Biomolecules Five Marks

1. Derive / Discuss / Elucidate / Prove the structure of glucose (in detail).

1. Elemental analysis and molecular weight determination show that the molecular formula of glucose is C6H12O6.

2. Complete reduction of glucose with concentrated hydriodic acid in the presence of red phosphorous produces n-hexane as the major product. This indicates that the six carbon atoms in the glucose molecule form an unbranched chain.

               HI/P

Glucose → CH3 – CH2 – CH2 – CH2 – CH2 – CH3

           Reduction                     n-Hexane

3. Glucose readily dissolves in water to give a neutral solution. This indicates that the glucose molecule does not contain a carboxyl ( - COOH) group.

4. Glucose reacts with hydroxylamine to form a monoxime or adds only one mole of HCN to give a cyanohydrin. This reaction indicates the presence of either an aldehyde (-CHO) or a ketone (> C= O) group.

5. Mild oxidation of glucose with bromine water gives gluconic acid. This indicates the presence of an aldehyde ( - CHO) group since only the aldehyde group can be oxidised to an acid, containing same number of carbon atoms. Since the six carbon atoms in glucose form a consecutive unbranched chain, the aldehyde group, must occupy one end of this chain.

6. Further oxidation of gluconic acid with nitric acid gives saccharic acid. This indicates the presence of a primary alcoholic (-CH2OH) group.

                           COOH                      COOH

             Br2/H2O    |                 HNO3        |

Glucose    →    (CHOH)4     →        (CHOH)4

               Mild      |                  Strong       |

             oxidation    CH2OH        oxidation  COOH

                          Gluconic acid            Saccharic acid

7. Glucose reduces Tollen’s reagent (ammoniacal solution of silver nitrate) to metallic silver or Fehling’s solution (basic solution of cupric ion) to red cuprous oxide. These reactions further confirm the presence of a aldehyde ( - CHO) group.

8. Glucose reacts with acetic anhydride in the presence of pyridine to form a penta acetate. This reaction indicates the presence of five hydroxyl ( - OH) groups in a glucose molecule.

9. Conclusion: From the above evidences we conclude that glucose is a penta hydroxy hexanal (an aldohexose).

10. Glucose can be represented by the following structure.

   CHO

   |

*CHOH

   |

*CHOH                      *C = Asymmetric carbon atom

   |

*CHOH

   |

*CHOH                      (2, 3, 4, 5, 6 penta hydroxy hexanal)

   |

  CH2OH

Glucose
 Or

1. Molecular Formula of glucose is C6H12O6 

2. Glucose + HI / P → n - Hexane.

This indicates the Six Carbon atoms in an Unbranched chain.

3. Glucose + water → Neutral solution.

This indicates the absence of a Carboxyl ( – COOH) group.

4. Glucose + Hydroxylamine → Monoxime

   Glucose + HCN → Cyanohydrin.                   

This indicates the presence of either an Aldehyde ( – CHO) or a Ketone ( > C = O) group.

5. Glucose + Bromine water → Gluconic acid.

This indicates the presence of an Aldehyde ( – CHO) group. That must occupy one end of the chain

 6. Glucose + Nitric acid → Saccharic acid.

This indicates the presence of a Primary alcoholic ( – CH2OH) group.

7. Glucose + Tollen’s reagent → Metallic Silver

    Glucose + Fehling’s solution →  Red Cuprous oxide.

These confirm the presence of an Aldehyde ( – CHO) group.

8. Glucose + Acetic anhydride / Pyridine → Penta acetate.

This indicates the presence of 5 – Hydroxyl ( – OH) groups

ஃ Glucose is a penta hydroxy hexanal (an aldohexose)

            From the above evidences Glucose can be represented by the following Structure

  CHO

   |

*CHOH

   |

*CHOH                      *C = Asymmetric carbon atom

   |

*CHOH

   |

*CHOH                      (2, 3, 4, 5, 6 - Penta hydroxy hexanal)

   |

  CH2OH
  Or 
1. Elemental analysis and molecular weight determination show that the molecular formula of glucose is C6H12O6

S. No	Experiment	Observation	Inference
2	Glucose + HI / Red P	n - Hexane	6 Carbon atoms in an Unbranched chain
3	Glucose + water	Neutral solution	Absence of a Carboxyl ( – COOH) group
4	Glucose + Hydroxylamine Glucose + HCN	Monoxime Cyanohydrin	Presence of either an Aldehyde ( – CHO) or a Ketone ( > C = O) group
5	Glucose + Bromine water	Gluconic acid	Presence of an Aldehyde ( – CHO) group. That must occupy one end of the chain
6	Glucose + Nitric acid	Saccharic acid	Presence of a Primary alcoholic ( – CH2OH) group
7	Glucose + Tollen’s reagent Glucose + Fehling’s solution	Metallic Silver Red Cuprous oxide	Confirm the presence of an Aldehyde ( – CHO) group
8	Glucose + Acetic anhydride / Pyridine	Penta acetate	Presence of 5 – Hydroxyl ( – OH) groups
ஃ Glucose is a Penta hydroxy hexanal (an Aldohexose)

