Q. 34 The number of NADP +molecules required to completely oxidize one molecule of glucose to 𝐂𝐎𝟐
through pentose phosphate pathway is ____ (correct to integer number).
The pentose phosphate pathway (PPP) oxidizes one glucose molecule to six CO2 molecules, producing 12 NADP+ molecules reduced to NADPH. This complete oxidation mode maximizes NADPH generation for biosynthetic needs.
Pathway Overview
The PPP consists of an oxidative phase and a non-oxidative phase. Glucose-6-phosphate enters the oxidative phase, yielding ribulose-5-phosphate, one CO2, and two NADPH per cycle. The non-oxidative phase recycles pentose phosphates back to glucose-6-phosphate, allowing repeated oxidation.
Complete Oxidation Mechanism
For full oxidation of glucose (C6) to 6 CO2, the pathway cycles six times through the oxidative phase. Each cycle consumes one NADP+ at glucose-6-phosphate dehydrogenase and one at 6-phosphogluconate dehydrogenase, totaling 12 NADP+ per glucose. No NADPH forms in the non-oxidative phase, confirming the count.
Correct Answer and Options
The answer is 12.
| Option | Description | Correct? |
|---|---|---|
| 6 | Matches one oxidative pass (partial oxidation to ribulose-5-P). Incorrect for complete oxidation. | No |
| 12 | Accounts for 6 cycles × 2 NADPH each, fully oxidizing to 6 CO2. Correct. | Yes |
| 2 | Only first step; ignores full pathway. Incorrect. | No |
| 24 | Overcounts; no basis in standard stoichiometry. Incorrect. | No |
Biological Significance
Cells use this mode in tissues like liver and adipose for NADPH in fatty acid synthesis. Regulation occurs via NADP+/NADPH ratio at the first step. This contrasts glycolysis, emphasizing PPP’s role beyond energy production.
