Biomolecules Class 12

Biomolecules Class 12


In these chemistry notes, we learn about biomolecules class 12 chemistry. Some state boards books also contain these notes in 11th class chemistry book i.e. Biomolecules class 11. We prepare chemistry notes as per biomolecules class 12 ncert book, that is good for both classes of students.
Cell is the basic unit in all living organisms. Cell has very complicated structure and contains a large number of lifeless molecules.

Biomolecules Definition

The Biomolecules are lifeless molecules which combine in specific manner to produce life or control biological reactions. We can also say that, Bio-molecule is a molecule that is produced by a living organism.

Biomolecules Chemistry

Biomolecules in chemistry may be simple molecule like water, minerals, salts, vitamins, hormones etc. or macro-molecules like carbohydrates, proteins lipids (fats and oils) etc. They play an important role in function of organisms. The chief constituents of our diet are carbohydrates and proteins.
Biomolecules Notes
Carbohydrates are major source of energy. Proteins are nitrogenous substances present in hair, skin, muscle, nails etc. which plays variety of role in proper functioning of the living organisms. Enzymes which catalyze chemical reaction that take place in cell are proteins. Lipids are a source of stored energy. RNA (Ribonucleic Acid) and DNA (Deoxyribonucleic Acid) present in the nucleus of the cell are responsible for the storing genetic characteristics and synthesis of proteins.

Biomolecules Class 12th

Biomolecules are optically active polyhydroxy aldehydes or polyhydroxy ketones or the compounds that can be hydrolysed to polyhydroxy aldehydes or polyhydroxy ketones.


  • Early chemist noticed that carbohydrates had molecular formula      which could be represented as Cx(H2O). They were considered to be hydrates of carbon hence the name was given as carbohydrates.
  • Later studies revealed that these compounds were not hydrates of carbon because they did not contain free water molecules.
  • Similarly the compounds which fit into this formula are not classified as carbohydrates. e.g. formaldehyde HCHO, acetic acid CH3COOH, fit into this formula but are not carbohydrates.
  • Rhamnose C6H12O5 does not fit into this formula but are carbohydrate. You read these Biomolecules class 12 notes of NCERT chemistry on

Classification of carbohydrates

On the basis of their hydrolysis products, carbohydrates are classified as:

Simple carbohydrates

  • Simple carbohydrates are also known as monosaccharides cannot be hydrolyzed further to any simpler compound. They are basic unit of all carbohydrates.
  • Monosaccharides are further classified as (1) aldoses, containing aldehyde (CHO) group (2) ketoses, containing keto group.
  • Depending upon number of carbon atoms and functional group, monosaccharides are further classified as triose having three carbon atoms, tetrose having four carbon atoms, pentose having five carbon atoms etc.

Complex carbohydrates

It is further classified as oligosaccharides and polysachharides


An oligosachharide is carbohydrate (sugar) which on hydrolysis gives two to ten monosachharides units.
Depending upon number of monosachharides units produced on hydrolysis oligosaccharides are further classified as:(On hydrolysis)
  • Disaccharides (C12H22O11) :
Sucrose        ——>           Glucose + Fructose
Maltose  ——>  Glucose + Glucose
Lactose   ——>  Glucose + Galactose
  • Trisaccharides (C12H32O6) :
Raffinose   ——>  Glucose + Fructose + Galactose
  • Tetrasaccharides (C24H42O21) :
Stachyose   ——>  Glucose + Fructose + Galactose + Galactose


A carbohydrate that on hydrolysis yields large number of monosaccharide units is called polysaccharide. It is non-sugar. Polysaccharides are tasteless, amorphous, insoluble in water and natural biopolymers of monosaccharides e.g. starch, cellulose, glycogen, gums etc.

Fischer Projection Formula

A tetrahedral carbon atom is represented in a fischer projection by two crossed lines. The horizontal line represents bonds coming out of the page and vertical lines represent bonds going into the page.

D- and L- Sugars

  • The dextorotatory enantiomer of glyceraldehyde occurs naturally and represented as (+) – glyceraldehyde. It is referred as D – glyceraldehyde.
  • The leavorotatory enantiomer of glyceraldehyde is represented as (-) – glyceraldehyde and referred as L – glyceraldehyde.
  • In fischer projection, if the hydroxyl group at lowest chirality center points to the right, the monosaccharide is referred to be D – sugar.
  • Whereas monosaccharide having the hydroxyl group at lowest chirality center pointing to the left is referred to be L – sugar. 


