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Glycogen is a glucose polymer resembling the starch in plants (so it is sometimes called "animal starch"). Glycogen is a highly branched glucose polymer. It is formed of small chains of 8 to 12 glucose molecules linked together with α (1→4) bonds. These small chains are in turn linked together with α (1→6) bonds. A single molecule of glycogen can be made of up to 120,000 molecules of glucose. It is stored in the form of granules in the cytosol.
These granules contain both glycogen and the necessary enzymes for its conversion into glucose. It is generated from glucose by the enzyme glycogen synthase. This process is called glycogenesis . The addition of a glucose molecule to glycogen takes two high energy bonds: one from ATP and one from UTPUridine 5'-triphosphate is uridine esterified with triphosphoric acid at its 5'-position..
Its breakdown into glucose, called glycogenolysisGlycogenolysis is the catabolism of glycogen (requiring removal of glucose unit from glycogen and addition of phosphate) thus producing glucose-1-phosphate. Glycogenolysis requires three enzymes : Phosphorylase Glucan Transferase Debranching enzyme., is mediated by the enzyme glycogen phosphorylase . Its highly branched nature allows for the quick retrieval of glucose molecules when needed.
As a carbohydrateCarbohydrates (literally hydrates of carbon are chemical compounds that act as the primary biological means of storing or consuming energy, other forms being fat and protein. Relatively complex carbohydrates are known as polysaccharides. Carbohydrates are meal is eaten and digested, blood glucose levels rise, and the pancreasThe pancreas is a retroperitoneal organ that serves two functions: exocrine it produces pancreatic juice containing digestive enzymes endocrine it produces several important hormones Anatomy The pancreas is a retroperitoneal organ located posterior to the secretes insulincarbon; green: oxygen; blue: nitrogen; pink: sulphur. The blue/purple ribbons denote the skeleton [-N-C-C-]n in the protein's amino acid sequence H-[-NH-CHR-CO-]n-OH where R is the part protruding from the skeleton in each amino acid. Insulin ( Latin insu. Glucose from the portal veinThe portal vein is a major vein in the human body draining blood from the digestive system and its associated glands. It is formed by the union of the splenic and superior mesenteric veins. It divides into a right and left branches, before entering the li enters the liver cells ( hepatocyteHepatocytes are epithelial cells that make up 60-80% of the cytoplasmic mass of the liver. These cells are are essential for life, and are involved in protein synthesis, protein storage and transformation of carbohydrates, synthesis of cholesterol, bile ss). Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase . Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. In this post-prandial or "fed" state, the liver takes in more glucose from the blood than it releases.
After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. About 4 hours after a meal, glycogen begins to be broken down to be converted again to glucose. Glycogen phosphorylase is the primary enzyme of glycogen breakdown. For the next 8-12 hours, glucose derived from liver glycogen will be the primary source of blood glucose to be used by the rest of the body for fuel.
Glucagon is another hormone produced by the pancreas and in many respects serves as a counter-signal to insulin. When the blood sugar begins to fall below normal, glucagon is secreted in increasing amounts. It stimulates glycogen breakdown into glucose even when insulin levels are abnormally high.