The polysaccharides are the most abundant carbohydrates in nature and serve a variety of functions, such as energy storage or as components of plant cell walls. Polysaccharides are very large polymers (long chains of molecules) composed of tens to thousands of monosaccharides joined together by glycosidic linkages. The three most abundant polysaccharides are starch, glycogen, and cellulose. These three are referred to as homopolymers because each yields only one type of monosaccharide (glucose) after complete hydrolysis. Heteropolymers may contain sugar acids, amino sugars, or noncarbohydrate substances in addition to monosaccharides. Heteropolymers are common in nature (gums, pectins, and other substances) but will not be discussed in detail in this textbook. The polysaccharides are not sweet tasting, and do not undergo mutarotation.



Starch is a mixture of two polymers: amylose and amylopectin. Natural starches consist of about 10%–30% amylase and 70%–90% amylopectin. Amylose is a linear polysaccharide composed entirely of D-glucose units joined by the α-1,4-glycosidic linkages we saw in maltose (see part (a) in the figure below). Experimental evidence indicates that amylose is not a straight chain of glucose units but instead is coiled like a spring, with six glucose monomers per turn (see part (b) in the figure below). When coiled in this fashion, amylose has just enough room in its core to accommodate an iodine molecule. The characteristic blue-violet color that appears when starch is treated with iodine is due to the formation of the amylose-iodine complex. This color test is sensitive enough to detect even minute amounts of starch in solution.

(a) A Haworth projection of amylose. Amylose is a linear chain of α-D-glucose units joined together by α-1,4-glycosidic bonds. (b) A spiral or coiled structure representing the 3-dimensional structure of amylose.


The structure of amylopectin is shown, which contains amylose chains with branches connected by alpha-1,6-linkages



starch dextrins maltose glucose








Like amylose, cellulose is a linear polymer of glucose. It differs, however, in that the glucose units are joined by β-1,4-glycosidic linkages, producing a more extended structure than amylose (part (a) of the figure below). This extreme linearity allows a great deal of hydrogen bonding between OH groups on adjacent chains, causing them to pack closely into fibers (part (b) of the figure below). As a result, cellulose exhibits little interaction with water or any other solvent. Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. Because cellulose does not have a helical structure, it does not bind to iodine to form a colored product.

(a) Structure of cellulose, which consists of D-glucose units connected by a beta-1,4-glycosidic linkage. Extensive hydrogen bonding keeps the glucose units in a rigid linear structure. (b) A electron micrograph of the cell wall of an alga shows the individual layers of cellulose fibers in parallel arrangement.

Cellulose yields D-glucose after complete acid hydrolysis, yet humans are unable to metabolize cellulose as a source of glucose. Our digestive juices lack enzymes that can hydrolyze the β-glycosidic linkages found in cellulose, so although we can eat potatoes, we cannot eat grass. The undigested cellulose in fibrous foods is passed through our digestive system as dietary fiber.


Concept Review Exercises

  1. What purposes do starch and cellulose serve in plants?
  2. What purpose does glycogen serve in animals?
  3. What monosaccharide is obtained from the hydrolysis of each carbohydrate?
    1. starch
    2. cellulose
    3. glycogen
  4. Indicate whether each polysaccharide is found in plants or mammals.
    1. starch
    2. cellulose
    3. glycogen
  5. Describe the similarities and differences between amylose and cellulose.
  6. Describe the similarities and differences between amylopectin and glycogen.


    1. glucose
    2. glucose
    3. glucose
    1. plants
    2. plants
    3. animals
  5. Amylose and cellulose are both linear polysaccharides of glucose units, but the glycosidic linkages between the glucose units differ. The linkages in amylose are α-1,4-glycosidic linkages, while the linkages in cellulose they are β-1,4-glycosidic linkages.

  6. Amylopectin and glycogen are both branched polysaccharides of glucose units, but glycogen is more branched than amylopectin. Both polysaccharides consist of linear chains of glucose linked by α-1,4-glycosidic linkages. The branches occur more often in glycogen and are connected to the linear chain by α-1,6-glycosidic linkages.


This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhDOpenstax which is licensed under CC BY 4.0. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction

This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J ScottSaylor which is licensed under CC BY-NC-SA 4.0. Access for free at http://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/index.html