Glucose and glycogen are both carbohydrates, but glucose is classified as a monosaccharide and sugar. As a single unit, it is a much smaller molecule. According to Virtual Chembook at Elmhurst College, glycogen is classified as a complex carbohydrate and starch, and it's made up of several glucose molecules.
Glucose can be rapidly metabolized to produce energy. It dissolves readily in water and can be readily transported throughout your body. It can be carried in your bloodstream as well as in the sap of plants. Glucose serves as a primary energy source for plants as well as animals. Joining different numbers of glucose units forms different types of carbohydrates, according to the Department of Chemistry at Imperial College in the U.K.
Disaccharides like sucrose and lactose consist of two linked glucose units, while polysaccarides consist of many more. In animals, glycogen is a large storage molecule for extra glucose, just as starch is the storage form in plants. Your liver and muscles synthesize glycogen and act as your main storehouses. Your stores can be broken down again to glucose for energy if necessary, and they can also provide structural support in various tissues in your body.
Glycogen as Storage
One glycogen molecule can consist of long chains of 1,700 to 600,000 glucose units. About 0.5 percent of the weight of your muscles and 5 percent of the weight of your liver are made up of glycogen. Unlike glucose, glycogen is not soluble in water and cannot pass in and out of cells unless it is broken down into smaller, more soluble units. Its usefulness as a storage molecule is largely due to this insolubility.
Your body normally regulates blood glucose levels very closely. After you eat carbohydrates, your body breaks them down to glucose, which then enters your bloodstream. What your cells do not require for energy is taken out of circulation and stored as glycogen by your liver, because the continued presence of circulating glucose can upset the balance of fluid between your cells and your blood.