Glycolysis is a catabolic process where the substrate glucose is broken down into smaller molecules- pyruvic acid and occurs anaerobically. The site for glycolysis is the cytoplasm. It is a stepwise process involving a series of ten enzymes to catalyze the processes. In it, some of the intermediates may get involved in other metabolic processes than respiration. Like DHAP formed can be used in the lipid synthesis as it is a source of glycerol to join fatty acids. It is the connecting link or common pathway between both types of respiration. In aerobic respiration, the oxygen molecule is not involved in the glycolysis pathway.

ATP obtained from ADP and a phosphorylated intermediate instead of inorganic phosphate, Pi, is referred to as substrate-level phosphorylation. This process is observed in the glycolysis and the amount of ATP generated is relatively low. The net result of glycolysis is 2 ATP with 2 NADH2 which is used as an energy source later in ETC.