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Chapter 1: Biological Molecules

PM&DC Syllabus
 Explain the shape of the benzene molecule (molecular orbital treatment).
 Define and classify biological molecules.
 Discuss the importance of biological molecules.
 Describe biologically important properties of water (polarity, hydrolysis, specific heat, water as solvent
and reagent, density, cohesion/ionization).
 Discuss carbohydrates: monosaccharaides (glucose), oligosaccharides (cane sugar, sucrose,
lactose), polysaccharides (starches, cellulose, glycogen).
 Describe proteins: amino acids, structure of proteins.
 Describe lipids: phospholipids, triglycerides, alcohol and esters (acylglycerol).
 Give an account of RNA. Discuss conjugated molecules (glycol lipids, glycol proteins).

NUMS Syllabus
a. Introduction to biological molecules d. Proteins
b. Water e. Lipids
c. Carbohydrates f. Conjugated molecules (glycolipids, glycoproteins)

Introduction to Biochemistry
 Biological molecules are different chemical
compounds of living beings.
 Biochemistry is the branch of biology which deals
with the study of chemical components and
chemical processes in living organisms.

Chemical composition of protoplasm

 Approximately 25 elements out of 92 naturally
occurring elements of earth are found in living
beings. These are called bio elements. Or biogenic
elements.
 Human body is composed of only 16 of these bio
elements.
 The six commonest bio elements that constitute
99% of protoplasm are called major bio elements.
 Minor bio elements are those that are found as less
than 1% whereas those that are found as less than Proportion of various bio elements in
0.01% of the protoplasm are called trace human body.
elements.
 They are also called dietary elements.
 The survival of an organism depends upon its ability to take up some chemicals from its environment
and use them to chemical of its living matter.
 The bio elements are combined with each other and can form thousands of different biomolecules
which may be:
1. Inorganic (water, minerals, carbon dioxide, acids, bases and salts)
2. Organic (carbohydrates, lipids, proteins and nucleic acids).

Proportion of various biomolecules in bacterial and mammalian cells

Biomolecules Bacterial Cell Mammalian Cell
Water 70% 70%
Protein 15% 18%
Carbohydrates 3% 4%
Lipids 2% 3%

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DNA 1% 0.25%
RNA 6% 1.1%
Other organic molecules (enzymes, hormones, metabolites) 2% 2%
Inorganic ions (Na+, K+, Ca++, Mg++, Cl-, SO42-) 1% 1%

Inorganic e.g., water, mineral

On the basis of
constituent elements
Organic e.g., carbohydrate
Classification of
bioelements (25) Major forming 99% of body including C, H, O, N, Ca, P

On the basis of
Minor forming 1% body e.g., S, Mg, etc.
presence

Trace / dietary forming about 0.1% e,g., Zn, Mn, etc.

Macro-organic molecules
 Carbohydrates are composed of C, H, O and provide fuel for the metabolic activities of the cell.
Carbohydrates are present in the cytoplasm of the cells.
 Proteins are the most abundant organic
compounds in protoplasm. Proteins are present in
the membranes, ribosomes, cytoskeleton and
enzymes of the cell. Metabolism
 Lipids are heterogenous groups of hydrophobic
compounds. Lipids are present in the membranes
and cytoplasm of the cell
 Nucleic acids (DNA and RNA) are most essential Anabolism Catabolism
organic compounds, for living organisms. The
nucleic acid DNA is present in the chromosome. The
nucleic acid RNA is present in the nucleoplasm and
cytoplasm. Energy is
Needs energy
released
1. C-H Hydrocarbon (potential source of
bond chemical energy for cellular activities)
2. C-N Peptide bond (forms proteins which are Simpler Larger
bond very important due to their diversity in compounds molecules
structure and function) join break
3. C-O Glyosidic bond (provides stability to
bond complex carbohydrate molecules)
Complex Smaller
compounds simpler
Metabolic reactions of cell are formed molecules
 All the reactions taking place within a cell are formed
collectively referred to as metabolism.
 Those reactions in which simpler substances are Such as
combined to form complex substances are called photosynthesis Such as
anabolic reactions. Anabolic reactions need respiration
energy.
 Energy is released by the breakdown of complex
molecules into simpler ones, such reactions are called catabolic reactions.

Condensation and hydrolysis

Condensation Hydrolysis
 Condensation is the combination of monomers  The hydrolysis is essentially the reverse of
into polymer by the removal of water. condensation i.e., the breakdown of a
polymer into its monomers by the addition of
water.

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 During condensation, a hydroxyl (OH -) group is  During hydrolysis, an (OH-) group from water
removed from one monomer and a hydrogen (- is attached to one monomer and (H) is
H) is removed from the other to make water and attached to the other monomer. All digestion
as a result a bond is synthesized between the reactions are example of hydrolysis.
monomers. Condensation is also called
dehydration synthesis.

Condensation and hydrolysis

Importance of Water
 Water is the most abundant compound in all organisms.
 It varies from 65 to 89 percent (FTB = 70%, BTB & KPK = 70 to 90%) in different organisms.
 Human tissues contain about 20 percent water in bone cells and 85 percent (FTB = 85 to 90%) in brain
cells.
 Seeds also contain 20% water.
 Blood contains 88% water.
 Cucumber contains 98% water.
 Jelly fish has exceptionally large amount of water i.e. 99% that’s why its body shows transparency.

Solvent properties
 Due to its polarity, water is an excellent solvent for polar substances and hence called universal
solvent.
 Ionic substances when dissolved in water, dissociate into positive and negative ions.
 Non-ionic substances having charged groups in their molecules are dispersed in water.
 It is because of this property of water that almost all reactions in cells occur in aqueous media.
 Nonpolar organic molecules, such as fats, are insoluble in water and help to maintain membranes
which make compartments in the cell.

