Bio Chemistry

BIOCHEMISTRY 3(2+1)

Lesson 7 : Carbohydrates - Classification- Structure, Function

Oligosaccharides and polysaccharides

Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide raffinose and the tetrasaccharide stachyose

Divided in two classes:

1. homoglycans - homopolysaccharides composed on one monosaccharide
2. heteroglycans- heteropolysaccharides made of more than one type of monosaccharide

Often classified according to their biological role:

1. Starch and glycogen - storage polysaccharides

• Both are homoglycans.
• Starch is storage form in plants and fungi.
• Glycogen is storage form in animals.
• Bacteria contain both.

Starch - mixture of amylose and amylopectin

Amylose is an unbranched polymer of 100-1000 D-glucose in an a-(1 --> 4) glycosidic linkage.


Amylopectin is a branched polymer a-(1--> 6) branches of residues in an a-(1 --> 4) linkage; overall between 300-6000 glucose residues, with branches once every 25 residues; side chains are 15-25 residues long

?-amylase is an endoglycosidase found in human saliva and also in plants, that randomly hydrolyzes the a (1--> 4) bond of amylose and amylopectin.

?-amylase is an exoglycosidase found in higher plants, that hydrolyzes maltose residues from non-reducing ends of amylopectin.

Glycogen - branched polymer of glucose residues with branches every 8-12 residues and containing as many as 50,000 glucose residues


2. Cellulose and chitin - structural polysaccharides

cellulose - straight chain homoglycan of glucose with ?-(1--> 4) linkages in alternating glucose molecules; ranges in size from 300-15,000 glucose residues. Extensive H- bonding exists within and between cellulose chains, making bundles or fibrils rigid.

Chitin - linear polymer of N-acetylglucosamine residues. Alternating 180o with ? - (1 --> 4) linkage with lots of H-bonding between adjacent strands.

Foods high in carbohydrates include breads, pastas, beans, potatoes, bran, rice, and cereals. Most such foods are high in starch. Carbohydrates require less water to digest than proteins or fats and are the most common source of energy in living things. Proteins and fats are necessary building components for body tissue and cells, and are also a source of energy for most organisms.

Carbohydrates are not essential nutrients in humans: the body can obtain all its energy from protein and fats. However, the brain and neurons generally cannot burn fat and need glucose for energy; the body can make some glucose from a few of the amino acids in protein and also from the glycerol backbone in triglycerides. Carbohydrate contains 3.75 and proteins 4 calories per gram, respectively, while fats contain 9 calories per gram. In the case of protein, this is somewhat misleading as only some amino acids are usable for fuel. Likewise, in humans, only some carbohydrates are usable for fuel, as in many monosaccharides and some disaccharides. Other carbohydrate types can be used, but only with the assistance of gut bacteria. Ruminants can even process cellulose, which is indigestible to other organisms.

Based on the effects on risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).