A basic goal of the respiratory pathways is to release the chemical energy of glucose or other nutrients and recapture that energy in the high energy bonds of ATP molecules. When glucose is fully oxidized, it is broken down to carbon dioxide and water. In the process, energy is released. It is possible to measure the energy released by a reaction in an instrument called a calorimeter. The slow step-wise controlled release of energy during glycolysis and respiration allows a significant proportion of the energy of the sugar to be converted to energy stored in ATP molecules.
Glycolysis is a universal metabolic pathway.
The diagrams on the next page show the 10 steps of glycolysis. Aerobic respiration extracts far more usable energy from the glucose molecule than glycolysis alone. If oxygen is not available, the pyruvate formed during glycolysis is sent along a different metabolic pathway. Anaerobic respiration requires no oxygen and produces no additional usable energy, but it does allow glycolysis to continue.