Polysaccharides

Some examples of commonly occurring polysaccharides:

• Starch
  
  
  Starch is a polysaccharide (polymer of D-glucose units) synthesized by plants. Some common food starches are derived from the high-starch content potato tuber and cereal grains (e.g., corn, wheat). A 3-glucose polymer unit is shown in the image above merely as an example of the polymeric nature of starch. Starch is highly variable in molecular weight (a few thousand to 500,000) due to the variable number of glucose "subunits". There are two primary forms of naturally occurring starch:

• Amylose which is the "straight chain" form containing exclusively a-1-4 glycosidic [shown above] linkages between glucose sub-units, and,

• Amylopectin which is the "branched chain" form containing both a-1-4 glycosidic linkages (within straight chain portions) and a-1-6 glycosidic linkages (at branch points; see image below).

  

Starch can generally be sequentially digested in the gastrointestinal tracts of animals by certain carbohydrases (a general term for enzymes which digest some type of carbohydrate; e.g. amylase [attacks a-1-4 linkages], glucanohydrolase [attacks a-1-6 linkages]) and absorbed as individual D-glucose units. The absorbed glucose can be utilized as an "immediate" energy source or as a storage energy source (glycogen or fat).

• Glycogen

  Glycogen is a polysaccharide of D-glucose units linked by both a-1-4 and a-1-6 glycosidic linkages. Thus, glycogen is similar to amylopectin as it is highly branched. However, the molecular weight of glycogen, while highly variable, can be as high as 100,000,000. Glycogen serves as an intermediate storage form of energy, primarily in liver (up to 10% of wet weight) and skeletal muscle (1-2% of wet weight).

• Cellulose

  

Cellulose is a polysaccharide of D-glucose units linked by b-1-4 glycosidic linkages. Cellulose is a common "structural" polysaccaride which provides rigidity and strength to the cell walls of plants. Cellulose "fiber" from wood or other plant sources is used in the manufacture of paper. Cotton fiber is nearly pure cellulose.

Animals do not produce the digestive enzyme (cellulase) necessary to digest cellulose to derive utilizable energy. Thus, herbivorous animals (including termites) generally have specialized regions of their gastrointestinal tracts (e.g., the rumenoreticulum in cows, the cecum and colon in horses) which provide a suitable environment ("fermentation vat") for specialized bacteria which produce cellulase to digest the cellulose. Once released by the action of cellulase, glucose is then fermented by the bacteria, thus providing energy for the bacteria, but also producing fermentation end-products (organic acids: acetic, propionic, butyric, lactic and others) which can be absorbed and utilized by the host animal as energy sources (propionic acid and lactic acid can, through gluconeogenesis by the liver, supply glucose for those tissues specifically requiring glucose as an energy source or substrate for other products [e.g., the lactose produced by the mammary gland and secreted in milk]).

This page updated by GRF.