What is Glycogen?
Glycogen is defined as a glucose storage molecule. Glucose is a monosaccharide (single sugar molecule) that the body uses for energy. Since energy is critical in maintaining the body's daily function, it is important to have a means of energy storage for use between periods of food consumption. Glycogen is one way the body chooses to store energy and the only way it chooses to store glucose. Glycogen belongs to a larger family of macromolecules (large molecules with a high number of atoms) called carbohydrates. Carbohydrates are a group of organic molecules that include sugars, starch, and cellulose.
Is glycogen a lipid? No, lipids are oils and fats. While fat is another form of energy used and stored by the body (via adipose tissue), sugar and fat are two different biomolecules that provide different amounts of energy and require vastly different forms of synthesis and metabolism.
What is glycogen made of? Glycogen is made of glucose. If glucose is a monosaccharide, then glycogen is a polysaccharide. Polysaccharides are long chains of monosaccharides bound together by glycosidic linkages. Therefore, glycogen is a long chain of multiple glucose molecules linked together by glycosidic bonds. Glycosidic linkages are covalent bonds (electrons are shared between atoms) that join one sugar to another.
Glycogen formation exists to store excess sugar that the body is not currently using. As mentioned, energy storage is critical for survival. In times of little to no food consumption (fasting) or high-energy demand (exercise), the body will be able to survive on energy stores. Typically, the body will deplete its glycogen stores before utilizing fat. This is because it is easier for the body to convert glycogen to energy than it is to convert fat.
How is glycogen formed? Glycogen production occurs through a process called glycogenesis. Glycogenesis is a series of biochemical reactions used to convert singular glucose molecules into glycogen macromolecules. This process is stimulated by the hormone insulin, which signifies that blood sugar levels are too high and must be stored to remove them from the blood.
Where is Glycogen Stored?
Once glycogen is formed, it is stored in either the liver or the muscles.
Glycogen is stored in skeletal muscles to allow for easy access during high-energy physical activities like exercise or sports. It is also stored in cardiac muscle to sustain the constant pumping of blood, and to a much lesser degree, smooth muscle to sustain organ function. Recall that glycogen is the preferred energy source because it is easier to break down than fat. It makes sense then that an easily accessible energy source would be placed among muscle cells that need it the most.
When muscles are in need of energy, glycogen is broken down by a process called glycogenolysis. Glycogenolysis is a series of biochemical pathways used to produce singular glucose molecules from glycogen storage. Glycogenolysis should not be confused with:
- Glycolysis: the conversion of glucose to a molecule called pyruvate for the overarching purpose of creating adenosine triphosphate (ATP), a high-energy molecule that drives many biochemical reactions
- Gluconeogenesis: the formation of glucose from non-carbohydrate sources
- Glycogenesis: the formation of glycogen from individual glucose molecules as a means of storage
The amount of glycogen stored in the liver is dependent upon blood glucose levels. Blood glucose levels are tightly controlled to keep the body healthy and are dependent upon food consumption and the body's energy needs at any given time.
As mentioned, the body releases a hormone called insulin when blood glucose levels are high. Insulin triggers glycogenesis in the muscles and liver so that excess sugar is removed from the blood and formed into glycogen. Alternatively, when blood glucose levels are too low, the body releases a hormone called glucagon. Glucagon triggers glycogenolysis in the muscles to provide sugar and support muscular function. It triggers glycogenolysis in the liver to replenish glucose in the blood. Insulin and glucagon are made and released by the pancreas.
So, what does glycogen do? It stores glucose, and it is either created or released under hormonal control to meet the body's energy needs.
The formation and breakdown of glycogen are more than just biological concepts or pathways. They have real implications in daily life, especially in times of dysfunction. Glycogen examples of disorder and depletion include:
- Diabetes is the most common glycogen-related disorder. Diabetes is the chronic state of high glucose levels either because the pancreas cannot make enough insulin due to a genetic defect (type I diabetes) or the pancreas, liver, and muscles are fatigued and no longer adequately responsive due to constant high blood sugar levels (type II diabetes). Therefore, the individual has lost their ability to regulate blood sugar. It should be noted that type II diabetes is not solely caused by a poor diet or lack of discipline. Chronic stress can also increase blood sugar levels and cause diabetes.
- Because exercise depletes glycogen stores, the body will try to replenish glycogen during periods of rest. Endurance athletes run the risk of glycogen depletion if they do not receive proper rest and maintain a nutritive diet that includes a healthy intake of carbohydrates. Furthermore, if glycogen stores are not replenished, the body may try to break down muscle to use protein as an energy source. This can lead to muscle damage, fatigue, or even extreme exhaustion.
Glycogen is a glucose storage molecule found in both liver and muscle tissue. It is a polysaccharide that is formed from excess sugar in the body. The process of converting a single sugar molecule into glycogen is called glycogenesis. Insulin is the hormone that helps with the conversion of glucose to glycogen.
When glucose is needed during an emergency situation, the process of converting glycogen to glucose is called glycogenolysis. Glycogenolysis occurs in muscle cells to provide energy for increased physical activity. It occurs in the liver to increase sugar levels in the blood. Glucagon is the hormone that helps with the conversion of glycogen to glucose.
Diabetes is the most common glycogen-related disorder. It occurs when either the pancreas cannot make insulin due to a genetic defect or when the pancreas, liver, and muscles are fatigued due to chronically high blood glucose levels. Either way, the body is unable to regulate the amount of glucose in the blood. Another glycogen-related disorder is glycogen depletion. This occurs when endurance athletes deplete glycogen stores, damage muscles, and experience extreme fatigue. Glycogen depletion is most often remedied by prolonged periods of rest and a healthy diet of sufficient carbohydrates.
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What is glycogen in simple terms?
Glycogen is a glucose storage molecule in the liver and muscle. It stores energy in the form of glucose for the body to use when needed.
What is the function of glycogen?
Glycogen stores glucose for release to muscles in times of physical activity. It releases glucose to the liver to increase blood glucose levels.
Do humans store glycogen?
Yes. Humans store glycogen in the liver and muscle. Glycogen is the body's way of storing multiple glucose molecules for future energy use.
Is glycogen a sugar?
Yes. Glycogen is a polysaccharide, meaning it is a chain of multiple sugar molecules. Multiple glucose molecules make up one glycogen macromolecule.
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