## Cori Cycle Overview The Cori cycle is a metabolic pathway that shuttles lactate from anaerobic tissues (muscle, RBC) back to the liver for gluconeogenesis. ### Key Components of the Cori Cycle **Key Point:** The Cori cycle is NOT energetically efficient — it actually costs the body ATP. Lactate produced in muscle is transported to liver, converted back to glucose via gluconeogenesis (which requires 6 ATP equivalents per glucose), and the glucose returns to muscle for re-glycolysis. ### Energy Cost Analysis | Process | Location | ATP Cost/Yield | |---------|----------|----------------| | Glycolysis (glucose → pyruvate → lactate) | Muscle | +2 ATP net | | Gluconeogenesis (lactate → glucose) | Liver | −6 ATP equivalents | | **Net cost per glucose cycle** | — | **−4 ATP** | **High-Yield:** The Cori cycle is metabolically expensive. It exists not for energy efficiency but for: 1. Lactate clearance during exercise 2. Maintenance of blood glucose during stress 3. Redistribution of metabolic load from muscle to liver ### Why Each Statement Is Correct (Except the Answer) - **Lactate transport to liver:** ✓ Correct. Lactate is released from muscle into blood and taken up by hepatocytes via monocarboxylate transporters (MCT1/MCT4). - **LDH catalyzes pyruvate ↔ lactate:** ✓ Correct. LDH is bidirectional; in muscle (anaerobic), it favors lactate formation; in liver, it favors pyruvate formation. - **Gluconeogenesis converts lactate to glucose:** ✓ Correct. Lactate enters gluconeogenesis via pyruvate carboxylase and PEPCK. - **Cori cycle is energetically efficient:** ✗ **WRONG.** The cycle costs net ATP; it is energetically expensive, not efficient. **Clinical Pearl:** During intense exercise, lactate accumulation reflects not just anaerobic metabolism but also the body's deliberate use of the Cori cycle to maintain blood glucose and clear lactate, even at an ATP cost. [cite:Lehninger Principles of Biochemistry Ch 20]
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