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Polysaccharides are chains of sugar units that form in configurations from tens to thousands of units long. They comprise the carbohydrate storage units for plants and animals, and they can also have structural roles in plants, fungi, insects, and crustaceans. Plants make starches as their storage units, while animals store glycogen. Starches and glycogen are homopolysaccharides, meaning that they have the same type of sugar throughout their chain.
The polymeric carbohydrate structure for glycogen and starch is made of chains of glucose linked by glycosidic bonds. These are bonds in which the adjoining –OH groups of glucose join together, forming water as a byproduct. These bonds are easily broken, making starch and glycogen ready sources of energy in the form of glucose.
Glycogen has thousands of units of glucose with many branches in its structure. There are two forms of starch, amylose and amylopectin. Amylose is an unbranched chain of hundreds of glucose units, while amylopectin is a chain of thousands of units that is branched. Most starch is comprised of amylopectin. Glycogen is very similar in structure to amylopectin, but differs by having more branches.
The advantages of having glucose stored in these polysaccharides is that individual molecules absorb water and take up a lot of space. These storage polymers are compact and do not absorb water. They are efficient ways are storing large amounts of glucose that can be readily metabolized, and do not affect the cell’s chemistry.
Starch is found in many plants, such as potatoes and cereal grains, and it is the major carbohydrate that humans consume. Both plants and humans produce the enzyme amylase to degrade starch. When starch is broken down to glucose, it causes blood glucose levels to increase. This leads to the secretion of insulin from the pancreas. Glycogen is synthesized after eating, when blood glucose levels increase.
Glycogen is stored primarily in the liver and muscle tissue, with a small amount stored in the brain. It acts as a fuel reserve. When blood sugars become low, the pancreas releases the hormone glucagon, which travels to the liver and triggers the conversion of glycogen to glucose. This process can also be activated by adrenaline as a stress response. The glucose is released into the blood, circulating to other tissues, such as the brain.
In the muscles, the stored glycogen is used internally and is not transferred to other cells. It serves as an immediate source of energy for the muscle cells. One advantage of obtaining energy from glycogen, rather than from fatty acids, is that the process can proceed in the absence of oxygen.
In addition to these vitally important storage polysaccharides, there are other structures that give rigidity to cells. They are also comprised of glucose, but in different types of linkages that are much more difficult to degrade. Cellulose helps gives plant walls their strength and contains beta-glucan units. Chitin is a component of fungal cell walls, and forms the outer skeletons of insects and crustaceans.
Another polysaccharide of importance is exopolysaccharide, which forms the mucilage around some types of bacteria. It can help the organisms invade cells. This is important both for the ability of certain types of bacteria to cause infections and to form nodules with some types of plants to fix nitrogen.