3.5: Carbohydrates - Polysaccharides
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- Ying Liu
- City College of San Francisco
Learning Objectives
- Describe the structures of starch, glycogen and cellulose
- Describe the functions of these polysaccharides
Polysaccharides
Polysaccharides , also called glycans, are large polymers composed of hundreds of monosaccharide monomers. Unlike mono- and disaccharides, polysaccharides are not sweet and, in general, they are not soluble in water. Like disaccharides, the monomeric units of polysaccharides are linked together by glycosidic bonds.
Polysaccharides are very diverse in their structure. Three of the most biologically important polysaccharides — starch , glycogen , and cellulose — are all composed of repetitive glucose units, although they differ in their structure (Figure \(\PageIndex{1}\)). Cellulose consists of a linear chain of glucose molecules and is a common structural component of cell walls in plants and other organisms. Glycogen and starch are branched polymers; glycogen is the primary energy-storage molecule in animals and bacteria, whereas plants primarily store energy in starch. The orientation of the glycosidic linkages in these three polymers is different as well and, as a consequence, linear and branched macromolecules have different properties.
Modified glucose molecules can be fundamental components of other structural polysaccharides. Examples of these types of structural polysaccharides are N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) found in bacterial cell wall peptidoglycan. Polymers of NAG form chitin, which is found in fungal cell walls and in the exoskeleton of insects.
Exercise \(\PageIndex{2}\)
What are the most biologically important polysaccharides and why are they important?
Query \(\PageIndex{1}\)
Key Concepts and Summary
- Polysaccharides , or glycans , are polymers composed of hundreds of monosaccharide monomers linked together by glycosidic bonds. The energy-storage polymers starch and glycogen are examples of polysaccharides and are all composed of branched chains of glucose molecules.
- The polysaccharide cellulose is a common structural component of the cell walls of organisms. Other structural polysaccharides, such as N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM), incorporate modified glucose molecules and are used in the construction of peptidoglycan or chitin.