Glucose is present in most of the sweet fruits, grapes, cane sugar, honey and in polysaccharides like starch and cellulose. It is also an essential component of human blood. Naturally occurring glucose is dextrorotatory and known as dextrose.

Preparation of glucose

  • From sucrose (laboratory method)
Glucose is prepared in the laboratory by hydrolysis of sucrose by boiling it with dilute hydrochloric acid or dilute sulphuric acid for about two hours.
C12H22O11 + H2O —–> C6H12O+C12H12O6
                                            Glucose    Fructose
  • From starch (commercial method)
When starch is boiled with dilute sulphuric acid at 393K under pressure it undergoes hydrolysis to give glucose. Excess of sulphuric acid is neutralized by adding chalk powder.
(C6H10O5n + nH2O —–> nC6H12O6

Open chain structure of glucose

Glucose was assigned the open chain structure given below, on the basis of following evidences. 
  1. The molecular formula of glucose was found to be C6H12O6 from the elemental analysis and molecular weight determination experiments.
  2. Action of HI: glucose prolonged heating with HI gives n-hexane, indicates that all the six carbon atoms are linked in straight carbon chain.
  3. Action of hydroxyl amine: Glucose reacts with hydroxyl amine in an aqueous solution to form glucose oxime. This indicates the presence of CHO group in glucose.
  4. Action of hydrogen cyanide: Glucose reacts with hydrogen cyanide to form glucose cynohydrin.
  5. Action of bromine water: Glucose on oxidation with bromine water gives gluconic acid, which shows that the carbonyl group in glucose is aldehyde group.
  6. Action of acetic anhydride: When glucose is heated with acetic anhydride in the presence of catalyst pyridine, glucose penta acetate is formed. It indicates that glucose is stable compound Action of dilute nitric acid: glucose as well as gluconic acid on oxidation with dilute nitric acid forms saccharic acid. This indicates the presence of a primary alcoholic group (-CH2OH) in glucose.
Spatial arrangement of different -OH groups in glucose as follows-

Cyclic structure of glucose

Open chain structure does not explain following reaction and facts.
  1. Glucose in spite having aldehyde group does not give condensation reaction with 2,4 dinitro- phenyl hydrazine, addition reaction with sodium bisulphite and gives negative test with schiff base.
  2. Glucose penta- acetatedoes not condense with hydroxyl amine indicating the absence of aldehyde group.
  3. Glucose is found to exist in two different crystalline form α and β, called anomers. On crystallization from hot and aqueous solution, α- glucose is obtained at 303K while β-glucose is obtained at 371K.
Cyclic six-membered structure of glucose are called pyranose structures, in analogy  with pyran. Pyran is a heterocyclic compound with one oxygen atom and five carbon atoms in the ring.

Haworth Projection Formula of Glucose Biomolecules

The substituents that are to right in a fischer projection formula. Orient the haworth projection formula with the ring oxygen at the back and the anomeric carbon at the right.

Haworth projection formula of glucose


Fructose is made by isomerization of glucose. Fructose is the sweetest. On the basis of different reaction it is shown that, fructose is penta-hydroxyl ketone. The open structure of fructose is as follows.

The cyclic five-membered structures of fructose are called furanose structures in analogy with furan in a heterocyclic compound with one oxygen atom and four carbon atoms in the ring.

Haworth Projection Formula of Fructose Biomolecules


A carbohydrate that on hydrolysis gives two same or different monosaccharide units is called disaccharide. In disaccharides, anomeric carbon of one monosaccharide molecule is bounded to a carbon of another monosaccharide molecule through an oxygen atom. Such a linkage between two monosaccharide molecules through oxygen is called glycosidic linkage. e.g. – sucrose, maltose, cellobiose, lactose.


Sucrose is known as cane sugar or common table sugar. In sucrose, C-1 of α-D-glucopyranose is linked to C-2 of β-D-fructofuranose by glycosidic linkage. On hydrolysis sucrose give equimolar mixture of dextrorotatory glucose and laevorotatory fructose. It is invert sugar.


