Question

In two turns of the Krebs cycle, the number of ${NADH}_{2}$ molecules produced is
(a)Six
(b)Five
(c)Four
(d)Three

Answer 1

Hint: Krebs’s cycle or citric acid is a cyclic process of 8 enzyme reactions which occur in mitochondria. It converts acetyl Co-A to carbon dioxide accompanied by the release of energy. It occurs in the mitochondrial matrix.

Complete answer:
Kreb's cycle is the second phase of aerobic respiration during which acetyl CoA condenses with OAA to form citrate which undergoes oxidative decarboxylation forming 3 molecules of ${NADH}_{2}$ and one molecule ${FADH}_{2}$. Since glycolysis produces two pyruvates and hence two acetyl CoA from one molecule of glucose, the net yield of NADH2 from the Krebs cycle for one glucose is 6.
So, the correct answer is option A.

Additional Information: 2 molecules of pyruvate are obtained from glycolysis which occurs in the cytosol. Now, this pyruvic acid molecule is acted upon by enzyme pyruvic dehydrogenase. The enzyme catalyses the addition of coenzyme-A by oxidative decarboxylation. One molecule of NADH is reduced in this reaction. In the next reaction, acetyl Co-A which is a 5C containing compound is converted to citrate (6C) by the action of enzyme citrate synthase. Citrate is then converted to Iso-citrate under the action of enzyme aconitase. For the next 2 reactions, succinate (4C) is obtained. Succinate is then converted to fumarate by a dehydrogenation reaction catalysed by enzyme fumarase.
This can be better understood from the diagram given below-

On the other hand, aldolase is an enzyme from the glycolytic pathway. It cleaves one molecule of fructose 1,6 bisphosphate into one molecule of 3 phosphoglyceraldehyde and one of dihydroxyacetone phosphate. So, aldolase is not an enzyme for Krebs’s cycle.
So, the correct answer is, ‘Six.’

Note: Glycolysis and Krebs’s cycle are linked processes which together complete cellular respiration. The end product of glycolysis is the production of 2 molecules of pyruvate. The cycle mentioned above is always operating in two turns i.e. one cycle per pyruvate molecule. After completion of glycolysis, Krebs cycle and electron transport chain, each glucose molecule yields 38 ATP molecules.