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Although there is vast diversity of living organisms. The chemical compositon and metabolic reactions of the organisms appear to be similar. The composition of living tissues and non-living matter also appear to be similar in qualitative analysis.Closer analysis reveals that the relative abundance of carbon, hydrogen and oxygen is higher in living system.

All forms of life are composed of biomolecules only. Biomolecules are organic molecules especially macromolecules like carbohydrates, proteins in living organisms. All living forms bacteria, algae, plant and animals are made of similar macromolecules that are responsible for life. All the carbon compounds we get from living tissues can be called biomolecules.


Biomolecules Definition

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Biomolecules are molecules that occur naturally in living organisms. Biomolecules include macromolecules like proteins, carbohydrates, lipids and nucleic acids. It also includes small molecules like primary and secondary metabolites and natural products. Biomolecules consists mainly of carbon and hydrogen with nitrogen, oxygen, sulphur, and phosphorus. Biomolecules are very large molecules of many atoms, that are covalently bound together.

Classes of Biomolecules

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There are four major classes of biomolecules: 
  • Carbohydrates
  • Lipids
  • Proteins
  • Nucleic acids


Carbohydrates are good source of energy. Carbohydrates (polysaccharides) are long chains of sugars. Monosaccharides are simple sugars that are composed of 3-7 carbon atoms. They have a free aldehyde or ketone group, which acts as reducing agents and are known as reducing sugars. Disaccharides are made of  two monosaccharides. The bonds shared between two monosaccharides is the glycosidic bonds. Monosaccharides and disaccharides are sweet, crystalline and water soluble substances.Polysaccharides are polymers of monosaccharides. They are unsweet, and complex carbohydrates.They are insoluble in water and are not in crystalline form. 

Example: glucose, fructose, sucrose, maltose, starch, cellulose etc.

Fructose Molecule

Lipids are composed of long hydrocarbon chains. Lipid molecules hold a large amount of energy and are energy storage molecules. Lipids are generally esters of fatty acids and are building blocks of biological membranes. Most of the lipids have a polar head and non-polar tail. Fatty acids can be unsaturated and saturated fatty acids. 

Lipids present in biological membranes are of three classes based on the type of hydrophilic head present:
  • Glycolipids are lipids whose head contains oligosaccharides with 1-15 saccharide residues.  
  • Phospholipids contain a positively charged head which are linked to the negatively charged phosphate groups. 
  • Sterols, whose head contain a steroid ring. Example steroid.
Example of lipids: oils, fats, phospholipids, glycolipids, etc.

Lipid Molecule

Proteins are heteropolymers of stings of 
amino acids. Amino acids are joined together by the peptide bond which is formed in between the carboxyl group and amino group of successive amino acids. Proteins are formed from 20 different amino acids, depending on the number of amino acids and the sequence of amino acids. 

There are four levels of protein structure:
  • Primary structure of Protein - Here protein exist as long chain of amino acids arranged in a particular sequence. They are non-functional proteins.
  • Secondary structure of protein - The long chain of proteins are folded and arranged in a helix shape, where the amino acids interact by the formation of hydrogen bonds. This structure is called the pleated sheet. Example: silk fibres. 
  • Tertiary structure of protein - Long polypeptide chains become more stabilizes by folding and coiling, by the formation of ionic or hydrophobic bonds or disulphide bridges, this results in the tertiary structure of protein.
  • Quaternary  structure of protein - When a protein is an assembly of more than one polypeptide or subunits of its own, this is said to be the quaternary structure of protein. Example: Haemoglobin, insulin. 

Amino Acid Molecule

Nucleic Acids

Nucleic acids are organic compounds with heterocyclic rings. Nucleic acids are made of polymer of nucleotides. Nucleotides consists of nitrogenous base, a pentose sugar and a phosphate group. A nucleoside is made of nitrogenous base attached to a pentose sugar. The nitrogenous bases are adenine, guanine, thyamine, cytosine and uracil. Polymerized nucleotides form DNA and RNA which are genetic material. 

Nucleic Acid

Functions of Biomolecules

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Carbohydrates provide the body with source of fuel and energy, it aids in proper functioning of our brain, heart and nervous, digestive and immune system. Deficiency of carbohydrates in the diet causes fatigue, poor mental function. 

Each protein in the body has specific functions, some proteins provide structural support, help in body movement, and also defense against germs and infections. Proteins can be antibodies, hormonal, enzymes and contractile proteins.

Lipids, the primary purpose of lipids in body is energy storage. Structural membranes are composed of lipids which forms a barrier and controls flow of material in and out of the cell. Lipid hormones, like sterols, help in mediating communication between cells. 

Nucleic Acids are the DNA and RNA, they carry genetic information in the cell. They also help in synthesis of proteins, through the process of translation and transcription.

Structure of Biomolecules

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Structure of biomolecule is intricate folded, three-dimensional structure that is formed by protein, RNA, and DNA. The structure of these molecules are in different forms, primary, secondary, tertiary and quaternary structure. The scaffold for this is provided by the hydrogen bonds within the molecule.

  • Primary structure of a biomolecule is the exact specification of its atomic composition and and the chemical bonds connecting the atoms. 
  • Secondary structure of the biomolecule is the three-dimensional form of biopolymers, secondary structure is defined by the hydrogen bonds of the biomolecules. 
  • Tertiary structure of the biomolecule is the three-dimensional structure,defined by its atomic coordinates, by the formation of hydrogen, ionic or sulphide bonds. 
  • Quaternary structure is the arrangement of multiple folds of complex, in a mutli-subunit complex. 
More topics in Biomolecules
Carbohydrates Amino Acids
Proteins Lipids
Nucleic Acids Enzymes
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