Maltose is obtained by partial hydrolysis of starch. In maltose C-1 of one α-D-glucopyranose is linked to C-4 of another α-D-glucopyranose molecule by glycosidic linkage. Thus maltose contains 1—>4α glycoside bond. Maltose is a reducing sugar.


Cellobiose is obtained by partial hydrolysis of cellulose. In cellobiose, C-1 of one β-D-glucopyranose is linked to C-4 of another β-D-glucopyranose molecule by glycosidic linkage. Thus cellobiose contains 1—>4β glycoside bond. Cellobiose is also reducing sugar.


Lactose is known as milk sugar. In lactose, C-1 of β-D-glycopyranose is linked to C-4 of β-D-glucopyranose by glycoside linkage. Thus lactose also contains 1—->4β glycoside bond. Lactose is also reducing sugar.

Polysaccharides (C6H10O5)n

A carbohydrate that on hydrolysis yields large number of monosaccharide unit is called polysaccharides. In polysaccharides, a large number of same or different monosaccharaides are linked together by glycosidic linkages.


The Starch is mainly found in cereal grains, roots, tubers, potatoes etc. it is an important part of our diet. Starch is polymer of α-D-glucopyranose and consists of two fractions, amylose and amylopectin.
Amylose is water soluble component and constitutes about 20% of starch. It is long chain unbranched polymer Amylopectin is insoluble in water and constitutes about 80% of starch. It is a branched chain polymer.


Cellulose is the principal component of plant structure viz. wood, cotton to support the weight of the plant. Cell wall of the plant is made up of cellulose. It is long chain unbranched polymer.
Cellulose contains 1—>4β glycosidic linkages like cellobiose. Humans cannot hydrolyze cellulose due to lack of enzyme required for hydrolysis, hence they cannot use it directly as a food.


The glycogen is stored in animal body in the form of glycogen. It also known as animal starch, because its structure is similar to amylopectin Which is fraction of starch. Glycogen is highly branched.
Importance of carbohydrates bio molecules
  1. Carbohydrates are essential for life in both plants and animals.
  2. They form major part of our food and store chemical energy in plants.
  3. They act as the major source of energy for all the animals.
  4. In the plants they are important constituent of supporting tissues e.g. cellulose in wood, cotton in clothes.
  5. The main source of energy of animals is glucose which is in the form of glycogen.


  1. Proteins are the biopolymers of a large number of α-amino acid and they are naturally occurring polymeric nitrogenous organic compound containing 16% nitrogen and peptide linkages (-Co-NH-)
  2. Successive amino acids are joined together to form protein.
  3. It is present in hair, skin, muscle.
  4. Common source of proteins is egg, fish, cereals, milk etc.
  5. Proteins on hydrolysis give α amino acid.
Peptide bond and polypeptide
  • The amide linkage (-Co-NH-) by which α-amino acids are joined in linear fashion in proteins is called peptide linkage.
  • Peptide linkage is an amide formed between –COOH and NH3 group by elimination of water.
  • When many molecules of amino acid joined together by peptide linkages they form polypeptide which is proteins.

Classification (on the basis of molecular shape)

  • Fibrous proteins: The proteins in which the polypeptide chains lie side by side to form fiber like structure. The polypeptide chain held together by hydrogen bond. Example: keratin ( in hair, skin, nails)
  • Globular proteins: The proteins have intramolecular hydrogen bonding and are folded to form spherical structures are called globular proteins. Example: haemoglobin (in blood)

Structure of proteins

Each structure of protein is more complex than previous one.

Primary structure

In primary structure, proteins contain polypeptide chain. Amino acids linked with each other in specific sequence in protein this sequence is known as primary structure.
Secondary structure
In a protein, long amino acid chain is coiled, looped, or folded then this Structure of protein is known as secondary structure. It has two different types 
  • A polypeptide chain gets coiled by twisting into right handed spiral is known as α-helix.
  • The polypeptide chains are converted into flat sheet by stretched out. The contraction results in a pleated called β-pleated sheet.

Tertiary structure

α-helix bend on twist and folding gives three dimensional structure is known as tertiary structure.

