Classification of Carbohydrates

Carbohydrates or saccharine (Greek: Sacharon, sugar) means "carbon hydrate" which is (CH,O), wherein n>3. Carbohydrates arc classified according to the number of basic sugar or saccharide unit.

Monosaccharides

Monosaccharides are the simplest form of carbohydrate known as simple sugar. Monosaccharides are classified according to their functional group and number of carbon atoms.

Aldoses: If the functional group is aldehyde, as in glucose, it is referred as aldose.

Ketoses: If the functional group is ketose, as in fructose, it is referred as ketoses. Trioses are the smallest monosacchirides of three carbon atoms. Those with four, five, six, seven carbon atoms are tetroses, pentoses, hexoses, and heptoses respectively. Six-carbon aldose is an aldohexose and of ketose is ketohexose.

The three main monosaccharides important in human nutrition are glucose, fructose and galactose.

Glucose: Naturally preformed sweet sugar, glucose is found only in few foods such as corn syrup. Digestion of starch produces glucose. In body, all other types of sugars are converted to glucose. It is also known as "dextrose". (Normal blood sugar level is about 70 - 1 10 mg /lo0 ml.) Glucose is the ultimate energy fuel, which is oxidized in cell to give energy.

Structure of Glucose

Fructose: It is the sweetest simple sugar. Fructose is found in fruits as honey. Fructose is converted into glucose in our body to provide energy.

Structure of Fructose

Galactose: It is not found in free-form in food, but is produced from lactose (milk sugar). Galactose is also changed to glucose for energy, which is a reversible reaction. During milk production, glucose is converted to galactose. Galactosemia, a genetic disorder, is due to the absence/ deficiency of the enzyme required for the conversion of galactose to glucose and thus galactose accumulates.

Structure of Galactose

Oligosaccharides

Oligosaccharides are composed of 2 -10 monosaccharides linked together. Those with two monosaccharides unit are called disaccharides. The three main disaccharides of nhvsiological impoflance are sucrose, lactose and maltose.

Sucrose: Sugarcane beet is a rich source of sucrose. It is a common disaccharide. Sucrose can be found in all molasses, some fruits and vegetables such as pineapple, carrot etc.

Lactose: Lactose is also known as milk sugar. During lactation, it is formed in the body from glucose. It is 116" as sweet as sucrose. Cheese, a milk product, has very little or no lactose.

Structure of Lactose

Maltose: Commercial malt products of starch breakdown and germinating cereal grain's are the rich source of maltose. It is an important metabolic carbohydrate and an intermediate product of starch digestion.

Structure of Maltose

Polysaccharides

Polysaccharides are made up of many saccharides (simple sugar) units. The most important polysaccharide in human nutrition is starch. Other polysaccharides are glycogen and dextrin. The bulk of animal diet is composed of non-digestable forms of dietary fiber e.g., cellulose. Polysaccharides are of two types:

Homopolysaccharides and Heteropolysaccharides.

i) Homopolysaccharides

They yield only one type of monosaccharide (glucose) upon complete hydrolysis. Some important homoploysaccharides are as follows: 

Starch: It is made up of many glucose units attached in a branch chain manner. Actually, it is a mixture of two polymers, amylase (10-20 per cent) and arnylopectin (80-90 per cent). It yields only glucose on digestion. Starch is the most important source of dietary carbohydrate in the world. It is an important part/component of human nutrition Ad health. In many countries, starch is the staple food material forming the bulk of the diet. Cereal grains, legumes, potatoes and many vegetables are rich in starch.

Dextrin: Dextrin is formed as an intermediate product in the break down of starch.

Starch to Glucose 

Glycogen: The storage carbohydrate in animals is glycogen, also known & animal starch. In fact, it is a polymer of glucose similar to amylopectin but with high branching and its branches are smaller. It is formed during cell metabolism and stored in small amounts in liver and muscle tissues. During fasting period such as sleep hours, glycogen provides immediate energy fuel or glucose for muscle action. Dietary carbohydrates are needed to maintain glycogen stores. Low dietary carbohydrate intake causes symptoms like fatigue, dehydration and excessive protein breakdown.

Cellulose: Cellulose is the main component of the framework of plant cell wall. The repeating unit is a disaccharide cellobiose which is made up of two units of D glucose joined by P- l,4 glucosidic linkage, It provides most of the substances labeled "crude fiber". The main food sources are stem and leaves of vegetables, seed and grain coverings, skin and hulls. Humans cannot digest cellulose. Human lack the necessary digestive enzymes which can break P- l,4 glucosidic linkage. Therefore, cellulose remains undigested. Non-cellulose fibers absorb water and have slow gastric emptying time. They are gum like water-soluble substances that aid in binding cholesterol and controlling its absorption. They provide bulk for normal intestinal muscle action to prevent colon pressure.

Structure of Cellulose

ii) Heteropolysaccharides

These are the polysaccharides, which on complete hydrolysis yield more than one particular type of components (sugar acids, amino sugars as noncarbohydrates). These are also referred as non-cellular portion of plant. Some of the important heteropolysaccharides are:

Hemicellulose: The constituents of hemicellulose are xylose, arabinose, uronic acid, glucose and galactose. It is more digestible than cellulose.

Chitin: It forms the structural element in lower plants and in invertebrates. Its subunits are 2-acetarnido-2-deoxy-D-glucopyranose linked by P-1,4, glycosidic bonds.

Pectin: 1t is composed of galactouronic acid chain with arabinose, glucose and xylose. It is used in fruit conserving industry.

Gums: The complete hydrolysis of gums yields arabinose, glucose, rhamnose, glucuronic and galacturonic acids. In nature, it occurs in the form of copper and magnesium salt.

Mucilages: According to source, their constituents differ. Mainly composed of arabinose, galactose, rhamnose and galacturonic acid.

Classification of Carbohydrates