Metabolic Pathways and Regulation · USMLE Step 1

Which pathway provides reducing equivalents (NADPH) and ribose-5-phosphate for biosynthesis and antioxidant defense rather than ATP?

  1. Glycolysis in the cytosol
  2. Citric acid cycle in mitochondria
  3. Pentose phosphate pathway (PPP)
  4. Fatty acid beta-oxidation
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Correct answer: Pentose phosphate pathway (PPP)

The pentose phosphate pathway generates NADPH (for biosynthesis and redox balance) and ribose-5-phosphate (for nucleotide synthesis), serving anabolic and antioxidant functions rather than directly producing ATP.

Difficulty: Medium Question 17 of 19

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