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57.7 FUNCTIONS OF CARBOHYDRATES

The carbohydrates perform many important functions in living bodies.

They act as biofuels to provide energy for functioning of living organisms.

In human system, all the carbohydrates except cellulose can serve as source of energy. Starch and various sugars which are taken as food are the first hydrolysed to glucose by the enzymes present in the digestive system.

Amylase

2(C6H10O5)n + nH2O                     →                nC12H22O11

Starch                                                                      Maltose

Maltase

C12H22O11 + H2O                     →                2C6H12O6

Maltose                                                           Glucose

Glucose on slow oxidation to carbon dioxide and water in the presence of enzymes liberates large amount of energy which ‘is used by the body for carrying out various functions.

C6H12O6 + 6O2     →     6CO2 + 6H2O + 2832 kJ

In order to fulfil the emergency requirements, our body also stores some of the carbohydrates as glycogen in the liver. Glycogen on hydrolysis gives glucose.

It may be noted that cellulose cannot be hydrolysed in our body because enzymes required for its hydrolysis are not present in our body. However, grazing animals are capable of hydrolysing cellulose to glucose. In these animals cellulolytic bacteria present in the rumen, break down cellulose with the help of enzyme cellulose and is subsequently digested and converted into glucose.

1.      They act as constituents of cell membrane of plants and bacteria.

2.     D-ribose and 2-deoxy-D-ribose are present in nucleic acids.

 

SOlVED EXAMPLES

Example 57.1. What are reducing and non-reducing sugars?

Solution. Carbohydrates which reduce Tollen’s reagent and Fehling solution are called reducing sugars, while those which do not reduce these are called non-reducing sugars. For example, glucose, fructose, maltose and lactose are reducing sugars while sucrose is a non-reducing sugar.

Example 57.2. Draw open chain structures of an aldopentose and an aldohexose:

Solution.

Example 57.3 Amylose and cellulose are both straight chain polysaccharides containing only D-glucose units. What is the structural difference between the two?

Solution. In amylose, Dvglucose units are joined together by α-glycosidic linkages involving C-l of one glucose molecule and C-4 of the next glucose molecule. In cellulose, D-glucose units are joined together by β-glycosidic linkages between C-l of one molecule and C-4 of the next glucose molecule.

SUMMARY

  • Carbohydrates are the polyhydroxy aldehydes or polyhydroxy ketones or the compounds which can be hydrolysed to such compounds.
  • Carbohydrates are classified as monosaccharides, oligosaccharides or polysaccharides.
  • The carbohydrates which can reduce Tollen’s reagent, Fehling’s solution or Bededict’s solution are known as reducing carbohydrates.

Glucose, fructose and lactose are reducing sugars sucrose is a non reducing sugar.

  • Carbohydrates which are sweet in taste are known as sugars. Fructose is the sweetest sugar.
  • Sucrose on hydrolysis gives glucose and fructose.
  • Lactose on hydrolysis gives glucose and galactose.
  • Maltose, starch and cellulose on hydrolysis give only glucose on hydrolysis.
  • All monosaccharides and disaccharides are soluble in water.
  • Starch, cellulose and glycogen are examples of polysaccharides.
  • Carbohydrates are the major source of energy for functioning of living beings. They are the constituents of cell membrane of plants.

 

EVALUATE YOURSELF

I. Objective Type Questions

Select the most appropriate choice from the options given as (a), (b), (c) and (d) after each question: -,

1.      Which of the following is an example of aldohexose?

a)      Ribose

b)      Fructose

c)      Sucrose

d)     Glucose.

2.      Maltose on hydrolysis gives

a)      α-D-glucose

b)      α and β-D-glucose

c)      glucose and fructose

d)     fructose only;

3.      Fructose has/is

a)      bitter taste

b)      sweet taste

c)      tasteless

d)     nothing certain.

4.      The constituent units of sucrose are

a)      lactose and glucose

b)      glucose and fructose

c)      galactose and glucose

d)     glucose and maltose.

5.      Which disaccharide is present in milk?

a)      Maltose

b)      Galactose

c)      Sucrose

d)     Lactose.

6.      Which of the following is a ketohexose ?

a)      Fructose

b)      Maltose

c)      Glucose

d)     Ribose.

7.                  The linkage that holds monosaccharide units together in a polysaccharide is called

a)      peptide linkage

b)      glycoside linkage

c)      ester linkage

d)     ionic linkage.

8.      Lactose on hydrolysis yields

a)      Glucose

b)      fructose

c)      glucose and fructose

d)     glucose and galactose.

9.      Which of the following carbohydrates is not a reducing sugar

a)      Sucrose

b)      Lactose

c)      Glucose

d)     Fructose.

 

II. Fill in the Blanks

10.  Complete the following sentences by supplying appropriate words:

                    i.            Sucrose on hydrolysis yields …………

                  ii.            ………… is the sweetest sugar.

                iii.            ………… (carbohydrate) is not digested in our body.

                 iv.            Two monosaccharide units in a disaccharide are held by ………… linkage

                  v.            Molecular formula of fructose is …………

 

III. Discussion Questions

11.  What are carbohydrates? How are they classified? Give examples. What are the important sources of glucose and fructose.

12.  What are reducing and non-reducing sugars?

13.  Give an example of aldohexose and an aldotriose.

14.  What is the name given to the linkage which holds together two monosaccharides units in a polysaccharide?

15.  Give the structure of:

                                i.            sucrose

                             ii.            α-D-fructose

                            iii.            maltose.

16.  Out of amylose and amylopectin which form of starch is soluble in water?

17.  Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.

18.  What products are expected when lactose is hydrolysed?

19.  Draw structures of α-D-fructose and β-D-glucose.

20.  What are the hydrolysis products of?

                                i.            sucrose

                              ii.            lactose

21.  What is glycogen?

22.  Describe the functions of carbohydrates in living organism.

23.  List some important uses of carbohydrates.

24.  Draw the structure of cellulose, What is the product of hydrolysis of cellulose?

Answers

I.  Objective Type Questions

1.      (d)

2.      (a)

3.      (b)

4.      (b)

5.      (d)

6.      (a)

7.      (b)

8.      (d)

9.      (a)

II.                Fill in the Blanks

10.  (i) glucose and fructose

(ii) Fructose

(iii) cellulose

(iv) glycoside

(v) C6H12O6