Konu Tarihi: 20.09.2010- 22:55

  

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CARBOHYDRATES

Carbohydrates are a large class of natural organic substances that includes sugars, starches and cellulose.
1. Structure & Function of Carbohydrates Carbohydrates are made up of C, O, H elements. They found in all cells. Organisms use carbohydrates as a source of energy and as a structural compounds in cell.
2. Kinds of Carbohydrates
a) Monosaccharides
b) Dissaccharides
c) Polysaccharides
a) Monosaccharides (Single Sugars) They are the simplest of the carbohydrates. The number of carbon atomy may change from three to eight. If sugar has 5 C; pentoses: RNA, DNA (ribose, deoxyribose) If sugar has 6 C; hexoses: glucose, galactose, fructose The three hexoses have the same formula, but different structurak formulas. Such compounds are called isomers.

 Glucose(grape sugar): It is a white crystalline solid. It is found in honey and sweet fruits. Glucose is a very important monosaccharide because it is produced by green plants in photosynthesis and other carbohydrates are converted into glucose in human body.
 Fructose (fruit sugar): It is sweet, water soluble crystalline solid. It is the sweetest of all simple sugars.
 Galactose: It doesn’t exist freely but in the structure of lactose (milk sugar).
b) Dissaccharides (Double Sugars) They formed when two monosaccharide molecules combine chemically by eliminating a water molecule. This kind of reaction is called dehydration synthesis because of the removal of the water molecules. The (C-O-C) bond that joind two monosaccharides is called a glucoside bond. In dehydration synthesis, the number of glucoside bonds is equal to the number of water molecules. Dissaccharides are less soluble in water and less sweet compared to monosaccharides. The important dissacharides are sucrose, maltose and lactose.
 Sucrose (Table Sugar): glucose + fructose = sucrose + H2O (Dehydration synthesis)
 Lactose (Milk Sugar): glucose + galactose = lactose + H2O (Dehydration synthesis)
 Maltose (Malt Sugar): glucose + glucose = maltose + H2O (Dehydration synthesis) Dissaccharides are broken down their contituent monosaccharides with the help of the water. This kind of reaction is called hydrolysis. sucrose + H2O = glucose + fructose (Hydrolysis) lactose + H2O = glucose + galactose (Hydrolysis) maltose + H2O = glucose + glucose (Hydrolysis)
c) Polysaccharides Polysaccharides also known as multiple sugars. They are large molecules formed the joining of monosaccharide units. They are less soluble in water and are not sweet in contrary to other carbohydrates. Starch, glycogen and cellulose are important polysaccharides.
 Starch (Plant Starch): It is made up of glucose units. It is storage products of plants. Many foods such as potatoes, rice, bread and pasta are rich in starch. The existence of starch can be determined bu lugol or iodine solution. Starch produces a dark-blue color with iodine.
 Glycogen (Animal Starch): It is also composed of glucose units. It is the primary storage form of glucose in the liver and muscles.
 Cellulose: It is the basic structural material of cell wall in plant cells. It is a large, linear molecule composed of 3000 or more glucose units.
3. Importance of Carbohydrates for Human Body Carbohydrates are the most frequently consumed foods by humans. They are the main source of energy. Some carbohydrates are used as structural material in cells. The excess of carbohydrates are converted into fats and stored in various parts of body like around the kidneys or under the skin. Two monosaccharides, ribose and deoxyribose are found in all cells. They form the carbohydrate component of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Polysaccharides function as structural components in certain animals. For example “Chitin” makes up the exoskeleton of insects.
4. Foods Rich in Carbohydrates
 Potatoes
 Rice
 Bread
 Cereals
 Many sweet fruits
 Sweets
 Nuts