9. From the above evidences Glucose can be represented by the following Structure

  CHO

   |

*CHOH

   |

*CHOH                      *C = Asymmetric carbon atom

   |

*CHOH

   |

*CHOH                      (2, 3, 4, 5, 6 - Penta hydroxy hexanal)

   | 

   CH2OH

2.  Discuss / Elucidate the structure of fructose in detail. Or How is the structure of fructose determined?

1. Elemental analysis and molecular weight determination show that the molecular formula of fructose is C6H12O6.

2. Complete reduction of fructose with concentrated hydriodic acid in the presence of red phosphorous gives n-hexane as the major product. This indicates that the six carbon atoms in the fructose molecule form a consecutive unbranched chain.

                  HI/P

Fructose → CH3 – CH2 – CH2 – CH2 – CH2 – CH3

              Reduction                     n-Hexane

3. Fructose readily dissolves in water to give a neutral solution. This indicates that the fructose molecule does not contain a carboxyl ( - COOH) group.

4. Fructose reacts with acetic anhydride in the presence of pyridine to form a penta acetate. This reaction indicates the presence of five hydroxyl ( - OH) groups in a fructose molecule.

5. Fructose reacts with hydroxylamine to form a monoxime or adds only one mole of HCN to give a cyanohydrin. This reaction indicates the presence of either an aldehyde ( — CHO) or a ketonic  ( > C = O) group.

6. Fructose is not oxidised by Bromine water indicating the absence of aldehydic group.

Oxidation of fructose with concentrated nitric acid yields a mixture of glycollic acid and tartaric acid. Since this oxidation occurs with the rupture of the carbon chain, the carbonyl group must be present as a ketone ( > C = O) group.

                                       COOH          CH2OH

                         HNO3      |                    |

Fructose + 4(O)   →    (CHOH)2  + COOH            +   3H2O

                                        |                  Glycollic acid

                                       COOH

                                    Tartaric acid

7. Partial reduction of fructose with sodium amalgam and water produces a mixture of two epimeric alcohols, sorbitol and mannitol, because a new asymmetric centre is being created at C–2. This confirms the presence of a ketonic ( > C = O) group.

8. When fructose is treated with HCN, it forms an addition product which upon hydrolysis and subsequent reduction with hydriodic acid and red phosphorous gives 2-methyl-hexanoic acid. This indicates that the ketone ( > C = O) group is adjacent to one of the terminal carbon atoms.

                  HCN             Hydrolysis                  Reduction

Fructose → CH2OH     →  CH2OH     →    CH3

                      |      CN              |      COOH          |     COOH

                      C ∕                      C ∕                        C ∕

                      |   \ OH               |  \  OH                 |  \   H

                    (CHOH)3          (CHOH)3              (CH2)3

                      |                         |                             |

                      CH2OH            CH2OH                 CH3

                                                                             2-Methyl-hexanoic acid

9. Conclusion: From the above evidences we conclude that fructose is a penta hydroxyl hexanone (a ketohexose)

10. Fructose can be represented by the following structure.

  CH2OH

   |

  C = O

   |

*CHOH                                 *C = asymmetric carbon atom

   |

*CHOH                                

   |

*CHOH                                 (1, 3, 4, 5, 6 pentahydroxy-2-hexanone)

   |

  CH2OH

Fructose
  Or

1. Molecular Formula of Fructose is C6H12O6

2. Fructose + HI / P → n - Hexane.

This indicates the Six Carbon atoms in a consecutive Unbranched chain.

3. Fructose + water → Neutral solution.

This indicates the absence of a Carboxyl ( – COOH) group.

4. Fructose + Acetic anhydride / Pyridine → Penta acetate.

This indicates the presence of 5 – Hydroxyl ( – OH) groups

5. Fructose + Hydroxylamine → Monoxime

   Fructose + HCN → Cyanohydrin.                   

This indicates the presence of either an Aldehyde ( – CHO) or a Ketone ( > C = O) group.

6. Fructose + Bromine water → No reaction.

This indicates the Absence of an Aldehyde ( – CHO) group.

 7. Fructose + Conc. Nitric acid → Mixture of Glycollic acid and Tartaric acid

     Rupture of the carbon chain.