Hydrogen bonding
 The polarity of water molecules makes them interact with each other.
 The charged regions on each molecule are attracted to oppositely charged regions on neighbouring
molecules, forming weak bonds.
 Since the positively charged region in this special type of bond
is always an H atom, the bond is called a hydrogen bond.
 This bond is often represented by a dotted line because a
hydrogen bond is easily broken.
 The presence of hydrogen bonds among water molecules
causes it to remain in liquid state rather than change to ice or
steam.
 Without hydrogen bonds, water would boil at -80 degree
Celsius and freeze at -100 degree Celsius.

Cohesion and adhesion

 Cohesion is the intermolecular attraction between similar
molecules while adhesion is the attraction between dissimilar
molecules.
 Cohesion is the attraction among the water molecules which
enables the water molecules to stick together and flow together. Hydrogen Bonding between
 Water molecules also have attraction to polar surfaces. This water molecules
attraction is called adhesion.
 Both cohesion and adhesion are due to hydrogen bonds among water molecules.

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 These properties of water enable it to act as transport medium.

High specific heat capacity

 Water has great ability of absorbing heat with minimum of change in its own temperature due to its
high specific heat capacity.
 The specific heat capacity of water i.e. the number of calories required to raise the temperature of 1g
of water from 15 to 16 degree C is 1.0 calorie (or 4.18 joules) which is relatively high.
 Reason of high specific heat is that much of the energy is used to break hydrogen bonds.
 Water thus works as temperature stabilizer for organisms in the environment and hence protects living
material against sudden thermal changes due to this property.
 Due to this property, hot water cools slowly while cool water gets hot slowly.

High heat of vapourization

 Water absorbs much heat as it changes from liquid to gas due to its high heat of vapourization.
 Heat of vaporization is expressed as calories absorbed per gram vaporized. The specific heat of
vaporization of water is 574 kcal/kg.
 This property plays an important role in the regulation of heat produced by oxidation.
 It also provides cooling effect to plants when water is transpired, or to animals when water is
perspired.
 This is high heat of vapourization of water that gives animals an efficient way to release excess body
heat in a hot environment.
 Evaporation of only two ml out of one litre of water lowers the temperature of the remaining 998 ml
by 1 degree C.

Hydrophobic exclusion
 Hydrophobic exclusion can be defined as reduction of the contact area between water and hydrophobic
substances which are placed in water.
 Biologically, hydrophobic exclusion plays key roles in
maintaining the integrity of lipid bilayer membranes.

Ionization of water
 The dissociation of a molecule into ions is called
ionization.
 When water molecule ionizes, it releases an equal
number of positive hydrogen and negative hydroxyl ions. Hydrophobic Exclusion.
 This reaction is reversible but an equilibrium is
maintained.
 At 25 degree C the concentration of each of H+ and OH - ions in pure
water is about 10-7 mole/litre.
 The H+ and OH - ions effect and take part in many chemical reactions
that occur in the cells.

Lower density of ice

 Ice floats on water. This is because ice is less dense than water.
 Water is the only substance whose solid form is less dense than
liquid.
 The reason of less density is that ice has a giant structure and show Lattice like arrangement of
maximum number of hydrogen bonding among water molecules; ice.
hence, they are arranged like a lattice.

Protection
 Water is an effective lubricant that provides protection against the damage resulting from friction.
 For example, tears protect the surface of eye from the rubbing of eyelids.
 Water also forms a fluid cushion around organs that helps to protect them from trauma.
 For example, pleural fluid around lungs, pericardial fluid around heart, CSF around brain.

Carbohydrates
 Carbohydrates are the most abundant biomolecule in nature.
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 The word carbohydrate literally means hydrated carbons.
 They are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the
same as in water i.e. 2:1.
 Their general formula is Cx(H2O)y where x is the whole number from three to many thousands whereas
y may be the same or different whole number.
 Chemically, carbohydrates are defined as “polyhydroxy aldehydes or ketones, or complex
substances which on hydrolysis yield polyhydroxy aldehyde or ketone subunits.”
 They are strong reducing agents.
 The sources of carbohydrates are green living things e.g. plants, cyanobacteria, algae and many
bacteria.
 These are the primary products of photosynthesis.
 One gram of carbohydrates gives 4.1 kcal of energy.
 Carbohydrates are commonly known as sugar or saccharides.
 Carbohydrates combine with lipids and proteins called glycoproteins and glycolipids.
 Both have structural role in extracellular matrix of animals and bacterial cell wall.
 Both are the conjugated molecule.

Classification of carbohydrates
Carbohydrates

Monosaccharides: single Oligosaccharides 2-10 sugar Polysaccharides containing more than

monomeric sugar units e.g sucrose 10 units

Homopolysaccharide Hetropolysacc
Reducing Non- reducing s having same type harides having
Aldoses e.g Ketoses e.g
sugar e.g sugars e.g of monomeric units. different type
glucose fructose
maltose Sucrose e.g starch , glycogen of monomers.
etc. e.g agar
Carbohydrates are classified into three groups which are:

Monosaccharides Oligosaccharides Polysaccharides

They consist of single They are composed of 2 to 10 They are composed of more
saccharide unit. saccharide units. than 10 monosaccharide units.
They are simplest They have less complex They have highly complex
carbohydrates; therefore, they structure, so upon hydrolysis structure, so upon hydrolysis
cannot be further hydrolyzed. they yield at least 2 and they yield at least 11
maximum 10 monosaccharides. monosaccharides.
They are highly soluble in water. They are less soluble in water. They are generally insoluble in
water.
They are sweetest among all They are less sweet in taste. They are tasteless.
carbohydrates.

