Glycolysis invests 2 ATP (at the hexokinase and phosphofructokinase-1 steps) and generates 4 ATP (2 each at the phosphoglycerate kinase and pyruvate kinase steps, since 2 triose molecules are produced per glucose), giving a net gain of 2 ATP per glucose molecule — the same net yield whether the pathway proceeds aerobically or anaerobically, since these steps occur before pyruvate's fate is determined.