Cori-Cycle

Cori Cycle – Significance of Cori Cycle

Cori Cycle

The Cori cycle, or glucose-lactate cycle, was found by Carl Ferdinand Cori and Gerty Theresa Radnitz, a husband-and-wife group, in the ’30s and ’40s of the last century. They showed the existence of metabolic cooperation between the skeletal muscle working under low oxygen conditions and the liver. In addition to skeletal muscle, this metabolic cooperation was also shown between other extrahepatic tissues and the liver.

Certainly, like the glucose-alanine cycle, the glucose-lactate cycle is active in between the liver and all those tissues that do not entirely oxidize glucose to CO2 and H2O, in which case pyruvate for conversion to lactate or, by transamination, to alanine would lack.

In addition to skeletal muscle cells, examples of cells that continually produce lactic acid are red blood cells, immune cells in the lymph nodules, proliferating cells in the bone marrow, and epithelial cells in the skin.

Remember that skeletal muscle produces lactate even at rest, although at a low rate.

Cori-Cycle

When working out, the skeletal muscles require glucose for energy. If the cells have adequate oxygen, the procedure of glycolysis will produce pyruvate and continue through the TCA cycle and the electron transport process to produce the needed energy currency in the form of ATP.

If the oxygen supply is insufficient, the pyruvate can continue through the process of fermentation, producing lactate and ethyl alcohol plus NAD+ to replenish the essential supply of NAD+ for continuing glycolysis. During intense exertion, glycogen stores in the muscle stores are mobilized and used to produce pyruvate, and if the oxygen supply is low, this contributes even more to the production of lactate in the cells.

The lactate cannot be utilized by the cell and is transferred out of the cell into the bloodstream, and a portion of it reaches the liver where it can go through gluconeogenesis to produce glucose to carry back to the cells.

Energy expense of the glucose-lactate cycle

The Cori cycle results in a net consumption of 4 ATP.

  • The gluconeogenic leg of the cycle takes in 2 GTP and 4 ATP per particle of glucose synthesized, that is, 6 ATP.
The ATP-consuming reactions are catalyzed by:
  • pyruvate carboxylase: one ATP;
  • phosphoenolpyruvate carboxykinase: one GTP;
  • glyceraldehyde 3-phosphate dehydrogenase: one ATP.
  • Given that 2 molecules of lactate are required for the synthesis of one particle of glucose, the net cost is 2 x 3 = 6 high energy bonds per molecule of glucose.

Alternatively, the glycolytic leg of the cycle produces just 2 ATP per particle of glucose.

Therefore, more energy is needed to produce glucose from lactate than that acquired by anaerobic glycolysis in extrahepatic tissues. This explains why the Cori cycle cannot be sustained forever.

Significance of Cori Cycle

The transfer of lactic acid formed in the muscle to the liver and transfer of glucose from the liver to the muscle is called Cori’s cycle.

  • Cori’s cycle aid in preventing acid build-up in a working muscle.
  • Working muscle produces lactic acid by anaerobic respiration. And it gets deprived of energy due to inadequate glucose. Cori’s cycle helps in the transfer of lactic acid to the liver and the production of glucose by gluconeogenesis. This glucose is sent to muscle for energy production.
  • Hence, Cori’s cycle helps in proper muscle functioning by supplying the required energy in the form of glucose.
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Indeed, this cycle allows the efficient performance of numerous extrahepatic cells at the expense of the liver and partially of the renal cortex.

Red blood cells

These cells, which do not have a nucleus, ribosomes, and mitochondria, are smaller than many other cells. Their small size permits them to pass through tiny blood vessels. Nevertheless, the absence of mitochondria makes them completely dependent on anaerobic glycolysis for ATP production. Then, the lactate is partially disposed of by the liver and renal cortex.