Monosaccharides
 Monosaccharides are only true carbohydrates.
 All carbon atoms in a monosaccharide except one, have a hydroxyl group.
 The remaining carbon atom is either a part of an aldehyde group or a keto group.
 Their general formula is CnH2nOn.

Classification of monosaccharides
 It may be on the basis of functional group or number of carbon atoms.
 On the basis of functional group, the monosaccharides containing aldehyde are called aldoses
while those containing ketone are called ketoses.
 The range of number of carbons in monosaccharides is 3 to 7. Hence,monosaccharides are
classified into five groups based upon number of carbon atoms i.e., trioses (3C), tetroses (4C),
pentoses (5C), hexoses (6C) and heptoses (7C).
 Tetroses are rare in nature and ocuur in some bacteria.
 Most common are pentose and hexose. Most common hexose is glucose.

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Glyceraldehyde

Dihydroxy
acetone

Class Aldoses Ketoses Functions

Trioses Glyceraldehyde Dihydroxy Intermediate in photosynthesis and cellular
(C3H6O3) acetone respiration.
Tetroses Ethryose Erythrulose Intermediates in bacterial photosynthesis.
(C4H8O4)
Pentoses Ribose, Ribulose Ribose and deoxyribose are components of DNA
(C5H10O5) Deoxyribose and RNA.
(C5H10O4) Ribulose is an intermediate in
Photosynthesis.
Hexoses Glucose, Fructose Glucose is respiratory fuel.
(C6H12O6) Galactose Fructose is an intermediate in respiration.
Galactose is component of milk sugar.
Heptoses Glucoheptose Sedoheptulose Intermediates in phoyosynthesis.
(C7H14O7)

Chemical structures of monosaccharides

Interconversion of ring form and

open chain structures. Fischer
projection on the left and Haworth
structure on the right.

 The open chain structure is known as Fischer projection.

 Normally these sugars are not found in open form in
solutions.
 When they are dissolved in water most of them (usually
pentoses and hexoses) are converted into ring chain
structure. This is known as Haworth structure.
Anomeric Isomers of glucose
 For e.g. ribose will form a five cornered ring called
ribofuranose and glucose will form a six cornered ring
called glucopyranose.
 All pentoses and ketohexoses like fructose are converted into furanose ring.

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 Second last carbon is called penultimate
carbon e.g., carbon 4 in ribose. Stereoisomers
 In ring structure formation,oxygen atom
reacts with penultimate carbon to link.
 Let us consider example of ribofuranose. Enantiomers Diastereomers Epimers
 When it is dissolved in water, the oxygen
atom from aldehyde group reacts with
second last carbon i.e., C4 in case of ribose.
 In this way oxygen atom forms a link between C1 and C4
while the OH group of C4 is shifted to C1.
 Each pentose or hexose molecule in ring structure exists
in either α or β form depending uponthe position of –H
and -OH group on C-1.
 These are known as Anomers
1. If -OH group is found downward on C-1 then it is
called α sugar
2. If-OH is present upward on C-1 then it is known as
β-sugar. Diastereoisomerism
 For example, consider ring structure of glucose.
 Diastereoisomers are sugars differeing in orientation at two carbons. (more than 1).
 For example, altrose is a diastereoisomers of glucose.
 Epimers are isomers of sugars that differ in the configuration at one carbon only.
 For example glucose and galactose differ at orientation at carbon no. 4 only

Enantiomers Diastereomers Epimers

These are non-superimposable These have different These have differed
image of each other. arrangement of H and –OH arrangement of H and OH
groups at more than one groups at only one asymmetrical
asymmetrical carbon atoms. carbon are called epimers.
These are not mirror images.
D and L isomers of glucose. d-Glucose and D-altrose. d-Glucose and d-mannose.

Comparison between structural isomers and sterisomers

 There are two forms of isomerism which are:

Structural isomers Stereoisomers

In structural isomers (also called constitutional In stereoisomers, the bond structure is same but
isomers) the atoms and functional groups are the geometrical positioning of atoms and
joined together in different ways. functional groups in space differs.
For example, glucose and fructose are structural For example, D-glucose and L-glucose.
isomers.
 Acetic acid,lactic acid and formaldehyde have the same formula as carbohydrates but they are not
carbohydrates.
 While rhamnose (C6H12O5)n does not match the carbohydrate formula but are carbohydrates.

Stereomerism in glucose
 Stereoisomers are molecules that have the same
molecular formula and differ only in how atoms
are arranged in 3D space.
 Enantiomers is a type of stereoisomers in which
molecules are non superimposable mirror-
images.
 An example of enantiomer is D and L glucose. D
sugars are right handed and L-sugars are left
handed molecules.

Laboratory manufatured (artificial) Epimers of glucose

sweeteners
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 Laboratory manufactured sugars are L sugars. On
theother hand the naturally occurring sugars in bodies
are D sugars.
 Proteins and cell receptors are designed to reactonly with
D sugars.
 For example, enzymes in your stomach can digest only
right-handed sugars.

Occurrence of glucose
 In free state, glucose is present in all fruits, being
abundant in grapes, figs, and dates. An example of enantiomers.
 Grapes contain as much as 27% glucose.
 Our blood normally contains 0.08% glucose. Human blood contains 100 mg of glucose per 100 ml of
blood. That’s why glucose is known as blood sugar.
 In combined form, it is found in many disaccharides and polysaccharides.
 Starch, cellulose and glycogen yield glucose on complete hydrolysis only.
 Glucose is naturally produced in green plants through photosynthesis.
 For the synthesis of 10g of glucose 717.6 kcal of solar energy is used. This energy is stored in the
glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in
the body.

