The pathways that can produce energy even in the absence of O2 is

In Glycolysis--in Absence of O2 (Anaerobic Phase) * In absence of O2, reoxidation of NADH at glyceraldehyde-3-P-dehydrogenase stage cannot take place in the electron-transpo chain. * But the cells have limited coenzyme. Hence to continue the glycolytic cycle NADH must be oxidised to NAD+. This is achieved by reoxidation of NADH by conversion of pyruvate to lactate (without producing ATP) by the enzyme lactate dehydrogenase. SECTION FOUR * It is to be noted that in the reaction catalysed by glyceraldehyde-3-P-dehydrogenase, therefore, no ATP is produced. In anaerobic phase per molecule of glucose oxidation 4 - 2 = 2 ATP will be produced. + 2 ATPRef: MN Chatterjea Textbook of Medical Biochemistry, 8th edition, page no: 332