Split reins are a piece of equipment used for communicating with a horse while it is being ridden. Split reins are actually two separate, individual reins that are used together, with one rein on each side of the horse's neck.
As discussed above, mitochondria are the sites of respiration, and generate chemical energy in the form of ATP by metabolizing sugars, fats, and other chemical fuels with the assistance of molecular oxygen.
A secondary function of mitochondria is to synthesize proteins for their own use. They work independently, and execute the transcription of DNA to RNA, and translation of RNA to amino acids (the building blocks of protein), without using any components of the cell.