Both processes produce ATP from substrates but the Krebs cycle produces many more ATP molecules than glycolysis! Every stage in each process is catalysed by a specific enzyme. In aerobic respiration both glycolysis and the Krebs cycle are involved whereas in anaerobic respiration only glycolysis takes place.
The flow diagram shows that every time a stage produces two hydrogen atoms, in the presence of oxygen, three ATP molecules are produced. The role of these hydrogen atoms is shown in the electron carrier system.
Electron carrier system
The main feature of the electron carrier or electron transport system is that three ATPs are produced every time 2H atoms are transported. It takes place in the mitochondria