MCQs with Answers

  • What is another name for the Cori cycle?
    • a) Glycolysis
    • b) Gluconeogenesis
    • c) Krebs cycle
    • d) Glucose-lactate cycle
    • Answer: d) Glucose-lactate cycle
  • Who discovered the Cori cycle?
    • a) Carl Ferdinand Cori
    • b) Gerty Theresa Radnitz
    • c) Both a and b
    • d) Neither a nor b
    • Answer: c) Both a and b
  • In addition to skeletal muscle, with which other tissues does the Cori cycle show metabolic cooperation?
    • a) Heart muscle
    • b) Lung tissue
    • c) Extrahepatic tissues
    • d) Brain cells
    • Answer: c) Extrahepatic tissues
  • Which cells continually produce lactic acid?
    • a) Nerve cells
    • b) Heart muscle cells
    • c) Red blood cells
    • d) Liver cells
    • Answer: c) Red blood cells
  • What is the primary purpose of the Cori cycle during exercise?
    • a) Breakdown of glucose
    • b) Production of lactate
    • c) Supply of glucose for energy
    • d) ATP synthesis in the liver
    • Answer: c) Supply of glucose for energy
  • How does the Cori cycle prevent acid build-up in working muscles?
    • a) Breakdown of lactate
    • b) Accumulation of glucose
    • c) Release of oxygen
    • d) Activation of mitochondria
    • Answer: a) Breakdown of lactate
  • What is the net consumption of ATP in the Cori cycle?
    • a) 2 ATP
    • b) 4 ATP
    • c) 6 ATP
    • d) 8 ATP
    • Answer: c) 6 ATP
  • Which enzyme catalyzes the ATP-consuming reactions in the gluconeogenic leg of the Cori cycle?
    • a) Pyruvate carboxylase
    • b) Phosphoenolpyruvate carboxykinase
    • c) Glyceraldehyde 3-phosphate dehydrogenase
    • d) Lactate dehydrogenase
    • Answer: b) Phosphoenolpyruvate carboxykinase
  • What is the significance of the gluconeogenic leg of the Cori cycle in terms of ATP consumption?
    • a) It produces ATP
    • b) It consumes 2 GTP and 4 ATP per glucose synthesized
    • c) It is not involved in ATP production
    • d) It consumes only 1 ATP per glucose synthesized
    • Answer: b) It consumes 2 GTP and 4 ATP per glucose synthesized
  • Why is the Cori cycle unable to be sustained forever?
    • a) It requires too much oxygen
    • b) It involves excessive glucose production
    • c) It has a net consumption of ATP
    • d) It relies on anaerobic glycolysis
    • Answer: c) It has a net consumption of ATP
  • What is the primary purpose of the Cori cycle in red blood cells?
    • a) ATP production in mitochondria
    • b) Glycogen storage
    • c) Dependent on anaerobic glycolysis for ATP
    • d) Lactate disposal
    • Answer: c) Dependent on anaerobic glycolysis for ATP
  • What is transferred from the muscle to the liver in the Cori cycle?
    • a) Glucose
    • b) Oxygen
    • c) Lactate
    • d) ATP
    • Answer: c) Lactate
  • How does the Cori cycle aid in muscle functioning during intense exertion?
    • a) By storing glycogen
    • b) By converting lactate to ATP
    • c) By preventing acid build-up
    • d) By activating mitochondria
    • Answer: c) By preventing acid build-up
  • What is the net cost in terms of high-energy bonds per molecule of glucose in the Cori cycle?
    • a) 2
    • b) 4
    • c) 6
    • d) 8
    • Answer: c) 6
  • Which cells are completely dependent on anaerobic glycolysis for ATP production?
    • a) Liver cells
    • b) Nerve cells
    • c) Red blood cells
    • d) Muscle cells
    • Answer: c) Red blood cells
  • What is the primary purpose of the Cori cycle in extrahepatic cells?
    • a) ATP production
    • b) Lactate synthesis
    • c) Supplying energy at the expense of the liver
    • d) Mitochondrial activation
    • Answer: c) Supplying energy at the expense of the liver
  • What is the primary product of fermentation in the absence of oxygen during the Cori cycle?
    • a) Lactate
    • b) Glucose
    • c) ATP
    • d) Pyruvate
    • Answer: a) Lactate
  • Which process contributes to the production of lactate during intense exertion in the Cori cycle?
    • a) Glycogenolysis
    • b) Glycolysis
    • c) Gluconeogenesis
    • d) Krebs cycle
    • Answer: b) Glycolysis
  • What is the net outcome of the Cori cycle in terms of energy expense?
    • a) Net production of ATP
    • b) Net consumption of ATP
    • c) Equal production and consumption of ATP
    • d) No ATP involvement
    • Answer: b) Net consumption of ATP
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FAQs related to Cori Cycle

