The free energy for the oxidation of glucose to CO2 and water is -686 kcal/mol and the free energy for the reduction of NAD to NADH is 53 kcal/mol. What is the main reason that only two molecules of NADH formed during glycolysis when it appears that as many as a dozen could be formed

in the process of  of glucose oxidation, it produces CO₂ and water and releases  -686kcal/mol of free energy and in the reduction reaction of NAD+ to NADH, it produce +53 kcal/mol of free energy. only two molecules of NADH is formed during glycolysis, because-

Most of the free energy available from the oxidation of glucose remains in the pyruvate form which is one of the products of glycolysis.

as the population of phytoplankton decreases, the amount of food for zooplancktons decreases