Cori Cycle: Purpose, Process, Importance, Key Steps, Regulations

What Is The Cori Cycle?

Cori cycle entails the formation of lactate after anaerobic respiration in the muscles into glucose in the liver. Vigorous exercises reduce oxygen within the muscle; in turn, glucose is catabolized to lactate which is where it is built and transported out to the liver.

In the liver, lactate is reversed back into glucose through a process referred to as gluconeogenesis. The newly formed glucose is then released into the bloodstream and taken back into the muscles to be used as an energy source, hence supporting the muscle activity during prolonged or heavy physical activity, while balancing the accumulation of lactic acid.

Components and Steps Of The Cori Cycle

The Cori Cycle is discussed below-

Glycolysis In Muscles

When glucose undergoes metabolism inside the muscle cells to pyruvate, the process is what produces Adenosine-5'-triphosphate for energy. This pyruvate converts to lactate during heavy exercises since it is anaerobic respiration.

Hexokinase converts glucose to glucose-6-phosphate, phosphofructokinase converts the fructose-6-phosphate to fructose-1,6-bisphosphate, and finally the pyruvate kinase converts phosphoenolpyruvate into pyruvate.

Lactate production

In the absence of sufficient oxygen, by the action of the enzyme lactate dehydrogenase, pyruvate is reduced to lactate. This step is the regeneration of NAD+ from NADH, which needs to be replenished to continue the glycolytic pathway.

Transport of Lactate to the Liver

All the lactate that diffuses into the bloodstream gets carried to the liver by the blood after it is produced in the muscles.

No definite enzymes that perform the transport. The whole process occurs with the help of the circulatory system.

Gluconeogenesis in the liver

The lactate dehydrogenase at the liver will then convert back the lactate to pyruvate. The pyruvate will undergo gluconeogenesis to produce glucose. The funny fact is that this path, in the thermodynamic sense, is almost the reverse of glycolysis; however, it uses different enzymes in some places to bypass the one-way steps of glycolysis.

Lactate dehydrogenase (lactate → pyruvate), pyruvate carboxylase (pyruvate → oxaloacetate), phosphoenolpyruvate carboxykinase (oxaloacetate → phosphoenolpyruvate), fructose-1,6-bisphosphatase (fructose-1,6-bisphosphate → fructose-6-phosphate), and glucose-6-phosphatase.

Transport of Glucose Back to Muscles

This newly formed glucose in the liver is passed on into the bloodstream. There are no specific enzymes directly involved in the transport; the process occurs in the circulatory system.

Importance Of The Cori Cycle

The importance of the Cori Cycle is discussed-

Energy Production During Intense Exercise

The apparent contribution of the Cori cycle is in the production of energy during strong exercise, when the body cannot afford to have sufficient free oxygen for aerobic respiration. This will make the process of glycolysis go on continuously because it reduces pyruvate to lactate in the muscles and hence can provide ATP, which will supply the rapid speed for muscle contraction under anaerobic conditions.

Prevention Of Lactic Acidosis

It is also involved in the prevention of lactic acidosis since, through the Cori cycle, it carries the lactate produced within the muscles to the liver and reformats it back to glucose. This way, it clears the excess amount of lactate from the blood circulation, hence preventing its accumulation and cancer acidosis due to a pH drop.

Maintenance Of Blood Glucose Levels

The Cori cycle helps maintain blood glucose levels by gluconeogenesis in the liver. During severe exercises, when the muscle glucose is spent in an overly rapid manner, the liver replenishes it from lactate. In this way, it provides for the continued supply of glucose to the muscle and other tissues, thus maintaining the total energy homeostasis and metabolic balance

Regulation Of The Cori Cycle

The regulation of the Cori Cycle is discussed-

Hormonal Regulation

Insulin: This hormone increases in a feeding state, increasing the uptake of glucose by tissues, which include muscles. Glucose is further directed to glycolysis and glycogenesis. With this insulin effect, there would be less dependence upon gluconeogenesis. That is to say, when the level of insulin is high, the Cori Cycle is low because less glucose would necessarily need to be replenished.

