Modern biological research has revealed strong evidence that the enzymes of the mitochondrial respiratory chain assemble into larger, supramolecular structures called supercomplexes, instead of the traditional fluid model of discrete enzymes dispersed in the inner mitochondrial membrane.  These supercomplexes are functionally active and necessary for forming stable respiratory complexes.
One supercomplex of complex I, III, and IV make up a unit known as a respirasome. Respirasomes have been found in a variety of species and tissues, including rat brain, liver, kidney, skeletal muscle, heart, bovine heart, human skin fibroblasts, fungi, plants, and C. elegans.