In the conversion of glucose into two molecules of lactate, the NADH generated earlier in the pathway is oxidized to NADT.+Why is it not to the cell's advantage to simply make more NAD so that the regeneration would not be necessary?++NAD synthesis requires ATP, so it is more energetically favorable to use NAD catalytically and couple its recyclingto lactate production.++Catalytic use of NAD would require the synthesis of a new molecule of NAD each time a molecule of pyruvate wasproduced from glucose.++Stoichiometric use of NAD would require the synthesis of a new molecule of NAD for every molecule of pyruvateconverted to lactate.++There is no cellular pathway to synthesize new NAD molecules, so recycling is the only means by which NADlevels are maintained.

NAD is an electron carrier. It carries electrons from substrate oxidation to the ETC to generate a proton gradient for ATP synthesis.Glycolysis cannot proceed from NAD. A byproduct of glycolysis is NADH. NAD is regenerated from NADH via the ETC in the presence of oxygen. In the absence of oxygen, ETC is halted and NAD is regenerated from NADH via the catalytic activity of lactate dehydrogenase. For the oxidation of a single glucose molecule, 2 NAD molecules are needed. Biosynthesis of NAD from scratch is energetically costly. Also, biosynthesis of NAD would disrupt the NAD:NADH ratio of the cell's cytoplasm which is important for regulating the cell's other biochemical pathways and maintaining the redox balance within the cell. So, the cells evolve to recycle NAD than synthesize NAD. NAD synthesis requires ATP, so it is more energetically favourable to use NAD catalytically and couple its recycling to lactate production.This statement is an accurate summarization of the explanation provided in Step 2 and is therefore valid. Catalytic use of NAD would require the synthesis of a new molecule of NAD each time a molecule of pyruvate was produced from glucose.This statement is partly correct because two molecules of NAD would be required per glucose molecule oxidised to two pyruvate. Stoichiometric use of NAD would require the synthesis of a new molecule of NAD for every molecule of pyruvate converted to lactate.Pyruvate to lactate conversion requires NADH not NAD, so this statement is incorrect. There is no cellular pathway to synthesize new NAD molecules, so recycling is the only means by which NAD levels are maintained. The Preiss-Handler pathway and the salvage pathway are two biochemical pathways that produce NAD. Therefore this statement is incorrect. The correct choice is: NAD synthesis requires ATP, so it is more energetically favourable to use NAD catalytically and couple its recycling to lactate production.