 It was clear already in the 1940s that this cannot be the only process that makes ATP, as it is anaerobic (doesn’t require oxygen) and not very efficient. The complete breakdown of food molecules requires oxygen. Hans Krebs showed that this occurs in a cyclic reaction pathway today known as the Krebs cycle. The Krebs cycle generates a compound called reduced NADH (nicotinamide adenine dinucleotide). The big question in biochemistry from the 1940s onward was how this compound was oxidized and how this oxidation was coupled to the phosphorylation of ADP. Naturally, biochemists searched for a high-energy intermediate as it was used in glycolysis. However, this intermediate proved to be extremely elusive.