Oligosaccharides
 These are comparatively less sweet in taste, and less soluble in water.
 On hydrolysis oligosaccharides yield from two to ten monosaccharides units
 The ones yielding two monosaccharides are known as disaccharides, those yielding three are known
as trisaccharides and so on.
Characteristics Sucrose Maltose Lactose
Common name Cane sugar Malt sugar Milk sugar
Occurrence/ Sugar cane, Germinating seeds, barley, 4-6% in cow milk
Sources sugar beet, fruits,
home sugar, honey in our digestive tract as a
result of starch digestion
Units α - Glucose and β -fructose Two alpha glucoses Beta galactose and
beta glucose
Linkage α-1,2-glycosidic α-1, 4-glycosidic β-1, 4-glycosidic
Nature Non-reducing sugar Reducing sugar Reducing sugar
Uses used as sweetener at homes. Used in brewing industry to It is an important
In plants sucrose is also called synthesize alcohol. energy source for
transport sugar as prepared young mammals.
food in plants is transported in
the form of sucrose because It Maltose is an intermediate
is very soluble and also disaccharide produced Used as milk
relatively unreactive during the breakdown of sweetener.
chemically. starch and glycogen.
 The covalent bond between two monosaccharides is called glycosidic bond.

Disaccharides
 Most common oligosaccharides are disaccharides.
 The general formula of disaccharides is C12H22O11.
 Most common disaccharides are sucrose, lactose and maltose.

Reducing and Non-reducing sugars

 Any carbohydrate which is capable of being oxidized and causes the reduction of other substances
without having to be hydrolyzed first is known as reducing sugar.

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 All monosaccharides and two of three types of disaccharides (maltose and lactose) are reducing
sugars.
 The third type of disaccharides, sucrose, and all
polysaccharides are non-reducing sugars.

Polysaccharides
 Polysaccharides are the most complex and the most
abundant carbohydrates in nature.
 They are usually branched and tasteless.
 They are formed by several (more than 10)
monosaccharides linked by glycosidic bond.
 Polysaccharides have high molecular weights and are
only sparingly soluble in water.

Types of polysaccharides
Polysaccharides can be classified as: Structure of various disaccharides.

Homopolysaccharides Heteropolysaccharides
The polysaccharides which are composed by the The polysaccharide which are composed by the
condensation of only one kind of condensation of different kind of monosaccharides
monosaccharides are called are called heteropolysaccharides.
homopolysaccharides.
e.g., starch, glycogen, cellulose, chitin. e.g., agar, pectin, peptidoglycan.
Some common homopolysaccharides are discused here

Starch
 Starch is formed by the condensation of hundreds of a-glucoses.
 It is called storage carbohydrate of plants. It is mainly stored in root, stem and seeds.
 It is found in roots, grains, seeds and tubers.
 Starch is digested in oral cavity and in small intestine by the enzyme amylase. Upon hydrolysis it
yields maltose first and then maltose is further digested by maltase enzyme and yields glucoses.
 It gives blue colour with iodine
solution. Unbranched chains.
 It is the main souce of Amylose
carbohydrates in diet for Soluble in hot water.
animals. Starch
 There are two types of Branched chains
starches i.e., amylose and
Amylopectin
Insoluble in water.
amylopectin.
1. Amylose:
 It is un-branched i.e., a linear chain of glucoses in which glucoses are attached together by α-1, 4-
glycosidic linkages.
 It is soluble in hot water and insoluble in cold water.

Structure of starches (Amylose and amylopectin)

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2. Amylopectin:
 It has branched structure i.e., a linear chain of glucoses having α-1,4 glycosidic linkages but more
chains of glucoses in the form of branches are also attached by α-1, 6-glycosidic linkages. (after 20
monomers, there is a branch).
 It is completely insoluble in both hot and cold water. Mnemonic: AmylopecTIn = Totally Insoluble

Glycogen
 Glycogen is also composed of a-glucoses.
 It is called storage carbohydrate of animals. It is found abundantly in liver and muscles, though found
in all animal cells. It is also found in fungi.
 It is also known as animal's starch.
 The digestion of glycogen is also quite similar to
that of starch. It yeilds maltose upon initial
hydrolysis and glucose after complete hydrolysis.
 It gives red colour with iodine solution.
 Structure of glycogen resembles with amylopectin
starch but glycogen has much more branching
than amylopectin (after 10 monomers,there is a
branch)
 It is insoluble in water. Structure of Glycogen

Cellulose (Fibre)
 Cellulose is the most abundant carbohydrate on earth.
 It is formed by condensation of hundreds of β-glucoses.
 It is structural carbohydrate of plants because it is the main constituent of their cell wall of plants
(and algae also).
 Cotton and paper are pure forms of cellulose.
 It is not digested by human digestive tract. In herbivores, it
is digested because of micro-organisms (bacteria, yeast
and protozoa) in their digestive tract which secrete
cellulase for digestion of cellulose.
 Upon intial hydrolysis, it yeilds cellubiose (a disaccharide
with beta 1,4-glycosidic linkage) and after complete
hydrolysis, it yeilds glucose.
 It gives no colour with iodine solution. Structure of cellulose
 Cellulose is added to iodine, the colour of iodine solution
remains intact that is light orange brown [SZABMU MDCAT-I 2024]s
 It is highly insoluble in water.
 Its structure resembles amylose starch as it is also unbranched
but it has β-1,4 glycosidic linkages between glucoses.

Chitin
 Chitin is second most abundant organic molecule on earth.
 It is a derivative of N-acetyl glucosamine monomers.
 It is a structural carbohydrate found in cell walls of fungi and in
the exoskeleton of arthropods.
 It is also known as fungal cellulose.
 Like cellulose, chitin is also undigestable.
Structure of chitin.
 It is unbranched and it has β-1,4 glycosidic linkage between
its monomers.

