BIOMOLECULES ,CHEMISTRY XII- NOTES ,FREE

BIOMOLECULES

Biomolecules are the organic compounds which form the basis of life, i.e., they build up the

living system and responsible for their growth and maintenance.The sequence that relates biomolecules to living organism is

Biomolecules → Organelles → Cells → Tissues → Organs → Living organism.

Carbohydrates

These are Optically active polyhydroxy aldehydes (aldcses) or ketones (ketoses) or compounds which on hydrolysis give these units are known as carbohydrates. They are also called saccharides(Latin Saccharum = sugar) due to sweet taste of simpler members.

Depending upon their behaviour towards hydrolysis, carbohydrates can be of following three

Types

(i)Monosaccharides

These cannot be hydrolysed to simpler molecules and further subdivided into tetroses, pentoses

or hexoses depending upon the number of carbon atoms. These are also called homopolysaccharides.            Aldotetroses  Erythrose, Threose

Aldopentoses            Xylose, Ribose,

Aldohexoses

Glucose, Galactose,

Ketohexoses-Fructose

All naturally occurring monosaccharides belong to D series.

(ii)Oligosaccharides

(Greek oligos = few). On hydrolysis, they generally give two to nine monosaccharides (same or

different) and are further classified as disaccharides, e.g., sucrose, maltose, lactose,

trisaccharides and so on. C₁₂H₂₂O₁₁is a disaccharide because it gives two monosaccharides.

 ‘’The bond formed between two monosaccharides is called a glycosidic bond and normally it is

(1, 4) bond.Sucrose is most abundant in plants and known as cane sugar or table sugar or invert sugar as equimolar mixture of glucose and fructose is obtained by hydrolysis of sucrose.

Trisaccharides Raffinose ‘’(C₁₈H₃₂O₁₆)

(iii)Polysaccharides

These are polymers of monosaccharides. Examples are starch, cellulose, glycogen, etc.

1. Starch,(C₆H₁₀O₅)_N

It is a polymer of a glucose and a major reserve food in plants. It turns blue with iodine. It is a

mixture of two components:

1.Amylose (20%), an unbranched water soluble polymer.

2.Amylopectin (80%), a branched water insoluble polymer.

Sources of starch are potatoes, wheat, rice, maize, etc.

2. Cellulose,(C₆H₁₀O₅)n

It is the most abundant and structural, polysaccharide of plants. It is important food source of

some animals It is a polymer of D (+) β-glucose.The chief sources of cellulose are wood (Contains 50% cellulose rest being lignin, resins, etc) and cotton (contains 90% cellulose rest being fats and waxes).

Several materials are obtained from cellulose:

1.Mercerised cotton---Cellulose treated with cone. sodium hydroxide solution acquire silky lustre. It is called mercerissd cotton.

2.Gun cotton----It is completely nitrated cellulose

(cellulose nitrate), highly explosive in nature and is used in the manufacture of smokeless gun powder, called blasting gelatin.

3.Cellulose acetate---It is used for making acetate rayon and motion picture films.

4.Cellulosexanthate---It is obtained by treating cellulose with sodium hydroxide and carbon

disulphide and is the basic material for VISCOSE rayon.

Oligosaccharides and heteropolysaccharides are also called heteropolysaccharides.

Reducing and Non reducing sugars

Based upon reducing and non reducing properties,

carbohydrates are classified as reducing and

Non reducing sugars. Carbohydrates reducing Fehling reagent or Tollen‟s reagent are termed as

reducing carbohydrates. e.g., All monosaccharides and disaccharides (except sucrose). But

carbohydrates which do not reduce such reagents are known as non reducing carbohydrates.

e.g., sucrose and polysaccharides.

Sugars and Non-sugars

On the basis of their, taste, carbohydrates are classified as sugars and non sugars. The

monosaccharides and oligosaccharides having sweet taste are collectively known as sugars.

Polysaccharides which are insoluble in water and not sweet in taste, are non sugars.

Glucose Dextrose, grape sugar, corn sugar, blood sugar (C₆H₁₂O₆).

Reaction of D-glucose is treated with the following reagents?

 (i)HI (ii)Bromine water (iii)HNO₃

 (i) When D-glucose is heated with HI for a long time, n-hexane is formed.

