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Glycogen: Meaning, Biological Significance, Structure, Glycogen and Starch Differences


What is Glycogen?

Glycogen is a polysaccharide made up of smaller units of glucose. This multibranched polymer is a form of energy reserve found in animals, bacteria and fungi. It is analogous to starch, the one found in plants. Glycogen and triglycerides are the two energy stores found in animals.

Glycogen contributes to about 5-6% of the liver’s fresh weight; in skeletal muscle, it is present in low concentrations of about 1-2%. Besides this, glycogen is also found in minute quantities in red blood cells, white blood cells, glial cells, and kidneys. In fact, the uterus also has some amount of stored glycogen with it, contributing to the embryo's growth.

In the case of starvation, glycogen stored in the liver and the skeletal muscles undergo glycogenolysis to form glucose for utilization. This helps in maintaining the blood glucose level by replenishing the missing glucose in the body.

Structure of Glycogen

Glycogen is found in the cytoplasm of the cells as dark granules of 10-40nm. It is present in the hydrated form in the liver and skeletal muscles.

Biological Significance of Glycogen

Glycogen plays a variety of roles in the biological system. Some of them are as follows:

1. In liver cells

  • Glycogen is primarily synthesized in the liver when the excess of glucose consumed is left unused; the glucose molecules then undergo a process of glycogenesis to produce glycogen in the body with the help of the enzyme glycogen synthase. These glycogen molecules formed are stored in the liver.
  • When the blood glucose level goes down beyond the threshold limit, the Islets of Langerhans of the pancreas secrete the hormone glucagon. This glucagon stimulates the enzymes needed to perform glycogenolysis, which is basically the breakdown of glycogen.
  • This glucose obtained from glycogen, is released into the bloodstream, and the blood glucose level is revived.
  • Apart from glucagon, hormones like cortisol, epinephrine and norepinephrine also stimulate the breakdown of glycogen.

2. In skeletal muscles

  • Muscle cells lack the enzyme needed for a complete breakdown of glycogen to glucose. So, the end-product formed by glycogenolysis in muscle is glucose- 1- phosphate, which is converted to glucose-6-phosphate for energy.
  • Glycogen in skeletal muscles provides energy only for the functioning of the muscles and not the entire body.

3. In uterus

  • Glycogen reserves in the uterus ensure the proper growth of the embryo, as glycogen is a good source of nutrition.
  • Glycogen starts to accumulate in the fetal cells about 26 weeks into gestation and helps in the synthesis of pulmonary surfactants.

4. Micro-organisms

  • In bacteria and fungi, glycogen serves as the main source of energy. For example, micro-organisms like yeast produce glycogen in response to an excess of carbon available.
  • Glycogen starts to accumulate in the fetal cells about 26 weeks into gestation and helps in the synthesis of pulmonary surfactants.The growth of the bacteria, sporulation of yeast are all found to be interlinked with glycogen storage.

Glycogen v/s Starch

Before differentiating between the two polysaccharides, it is important to note their similarities. Both glycogen and starch are white powders in their dried form. They are energy stores and are both composed of the monosaccharide glucose. In addition, glycogen and starch are both sparingly soluble in water at room temperature.

Glycogen Starch
Glycogen is the storage form of carbohydrates found in animals, bacteria and fungi. Starch is the storage form of carbohydrates found in plants
Glycogen is composed of a single molecule. Starch is a mixture of amylose and amylopectin
Glycogen has a multi-branched structure. Starch can form linear, branched as well as coiled structures.
Glycogen is found in the cells in the form of dark granules Starch is found in the cell as grains
The site of storage for glycogen is the liver and skeletal muscles The site of storage for starch is amyloplast.
Glycogen has no commercial value, as it is not used at the industrial level. Starch is used in the paper and textile industry
The molar mass of the glycogen molecule is 666.577 g/mol The molar mass of starch is variable and hence not determined.





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