Introduction to chemistry of carbohydrates
Carbohydrates are the most abundant macromolecules in nature. They are the main source and storage of energy in the body. They serve also as structural component of cell membrane. The general molecular formula of carbohydrate is CnH2nOn or (CH2O)n, where n > 3.
Chemically, they contain the elements like carbon, hydrogen and oxygen. Thus, they are Carbon compounds that contain large quantities of Hydroxyl groups (-OH).
Carbohydrates in general are polyhydroxy aldehydes or ketones or compounds which give these substances on hydrolysis.
1. Chemistry of Carbohydrates
1.4. Homopolysaccharides
Homopolysaccharides
Example of Homopolysaccharides: Starch, glycogen, Cellulose and dextrins.
Starch
It is one of the most important storage polysaccharides in plant cells. It is especially abundant in tubers, such as potatoes and in seeds such as cereals.
Starch consists of two polymeric units made of glucose called Amylose and Amylopectin but they differ in molecular architecture.
Amylose is unbranched with 250 to 300 D-Glucose units linked by α-(1, 4) linkages Amylopectin consists of long branched glucose residue (units) with higher molecular weight.
The inner part of glucose units in amylopectin are joined by α-(1,4) glycosidic linkage as in amylose, but the branch points of amylopectin are α- (1,6) linkages. The branch points repeat about every 20 to 30 (1-4) linkages.
Glycogen
- Glycogen is the main storage polysaccharide of animal cells (Animal starch).
- It is present in liver and in skeletal muscle.
- Like amylopectin glycogen is a branched polysaccharide of D-glucose units in α - (1, 4) linkages, but it is highly branched.
- The branches are formed by α - (1,6) glycosidic linkage that occurs after every 8 -12 residues. Therefore, liver cell can store glycogen within a small space. Multiple terminals of branch points release many glucose units in short time.
Cellulose
Cellulose is the most abundant structural polysaccharide in plants. It is fibrous, tough, water insoluble. Cellulose is a linear unbranched homopolysaccharide of 10,000 or more D- glucose units connected by β-(1, 4) glycosidic bonds. Humans cannot use cellulose because they lack of enzyme (cellulase) to hydrolyze the β-( 1-4) linkages.
Dextrins
These are highly branched homopolymers of glucose units with α-(1, 6), α-(1, 4) and α-(1, 3) linkages. Since they do not easily go out of vascular compartment they are used for intravenous infusion as plasma volume expander in the treatment of hypovolemic shock.