Glucagon: In fasting states or sustained exercise, its levels go higher. High levels of this hormone stimulate gluconeogenesis in the liver. Glucagon favours glucose synthesis from lactate and thus enhances the activity of the Cori Cycle to maintain blood glucose and hand energy to muscles.

Role Of The Cori Cycle In Fasting And Feeding States

Fasting Organs: Since glucose is an essential fuel for vital organs and muscles, the need for more glucagon to be secreted is facilitated because of the low levels of glucose in the body during fasting. In this situation, the Cori Cycle works actively to produce glucose from lactate so that the continuous supply is maintained to the vital organs and muscles thereby maintaining blood glucose levels and providing energy wherein the possibility of deficiency in carbohydrate availability exists.

Feeding State: This is the state post-feeding when there is more glucose in the system. Insulin is secreted to support uptake and storage. Here, the Cori Cycle needs to be encouraged less to be felt because, at this point, there are ample supplies of glucose through dietary intake. This cycle reduces activity where the body is focused on storing the excess glucose as glycogen in the liver and muscles.

Cori Cycle NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Steps of Cori Cycle

• Regulation of Cori Cycle

Practice Questions for NEET

Q1. The Cori’s cycle is also known as

1. Fructose-lactate cycle

2. Glucose-lactate cycle

3. Fructose-fumarate cycle

4. Sucrose-fumarate cycle

Correct answer: 2) Glucose-lactate cycle

Explanation:

Also called as glucose-lactate cycle is metabolic cooperation between the skeletal muscle working under low oxygen conditions and the liver. The Cori cycle, or glucose-lactate cycle is metabolic cooperation between the skeletal muscle working under low oxygen conditions and the liver.

Hence, the correct option is the 2) glucose-lactate cycle.

Q2. Which of the following is not an event included in Cori’s cycle?

1. The conversion of glucose to lactic acid by anaerobic glycolysis in skeletal muscle

2. The conversion of lactate to glucose by hepatic gluconeogenesis;

3. The diffusion of glucose from the hepatocytes into the bloodstream

4. The diffusion of lactate from muscle cells into the bloodstream by which it is transported to skeletal muscle

Correct answer: 4) The diffusion of lactate from muscle cells into the bloodstream by which it is transported to skeletal muscle

Explanation:

Cori's cycle (also known as the lactic acid cycle) describes the process through which lactate produced by anaerobic glycolysis in muscles is transported to the liver and converted back to glucose. The following events are part of the cycle:

1. Lactate conversion to pyruvate – In the liver, lactate is converted into pyruvate, which is then used to produce glucose.

2. Glucose production (gluconeogenesis) – Pyruvate is used in gluconeogenesis to produce glucose, which is then transported back to muscles.

3. Lactate formation in muscles – During intense exercise, muscles produce lactate through anaerobic glycolysis.

4. Glucose consumption by muscles – Muscles consume glucose for energy, which is converted into pyruvate and, under anaerobic conditions, into lactate.

The event not included in Cori’s cycle would be the direct oxidation of glucose in muscles (since it doesn't involve lactate formation).

Hence, the correct answer is option 4) The diffusion of lactate from muscle cells into the bloodstream by which it is transported to skeletal muscle.

Q3. The muscle fibers that contract slowly are

1. Red muscle fibres

2. White muscle fibres

3. Both a and b

4. None of these

Correct answer: 1) Red muscle fibres

Explanation:

Red muscle fibers, also known as slow-twitch fibers, contract slowly but are highly resistant to fatigue. They are rich in myoglobin, mitochondria, and blood supply, which enable sustained aerobic energy production. These fibers are well-suited for endurance activities like walking, running long distances, or maintaining posture, as they rely on oxidative metabolism for energy.

Hence, the correct answer is option 1) Red muscle fibers.

Also Read:

• MCQs on Cori Cycle

• Axial Skeleton System
• Synovial Joints

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