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Biological Molecules

The biological molecules or Biomolecules are involved in metabolic activities of living bodies such as plants and animals. They are usually polymers or macromolecules which are composed by the polymerisation of monomer units. For example, proteins are composed of amino acids which are bonded with each other through peptide linkage. Peptide linkage is an amide linkage that is formed by condensation polymerisation with the elimination of water molecules. The monomer units of Biomolecules are organic molecules such as amino acids contain –NH2 and –COOH group whereas monosaccharide units are polyhydroxy aldehyde and ketones.

Nucleic acids have 3 major components; phosphate group, nitrogenous base and a five membered sugar molecule. Biological molecules play an important role in the survival of living cells. For example, nucleic acids are involved in the transfer of genetic information and synthesis of proteins. Carbohydrates provide energy to living cells for their activities. Proteins are involved in different enzymes and structural units. Some of the Biomolecules like antibodies fight against foreign substances which are harmful for living cells. Let’s discuss some more important roles of biological molecules in the living bodies.

The relation between biomolecules and living cells present in the organism is as follows.

relationship sequence of biomolecules and living cells


What are Biological Molecules?

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"Organic complex molecules which build the living system and maintain the growth and reproduction of living system are called as bio molecules." Bio molecules can be small molecules like vitamin and hormone or can be bulky polymers like carbohydrates, lipids, proteins etc.
Most of the bio molecules in living cell have some broad functions like,

  1. Some bio molecules are essential to cellular and body structure.
  2. Some bio molecules act as energy-rich fuels during cellular respiration.
  3. Some of these organic molecules used to carry information of controlling growth and biological characteristics from one generation to another.
  4. Some bio molecules are good catalytic agents for the various chemical processes occur in cell and living body.

Generally these bio molecules are composed of certain elements carbon, hydrogen, oxygen with nitrogen and sulfur, phosphorous. Out of these elements, carbon has a ability to make covalent bonds to all of these atoms and mainly serve as backbone of bio molecules. Because of the catenation property of carbon atom long chain carbon atoms can form complex structures of bio molecules. The presence of specific functional group, these organic molecules can show some specific chemical properties.

Some common functional groups are as follows.

  • Hydroxyl group (-OH),
  • -SH group
  • Carbonyl group (-C=O-)
  • Aldehyde group (-CHO) and
  • Amide function (H2N-CO-).
  • Some ionizing functions make molecules very polar like carboxylate ion (-COO-), ammonium ion (-NH3+).

The presence of less polar aliphatic groups like -CH3 or CH3-CH2 and aromatic groups increase the
hydrophobicity of a molecule. Some functional groups are not stable and readily oxidized or reduced to stable one in the presence of enzymes.

Functional Group Structural Formula Example Found In
Hydroxyl Hydroxyl Hydroxyl carbohydrates,proteins,nucleic acids,lipids
Carbonyl Carbonyl
Carbonyl carbohydrates, nucleic acids
Carboxyl Carboxyl Carboxyl proteins, lipids
Amino Amino Amino proteins, nucleic acids
Sulfhydryl Sulfhydryl Sulfhydryl proteins
Phosphate Phosphate
Phosphate nucleic acids
Methyl Methyl Methyl proteins

Fig: Functional groups in biomolecules

4 Biological Molecules

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There are more than thousand of bio molecules involve in living organisms in various biological
processes. Some are vitamins, hormones, enzymes, ATP, carbohydrates, lipids, nucleic acids and proteins. Out of these main carbohydrates, proteins, lipids and nucleic acid are main bio molecules which play a vital role in living systems and mainly responsible for almost all biochemical processes.

4 Biological Molecules
These four bio molecules are basically polymers made up of simple organic compounds which are act as monomer units. Four important bio molecules with their monomer units are as follows.

Simple organic compounds Biomolecules Example
Monosaccharides Polysaccharides Sucrose, cellulose, starch
Fatty Acids and Glycerol Simple and Complex Lipids Phospholipids
Amino Acids Proteins Keratin, Albumin
Nucleotides Nucleic Acids DNA , RNA

Table: Four biomolecules with their monomer units

Monomer units of bio molecules are simple organic compounds with moderate molecular mass which further joined to form large and bulky molecule called as polymer. The formation of bio molecules generally precedes through either dehydration or condensation reactions between monomer units. The decomposition of bio molecules is also proceeds through hydrolysis reaction in the presence of certain enzymes.

Hence the formation as well as cleavage of bio molecules is an enzyme catalyzed reaction. These monomeric chains can be arranged in three dimensional space to form specific conformation of bio molecules.


Classes of Biological Molecules

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All bio molecules are specific in their chemical and physical properties. These organic molecules are playing an important role in living systems. They are quit differ from each other with respect to their conformation and reactivity towards other biological molecules.

