Polysaccharides are large carbohydrate molecules crucial for energy storage in living organisms. In animals, glycogen is the primary molecule used for this purpose. This complex carbohydrate allows the body to quickly access glucose when energy demands increase.
What is Glycogen
Glycogen is a complex carbohydrate, or polysaccharide, composed of many individual glucose units linked together. Its highly branched structure, similar to amylopectin in plants but more extensive, allows for efficient storage of numerous glucose molecules within a compact form. Glycogen is often called “animal starch” due to its analogous energy storage role compared to starch in plants.
The branched structure provides a significant advantage for rapid energy release. Each branch offers multiple points where enzymes can access and cleave off glucose units simultaneously. This design ensures glucose can be quickly mobilized and supplied to cells when the body requires immediate energy, such as during physical activity or between meals.
Glycogen Storage Locations
Glycogen is stored primarily in two main locations: the liver and skeletal muscles. The liver stores a higher concentration of glycogen, about 5–6% of its fresh weight, translating to roughly 100–120 grams in an adult. Skeletal muscles store glycogen at a lower concentration, typically 1–2% of muscle mass. However, since the total mass of skeletal muscles is much greater than the liver’s, approximately 75% of the body’s total glycogen is found within the muscles, amounting to around 400 grams in an adult.
Liver glycogen plays an important role in maintaining stable blood glucose levels for the entire body. When blood sugar drops, the liver breaks down its glycogen stores and releases glucose into the bloodstream, providing fuel for organs like the brain. Muscle glycogen primarily serves as an immediate energy source for the muscle cells themselves, especially during physical activity. Muscle cells lack the necessary enzyme to release glucose into the bloodstream, so their glycogen reserves are utilized for local energy needs.
Glycogen’s Role as Energy
Glycogen serves as a dynamic energy reserve, continuously undergoing synthesis and breakdown to meet the body’s fluctuating energy demands. When glucose is in excess, such as after a carbohydrate-rich meal, the body converts it into glycogen for storage through glycogenesis.
When the body requires energy, particularly during fasting or intense physical activity, glycogen is broken down into glucose through glycogenolysis. This rapid breakdown releases glucose, which cells use to produce adenosine triphosphate (ATP), the primary energy currency of the cell. Quickly mobilizing glucose from glycogen stores supports sustained activity and vital bodily functions.
Regulating Glycogen Levels
The body maintains precise control over glycogen levels through a sophisticated interplay of hormones, primarily insulin and glucagon. These hormones, produced by the pancreas, act in opposition to ensure blood glucose levels remain within a healthy range. This system helps prevent both excessively high and dangerously low blood sugar.
Insulin is released when blood glucose levels rise, typically after a meal. It promotes glucose uptake by cells and stimulates glycogenesis, encouraging glucose conversion into glycogen for storage in the liver and muscles. Glucagon is released when blood glucose levels fall. Its primary action is to stimulate glycogenolysis in the liver, breaking down stored glycogen and releasing glucose into the bloodstream to raise blood sugar. This hormonal balance ensures a continuous and regulated energy supply, adapting to the body’s changing metabolic state.