In biology, membranes are barriers for the transport of ions and polar substances. Biological membranes are even electric insulators. These properties have allowed nature to use mitochondrial and bacterial membranes for energy transduction via electric voltages (potentials) and ion gradients. Cytochrome c oxidase is an enzyme which transfers electrons from cytochrome c onto oxygen and consumes protons to form water as a product. This reaction creates an electric voltage and a pH difference across the membrane, because cytochrome c delivers its electrons from the outer surface of the membrane whereas the protons originate form the inner surface of the mitochondria or bacteria. In addition, the enzyme translocates (“pumps”) four protons from the inner to the outer surface per reaction cycle enhancing both electric voltage and the pH difference. This so-called “electrochemical proton gradient” drives protons back via the ATP-synthase leading to the synthesis of the universal biological energy carrier adenosine-5’-triphosphate (“ATP”) from adenosine-5’-diphosphate (“ADP”) and inorganic phosphate. The reaction catalysed by the cytochrome c oxidase is understood insufficiently and the subject of controversial discussions. The active site of the enzyme, where oxygen is reduced and water is formed, consists of a heme-iron and a copper atom. It is located in the center of the membrane. There are proton transfer pathways in the enzyme which allow and control the access of protons, required for water formation, to active site. One of these pathways is also used for protons to be pumped. However, it is e.g. unclear which chemical entity is bound to the active site when the enzyme is in its oxidized form. The author’s view, based on X-ray structures of the enzyme, will be presented.