Main Difference – Starch vs Cellulose vs Glycogen
Starch, cellulose, and glycogen are three types of polymeric carbohydrates found in living cells. Autotrophs produce glucose as the simple sugar during photosynthesis. All these carbohydrate polymers, starch, cellulose, and glycogen, are made up of joining glucose monomer units together by different types of glycosidic bonds. They serve as chemical energy sources as well as the structural components of the cell. The main difference between starch, cellulose and glycogen is that starch is the main storage carbohydrate source in plants whereas cellulose is the main structural component of the cell wall of plants and glycogen is the main storage carbohydrate energy source of fungi and animals.
This article explores,
1. What is Starch – Structure, Properties, Source, Function 2. What is Cellulose – Structure, Properties, Source, Function 3. What is Glycogen – Structure, Properties, Source, Function 4. What is the difference between Starch Cellulose and Glycogen
What is Starch
Starch is the polysaccharide synthesized by green plants as their main energy store. Glucose is produced by photosynthetic organisms as a simple organic compound. It is converted into insoluble substances like oils, fats, and starch for storage. Insoluble storage substances like starch do not affect the water potential inside the cell. They may not move away from the storage areas. In plants, glucose and starch are converted into structural components like cellulose. They are also converted into proteins which are required for the growth and repair of the cellular structures.
Plants store glucose in staple foods like fruits, tubers like potatoes, seeds like rice, wheat, corn, and cassava. Starch occurs in granules called amyloplasts, arranged into semi-crystalline structures. Starch is composed of two types of polymers: amylose and amylopectin. Amylose is a linear and helical chain but amylopectin is a branched chain. Around 25% of starch in plants are amylose while the rest is amylopectin. Glucose 1-phosphate is first converted into ADP-glucose. Then ADP-glucose is polymerized via 1,4-alpha glycosidic bond by the enzyme, starch synthase. This polymerization forms the linear polymer, amylose. The 1,6-alpha glycosidic bonds are introduced to the chain by starch branching enzyme that produces amylopectin. Starch granules of rice are shown in figure 1.
Figure 1: Starch granules in rice
What is Cellulose
Cellulose is the polysaccharide which is made up of hundred to many thousands of glucose units. It is the major component of the cell wall of plants. Many algae and oomycetes also use cellulose to form their cell wall. Cellulose is a straight chain polymer in which 1,4-beta glycosidic bonds are formed between glucose molecules. Hydrogen bonds are formed between multiple hydroxyl groups of one chain with neighboring chains. This allows the two chains to be held together firmly. Likewise, several cellulose chains are involved in the formation of cellulose fibers. A cellulose fiber, which is made up of three cellulose chains, is shown in figure 2. Hydrogen bonds between cellulose chains are shown in cyan color lines.
Figure 2: A cellulose fiber
What is Glycogen
Glycogen is the storage polysaccharide of animals and fungi. It is the analogue to starch in animals. Glycogen is structurally similar to amylopectin but highly branched than the latter. Linear chain forms via 1,4-alpha glycosidic bonds and branches occur via 1,6-alpha glycosidic bonds. Branching occurs in every 8 to 12 glucose molecules in the chain. Its granules occur in the cytosol of cells. Liver cells, as well as the muscle cells, store glycogen in humans. Once needed, glycogen is broken down into glucose by glycogen phosphorylase. The process is called glycogenolysis. Glucogon is the hormone which stimulates glycogenolysis. 1,4-alpha glycosidic and 1,6-alpha glycosidic linkages of glycogen are shown in figure 3.
Figure 3: Bonds in glycogen
Difference Between Starch Cellulose and Glycogen
Starch: Starch is the main storage carbohydrate source in plants.
Cellulose: Cellulose is the main structural component of the cell wall of plants.
Glycogen: Glycogen is the main storage carbohydrate energy source of fungi and animals.
Starch: The monomer of starch is alpha glucose.
Cellulose: The monomer of cellulose is beta glucose.
Glycogen: The monomer of glycogen is alpha glucose.
Bond Between Monomers
Starch: The 1,4 glycosidic bonds in amylose and 1,4 and 1,6 glycosidic bond in amylopectin occur between monomers of starch.
Cellulose: 1,4 glycosidic bonds occur between the monomers of cellulose.
Glycogen: 1,4 and 1,6 glycosidic bonds occur between the monomers of glycogen.
Nature of the Chain
Starch: Amylose is an unbranched, coiled chain and amylopectin is a long branched chain, of which some are coiled.
Cellulose: Cellulose is a straight, long, unbranched chain, which forms H-bonds with adjacent chains.
Glycogen: Glycogen is a short, many branched chains of which some chains are coiled.
Starch: The molecular formula of starch is (C6H10O5)n
Cellulose: The molecular formula of cellulose is (C6H10O5)n.
Glycogen: The molecular formula of glycogen is C24H42O21.
Starch: Molar mass of starch is variable.
Cellulose: Molar mass of cellulose is 162.1406 g/mol.
Glycogen: Molar mass of glycogen is 666.5777 g/mol.
Starch: Starch can be found in plants.
Cellulose: Cellulose is found in plants.
Glycogen: Glycogen is found in animals and fungi.
Starch: Starch serves as a carbohydrate energy store.
Cellulose: Cellulose is involved in the building of cellular structures like cell walls.
Glycogen: Glycogen serves as a carbohydrate energy store.
Starch: Starch occurs in grains.
Cellulose: Cellulose occurs in fibers.
Glycogen: Glycogen occurs in small granules.
Starch, cellulose, and glycogen are polysaccharides found in organisms. Starch is found in plants as their major storage form of carbohydrates. Linear chains of starch are called amylose and when branched they are called amylopectin. Glycogen is similar to amylopectin but is highly branched. It is the major carbohydrate storage form in animals and fungi. Cellulose is a linear polysaccharide, which forms hydrogen bonds among several cellulose chains to form a fibrous structure. It is the major component of the cell wall of plants, some algae, and fungi. Thus, the main difference between starch cellulose and glycogen is their role in each organism.
Reference:1. Berg, Jeremy M. “Complex Carbohydrates Are Formed by Linkage of Monosaccharides.” Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 17 May 2017. .
Image Courtesy:1. “Rice starch – microscopy” By MKD – Own work (CC BY-SA 3.0) via Commons Wikimedia2. “Cellulose spacefilling model”By CeresVesta (talk) (Uploads) – Own work (Public Domain) via Commons Wikimedia 3. “Glycogen” (Public Domain) via Commons Wikimedia