This indicates the presence of a Ketone ( > C = O) group.

8. Fructose + Sodium amalgam / Water → Mixture of two Epimeric alcohols, Sorbitol and Mannitol

A new Asymmetric centre is being created at C – 2

This indicates the presence of a Ketone ( > C = O) group

9. Fructose + HCN → Cyanohydrin

    Cyanohydrin + Hydrolysis and subsequent Reduction with HI / Red P → 2-Methyl hexanoic acid.

This indicates the presence of Ketone ( > C = O) group is Adjacent to one of the Terminal carbon atoms.

         ஃ Fructose is a Penta hydroxy hexanone (an Ketohexose)

10. Fructose can be represented by the following structure.

  CH2OH

   |

  C = O

   |

*CHOH                                 *C = asymmetric carbon atom

   |

*CHOH                                

   |

*CHOH                                 (1, 3, 4, 5, 6 pentahydroxy-2-hexanone)

   |

  CH2OH

Fructose
  Or 
1. Elemental analysis and molecular weight determination show that the molecular formula of fructose is C6H12O6

S. No	Experiment	Observation	Inference
2	Fructose + HI / Red P	n - Hexane	6 Carbon atoms in a consecutive Unbranched chain
3	Fructose + water	Dissolves in water Neutral solution	Absence of a Carboxyl ( – COOH) group
4	Fructose + Acetic anhydride / Pyridine	Penta acetate	Presence of 5 – Hydroxyl ( – OH) groups
5	Fructose + Hydroxylamine Fructose + HCN	Monoxime Cyanohydrin	Presence of either an Aldehyde ( – CHO) or a Ketone ( > C = O) group
6	Fructose + Bromine water	No reaction	Absence of an Aldehyde ( – CHO) group.
7	Fructose + Conc. Nitric acid	Mixture of Glycollic acid and Tartaric acid Rupture of the carbon chain	Presence of a Ketone ( > C = O) group
8	Fructose + Sodium amalgam / Water	Mixture of two Epimeric alcohols, Sorbitol and Mannitol A new Asymmetric centre is being created at C – 2	Confirms the presence of a ketone ( > C = O) group
9	Fructose + HCN Hydrolysis of Cyanohydrin formed and subsequent Reduction with HI / Red P	Forms Cyanohydrin 2-Methyl hexanoic acid	Presence of Ketone ( > C = O) group is Adjacent to one of the Terminal carbon atoms
ஃ Fructose is a Penta hydroxy hexanone (an Ketohexose)

10. Fructose can be represented by the following structure.

  CH2OH

   |

  C = O

   |

*CHOH                                 *C = asymmetric carbon atom

   |

*CHOH                                

   |

*CHOH                                 (1, 3, 4, 5, 6 pentahydroxy-2-hexanone)

   |

  CH2OH

Fructose

3. How are carbohydrates classified? Give example for each. Or Outline the classification of carbohydrates giving example for each.

Carbohydrates , also known as Saccharides, may be classified into two broad groups.

1. Sugars and 2. Non-sugars or Polysaccharides.

1. Sugars:

Sugars are sweet crystalline substances and soluble in water.

i) Monosaccharides: Polyhydroxy aldehydes or polyhydroxy ketones which cannot be hydrolysed into simpler sugars.

They may again be classified according to the nature of carbonyl group.

a) Aldoses, which contain an aldehyde group ( — C — H )

                                                                              ||

                                                                             O

b) Ketoses, which contain a keto group ( — C —  )

                                                                      ||

                                                                     O

The aldoses and ketoses are further divided into sub-groups on the basis of the number of carbon atoms in their molecules, as trioses, tetroses, pentoses, hexoses, heptoses, octoses, etc.

Thus monosaccharides are generally referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses etc.

Ex:

Glucose, Galactose - Aldohexoses

Fructose - Ketohexoses
Ribose - Aldopentose (a monosaccharide containing five carbon atoms) that, in its open chain form, has an aldehyde functional group at one end.
ii) Oligosaccharides: Sugars that yield two to ten monosaccharide molecules on hydrolysis and are thus again classified into various groups depending upon the number of monosaccharide units formed on hydrolysis.

a) Disaccharides: Sugars which on hydrolysis give two molecules of the same or different monosaccharides.

Ex:

C12H22O11 + H2O → C6H12O6 + C6H12O6

Sucrose                            Glucose        Fructose

formed by two molecules of Glucose linked by Beta-D-O glycosidic bonds

Lactose + H2O → Glucose + Galactose

Maltose + H2O → 2Glucose

 Isomaltose formed by two molecules of Glucose

Cellobiose formed by two molecules of Glucose linked by Beta-D-O glycosidic bonds

b) Trisaccharides: Sugars which on hydrolysis give three molecules of the same or different monosaccharides.

