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Chapter 13 – Biomolecules

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:


You know that every organism that you see around you is made up of atoms, elements, and molecules even though they differ in structure. Such carbon-containing organic compounds that are present in living organisms and interact with each other to carry out life processes are called biomolecules. Today we learn about biomolecules and their types. Different types of biomolecules present in living organisms comprise complex molecules like carbohydrates, proteins, lipids, nucleic acids, etc. Biomolecules in living organisms constitute a significant part of the cell, i.e., cell wall, cell membrane, cell organelles, etc. Biomolecules is an important chapter of chemistry, and it is easier to remember once you understand the concept, so let’s dive in.

Whatever we eat in daily life usually consists of chapati, rice, vegetables, pulses, meat, etc. Do you know what the major biomolecule is present in these food items? It is a carbohydrate. Carbohydrates are the macromolecules containing carbon, hydrogen and oxygen with the general formula Cx(H2O)y. Some carbohydrates taste sweet in taste, so they are called sugars.

(*Sugar used in the home – sucrose. Sugar present in milk – lactose. Lactose intolerant people cannot digest lactose present in milk)

Definition: Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.

Classification of Carbohydrates

Carbohydrates are classified either as:

  1. Reducing or non- reducing sugar, or
  2. Monosaccharides, Oligosaccharides, Polysaccharides – based on the nature of hydrolysis

(* Hydrolysis means the breakdown of complex substances into simpler substances in the presence of water molecules. Example of hydrolysis – Sucrose + H2O —> Glucose + Fructose)

Functional groups present in carbohydrates can be either aldehyde or ketone. In Oligosaccharides and Polysaccharides, monomers of carbohydrates are linked to each other by a glycosidic bond.

Carbohydrates that cannot be hydrolysed further to give polyhydroxy aldehyde or ketone are called monosaccharides. They are the smallest or basic units of carbohydrates—example – Glucose, fructose, galactose, ribose, etc. Glucose is an aldohexose (aldehyde containing 6-Carbon carbohydrate) and is also known as dextrose. It is the monomer of starch, cellulose, and sucrose. Fructose also has the molecular formula C6H12O6 and is called Furanose.

Those carbohydrates which can yield 2-10 monosaccharide units on hydrolysis are called oligosaccharides. They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides they provide on hydrolysis.

  • Disaccharides – Sucrose, maltose, lactose
  • Trisaccharides – Raffinose
  • Tetrasaccharides – Stachyose
  • Pentasaccharide – Barbascose

Carbohydrates that can yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Example – Starch, glycogen, cellulose, etc. They are not sweet in taste, hence called non-sugars.

Carbohydrates can be further classified as reducing or non-reducing sugars. Carbohydrates which can reduce Fehling’s solution and Tollens reagents are called reducing sugars. Example – maltose, glucose, lactose, etc. All monosaccharides are reducing sugars.

In disaccharides which cannot reduce Fehling’s solution and Tollens reagents due to the absence of free functional groups (aldehydes or ketones), they are called non-reducing sugars. Example – Sucrose.

Pulses, grains, milk, and fish are also called bodybuilding food items since they comprise mainly proteins. Proteins are abundant biomolecules and polymers of α- amino acids. Common sources of proteins found around us are milk, cheese, pulses, peanuts, fish, meat, etc. Amino acids contain two functional groups – amino (-NH2) and carboxyl (-COOH) groups. Amino acids are linked to each other by peptide bonds to form proteins.

  • The amino acid is sweet in taste – Glycine.
  • The amino acid derived from cheese – Tyrosine.

In aqueous solution of amino acids, zwitterion is formed. It is formed when the Carboxyl group loses a proton, and the amino group accepts a proton, giving rise to a dipolar ion.

Properties of amino acids

  1. Colourless and crystalline solids
  2. Water-soluble
  3. High melting point
  4. Behave like salts due to the presence of acidic (-COOH) and basic groups (-NH2) in the same molecule

Amino acids are classified as:

  1. Acidic, basic or neutral – Based on the number of carboxyl and amino group
  2. Essential or non-essential – Based on whether they are produced in the body or not

Neutral amino acids: number of amino group = number of the carboxyl group

Acidic amino acids: number of amino group = number of the carboxyl group

Basic amino acids: number of amino group > number of the carboxyl group

Non-essential amino acids – These amino acids can be synthesised in the human body hence not required much in the diet. Example – Serine, glutamic acid, Alanine, etc.

