The primary polysaccharide that animals store for energy in the liver is glycogen. Often referred to as "animal starch," glycogen is a large, highly branched polymer of glucose molecules. It serves as a vital short-term energy reserve, ensuring the body has a readily available source of glucose to maintain normal function, especially for critical organs like the brain. The liver's ability to store and release glucose from glycogen is a central component of glucose homeostasis, the process by which the body regulates blood sugar levels.
What is Glycogen?
Glycogen is a complex carbohydrate constructed from many individual glucose units linked together. The molecule's key structural feature is its extensive branching, with glucose units joined by both alpha-1,4 and alpha-1,6 glycosidic bonds. This highly branched structure is crucial for its function as a rapid energy source because it provides a large number of terminal ends for enzymes to act on simultaneously. This allows for the swift release of glucose during periods of high energy demand or low blood sugar. Glycogen is stored in the cytoplasm of cells in granules that also contain the enzymes needed for its metabolism. While the liver stores the highest concentration of glycogen by organ weight, the skeletal muscles hold the majority of the body's total glycogen reserves due to their much larger mass.
The Storage and Release of Glycogen
Glycogen metabolism is a tightly regulated process controlled by hormones to ensure the body's energy needs are met.
Glycogenesis (Glycogen Synthesis)
- Following a meal rich in carbohydrates, blood glucose levels rise.
- The pancreas releases the hormone insulin in response to this increase.
- Insulin signals liver and muscle cells to take up the excess glucose from the bloodstream.
- The enzyme glycogen synthase is activated, adding glucose units to the growing glycogen chains.
Glycogenolysis (Glycogen Breakdown)
- During fasting or between meals, blood glucose levels begin to fall.
- The pancreas releases the hormone glucagon, which counteracts insulin's effects.
- Glucagon signals the liver to break down its stored glycogen.
- The enzyme glycogen phosphorylase removes glucose units from the branched glycogen molecule.
- In the liver, the enzyme glucose-6-phosphatase removes a phosphate group from glucose-6-phosphate, allowing free glucose to be released into the bloodstream to raise blood sugar levels.
Liver vs. Muscle Glycogen
An important distinction exists between how glycogen is used in the liver and the muscles.
- Liver Glycogen: This reserve acts as a central glucose supply for the entire body. The liver's glycogen stores are released to maintain normal blood glucose levels during fasting to ensure the brain and other tissues have a constant fuel supply.
- Muscle Glycogen: Muscle glycogen is selfishly used by the muscle cells themselves. Muscle cells lack glucose-6-phosphatase, so they cannot release glucose into the bloodstream. Instead, they use their glycogen stores to provide energy for muscle contraction during exercise.
Glycogen vs. Other Storage Carbohydrates
To understand glycogen's role, it is useful to compare it to other important polysaccharides.
| Feature | Glycogen | Starch | Cellulose |
|---|---|---|---|
| Organism | Animals, Fungi | Plants | Plants, Algae, Oomycetes |
| Function | Short-term energy storage | Long-term energy storage | Structural component (cell walls) |
| Structure | Highly branched polymer of alpha-glucose | Less branched (amylopectin) and unbranched (amylose) polymer of alpha-glucose | Unbranched polymer of beta-glucose |
| Digestibility | Easily broken down by animals | Easily digestible by animals (amylose and amylopectin) | Indigestible by most animals (dietary fiber) |
| Solubility | Water-soluble granules | Insoluble starch granules | Insoluble fibers |
Glycogen Depletion and Fatigue
For athletes and individuals performing prolonged, intense physical activity, glycogen depletion is a well-known phenomenon. When the body's glycogen stores become exhausted, performance declines rapidly, a state often called "hitting the wall" or "bonking". At this point, the body must rely more heavily on fatty acids and gluconeogenesis for energy, a slower process that can't sustain high-intensity efforts. Proper nutritional strategies, such as carbohydrate loading, can maximize glycogen stores before an event, while consuming carbohydrates during exercise helps to maintain them.
Disorders of Glycogen Metabolism
Inherited metabolic diseases known as glycogen storage diseases (GSDs) can disrupt the body's ability to create, store, or break down glycogen. These rare genetic disorders are caused by mutations in the enzymes involved in glycogen metabolism. Depending on the specific enzyme deficiency and the tissue affected (e.g., liver, muscle), GSDs can lead to symptoms such as hypoglycemia, enlarged liver, and muscle weakness. For example, in Von Gierke's disease (GSD type I), the liver cannot release free glucose from its stored glycogen, leading to severe hypoglycemia during fasting.
Conclusion
Glycogen is the critical polysaccharide that animals store for energy in the liver. As a highly branched polymer of glucose, it provides a fast-acting, short-term energy reserve essential for maintaining blood sugar levels and fueling bodily functions. Regulated by the hormones insulin and glucagon, this system allows the body to efficiently manage its energy resources in response to feeding and fasting cycles. While liver glycogen serves the whole body, muscle glycogen is reserved for localized use, highlighting the specialized roles of this crucial molecule throughout the animal kingdom. National Institutes of Health (NIH)