Remember iodine test

Examples of homopolysaccharides (mnemonic = CGS)

Cellulose Glycogen Starch

CN (No colour with iodine) GR (Red colour with iodine) BS (Blue colour by starch)

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Proteins
 Word “protein” has been derived from greek word proteios meaning prime or first.
 Proteins are the most abundant organic compounds to be found in cells and comprise over 50% of
their total dry weight.
 One gram of protein gives 4.6 kcal of energy.
 All proteins contain C, H, O and N, while some contains P, S. Few proteins have Fe, I and Mg
incorporated into the molecule.

Structure and Composition of proteins

 Chemically proteins can be defined as polymers of amino acids or polypeptide chains.
 The number of amino acids varies from a few to 3000 or even more in different proteins.
Amino acids
 About 170 types of amino acids have been found to occur in cells
and tissues.
 Of these, about 25 are constituents of proteins.
 Most of the proteins are however, made of 20 types of amino
acids.
 All the amino acids have an amino group (-NH₂) and a carboxyl General structure of an
group (-COOH) attached to the same carbon atom also known as amino acid
alpha carbon.
 The general formula of amino acids is:
 -R group is a variable group. It may be –H in glycine or –CH3 in alanine.

Peptide linkage
 Amino acids are linked together to form polypeptides chains.The linkage between the hydroxyl group
of carboxyl group of one amino acid and the hydrogen of amino group of another amino acid release
H₂O and C-N link to form a bond called peptide bond.The resultant compound has two amino acid
subunits and is a dipeptide.
 A dipeptide has two ends; one is called amino or -N terminal end while other is called carboxylic acid
or -C terminal end.
 A new amino acid can be added in this chain from its carboxylic acid or -C terminal end in the same
way.
 Similarly, when several amino acids are linked together by many peptide bonds, the polypeptide chain
is formed.

Formation of a dipeptide and peptide bond

Structure of proteins
There are four levels of organization of a protein molecule.
Protein organization

Primary structure Secondary structure Tertiary structure Quaternary structure

Primary structure
 The primary structure comprises the number and sequence of amino acids in a protein molecule.

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 F. Sanger in 1951 was the first scientist who determined the sequence of amino acids in a protein
(insulin).
 Insulin is composed of 51 amino acids(49 peptide linkages) in two chains. One of the chains had 21
amino acids and the other had 30 amino acids and they were held together by disulphide bridges.
 Similarly, Haemoglobin is composed of 574 amino acids (570 peptide linkages) in four chains, two
alpha and two beta chains. Each alpha chain contains 141 (282 in both) amino acids, while each beta
chain contains 146 amino acids (292 in both).
 The size of a protein molecule is determined by the type and number of amino acids comprising that
particular protein molecule.
Hb (574 amino acids)

Two alpha chains Two beta chains

141 amino acids 141 amino acids 146 amino acids 146 amino acids

 To calculate the number of possible peptides of a given length from a set of amino acids, you use the
formula: Number of peptides = (number of amino acids)(length of peptide)
 For example, Number of different dipeptides from five different amino acids is 52 = 25 dipeptides.

Structural conformations in proteins

Secondary structure
 The polypeptide chains then usually coil into α-helix, or into some other regular configuration which is
a secondary structure.
 One of the secondary structures, α-helix involves a spiral formation of polypeptide chain.
 It is a very uniform geometric structure with 3.6 amino acids in each turn (36 in 10 turns) of the helix.
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In chemistry, select option that there are 27 amino acid units for each turn of the helix.
 The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in
successive turns of the spiral.
 β-pleated sheet is formed by folding back of the polypeptide.

Tertiary structure
 Usually a polypeptide chain bends and folds upon itself forming a globular shape or 3-D structure.
This is the proteins' tertiary conformation.
 It is maintained by three types of bonds, namely ionic, hydrogen, and disulfide (-S-S-).
 Disulphide bridges are present in antibodies and are formed by the presence of cysteine.
 In aqueous environment the most stable tertiary conformation is that in which hydrophobic amino acids
are buried inside while the hydrophilic amino acids are on the surface of the molecule.
 Metal ion coordination can be seen from tertiary structure onwards, e.g Fe2+ as heme group.

Quarternary structure
 When two or more polypeptide tertiary chains are aggregated and held together by hydrophobic
interactions, hydrogen and ionic bonds, the specific arrangement is the quaternary structure.
 For e.g, hemoglobin.

Significance of sequence of amino acids

 The sequence of amino acids in a protein is determined by order of nucleotides in DNA.
 The arrangement of amino acids in a protein molecule is highly specific for its proper functioning.
 The best example is the sickle cell hemoglobin of human beings.
 Normal RBCs are disc-shaped and look like doughnuts without holes in the centre.
 But in case of sickle cell anemia, body makes sickle or crescent shaped RBCs. They contain abnormal
hemoglobin called sickle haemoglobin (HbS).
 Sickle cell anemia is caused by a mutation (point mutation) and type of mutation is substitution. In β-
globin gene in which only one nucleotide is replaced by another which causes a change in amino acid
sequence of both β-chains of haemoglobin.
 Sickle cell HbS shows only one difference from normal HbA i.e., polar glutamic acid is replaced by non-
polar valine at position number six in both β-chains.
 Hence, hemoglobin fails to carry any or sufficient oxygen, hence leading to death of the patient.