Ex:

C18H32O16         +                2H2O → C6H12O6 + C6H12O6 + C6H12O6

Raffinose also called Melitose                 Galactose      Glucose        Fructose

c) Tetrasaccharide: Carbohydrate which gives upon hydrolysis four molecules of the same or different monosaccharides.

Ex:

C24H42O21+ 3H2O → 2C6H12O6 + C6H12O6 + C6H12O6

Stachyose                           Galactose      Glucose        Fructose
2. Non-sugars (or) Polysaccharides:

Non-sugars are tasteless amorphous substances and insoluble in water.

These are carbohydrates which involve a large number of monosaccharide units linked to each other by oxide bridges. These linkages are called glycosidic linkages.
The common and widely distributed polysaccharides correspond to the general formula (C6H10O5)n

Chemically, polysaccharides are long chain polymers of monosaccharides.

They get hydrolysed to give monosaccharides.

                                    H+

a) Homopolysaccharides: The polysaccharides which contain only one type of monosaccharides.

Ex:

Starch (formed by two different kinds of molecules:  Amylose and Amylopectin)

Dextrin (intermediate product of starch digestion)

Cellulose

Glycogen

Chitin

Inulin

(C6H10O5)n + nH2O → nC6H12O6

Starch                                    Glucose
b) Heteropolysaccharides: The polysaccharides which contain two or more types of monosaccharides or their derivatives. They are also called as Heteroglycans.

Ex:

Glycoprotiens (combinations of carbohydrates and proteins),

Glycolipids (combinations of carbohydrates and lipids)

Connective-tissue polysaccharides

The blood-group substances

Mucopolysaccharides (made of repeating units of sugar derivatives like amino sugars and uronic acids). These are known as glycosamino glycans (GAG). Ex:

Hyaluronic acid (Hyaluronate) (formed by thousands of alternative units of N-acetyl glucosamine and glucuronic acid)

Chondroitin sulphate

Dermatan sulfate

Keratan sulfate

Heparin

Classification of the carbohydrates may be summarized as:

4. Mention the biological functions of lipids

1. Fats and oils act as storage of energy in plants and animals. In the animal body fats are stored in fatty tissues which are almost pure fat.

Fat give about 2¼ times as much energy as carbohydrates or proteins.

Fat is a poor conductor of heat. Hence, fat layer under skin serves to prevent losses of heat from the body.

2. Wax acts as a protective agent on the surfaces of animals and plants. Waxy coating on the surface of plants and fruits protects them from excessive loss of moisture and becoming infected with fungi and bacteria.

3. Phospholipids like lecithins and cephalins play a greater role in biosystem.

a) The lecithins are required for normal transport and utilisation of other lipids, especially in the liver. Lecithin aids in the organisation of the cell structure.

b) Cephalins are found in the brain. Cephalins have been implicated in the process of blood coagulation.

4. Galactolipids occur in considerable amount in the white matter of the brain and of all nervous tissue.

5. The presence of galactose in the glycolipids suggests the importance of milk sugar in the diet of infants and children during the development of the brain and nervous system.

5. What is a peptide bond? Explain / Illustrate the formation of a peptide bond in glycyl alanine. Draw the structures of glucose and fructose.

Peptide linkage or bond

The bond formed between two amino acids by the elimination of a water molecule is called a peptide linkage or bond.

–  C – OH        +     H – N –           →       – C – N –         +      H2O

     ||                                 |                               ||      |

    O                               H                              O     H

  Carboxyl group         Amine group of          Peptide bond

of one amino acid       other amino acid

The product formed by linking amino acid molecules through peptide linkages, – CO – NH –, is called a peptide.

Formation of a peptide bond in glycyl alanine                                                  

                                                    CH3                                                             CH3

                                                    |                                                                    |

NH2 – CH2 – COOH + H2N – CH – COOH → H2N– CH2 – CO –NH – CH – COOH

Glycine                               Alanine                              Glycylalanine (a dipeptide)

Structures of glucose and fructose

  CHO                                     CH2OH

   |                                             |

*CHOH                                  C = O

   |                                            |

*CHOH                                *CHOH

   |                                            |

*CHOH                                *CHOH

   |                                            |

*CHOH                                *CHOH

   |                                            |

  CH2OH                                CH2OH

Glucose                                   Fructose
 6. Distinguish between Glucose and Fructose

 Courtesy : Chemistry Today Magazine 

7. Write notes on Polysaccharides.

8. Write about Zwitter ion and iso-electric point of amino acid.
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