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Glycogen is a form of glucose stored in the liver and skeletal muscles, which are responsible for producing energy for the body. The body breaks down most carbohydrates from food and converts them into glucose. This glucose is then stored in the muscles and liver in the form of glycogen. During exercise, glycogen in the muscles is broken down to release glucose, which is then used as fuel for the muscles. The liver also breaks down glycogen into glucose, which is then sent through the bloodstream to be used as fuel for other organs.
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What You'll Learn
Glycogen is stored in the muscles and liver
Glycogen is a form of glucose storage in mammals. In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. The liver has a higher glycogen concentration, but as the liver is much smaller (around 1.5 kg) than skeletal muscle, the total amount of glycogen stored in the liver is less.
The liver of an adult weighing 1.5 kg can store roughly 100–120 grams of glycogen, which can make up 5–6% of the organ's fresh weight. In skeletal muscle, glycogen is found in a low concentration (1–2% of the muscle mass). The skeletal muscle of an adult weighing 70 kg stores roughly 400 grams of glycogen. However, glycogen stores in skeletal muscles are limited because an efficient feedback-mediated inhibition of glycogen synthase prevents accumulation.
Glycogen in the liver is used to constantly replenish the 4 grams of glucose circulating in the blood. Liver glycogen stores serve as a store of glucose for use throughout the body, particularly by the central nervous system. The human brain consumes approximately 60% of blood glucose in fasted, sedentary individuals. In contrast, glycogen in skeletal muscle is mainly used as an immediate source of energy for that muscle rather than being used to maintain physiological blood glucose levels.
Glycogen in skeletal muscle can be broken down to lactate, which can be transported to the liver and via gluconeogenesis in the liver contribute to maintaining euglycemia (normal blood glucose concentration). During high-intensity exercise, muscle glycogen can supply 40 mmol glucose/kg wet weight/minute, whereas blood glucose can only supply 4–5 mmol.
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It is a reserve of phosphorylated glucose for muscle cells
In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. It is a reserve of quickly available phosphorylated glucose for muscle cells. This is particularly important during exercise, when muscle glycogen is an important fuel source. Inadequate glycogen availability results in reduced endurance exercise capacity and an inability to continue exercising.
Muscle glycogen is stored in skeletal muscle cells in the form of β particles. It is not shared with other cells because, unlike liver cells, muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the bloodstream. Instead, glycogen stored in muscle cells is available solely for internal use.
During exercise, muscle glycogen is used as a fuel source for muscle cells, providing more than 50% of the total energy requirements during prolonged moderate to high-intensity exercise. It is particularly important during high-intensity aerobic activity, such as brisk walking, jogging, or running, and during anaerobic activity, such as weightlifting and isometric exercise.
The breakdown of glycogen into glucose is catalysed by the enzyme glycogen phosphorylase, which releases glucose-1-phosphate from the linear chains of glycogen. This glucose-1-phosphate is then converted to glucose-6-phosphate, which can be used for metabolism.
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Glycogenolysis is used during high-intensity exercise
Glycogen is a branched, glucose polymer and the storage form of glucose in cells. It is stored in the muscles and liver, with the majority of glycogen stored in skeletal muscles. During exercise, muscle glycogen is an important fuel source, particularly for high-intensity exercise.
During high-intensity exercise, the rate of ATP hydrolysis increases, leading to a greater accumulation of ADP, AMP, and Pi. This increased accumulation of Pi can enhance glycogenolysis, as it provides more of the substrate required for the reaction. Additionally, the increased accumulation of free ADP and AMP can fine-tune the activity of phosphorylase a through allosteric regulation.
Glycogenolysis is the breakdown of muscle glycogen to glucose-1-phosphate, which is catalysed by the enzyme glycogen phosphorylase. The rate of glycogenolysis is influenced by exercise intensity, with higher intensity exercise leading to increased glycogenolysis. This is due to the increased demand for energy during high-intensity exercise, which requires a rapid supply of ATP. Muscle glycogen can provide a higher rate of substrate for ATP synthesis compared to blood glucose, making it a crucial energy source during high-intensity exercise.
Furthermore, during high-intensity exercise, the conversion of phosphorylase b to its active form, phosphorylase a, may contribute to the activation of glycogenolysis. This conversion is potentially mediated by the increased release of epinephrine (adrenaline) during vigorous exercise, which can lead to the phosphorylation of phosphorylase b. However, it is important to note that the percentage of phosphorylase in the active form does not always increase with exercise intensity and may even decrease after a short period of high-intensity exercise.
In summary, glycogenolysis is crucial during high-intensity exercise as it provides a rapid supply of ATP through the breakdown of muscle glycogen. The increased demand for energy during high-intensity exercise results in enhanced glycogenolysis, making it an essential process for maintaining energy levels in active muscle cells.
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Liver glycogen breaks down into glucose for all cells
The human body's main source of energy is glucose, which comes from the carbohydrates in the food and fluids we consume. Glucose is a simple sugar, and when the body has excess glucose, it stores it as glycogen for later use. The body creates glycogen from glucose through a process called glycogenesis.
Glycogen is stored in the liver and skeletal muscles, with small amounts in the brain and other tissues. The liver stores glycogen in a higher ratio compared to its mass, but the muscles store more by total weight because they have a greater mass. About three-quarters of the body's glycogen is found in the muscles, and the liver stores roughly 100-120 grams of glycogen.
The liver breaks down glycogen to maintain adequate blood glucose levels, whereas muscles break down glycogen to maintain energy for contraction. This process is called glycogenolysis, and it is activated by the hormone glucagon, which is released by the pancreas when blood glucose levels fall too low. The liver breaks down glycogen into glucose, which enters the bloodstream and can be used by all cells and tissues in the body.
In contrast, glycogen breakdown in the muscles releases glucose that can only be used by the muscles. The muscles' glycogen serves as a source of metabolic fuel for muscle tissue, especially during exercise. The muscles' glycogen is in the form of β particles, and since the muscles lack glucose-6-phosphatase, they cannot release glucose into the bloodstream.
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Muscle glycogen is a local energy substrate for exercise
Muscle glycogen is a reserve of phosphorylated glucose for muscle cells. It is stored in skeletal muscles, which account for around 40-50% of body weight. The liver has a higher concentration of glycogen, but as it is much smaller than skeletal muscles, the total amount of glycogen in the liver is much less. During exercise, the body breaks down carbohydrates from food and converts them into glucose. This glucose is then used as fuel for the body's cells.
The rate of glycogen degradation depends on the intensity of exercise. During all-out exercise, glycogen can release glucose at a much faster rate than during low-intensity exercise. This is important for athletes, as the ability to train day after day depends on the restoration of muscle glycogen stores.
Muscle glycogen is also important for resistance training, as repeated contractions of near-maximal loads stimulate glycogenolysis, resulting in a reduction in glycogen stores of 25-40%. After exercise, the rate of glycogen synthesis increases to replenish glycogen stores, and blood glucose becomes the substrate.
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Frequently asked questions
Glycogen is the stored form of a simple sugar called glucose. It is a fuel source stored in the cytosol of cells.
Glycogen is stored in the muscles and liver. In the human body, most glycogen is stored in skeletal muscles (350-500 g) and the liver (80-100 g).
Glycogen in muscles serves as a form of energy storage for the muscle itself. It is broken down to release glucose into the bloodstream to be used as fuel for the cells.