Point mutation in sickle cell haemoglobin

Classification of proteins

Fibrous proteins Globular proteins

1. They consist of molecules having one or 1. These are spherical or ellipsoidal or globular due
more polypeptide chains in the form of to multiple folding of polypeptide chains.
fibrils or filaments.
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2. Secondary structure is most important in 2. Tertiary structure is most important in them.
them.
3. They are insoluble in aqueous media. 3. They are soluble in aqueous media such as salt
solution, solution of acids or bases, or aqueous
alcohol.
4. They are non-crystalline and are elastic in 4. They are crystalline and inelastic.
nature. 5. (mnemonic:GC= Globular Crystalline)
5. They perform structural roles in cells and 6. They disorganize with changes in the physical and
organisms. physiological environment.
Examples Examples
Silk fiber (from silk worm, spiders web) Actin Enzymes, antibodies, hormones, hemoglobin,
and myosin (in muscle cells) myoglobin, channel proteins, albumen of egg white
Fibrin (of blood clot) and proteins of cell membranes.
Keratin (of nails, hair, fur, outer skin) Collagen
(in skin, ligaments, tendons, bones and in
cornea of eyes).

Note: Myosin, a fibrous protein has enzymatic activity due to its globular heads.

Role of proteins
Some proteins have structural roles while some have functional roles.
 Following is the list of structural proteins.

Types Roles of proteins

Collagen It establishes the matrix of bones and cartilage.
Elastin Elastin provides support for connective tissues such as bones and cartilage.
Keratin It strengthens protective coverings such as hair, nails, quils, feathers, horns and beaks.
Histone It arranges the DNA into the chromosome.

 Following is the list of functional proteins.

Types Roles of proteins

1. Enzymes Most enzymes are proteins which control metabolism i.e., they speed up
biochemical reactions.
2. Hormones Some hormones are protein in nature which are involved in regulation of
physiological activities.
3. Antibodies These proteins are produced by WBCs in response to antigens and
provide immunity.
4. Haemoglobin It is found in RBCs and is involved in the transport of oxygen mainly and
carbon dioxide to some extent.
5. Fibrinogen It is found in blood plasma and is involved in the blood clotting process.
6. Ovalbumin and casein Ovalbumin is found in egg whites and casein is a milk based protein.

Most abundant proteins

 The most abundant protein:
1. Earth = rubisco 2. Human = collagen type (I) 3. Blood = albumin
Note
 In plants, proteins are stored in most seeds for future need of the embryos. e.g., bean, pulses, pea etc.

Lipids
 Lipid is the collective name for variety of organic compounds such as fats, oils, waxes and fat-like
molecules (steroids) found in the body.
 Therefore, it is defined as a heterogeneous group of organic compounds.
 Most lipids are non-polar and are insoluble in water (hydrophobic) but soluble in organic solvent such
as acetone, alcohol, and ether etc.

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 Lipids are composed of carbon, hydrogen and oxygen. However, they have relatively less oxygen in
proportion to carbon and hydrogen.
 For instance, tristearin is a simple lipid which shows molecular formula as C57H110O6.
 Due to high contents of carbon and hydrogen, they store double amount of energy than carbohydrates.
 One gram of lipid gives 9 kcal of energy.
 Bloor proposed the term Lipid in 1943.

Functions of lipids
 Lipids are components of cellular membranes (e.g, phospholipids and cholestrol).
 They act as energy stores (e.g, triglycerides).
 Some hormones are lipids For e.g., auxins, gibberellins and cytokinins among plant hormones and
aldosterone and sex hormones among animal hormones.
 Lipids are also involved in protection, insulation, waterproofing and buoyancy.
 Waxes, in the exoskeleton of insects. and cutin, an additional protective layer on the cuticle of
epidermis of some plant organs e.g. leaves, fruits, seeds etc., are some of the main examples.

Classification of lipids
 Lipids have been classified as acylglycerols, waxes, phospholipids, sphingolipids, glycolipids and
terpenoid lipids.

Acylglycerols
 The most abundant lipids in living things are acylglycerols.
 Acylglycerols can be defined as esters of
glycerol (alcohol) and fatty acids.
 An ester is the compound produced as the
result of a chemical reaction of an alcohol
with acid and a water molecule is released
such a reaction is called esterification.
 Glycerol is a trihydroxy alcohol which
contains three carbons, each bears an OH
group.
 A fatty acid is a type of organic acid
containing one carboxylic acid group
attached to a hydrocarbon.
 Fatty acids contain even number of carbons
from 2 to 30.
 Each fatty acid is represented as R-COOH, Formation of a triacylglycerol (in the presence
where R is a hydrocarbon tail. of H+ catalyst) - FTB
1. When one glycerol molecule combines
chemically with one fatty acid, a
monoacylglycerol
(monoglyceride) is formed.
2. When two fatty acids combine
with one glycerol a
diacylglycerol is formed.
3. Similarly, three fatty acids + one
glycerol molecule =
triacylglycerol.
 Triacylglycerols are also called
neutral lipids as all three charge
bearing OH groups are occupied by
three fatty acids.
 The most widely spread acylglycerol
is triacylglycerol.
 In the formation of acylglycerols, OH
is released from alcohol and H from
acid which are together released as
water molecule (condensation) . (But In the presence of enzyme catalyst - PTB
according to FTB, OH is taken from
acid and H from alcohol).
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Reason: In laboratory/in vitro/chemistry, OH comes from acid because there is H+ catalyst and it is
nucleophilic substitution of carboxylic acid. But in vivo/in cell/in biology, ther reaction is carried out by
enzymes and in its mechanism, enzyme removes OH from alcohol and H from acid.
Acylglycerols

Monoacyglycerol Diacyglycerol Triacyglycerol

Glycerol + 1 fatty acid Glycerol + 2 fatty acids Glycerol + 3 fatty acids

Fatty acids properties

 About 30 different fatty acids are found.
 Acetic acid (2C) and butyric acid (4C) are simplest fatty acid.
 Palmitic acid (16C) and stearic acid (18C) are most common fatty acids.
 Some properties of fatty acid are increased with an increase in number of carbon atoms, such as
melting point, solubility in organic solvent and hydrophobic nature.
 For e.g., Palmitic acid (C16) is much more soluble in organic solvents and has higher melting point than
butyric acid (C4).
 Most fatty acids in plants contain 16-18 carbon per molecule
 In animals the fatty acids are straight chains, while in plants these may be branched or ringed.
 Fatty acids may be saturated or unsaturated.