 (ii) When D-glucose is treated with Br₂ water, D- gluconic acid is produced.

 (iii) On being treated with HNO₃, D-glucose get oxidised to give saccharic acid.

Reactions of D-glucose which cannot be explained by its open chain structure.

(1) Aldehydes give 2, 4-DNP test, Schiff's test, and react with NaHSO₄to form the hydrogen sulphite addition product. However, glucose does not undergo these reactions.

(2) Glucose doent react with hydroxyl amine.which inbdicate that CHO group is absent from compound.

(3) Glucose exist in two anomeric form alpha and beta form

Anomers

Glucose having (i) configuration about C-1is the α-glucose and having (ii) configuration about C-1is β-

glucose.The carbon C-1is known as anomeric carbon and these compounds are called anomers. Both the forms are optically active. ex-D-glucosehas specific rotation +111.5° and β-D-glucose has specific rotation + 19.5°.

Mutarotation

When either of the two forms of glucose is dissolved in water, there is a spontaneous change in specific rotation till the equilibrium value of +52.5°. This is known as mutarotation.

Properties of glucose Glucose has one aldehyde group, one primary hydroxyl (-CH₂OH) and four secondary hydroxyl (-CHOH) groups and gives the following reactions:

These reactions confirm the presence ofa carbonyl group in glucose.

(iii) Glucose reduces ammoniacal silver nitrate solution (Tollen‟s reagent) to metallic silver and

also Fehling‟S solution or Benedict solution to reddish brown cuprous oxide (Cu₂O) and itself

gets oxidised to gluconic acid. This confirms the presence of an aldehydic group in glucose.

(iv) With mild oxidising agent like bromine water,glucose is oxidised to gluconic acid. Glucose on oxidation with nitric acid gives saccharic acid.

9 . Fructose does not reduce Bromine Water.

Epimers

Monosaccharides differing in configuration at a carbon other than anomeric carbon are called

epimers, e.g., glucose and galactose differ in configuration at C-4, hence called epimers.

Osazones

Monosaccharides and reducing disaccharides react with excess of phenyl hydrazine to form

crystalline substances , known as osazones glucose and fructose give same osazone.

Molisch Test for Carbohydrates

 In aqueous solution of compound add solution of α-naphthol in alcohol and then cone. H₂SO₄ along the walls of the test tube. Purple coloured ring is obtained at the junction.

Amino Acids

The compounds containing amino group (-NH2) and carboxylic group (-COOH) are called amino acids.R = H, alkyl or aryl group. Except glycine (H2N.CH2COOH), others are optically active in

nature.

Classification of Amino Acids

Essential and Non-essential Amino Acids

Human body can synthesise ten amino acids, called non-essential amino acids. The remaining ten amino acids required for protein synthesis are not synthesised by body and are called essential amino acids.

Essential amino acids are;-

1.Phenylalanine       2.Histidine     3.Tryptophan             4.Valine         5.Methionine 6.Threonine

7.Arginine      8.Leucine       11Isoleucine             10.Lysin

Structure of Proteins

(a) Primary structure

It simply reveals the sequence of amino acids.

(b) Secondary structure

Alpha helix and beta plates are two different secondary structures of protein.

Alpha helix is a right handed-coiled or spiral conformation of polypeptide chains. In alpha helix, every backbone N-H group donates a hydrogen bond to the backbone C=O group, which is placed in four residues prior. Here, hydrogen bonds appear within a polypeptide chain in order to create a helical structure. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds; they form a generally twisted, pleated sheet. In contrast to the alpha helix, hydrogen bonds in beta sheets form in between N-H groups in the backbone of one strand and C=O groups in the backbone of the adjacent strands. This is the main difference between Alpha Helix and Beta Pleated Sheet.

Proteins are made up of polypeptide chains, and they are divided into several categories such as primary, secondary, tertiary, and quaternary, depending on the shape of a folding of the polypeptide chain. α-helices and beta-pleated sheets are the two most commonly encountered secondary structures of a polypeptide chain.

A Hemoglobin molecule, which has four heme-binding subunits, each made largely of α-helices.

Beta pleated sheets are another type of protein secondary structure. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet. Generally, a beta strand contains 3 to 10 amino acid residues, and these strands are arranged adjacent to other beta strands while forming extensive hydrogen bond network