Net gain of ATP in glycolysis

During the anaerobic (oxygen deficient) condition, when one molecule of glucose is conveed into 2 molecules of lactate, there is a net yield of 2 molecules of ATP. 4 molecules of ATP are synthesized by the 2 substrate level phosphorylations (steps 6 and 9). But 2 molecules of ATP are used in the steps 1 and 3, hence the net yield is only 2 ATP (Table 9.4). The whole reaction is summarized as Glucose + 2 Pi + 2 ADP --> 2 Lactate + 2 ATP iv.But when oxygen is in plenty, the two NADH molecules, generated in the glyceraldehyde3-phosphate dehydrogenase reaction (step 5), can enter the mitochondrial electron transpo chain for complete oxidation. As each NADH provides 2.5 ATPs, this reaction generates 2.5 x 2 = 5 ATPs. Thus when oxygen is available, the net gain of energy from the glycolysis pathway is 7 ATPs (Table 9.5).Hence the ATP yield from glycolysis is different in anaerobic and aerobic conditions.Ref: DM Vasudevan, 7th edition, page no: 112