Some common properties of bio molecules are as follows.
  1. Bio molecules are typically large molecules form due to polymerization of smaller subunits called as monomers.
  2. The backbone of almost all bio molecules is primarily consisting of carbon-carbon bond with the presence of certain functional group.
  3. Bio molecules show structural as well as stereo isomerism. All α-amino acids except glycine are optically active due the presence of chiral carbon atom.
  4. Bio molecules are very specific for reactant orientation and reactions conditions in any biochemical reaction.
  5. They have tendency to form different bonds hydrogen bond, disulphide bond, ionic and van der wall interaction with in the same molecular chain, thus show special orientation in three dimensional space which effect their reactivity. For example; the tertiary and quaternary structure of protein show different bonds between amino acids chains.
Classes of Biological Molecules

Proteins are high molecular mass bio polymers of amino acids. They are essential chemical
substances for the growth and reproduction of living organisms. They required for building new cells and repair of the worn out ones. Proteins act as structural materials, transport agent as well as metabolic regulators in many biological processes.

Almost all proteins are composed of twenty α-amino acids in which both functional groups, amino group (-NH2) and carboxyl group (-COOH) bonded on same carbon atom. Since both functional groups in amino acid are opposite in nature, one is basic (amino group) and acidic group (carboxyl group), thus they exist in the form of zwitterion in which two opposite charges are present in same molecule. All amino acids exist as zwitterion and have a certain isoelectric point which is a characteristic pH value for each amino acid.

Amino Acid and Zwitterion

The presence of different alkyl and aryl groups in amino acids, they show various chemical and physical properties and involve in the formation of proteins. Different amino acids with their properties are as follows.

Nonpolar Side ChainsPolar Side chainsCharged Side Chains

Two amino acids joined together by peptide linkage (-C(O)-NH-) which form due to condensation reaction between amino group of one amino acid and carboxyl group of another amino acid. The sequence of amino acid makes the backbone of protein as there are two terminals N-terminal and C-terminal which can further involve in peptide linkage to form polymeric chain.

Peptide Bond

The amino acid sequence and arrangement of amino acid in polypeptide chains make different
levels of organization in proteins.
  1. Primary Structure: Amino acids are arranged in a regular sequence in the polypeptide chain and that sequence is known as primary structure of proteins.
  2. Secondary Structure: The folding and coiling of primary structure of proteins make secondary structure. These folding get stabilized by hydrogen bonding between functional groups of amino acid. Two types of secondary structure are possible; alpha helix and beta pleated sheet.
  3. Tertiary Structure: The three dimensional structure which form due to bond formation between alkyl groups of amino acids is known as tertiary structure of proteins. It can be globular compact and fibrous elongated structure depends upon the bonding in proteins.
  4. Quaternary Structure: It is a three dimensional structure forms due to interactions between polypeptide chains, side chain of amino acid and hydrogen, ionic interactions.
Primary Structure
Secondary Structure
Tertiary Structure and Quaternary Structure
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  1. These bio molecules are widely distributed in animal as well as plant kingdom.
  2. Carbohydrates are originally a compound of class containing carbon, oxygen and hydrogen with the general formula Cx(H2O)y. Carbohydrates are batter known as saccharides which are polymers of monosaccharides.
  3. Monosaccharides are simplest units of carbohydrates which cannot further hydrolyzed to smaller molecules with general formula (CH2O)n.
  4. Oligosaccharides contain 2-10 monosaccharide molecules. For example; lactose, raffinose, etc. On the basis of number of monosaccharide units, oligoscharides can be named as disaccharide, Trisaccharide etc.
  5. When a large number of monosaccharide units joined together to form bulky molecule, it termed as polysaccharide like starch, glycogen etc.


Carbohydrates act as bio fuels and reserve food material in living organisms. They are main constituent of other bio molecules like DNA, RNA, ATP etc. Polysaccharides like cellulose acts as chief structural material of the cell wall of bacteria and plants. Carbohydrates act as raw material in many industries like paper, lacquer and breweries.
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  1. Lipids are a class of compounds include a variety of molecular types, like fats, oils, steroids, and waxes. It is not a polymer like other bio molecules carbohydrates and proteins.
  2. Hence lipids are composed of hydrogen and carbon atoms bonded with non-polar covalent bonds and hydrophobic in nature.
  3. Lipids are generally insoluble in water because of the presence of high proportion of non polar C – H bonds.
  4. Lipids can be classified as triglycerides, phospholipids, steroids, eicosanoids, sterols, vitamins (D, E, K) and waxes.
  5. Out of all these lipids; triglycerides are most common lipids form by esterification of fatty acids with glycerol in 3:1 ratio and also called as natural fats.
  6. Fatty acids are higher carboxylic acids, can be saturated and unsaturated. Glycerol is a trihydric alcohol with three carbon atoms.

Other lipid like phospholipids are composed of one glycerol, two fatty acids molecules with a phosphate group. The presence of phosphate group makes a polar head which is soluble in water with a non polar tails which is hydrophobic in nature. Because of this type of structure they have tendency to form micelles or lipid bilayers. Micelles are the cluster of phospholipid molecules in which the long hydrocarbon chain oriented the inside and hydrophilic regions exposed to the water environment.

Other lipids like waxes are composed of long-chain fatty acids and long-chain alcohols excluding glycerol. Steroids are made up of four interconnected rings of carbon atoms and mainly involve in formation of cell membranes as well as in the regulation of metabolism. Some common examples of steroids are testosterone and estrogens, Cholesterol and vitamin-D.

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