Polysaccharides – types, representatives and main important properties. Heteropolysaccharides.
Polysaccharides are biopolymers that contain many monosaccharide units.
The starches, glycogen, and cellulose are built by one type of building block – D-glucose unit. Such Polysaccharides consisting of one type of monosaccharide are collectively homopolysaccharides.
I. Cellulose- (C6H10O5)n, is a polysaccharide consisting of a linear chain of several hundred to over ten thousand (1,4) linked D-glucose units. Cellulose has no taste (anyway humans cannot use it as food); is odorless; hydrophilic insoluble in water and most organic solvents; is chiral and is biodegradable.
II. Starches: amylose, amylopectin. Starches is a group name for polysaccharides produced by all green plants as an energy store. Starches are the most important carbohydrates in the human diet consumed with potatoes, wheat, maize (corn), rice, etc.
Pure starch is a white, tasteless, and odorless powder that is insoluble in cold water or alcohol. Starch consists of two types of molecules: the water-soluble linear and helical
amylose and the water-insoluble branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin.
Amylose is a linear polymer of glucose linked by (1,4) glycosidic bonds. The polymeric chain can be made of several thousand D-glucose units (n=300-3000).
Amylopectin. is the water-insoluble fraction of starches. Like amylose, amylopectin is a polysaccharide of (1,4) linked D-glucose units. Instead of being a continuous length of (1,4) units, amylopectin is branched
III. Glycogen. The amylopectin counterpart in animals is glycogen which has the same composition and structure, but with more extensive branching via (1,6) glycosidic bonds that occurs every 8 to 12 glucoses units. The main polysaccharide chain contains many D-glucose units joined by (1,4) glycosidic bonds.
Glycogen is a very large polysaccharide. Nature utilizes mainly two features of such large structure:
- The large size (Mw>1x107) of glycogen prevents it from leaving the storage cell because the molecule has properties of a colloid;
- The storage of tens of thousands of glucose molecules into one molecule greatly relieves the osmotic problem for the storage cell. Such pressure would be caused by the attempted storage of many individual glucose molecules.
The highly branched nature of glycogen allows hydrolytic enzymes to have many -chains ends are available from which glucose molecules can be hydrolyzed.
- Glucose is the source of “ready energy” for the body;
- Long chain fatty acids of triacylglycerols are used for long term energy storage.