  1. What is the Cori cycle, and who discovered it?
    • The Cori cycle, also known as the glucose-lactate cycle, was discovered by Carl Ferdinand Cori and Gerty Theresa Radnitz in the ’30s and ’40s of the last century.
  2. Where does metabolic cooperation occur in the Cori cycle?
    • Metabolic cooperation occurs between skeletal muscles working under low oxygen conditions and the liver. It is also observed between other extrahepatic tissues and the liver.
  3. Which cells continually produce lactic acid in the Cori cycle?
    • Examples of cells that continually produce lactic acid include red blood cells, immune cells in the lymph nodules, proliferating cells in the bone marrow, and epithelial cells in the skin.
  4. Does skeletal muscle produce lactate only during exercise?
    • No, skeletal muscle produces lactate even at rest, although at a low rate.
  5. What happens in the Cori cycle during exercise when oxygen supply is adequate?
    • If the cells have adequate oxygen during exercise, glycolysis produces pyruvate, which continues through the TCA cycle and the electron transport process to produce ATP.
  6. What occurs in the Cori cycle if oxygen supply is insufficient during intense exertion?
    • In the absence of sufficient oxygen, pyruvate continues through fermentation, producing lactate and ethyl alcohol plus NAD+ to replenish the essential supply of NAD+ for continuing glycolysis.
  7. What is the net consumption of ATP in the Cori cycle?
    • The Cori cycle results in a net consumption of 4 ATP.
  8. Which reactions in the gluconeogenic leg of the Cori cycle consume ATP?
    • The ATP-consuming reactions in the gluconeogenic leg are catalyzed by pyruvate carboxylase (one ATP), phosphoenolpyruvate carboxykinase (one GTP), and glyceraldehyde 3-phosphate dehydrogenase (one ATP).
  9. Why can’t the Cori cycle be sustained forever?
    • The Cori cycle cannot be sustained forever because it requires more energy to produce glucose from lactate than that acquired by anaerobic glycolysis in extrahepatic tissues.
  10. What is the significance of Cori’s cycle in preventing acid build-up in working muscles?
    • Cori’s cycle transfers lactic acid formed in the muscle to the liver, preventing acid build-up. The liver then produces glucose by gluconeogenesis, supplying energy to the muscle.
  11. How does Cori’s cycle contribute to proper muscle functioning during intense exertion?
    • Cori’s cycle helps in the transfer of lactic acid to the liver and the production of glucose by gluconeogenesis. This glucose is then sent to the muscle for energy production, preventing energy deprivation and maintaining proper muscle functioning.
  12. Which cells are completely dependent on anaerobic glycolysis for ATP production?
    • Red blood cells, which lack a nucleus, ribosomes, and mitochondria, are completely dependent on anaerobic glycolysis for ATP production.
  13. What is the role of the renal cortex in the Cori cycle?
    • The Cori cycle allows the efficient performance of numerous extrahepatic cells at the expense of the liver and partially of the renal cortex.
  14. How do red blood cells dispose of lactate in the Cori cycle?
    • Red blood cells, lacking mitochondria, dispose of lactate partially by the liver and renal cortex.
  15. Is the Cori cycle active only during exercise?
    • No, the Cori cycle is active not only during exercise but also at rest, maintaining a balance of energy in various cells.
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Summary: Cori Cycle – Significance of Cori Cycle

The Cori Cycle, discovered by Carl Ferdinand Cori and Gerty Theresa Radnitz, elucidates metabolic cooperation between skeletal muscles under low oxygen conditions and the liver. Similar to the glucose-alanine cycle, it operates between the liver and tissues that don’t fully oxidize glucose, leading to lactate or alanine production.

During exercise, if oxygen is sufficient, glycolysis produces ATP through the TCA cycle. In insufficient oxygen conditions, pyruvate proceeds to fermentation, generating lactate. This lactate, transferred to the liver, undergoes gluconeogenesis, producing glucose returned to the cells.

The Cori cycle incurs a net ATP consumption of 4, with the gluconeogenic leg demanding 2 GTP and 4 ATP per glucose synthesized. Key ATP-consuming reactions involve pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and glyceraldehyde 3-phosphate dehydrogenase. As two lactate molecules are needed for one glucose, the net cost is 6 high-energy bonds.

The cycle’s significance lies in preventing acid buildup in working muscles. It transfers lactic acid from muscle to the liver, where gluconeogenesis produces glucose for energy. This process ensures proper muscle function and enables various extrahepatic cells to operate efficiently, though it partially utilizes the liver and renal cortex.

Red blood cells, lacking nuclei and mitochondria, depend on anaerobic glycolysis for ATP. The Cori cycle aids in disposing of lactate by the liver and renal cortex. Overall, this cycle highlights a complex metabolic interplay crucial for energy balance in diverse physiological conditions.