Classification of carbohydrates

The word ‘carbohydrate’ was coined more than 100 years ago to disrobe a large group of compounds of the general formula Cn(H2O)n. Sugars, starches and fiber are the main forms in which carbohydrates occurs in food.

Carbohydrates can be classified based on their chemical structure and/or based on their physiological effects.  The classification of carbohydrate is a complicated subject due to in part to the numerous classification schemes or systems used in the past.

There are three main groups, monosaccharide, or simple sugars; oligosaccharides, of which the most prevalent in nature are the disaccharides; and polysaccharides the most complex of the carbohydrates.

All carbohydrates must be reduced to simple sugars (monosaccharides in the intestine before they can be absorbed into the bloodstream).

The most basic or simple form of carbohydrate is the monosaccharide. Typical monosaccharide are of the empirical formula (CH2O)n where n is a value between 3 and 9. The basic or archetypal monosaccharide is the six-carbon simple carbohydrate glucose. Glucose, a simple sugar, is the form in which carbohydrates circulate in the bloodstream.

Polysaccharides are large polymers of monosaccharides. Polysaccharides may consist of straight chains or branched chains having hundreds or ten thousands of monosaccharides joined together through glycosidic bonds.
Classification of carbohydrates

Glycogen in human nutrition

During and immediately after a meal, glucose is converted in the liver into the storage polysaccharide glycogen by a process known as glycogenesis.

Although the total quantity of glycogen in the human body is low, considerably less than one-tenth percent of the total body weight, its role is primarily that of a storage carbohydrate, similar to the role of starch in plants cells. It occurs predominantly in the liver where it is important in the homeostatic mechanism regulating glucose level of the blood.

Glycogen is a branched chain polymer of 6,000 to 30,000 glucose units, that contains two types of glycosidic linkages, extended chains of alpha1-- 4 linked glucose residues with alpha 1-- 6 branches spaced about every four to six residues along the alpha 1-- 4 chain. It is similar to amylopectin in structure but is more highly branched. The average chain length is only 10 to 24 glucose units with 3 to 4 glucose units between branching points.

The highly branched structure of glycogen makes it possible for several glucose residues to be released at once to meet energy needs. 

Glycogen is stored in two tissues. In the liver, glycogen is stored for the short-termed maintenance of blood glucose. In muscle, glycogen is stored as a source of energy. Muscle glycogen is estimated to have a molecular weight of about 1000000 where as the liver of glycogen molecule is much larger, approximately 5 X 1000000. Both molecules, however, constantly change in size as glucose molecules are added or removed.

Glycogen plays an important role in the glucose cycle. The release of glycogen stored in the liver is triggered by low levels of glucose in blood. Liver glycogen breaks down to glucose-6-phosphate, which is hydrolyzed to give glucose.

The release of glucose from the liver by these breakdowns of glycogen replenishes the supply of glucose in the blood.

The most common disease in which glycogen metabolism becomes abnormal is diabetes, in which, because of abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted.

Several hereditary defects have been identified in the synthesis and catabolism of glycogen including: Gierke’s disease, Type II glycogen disease, Type III glycogen, Type IV disease, and McArdle’s disease.
Glycogen in human nutrition

What does polysaccharide mean?

Polysaccharides are a class of biopolymers constituted with simple sugar monomers. It represents most of the structural and energy reserve carbohydrates found in nature.

Unlike protein, polysaccharides generally do not have definite molecular weight.

Many polysaccharides exist in the plant and animal kingdoms. However, only a few of these are known to be significant in mammalian nutrition, either as dietary constituents or as human cell metabolites.

The most common digestible polysaccharides in plant is starch, a polymer of glucose. It is composed of amylose and amylopectin. Starch is the energy compound stored predominantly in seeds and tubers. Glycogen is the animal counterpart of starch, but with shorter, more numerous branches.

The composition of starches also differs somewhat, but all types contain both amylase, a straight chain polymer of glucoses, and amylopectin, a branch chain polymer.

The average chain contains 20 to 25 glucose units with approximately 5 to 8 glucose molecules between branching points within the chain. On hydrolysis in the intestinal tract, starch yields dextrin and maltose and, eventually glucose.

Cellulose, the most abundant polysaccharide, is the structural component of plant tissues. It is the structural component of plant cell was, which consist of long chain of glucose residues.

Cellulose is not attacked by digestive enzymes of the human and although it provides bulk to the diet it does not contribute significantly to the nutrition of body cells.

It is fully permeable to water and solutes and therefore does not affect exchange of materials between the cells and the environment. Cellulose tends to be affected little by usual acid hydrolysis and requires the action of strong mineral acids.

Plants also contain indigestible hemicellulose, which are unrelated chemically to cellulose and are homopolysaccharides containing D-xylose. Homopolysaccharides are defined as polysaccharides formed from only one type of monosaccharide.

Pectin, present in fruit, is an indigestible heteropolymer and contains arabinose, galactose, and galacturonic acid. In general, heteropolysaccharides contain two or more different monosaccharides.
What does polysaccharide mean?