Question

How many ATP molecules are released when 1 molecule of glucose is oxidised in our liver cells?

• A. 36
• B. 38
• C. 2
• D. 8

Answer 1

The correct option is B 38

The process of aerobic respiration of glucose can be considered in four stages.
(1) First stage is glycolysis. Net gain of ATP is 2 molecules. Also there is synthesis of 2 molecules of reduced coenzymes NADH2NADH2. Each NADH2NADH2 gives 3 ATP during oxidative phosphorylation. Thus the total gain in glycolysis is 8 ATP molecules.
(2) Second stage is oxidative decarboxylation of pyruvic acid to release acetyl CoA. During this reaction one molecule of reduced coenzyme NADH2NADH2 is produced. As two molecules of pyruvic acid get oxidatively decarboxylated to two molecules of acetyl CoA, there is net synthesis of two molecules of reduced coenzyme NADH2NADH2 or an equivalent of 6 ATP molecules.
(3) Third stage is Krebs cycle. During each turn of Krebs cycle, there is synthesis of of 3 molecules of reduced coenzyme NADH2NADH2, 1 molecule of reduced coenzyme FADH2FADH2 and 1 molecule of GTP by substrate level phosphorylation. As each FADH2FADH2 gives 2 molecules of ATP in oxidative phosphorylation and each molecule of NADH2NADH2 gives 3 molecules of ATP, the net gain of ATP in one turn of Krebs cycle is 12. During aerobic respiration of one molecule of glucose, the Krebs cycle turns twice, thus there is net synthesis of 24 ATP.
(4) The last step is mitochondrial electron system, which oxidises the reduced coenzymes NADH2NADH2 and FADH2FADH2and produces ATP by oxidative phosphorylation.
Thus, the total gain of ATP in steps 1, 2, and 3 is 8 + 6 +24 i.e., 38 ATP molecules.