CARBOHYDRATES

CARBOHYDRATES

Carbohydrates are synthesized by all green plants using solar energy, water  from the soil and carbon dioxide from the air. This complex process is called photosynthesis, the prefix photo indicates the importance of sunlight in this process. Plants are thus the primary source of food in the world.

Carbohydrates contain carbon, hydrogen and oxygen. The suffix hydrate indicates that water and oxygen occur in the same proportion as in water.

The members of the simplest class of carbohydrates have a single unit--- monosaccharide (mono or one, saccharine or sugar containing). Glucose is an example of this class. The disaccharides contain two sugars linked together to form a chain. Cane or beet sugar (sucrose), milk sugar (lactose), and maltose (malt sugar) are members of this class. Carbohydrates made up, of long chains of sugars are called polysaccharides (poly—many). Among them are starch, dextrin, glycogen, cellulose, hemicelluloses, pectin, plants gums and mucilage’s, presents the types and sources of carbohydrates occurring in nature.

Simple carbohydrates include mono and disaccharides. These are small molecules, which dissolve in water and are absorbed very quickly in the body. In contrast, starches and dietary fibers are very large, complex molecules, containing several hundred small sugar units and are absorbed slowly. Therefore polysaccharides are referred to as complex carbohydrates. Individual members of each of these classes (starch, pectin’s, gums, mucilage, cellulose, hemicelluloses, lignin) differ in the type of small units they contain and in the way these units are  united in the molecule.

Glucose, the most common monosaccharide, is present in honey, fruits and corn syrup. After absorption of food it is transported through blood and hence is present in blood as an easily available source of energy.

Sucrose is the ordinary sugar available in the grocery store and used in beverages and food preparations as a sweetener. As you can note from the table, it is mainly manufactured from cane sugar in the tropics and sugar beets in the temperature regions. It is also present in molasses, honey, fruits and vegetables. Sucrose is hydrolyzed to a mixture of equal amounts of glucose and fructose by action on digestive enzymes or when boiled with acid (Such as citric acid from fruits). This mixture of glucose and fructose is called invert sugar and it is used in preparation of candies and icings.

Maltose does not occur in free form in nature. It is formed during sprouting of grains or in digestion of starch by action of enzymes. It contains two glucose molecules. Lactose is the sugar inmilk. It yields glucose and galactose on hydrolysis by lactose. It is less soluble than sucrose and maltose and less sweet than glucose. Persons, who have lactase insufficiency, cannot utilize lactose and hence have to restrict their intake of milk products. Children, who are born without the liver enzyme lactase, cannot digest milk and have to be fed soya milk instead. 

Table: Forms and Sources of Carbohydrates

Carbohydrates	Main food sources	Remarks
Polysaccharides
Glucose & hemi-cellulose	Stalks & leaves of vegetables	Indigestible
	Outer covering  of seeds	Indigestible
Pectin	Fruits	Indigestible
Gums & mucilage	Plant secretions & seed exudates	Indigestible
Starch & dextrin	Grains, legumes & tubers	Digestible
Glycogen	Meats & Sea-foods	Digestible
Disaccharides
Sucrose	Cane & beet sugar, molasses	Digestible
lactose	Milk & milk products	Digestible
Maltose	Malt products, some breakfast cereals	Digestible
Mono saccharine
Glucose	Fruits, honey, corn syrup	Digestible
Fructose	Fruits, honey	Digestible

Starch is found in cereal grains, legumes and in tubers. Glycogen is found in  meat and seafood. These are digestible polysaccharides.

Cellulose and hemicelluloses, pectin, gums and mucilage are indigestible polysaccharides. Celluloses are found in stalks and leaves of vegetables and outer coverings of seeds. Pectin are present in fruits and gums and mucilage are part of plant exudates and seeds.  

     Table: Comparative sweetness of sugars

Sugar	Sweetness value
Fructose	173
Invert sugar	130
Sucrose	100
Glucose	74
Galactose	32
Maltose	32
Lactose	16