Quaternary structure

The spatial arrangement of polypeptide chain with respect to each other is known as quaternary structure.

Amino acids

  • Α-amino acids are derivatives of carboxylic acids obtained by replacing α-H by amino group.
  • Amino acids contain two functional groups –COOH and -NH2. On hydrolysis protein gives about 30 amino acids. All amino acids have trivial names. E.g. Gly, Val, Glu, Met etc.
  • Amino acids are crystalline, colourless, non volatile, high melting solid. In aqueous solutions it behaves like salts.

Amino Acids Classification

Amino acids are classified into –

Acidic amino acid

If numbers of carboxyl groups are more than amino groups the amino acids are acidic. Example-glutamic acid.

Basic amino acid

If numbers of amino groups are more than carboxyl groups, then amino acids are basic in nature. Example-lysine.

Neutral amino acids

The amino acid having equal number of carboxyl and amino groups are called neutral amino acids. Example-valine


All biological or bio-catalysts which catalyze the reaction in living organisms is called enzymes. All enzymes are protein. Enzymes help in the digestion of food and absorption of certain molecule producing energy. Enzyme maltase converts maltose to glucose.
    C12H22O11 ( Maltose ) —–> C6H12O6 ( Glucose )


Now we learn about lipids. Lipids form naturally occurring biomolecules of a diverse group having limited solubility in water and can be isolated from organism by extraction with non polar solvents. Also, Lipids have variety of function in living organisms.

Lipids Classification for Biomolecules chapter class 12

The Lipids are classified as

Complex lipids

Complex lipids are esters of long chain fatty acid and can be hydrolyzed. They include triglycerides (animal fats and vegetable oils), glycolipids, phospholipids and waxes. 

Simple lipids

Simple lipids do not have ester linkages and they cannot be hydrolyzed. They include steroids, tarpenes and prostaglandins.


Hormones are the secretion of endocrine glands (ductless glands). They regulate vital functions of body and produced in very small quantity like enzymes. They have low molecular weight and are easily diffusible.

Functions of hormones

  1. The hormone thyroxine is secreted by thyroid gland in the neck to increase the rate of energy exchange and consumption of oxygen.
  2. Insulin is peptide hormone, secretes in pancreas and controls carbohydrate metabolism by increasing glycogen in muscles and oxidation of glucose in tissue and also lowers the blood sugar.


Vitamins are organic compounds which are required in the diet in small quantity to perform biological function and proportionate growth in living beings. Enough supply of vitamins is obtained from a well balanced diet.

Vitamins Classification

Depending upon the solubility

Water soluble vitamins (B and C)

Vitamins B and C are soluble in water. They are readily excreted in urine, have low toxicity and cannot be stored in body.

Fat soluble vitamins (A, D, E, K and P)

Vitamins A, D, E, K and P are soluble in oils and fats and are stored in the body in liver and in tissue.

Depending upon chemical structure

  • First, Vitamins of aliphatic series e.g. vitamin C
  • Second, Vitamins of aromatic series e.g. vitamin K
  • Third, Vitamins of alicyclic series e.g. Vitamin A
  • Fourth, Vitamins of heterocyclic series e.g. vitamin B


Nucleus is present in every cell of living organisms. Nucleus has acidic properties and hence it was named as nucleic acid. It is the substance of heredity. Nucleotide is the structural unit of nucleic acid. It has three components they are as follows.
  1. Sugar: It has deoxyribose sugar in DNA and ribose sugar in RNA. It is pentagonal ring having five carbon atoms.
  2. Phosphate group: it has three active –OH groups of which two groups are involved in the formation of strand.
  3. Nitrogen base: These are cyclic compound named as adenine (A) and guanine (G) called purines and are double ring compounds, whereas thymine (T), cytosine (C), and uracil (U) are called as pyrimidines and are single ring compounds.

DNA (Deoxyribonucleic Acid)

It has deoxyribo sugar. DNA stores the genetic information of an organism as sequence of base A, G, T, and C. uracil base is absent in DNA.

RNA (Ribonucleic Acid)

RNA is produced on DNA template by the process called transcription. It has ribose sugar. RNA is involved in protein synthesis in most of the organisms. mRNA, rRNA and tRNA are three types of RNA.  The uracil base is present in RNA molecule.
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