Essential amino acids – These amino acids cannot be synthesised inside our body and must be obtained through diet. Example – Lysine, Phenylalanine, Methionine, Tryptophan, Histidine, etc.

Classification of Proteins

Proteins can be classified into two types based on the following  characteristics:

  1. Fibrous proteins – Fibrous proteins are formed when the polypeptide chains running parallel are held together by hydrogen and disulphide bonds. Example – Actin and Myosin.
  2. Globular proteins – Globular proteins are formed as a result of coiling of chains of polypeptides, thus giving a spherical shape—example – Albumin, insulin, etc.

Structure of proteins

Structure and shape of proteins can be studied at four different levels based on the complexity of the structure, i.e.

  1. Primary structure – In this structure, amino acids in each polypeptide chain are linked with each other in a specific sequence.
  2. Secondary structure – Due to the regular folding of the backbone of the polypeptide chain, the secondary structure is formed. It can be an α-helix or β-pleated sheet.
  3. Tertiary structure – Further folding of the secondary structure leads to the formation of the tertiary structure of proteins.
  4. Quaternary structure – The spatial arrangement of 2 or 3 subunits of proteins or polypeptide chains with respect to each other forms the quaternary structure.

Nucleic acids are responsible for the transfer of characters from parents to offspring. Chromosomes present inside the nucleus of the cell are made up of proteins, and another type of biomolecules called nucleic acids. Nucleic acids are classified into deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). 

RNA = Ribose sugar + phosphoric acid + nitrogenous bases

DNA = Deoxyribose sugar + phosphoric acid + nitrogenous bases

Structure of nucleic acids

Nucleic acids are long-chain polymers of nucleotides hence called polynucleotides. A nucleoside is a unit formed by the attachment of a nitrogenous base to 1′ position of the sugar. Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar.

(a)   Nucleoside = Pentose sugar + Nitrogenous base

(b)   Nucleotide = Nucleoside + Phosphoric acid

Nitrogenous bases are –  

  1. Adenine (A)
  2. Guanine (G)
  3. Thymine (T)
  4. Cytosine (C) 1
  5. Uracil (U)

Purine – Contains two nitrogen-containing heterocyclic rings (Adenine and Guanine)

Pyrimidine – Contains one nitrogen-containing heterocyclic ring (Thymine, Cytosine, Uracil)

DNA contains bases A, G, T and C while RNA contains bases A, G, T, C and U. DNA is a double-stranded molecule while RNA is a single-stranded molecule. The two strands of DNA are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bonds with thymine while Cytosine forms hydrogen bonds with guanine.

Double Helix structure of DNA was given by Watson and Crick.

When you eat groundnut or apply oil on your scalp, do you observe that it has a different nature than other biomolecules? This is due to the presence of Lipids in them. Lipids are water-insoluble, or hydrophobic long-chain molecules made up of fatty acids and glycerol. Lipids can be monoglycerides, diglycerides or triglycerides. Lipids are usually called as fats or oils (*Oils have lower melting point than fats hence don’t freeze in winters)

The fatty acid has a carboxyl group attached to the R group. R groups can be methyl (-CH3), ethyl (-C2H5), propyl (-C3H7) or higher groups. Fatty acids are long-chain molecules.

16 carbon fatty acid – Palmitic acid

20 carbon fatty acid – Arachidonic acid

Classification of fatty acids

Fatty acids can be classified as:

  1. Saturated fatty acids – without double bond, i.e., C-C bond.
  2. Unsaturated fatty acids – with one or more C=C double bonds.

Some lipids which have phosphorus and a phosphorylated organic compound are called phospholipids. They are found in cell membranes. Example – Lecithin.

Theoretical knowledge of the biomolecules will only take you halfway but to understand the concepts thoroughly, it will be better if you can watch out about them. You can find interesting and informative content on the platform providing you with good animated content. One such useful app for you is MSVgo.

MSVgo is an e-learning app which has been developed to embark conceptual learning in the students from grade 6-12. MSVgo has been providing the students with a core understanding of the concepts. It is a video library which is a wondrous collaboration of concepts with animations and explanatory visualisation. This app contains high-quality videos based on the curriculum, ICSE, ISC, IGCSE and IB curriculum in India. You must check out the videos on MSVgo to understand concepts in-depth on this topic.

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