Biomolecules notes for CBSE Exams, we discuss below topics in this Chapter: Before that click on the download button for a download PDF file of Biomolecules notes for CBSE Exams.

  • Carbohydrates
  • Sugar
  • Glucose
  • Proteins
  • Enzymes
  • Hormones
  • Nucleic Acids
  • Vitamines


General formula: Cx(H2O)y

Sugars or Saccharides: They are optically active polyhydroxy aldehydes or ketones.


Carbohydrates are classified as either reducing or non-reducing sugars:


On the basis of their behavior towards hydrolysis:



X Glucose (C6H12O6): An aldohexose as it contains six carbon atoms and Aldehydic Group.

– Glucose Structure:

Glucose structure

– Glucose Preparation:

Glucose Preparation

Glucose was assigned an open-chain structure on the basis of the following evidence:

Glucose open-chain structure

– Chemical reactions:

Glucose Chemical reactions
Glucose Chemical reactions

Fructose (C6H12O6): A ketohexose as it contains six carbon atoms and a ketonic group.

– Structure:

Mutarotation: The change in specific rotation of an optically active compound with time to an equilibrium value is called mutarotation.

Disaccharides and Polysaccharides


Proteins: They are the biomolecules of the living system made up of nitrogenous organic compounds by condensation polymerization of a-amino acids.

Classification of Amino Acids:

Classification of Amino Acids


X In aqueous solution, the carboxylic group can lose a proton and the amino group can accept a proton giving rise to a dipolar ion known as zwitter ion. This is neutral but contains both positive and negative charges.

Since these form salts with acids as well as with bases, their chemical reactions are similar to primary amines and carboxylic acids.

Isoelectric point: The pH at which dipolar ion (zwitterion) exists as a neutral ion, i.e., +ve and –ve charge is equal and it does not migrate to either electrode, is called isoelectric point. The amino acids have the least solubility in water at an isoelectric point which helps in their separation.

Except for glycine, all other naturally occurring a-amino acids are optically active because they contain a chiral, asymmetric carbon atom.

They exist in both D- and L-forms. Most naturally occurring a-amino acids have L-configuration.

Peptides and their classification:

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

Peptide bond

The products formed by the linking of amino acids by peptide linkage are known as peptides.

Peptides are further divided into di, tri, tetra depending upon the number of amino acids combined.

Oligopeptide: It contains anywhere between 2-10 amino acids.

Polypeptides: Structures with more than ten amino acids are known as polypeptides. Where R, R, R′′ may be the same or different.

A polypeptide with more than a hundred amino acid residues, having molecular mass higher than 10,000 u is called a protein.

Classification of proteins: On the basis of molecular structure, proteins are classified as:

Proteins Structure:

Primary structure: It refers to the number and linear sequence of amino acids held together by peptide bonds.

Primary structure

Secondary structure: It is due to the folding or coiling of the peptide chain. It is mainly of two types:

– α-helix: These coils are stabilized by hydrogen bonds between the carbonyl oxygen of the first amino acid to the amide nitrogen of the fourth amino acid.

– β-pleated sheet structure: b-pleated sheet structure is formed when hydrogen bonds are formed between the carbonyl oxygens and amide hydrogens of two or more adjacent polypeptide chains. The bonding in – β-pleated sheet structure is intermolecular H-bonding. The structure is not planar but is slightly pleated. Silk fibroin is rich in b-pleated sheets.

β-pleated sheet structure

Tertiary structure: It represents the overall folding of the polypeptide chains, i.e., further folding of the secondary structure and the bonds responsible for such interaction are hydrophobic interactions, hydrogen bonds, ionic interactions, van der Waals’ forces, and Disulphide bonds.

Quaternary structure: The spatial arrangement of the subunits (two or more polypeptide chains) with respect to each other.

Denaturation of proteins:

When a protein in its native form, is subjected to physical changes like change in temperature or chemical changes like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called the denaturation of the protein.

The denaturation causes change in secondary and tertiary structures but the primary structure remains intact e.g., coagulation of egg white on boiling, curdling of milk, the formation of cheese when an acid is added to milk.


The enzymes are biocatalysts produced by living cells that catalyze biochemical reactions in living organisms. Chemically, enzymes are naturally occurring simple or conjugated proteins. Some enzymes may be non-proteins also.


Hormones: They are the molecules that act as intercellular messengers and are poured directly into the bloodstream by endocrine glands.

Types of hormones:

Steroids: Estrogens and androgens

Polypeptides: Insulin and endorphins

Amino acid derivatives: Epinephrine and norepinephrine.


Vitamins: These are complex organic molecules that cannot be produced by the body and must be supplied in small amounts in the diet to carry out essential metabolic reactions that are required for normal growth and maintenance of the body.

Vitamins Classification:

Water-soluble vitamins: Soluble in water. Must be supplied regularly in the diet as they are regularly excreted in urine (except vitamin B12)

e.g., Vitamin– B1, B2, B6, B12 and C.

Fat-soluble vitamins: Soluble in fat and oils. Stored in liver and adipose tissues e.g., Vitamin – A, D, E and K.

Deficiency of more than one vitamin in the body causes avitaminosis while excess intake of vitamins(A and D)may cause hypervitaminoses.


Nucleic acids are the polymers of nucleotides present in the nucleus of all living cells and play an important role in the transmission of the hereditary characteristics and biosynthesis of proteins.

Nucleic acids

Types of nucleic acids:

Types of nucleic acids

Chargaff’s rule: The amount of purine bases is always equal to that of pyrimidine bases. Purine base of one strand of DNA molecule pairs with pyrimidine base of the other strand. Adenine (A) pairs with thymine (T) through two H-bonds (A = T) and guanine (G) pairs with cytosine (C) through three H-bonds (G C). In the case of RNA, adenine (A) pairs with uracil (U), (A U).

Read more about Chargaff’s Rule

Replication: It is the process by which a single DNA molecule produces two identical copies of itself.

Protein synthesis: It occurs in two steps:

Transcription: It is the process of synthesis of RNA.

Translation: The synthesis of proteins occurs in the cytoplasm of the cell. The m-RNA directs protein synthesis with the help of r-RNA and t-RNA.

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