Saturated fatty acids Unsaturated fatty acids

1. They contain no double bond. 1. They contain upto six(1-6) double bonds.
2. Saturated fatty acids tend to be solid at 2. Unsaturated fatty acids tend to be liquid at room
room temperature. temperature.
3. They have higher melting point. 3. They have lower melting point.
4. They contain more energy due to more C- 4. They contain less energy due to less C-H bonds.
H bonds.
5. They are more common in animal lipids 5. They are more common in plant lipids which are
which are called fats. called oils.
Exapmles: Examples:
Lauric acid, myristic acid, palmitic acid, stearic Oleic acid, linoleic acid, linoleneic acid, arachidonic
acid. acd.

 Fats and oils are lighter than water.

 These have specific gravity of 0.8.
 They are non-crystalline but some can be crystallized under specific conditions.
 Some common fatty acids are given in following table.

Name Typical No. of Condensed Formula Melting Point Type

Source Carbon (°C)
Palmitic Most fats 16 CH3(CH2)14COOH 63 Saturated
and oils
Stearic Most fats 18 CH3(CH2)16COOH 70 Saturated
and oils
Butyric Butter 4 CH3(CH2)2COOH -8 Saturated
acid and dairy
products.
Oleic Olive oil 18 CH3(CH2)7CH=CH(CH2)7COOH 4 Unsaturated
Linoleic Vegetable 18 CH3(CH2)4CH=CHCH2CH=CH( -5 Unsaturated
oils CH2)7COOH
Note
 Stearin is found in beef and mutton.
 Linolin (C57H104O6) is found in cotton seed have linoleic acid.

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Waxes
 Waxes are highly hydrophobic compounds.
 There are two types of wax.
 Natural waxes are simple lipids.
 They are typically esters of long chain fatty acids and long chain alcohols.
 They are solid at room temperature because they have high melting point.
 They are stable compound/inert and resistant to degradation and atmospheric oxidation.
 They are widespread as protective coatings on fruit and leaves and protect plants from water loss and
abrasive damage.
 They also provide water barrier for insects, birds and animals like sheep.
 For example:
1. bees wax (the most common animal wax and found in honeycomb)
2. cutin (on leaf surfaces and fruits) of plants)
3. suberin (in plant roots)
4. lanolin (found in sheep wool)
 Synthetic waxes which are generally derived from petroleum or polyethene.
 For e.g., paraffin wax which is used to make candles.
 Formula of wax is CH3(CH2)4COO(CH2)29CH3.
 The most common plant wax is epicuticular wax.

Phospholipids
 These are the most abundant lipids.
 Phospholipids are derivatives of phosphatidic acid.
 A phosphatidic acid molecule is most similar to diglyceride that it contains a glycerol, two fatty acids
esterified with first and second OH groups of glycerol and a phosphate group esterified with third OH
group of glycerol.
 A phospholipid is formed when phosphatidic acid combines with one of the four organic compounds
such as :
1. choline (a nitrogenous base) 2. ethanolamine (an amino alcohol)
3. inositol (an amino alcohol) 4. serine (an amino acid).
 Most common type of phospholipid is phosphatidylcholine also called lecithin in which choline is
attached to phosphate group of phosphatidic acid.
 Phospholipids are major compononets of cell membranes in the form lipid bilayer because they are
amphiphilic.
 One end of the phospholipid molecule, containing the phosphate group and additional compound (head
region) is hydrophilic i.e, polar and readily soluble in water.
 The other end, containing the fatty acid side chain (tail region) is hydrophobic i.e. non-polar.
 All lipids are hydrophobic except phospholipids head.
 Phospholipid protein complex are present in milk, blood, cell nucleus and eggs.

Terpenoids
 Terpenoids are a very large and diverse group of organic compounds.
 They are the only polymers among the class of lipids and are non-fatty acid lipids.
 The building block of terpenoids is a five-carbon isoprenoid unit.

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 Classification of terpenoids: Isoprenoid unit by condensation in
different ways give rise to compounds such as rubber,
carotenoids, steroids and terpenes.

Terpenes
 These are polymers of isoprene units. C5H8
 Two isoprene units form a monoterpene (C10H16) e.g, menthol,
four form a diterpene (C20H32) e.g., vitamin A, phytol (chlorophyll
tail) and six form a triterpene (C30H48) e.g., ambrein. Isoprenoid unit
 Natural rubber is a polyterpene.
 Terpenes help in oxidation-reduction reaction.

Steroids
 Steroids are non-fatty acids lipids of high molecular weight
which can be crystalline.
 A steroid nucleus consists of 17 carbon atoms arranged in
four attached rings.
 Three of the rings are six sided, and the fourth is five sided.
 These structures are synthesized from isoprene units.
 Example, cholesterol is the structural component of cell
membrane and brain tissue.
 Sex hormones like estrogen, progesterone in female and Steroid Nucleus
testosterone in male are steroids in nature.
 Vitamin D which regulates calcium metabolism and bile salts which emulsify fats are steroids.

