Question

During which stage in the complete oxidation of glucose are the greatest number of $ATP$ molecules formed from $ADP$.
A ) Glycolysis
B ) Krebs cycle
C ) Conversion of pyruvic acid to acetyl CoA
D ) Electron transport chain

Answer 1

Hint: When one glucose molecule is oxidised, 38 $ATP$ molecules are obtained. The carbon atoms of glucose are converted to carbon dioxide and are no longer available to cells. Out of four given options, identify the option, in which the by-products \[NADH_2\] and \[FADH_2\] (from other options) enter the chain and produce $ATP$ molecules.

Complete answer:
In the glycolysis stage, two molecules of \[NADH_2\] and four molecules of $ATP$ are formed. Two \[NADH_2\] molecules go to the electron transport chain. During oxidative decarboxylation, no $ATP$ molecule is formed. Two molecules of pyruvate form two molecules of \[NADH_2\]. These two molecules of \[NADH\] enter the electron transport chain.
In Krebs cycle, two molecules of $ATP$, six molecules of \[NADH_2\]and two molecules of \[FADH_2\]are formed from two acetyl molecules. $NADH_2$ and \[FADH_2\] molecules of the electron transport chain. There, two \[NADH_2\] molecules and two \[FADH_2\] molecules pass to electron carriers and give three and two molecules of $ATP$ respectively.
Thus, during glycolysis, $4ATP$ molecules are formed. In the Krebs cycle, two $ATP$ molecules are formed. From the electron transport chain, $34ATP$ molecules are formed. $40$ molecules and $2$ molecules of $ATP$ are consumed during glycolysis. Hence, $38$ molecules of $ATP$ are gained in one complete oxidation of glucose molecules.

Hence, the correct answer is option D ) Electron transport chain.

Note: Do not ignore the number of $ATP$ molecules consumed during the glycolysis. If this number is ignored, then the calculated number of $ATP$ molecules formed during complete oxidation of glucose will be higher than actual value.