Ubiquinone is a small carrier molecule with a long isoprene-derived tail related to the phytol tail of chlorophyll, to b-carotene, and to the other terpene derivatives discussed in Chapter 20. The working head of ubiquinone is a quinone ring that can accept two hydrogen atoms at the para positions and give them up again. From NADH to ubiquinone, reduction involves the transfer of hydrogen atoms. Beyond ubiquinone, the reducing hydrogens are split into protons, which are released into the solution, and electrons, which travel through the cytochromes from one heme iron atom to the next. Each cytochrome molecule is reduced to the Fe (II) state by the one before it, and then reoxidized to Fe(III).

All of the foregoing respiratory reactions take place inside mitochondria within the cell. The components of the respiratory chain are embedded in the inner mitochondrial membrane, and are organized into four complexes. Complex 1 contains a flavoprotein (a nonheme iron protein of uncertain function) and phospholipid, and has a molecular weight of around 600,000. Complex III (270,000 molecular weight) contains cytochromes c and c₁, more nonheme iron protein, and phospholipids. Complex IV (eytochrome oxidase) has a weight of 200,000 and contains cytochromes a and a₃, copper atoms, and phospholipids. Each pair of complexes is connected by a mobile shuttle, ubiquinone between complexes I and III, and the small cytochrome c molecule between III and IV.