Advantages of Glycogen: Compared to Glucose and Fat
In organisms, there are two substances used for storing energy: glycogen and fat. When completely oxidized to carbon dioxide and water, fat produces more energy for cells, but consumes more oxygen and time. Therefore, it is a medium-powered system that only meet the moderate-intensity exercises, such as jogging. However, glycogen also has its advantages. It is a temporary storage material that support life activities as quickly as possible. It is indispensable for high-intensity exercises that require rapid energy release, such as a 400-meter sprint. Glycolysis even generates ATP from it without oxygen. Although anaerobic glycolysis uses just 2% of the total glucose energy, it is crucial for organisms living in oxygen-poor environments, such as intestinal parasites and bivalves in lake and ocean sediments (The glycogen in oysters is as high as 5%.).
Glucose is stored in cells as a polymer and not a monomer due to different osmotic pressure of these two forms. Insoluble granular glycogen reduces the osmolarity in cytoplasm. It contributes to a concentration about 10⁻⁵mMol/L. If all glucose unit is dissolved in cytoplasm, the concentration would be 400 mMol/L. An osmotic pressure of 40 million times would allow a great deal of water to enter the cell resulting in rupture.
Distribution and Biological Function of Glycogen
Bacteria evolved anaerobic glycolysis to access energy stored in glucose during an era when Earth lacked oxygen, so it is an older energy system than fat. This explains why it can be found in almost all organisms, from bacteria and archaea to eukaryotes. However, it is important to note that in eukaryotes, only animals, protist and fungi have glycogen. It may be that plants have found a more suitable choice during evolution. Starch serves as substitution in plants.
Granular glycogen is stored in cytoplasm, especially abundant near mitochondria. Each granule contains tens of thousands of glucoses and enzymes that catalyze both synthesis and degradation. When there is sufficient nutrient, glucose is synthesized into glycogen by cell. When organisms are hungry and exercising vigorously, glycogen is broken down to supply ATP. Breakdown by lysosomes is another metabolic pathway in multicellular organisms. About 10% of glycogen is engulfed by lysosomes and its straight and branched chains are destroyed by acid alpha-glucosidase.
In animals, it is primarily found in liver and skeletal muscles, and there is also a little in nerve cells. Because most cells do not store energy, and they obtain glucose from blood and tissue fluid, the liver stores about 100g glycogen to maintain blood sugar stability for about 12 hours without any food intake. They are present in cytoplasm as large granules. Too small granules could lead to unstable blood sugar levels. For example, brain is our most energy-consuming organ by which about 120g of glucose is used per day, approximately 20% of body's total consumption. About 4.5g of glucose is in an adult blood. It merely keeps the brain working for an hour. Before that happens, the brain could become sluggish or even shut down (fainting or death).
Muscle is a very "selfish" tissue—glucose can enter, but it can’t leave. Self-sufficiency" ensures that muscle has an immediate and reliable source of fuel during exercise, but it also means that once glycogen is stored in muscle, it can’t be shared with other tissues and organs. Compared to liver glycogen, its smaller size facilitates the rapid breakdown of muscle glycogen. In some intense activities, they need to be mobilized within one minute as quickly as possible. Sometimes anaerobic glycolysis is also necessary, but this lasts merely for a very short time due to extremely inefficient energy utilization. Another function is to regulate blood sugar. An adult has about 300-500 grams of muscle glycogen, but an well-trained elite athlete has approximately twice, far more than the liver. Therefore, people who exercise regularly and have well-developed muscles rarely have diabetes.