or QCR7-2), UCRQ-1 (or UCRQ-2), QCR9, UCRY and probably QCR6-1 (or QCR6-2) (PubMed:18189341, PubMed:18305213). The complex exists as an obligatory dimer and forms supercomplexes (SCs) in the inner mitochondrial membrane with NADH-ubiquinone oxidoreductase (complex I, CI), resulting in different assemblies (supercomplexes SCI(1)III(2) and SCI(2)III(4)) (PubMed:12970493). A stretch of 270 kb of the mitochondrial genome is duplicated within the centromere of chromosome 2 resulting in the duplication of the gene. The expression of this duplicated gene (At2g07727) is not demonstrated. It is also probably not RNA edited and therefore differs in all the positions known to be edited. 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 protein contains only eight transmembrane helices, not nine as predicted by bioinformatics tools.