Cellular respiration involves a series of metabolic pathways that convert glucose into usable energy. Two central pathways in this process are glycolysis and the Krebs cycle (also known as the citric acid or TCA cycle). Understanding the molecular link between these pathways is essential for grasping how cells efficiently extract energy from glucose.

This article explains which compound serves as the connecting link between glycolysis and the Krebs cycle, detailing its formation and role in metabolism.

Glycolysis: From Glucose to Pyruvate

Glycolysis is the anaerobic breakdown of glucose (a six-carbon sugar) into two molecules of pyruvate (each containing three carbons). This occurs in the cytosol and yields a net gain of 2 ATP and 2 NADH molecules per glucose.

Pyruvate is the end product of glycolysis, but it cannot directly enter the Krebs cycle. Instead, it undergoes a critical conversion before entering the mitochondria.

The Link Reaction: Conversion of Pyruvate to Acetyl CoA

Before entering the Krebs cycle, pyruvate is transported into the mitochondrial matrix where it undergoes oxidative decarboxylation catalyzed by the pyruvate dehydrogenase complex.

• Process:
  Pyruvate (3 carbons) loses one carbon as CO₂ and is oxidized, producing NADH.

• Product:
  The remaining two-carbon fragment attaches to coenzyme A (CoA), forming acetyl CoA.

This acetyl CoA is the key intermediate that links glycolysis to the Krebs cycle.

Acetyl CoA: The Entry Molecule for the Krebs Cycle

Acetyl CoA enters the Krebs cycle by combining with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule), initiating the cycle of reactions that fully oxidize the acetyl group to CO₂.

Through the Krebs cycle, high-energy electron carriers NADH and FADH₂ are produced, which later fuel ATP synthesis via oxidative phosphorylation.

Why Other Options Are Not the Link

• Pyruvic Acid (Pyruvate):
  Although pyruvate is the product of glycolysis, it must be converted to acetyl CoA before entering the Krebs cycle.

• Glucose:
  Glucose is the initial substrate for glycolysis, not the connecting intermediate.

• Oxaloacetic Acid (Oxaloacetate):
  Oxaloacetate is a Krebs cycle intermediate but not the link between glycolysis and the Krebs cycle.

Summary Table

Conclusion

Acetyl CoA is the crucial compound linking glycolysis and the Krebs cycle. It is formed by the oxidative decarboxylation of pyruvate and serves as the substrate that enters the Krebs cycle for complete oxidation and energy extraction.

Correct answer: (1) Acetyl CoA