Carbohydrates

BIOCHEMISTRY

1. CARBHOHYDRATES Dr. P. SUGANYA ASSISTANT PROFESSOR Department of Biotechnology SRI KALISWARI COLLEGE (Autonomous), Sivakasi

2. Definition Carbohydrates  are most abundant biomolecules in the nature Carbohydrates  are molecules that contain  carbon, hydrogen,  and  oxygen . There are twice as many hydrogen atoms as carbon or oxygen atoms. The  general formula  for a carbohydrate can be written as C x (H 2 O) y . They act as the source of energy (e.g. glucose), as a store of energy (e.g. starch and glycogen) and as structural units (e.g. cellulose in plants and chitins in insects).

3. Functions Carbohydrates are involved in various functions : They also act as precursor for many organic compounds. It also involved in structural components of many organisms. It also involves in immediate source of energy demands of the body by serving as the storage form of energy (glycogen). They also involved in the structural and cellular functions of cell such as- fertilization, cell growth and adhesion.

4. Classifications

5. Carbohydrates are commonly referred to as a  saccharides  ( greek : sakcharon means sugar). Carbohydrates are classified into three major groups depending upon whether these undergo hydrolysis and if so on then the number of products formed by them. Monosaccharides Oligosaccharides Polysaccharides

6. Most carbohydrates are  polymers . Polymers are large, complex molecules composed of long chains of monomers. Monomers  are small, basic molecular units. Carbohydrates can be divided into  three groups : monosaccharides , disaccharides, and polysaccharides.

7. Monosaccharides – Structure, Properties, and Examples

8. Greek: mono means one. Monosaccharides  are simple sugars in which there are one oxygen atom and two hydrogen atoms for each carbon atom present in the molecule. They have  general formula  as (CH 2 O) n . Monosaccharides are  reducing sugars . The test for reducing sugar is called  Benedict’s test . They are  sugars , which taste sweet, are soluble in water and are insoluble in non-polar solvents. They exist in  straight chains  or  in the ring  or  cyclic forms .

9. They are classified according to the number of carbon atoms in each molecule as  trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7)  and so on. The names of all sugars end with  - ose . Examples:  Glyceraldehyde (triose), Erythrose ( tetrose ), Ribose (pentose), Glucose (hexose), Fructose (hexose), Galactose (hexose), Sedoheptulose ( heptose ), etc. They are used as a source of energy in respiration. They are important building blocks for large molecules.

10. Disaccharides – Structure, Properties , and Examples

11. Disaccharides  are made up of two monosaccharides joined together by a  condensation reaction . The condensation reaction  is the joining of two molecules with the formation of a new chemical bond and a water molecule is released when the bond is formed. A  glycosidic bond  is formed between two monosaccharides . If carbon 1 on one monosaccharide joins to carbon 4 on another monosaccharide, it is called a 1,4-glycosidic bond. Disaccharides are the most common, among oligosaccharides. It consists of two monosaccharides units which is held together by glycosidic bond. It is crystalline, soluble in water and sweet in taste.

12. Examples:   Maltose  is formed from two α-glucose molecules joined together by a glycosidic bond.  Sucrose  is formed from a condensation reaction between a glucose molecule and a fructose molecule.  Lactose  is formed from glucose and a galactose molecule. Sucrose is a  non-reducing sugar . Disaccharides can be split apart into two monosaccharides by breaking the glycosidic bond by adding water molecules, which is known as  hydrolysis reaction . The water provides a hydroxyl group (-OH) and hydrogen (-H), which helps the glycosidic bond to break. Sucrose is the transport sugar and Lactose is the sugar found in milk which an important constituent of the diet of young mammals.

13. Polysaccharides – Structure , Properties, and Examples Polysaccharides  are polymers formed by combining many monosaccharide molecules (more than two) by condensation reactions. Molecules with 3-10 sugar units are known as  oligosaccharides  while molecules containing 11 or more monosaccharides are  true polysaccharides . Polysaccharides  do not taste sweet . Because their molecules are so enormous, the majority of polysaccharides  do not dissolve in water . Polysaccharides made solely from one kind of monosaccharides are called  homopolysaccharides  (Starch) while those made of more than one monomer are called  heteropolysaccharides   (Hyaluronic acid).

14. Greek: poly means many. They are generally polymers of monosaccharide units with high molecular weight. They are tasteless and most often form colloids with water. It is linear as well as branched polymer. It is of two types: Homopolysaccharides : it yields only a single type of monosaccharide on hydrolysis. Heteropolysaccharides : it yields a mixture of a few monosaccharides or their derivatives on hydrolysis

15. Starch  is made up of long chains of α-glucose (Amylose and Amylopectin).  Glycogen  is made of α-glucose linked together by glycosidic bonds. Cellulose is also made of many β-glucose molecules linked by glycosidic bonds between carbon 1 and carbon 4. Starch  is the main energy storage materials in plants.  Glycogen  is the main energy storage materials in animals.  Cellulose  is the major component of cell walls in plants. The test for starch is called an  Iodine test .

16. References CGP. 2015. A-Level Biology Exam Board: AQA. Complete Revision and Practice. Original material by Richard Parsons. Glenn Toole and Susan Toole. 2015. AQA Biology for A-Level. 2nd Edition. Oxford University Press. https://microbenotes.com/carbohydrates/