How many ${\mathrm{ATP}}_{\mathrm{}}$ molecules are required to synthesize one complete ${\mathrm{glucose}}_{\mathrm{}}$ molecule during gluconeogenesis?

• A. $4 {\mathrm{ATP}}_{\mathrm{}}$ molecules
• B. $6 {\mathrm{ATP}}_{\mathrm{}}$ molecules
• C. $12 {\mathrm{ATP}}_{\mathrm{}}$ molecules
• D. $2 {\mathrm{ATP}}_{\mathrm{}}$ molecules

Answer: (C)

Verified step by step guidance

1. Understand that gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors, which is essentially the reverse of glycolysis.
2. Recognize that gluconeogenesis requires energy input in the form of ATP and GTP to drive the endergonic reactions that are not simply the reverse of glycolysis.
3. Identify the key steps in gluconeogenesis that require ATP: the conversion of pyruvate to phosphoenolpyruvate (PEP) and the conversion of 3-phosphoglycerate to 1,3-bisphosphoglycerate.
4. Note that the conversion of pyruvate to PEP involves two steps: pyruvate carboxylase converts pyruvate to oxaloacetate using 1 ATP, and phosphoenolpyruvate carboxykinase converts oxaloacetate to PEP using 1 GTP. This occurs twice for each glucose molecule, consuming 2 ATP and 2 GTP in total.
5. Calculate the total ATP requirement: 4 ATP are used in the conversion of 3-phosphoglycerate to 1,3-bisphosphoglycerate (2 ATP per molecule, twice for each glucose), and 2 ATP are used in the conversion of pyruvate to PEP, resulting in a total of 6 ATP and 2 GTP, which is equivalent to 12 ATP molecules when considering the energy equivalence of GTP to ATP.