Prostaglandins
 The name prostaglandins is derived from prostate gland because it was first isolated from seminal fluid
in 1935.
 It was believed to be part of prostatic secretions.
 They are found in every mammalian tissue and act as local hormones.
 They sensitize spinal neuron for pain.
 Thermoregulatory centre of hypothalamus to regulate fever.
 They are derivatives of arachidonic acid.
 Every prostaglandin contains 20 carbon atoms including a 5-carbon ring.
 Aspirin like drugs inhibit synthesis of prosatglandin.

Nucleic acids
 Nucleic acids were first isolated (in 1869) by a Swiss physician, Fredrick miescher from the nuclei of
pus cells (white blood cells) and sperm of salmon fish.
 He named this molecule as nuclein, because it was located in the nucleus.
 The basic structure and chemical nature of nuclein was determined (in 1920) and was renamed as
nucleic acid because of its acidic nature.
 Nucleic acids are of two types, DNA and RNA.
Nucleic Acids

DNA contain deoxy ribose RNA contains ribose

tRNA transfers Amino rRNA catalytic part of mRNA transfers information about
acids ribosomes amino acid sequence

 DNA occurs in chromosomes, in the nuclei of the cells and in much lesser amounts in mitochondria
and chloroplasts. RNA is present in the nucleolus, in the ribosomes, in the cytosol and in smaller
amounts in other parts of the cell.
 Both are linerar, unbranched polymers of nucleotides.
 DNA is stable long-term protein and nucleic acid but RNA degrades within 30 minutes in cell.

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Constituents of nucleotide

 The partial hydrolysis of nucleic acids yield compounds known as nucleotides or nucleosides.
 While, complete hydrolysis yields a mixture of bases, pentose sugars and phosphate ions.
 Nucleotides of DNA are deoxyribonucleotides while that of RNA are ribonucleotides.
 Both nucleotides consists of three parts
1. a pentose sugar 2. a phosphate
3. a nitrogen containing ring structure called base.
 The pentose sugar in deoxyribonucleotides is deoxyribose and in ribonucleotides is ribose.
 Deoxyribose has same structure as ribose except it has one oxygen removed from OH at carbon 2.

 Di-deoxyribonucleoside triphosphates are

used to terminate DNA synthesis at different
sites in Sanger’s method.
 Phosphoric acid is a common component of
both nucleotides which provides acidic
properties to DNA and RNA.
 The nitrogen containing heterocyclic
structures are called bases because of
unshared pair of electron on nitrogen atoms,
which can thus acquire a proton.
 There are two major classes of nitrogenous
bases i.e., single six cornered ring,
pyrimidine and double ring (a 6 cornered ring
attached to a 5 cornered ring)purines.
 Pyrimidine bases are of three types i.e.,
cytosine (C), thymine (T) and uracil (U).
Thymine is only found in DNA while the uracil
is only found in RNA.
 Purine bases are also two types i.e., adenine (A) and guanine (G).

Formation of nucleotide
 During the formation of a typical nucleotide, firstly nitrogenous base is linked with 1' carbon of pentose
sugar. Such combination is called nucleoside.

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 The bond that combines the base with sugar is called glycosidic bond (formed between C'1 of pentose
sugar and N9 of purines or N1 of pyrimidines)
 When a phosphoric acid is linked with 5' carbon of pentose sugar of a nucleoside, the nucleotide is
formed.
 A nucleotide with one phosphoric acid is called nucleoside monophosphate, with two phosphoric acids
is called nucleoside diphosphate and with three phosphoric acids is called nucleoside triphosphate.
 The nucleotides which take part in the formation of DNA or RNA polymer must contain three
phosphates but during their incorporation into DNA or RNA polymer each nucleotide losses its two
terminal phosphates.

Nitrogenous RNA (Ribo- RNA (Ribo- DNA (Deoxy- DNA (Deoxy-

Base nucleosides) nucleotides) ribonucleosides) ribonucleotides)
Adenine Adenosine AMP, ADP, ATP d-Adenosine dAMP, dADP, dATP
Guanine Guanosine GMP, GDP, GTP d-Guanosine dGMP, dGDP, dGTP
Cytosine Cytidine CMP, CDP, CTP d-Cytidine dCMP, dCDP, dCTP
Uracil/Thymine Uridine UMP, UDP, UTP d-Thymidine dTMP, dTDP, dTTP

Phosphodiester linkages
 The bond formed between phosphoric acids (H3PO4) and hydroxyl (OH) groups of pentose sugar is
called ester linkage.
 In a polynucleotide chain one phosphoric acid is attached to the OH group of carbon no. 3 of pentose
sugar in front while carbon No 5 of another pentose sugar behind it forming phosphodiester
linkage(C-O-P-O-C) between consecutive nucleotides.
 When one nucleotide joins to other in formation of polynucleotide, the molecule that is released is not
water but pyrophosphate (two phosphate groups bound together). When pyrophosphate is cleaved by
the addition of water, a great deal of free energy is released.
 In this way nucleotides begin to link by phosphodiester bonds and a polymer of nucleotides
(polynucleotide) is formed.

Formation of a phosphodiester bond

Ribonucleic acid (RNA)

 RNA is a polymer of ribonucleotides.
 Unlike DNA, RNA molecule is generally single stranded and does not form a double helix except Reo
virus.

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 But some RNA molecules have regions in which hydrogen bonds between A = U and G = C bases are
formed between different regions of the same molecule thus coiled itself look like double stranded hair-
pin loops.
 RNA is mostly present in cytoplasm but synthesized within nucleus by using one strand of DNA as
templane through transcription.
 RNA polymerase enzyme is used in transcription. In eukaryotes, there are three types of RNA
polymerases.
1. RNA polymerase I transcribes rRNA Types of RNA
2. RNA polymerase II transcribes mRNA
3. RNA polymerase III transcribes tRNA.
 About 97% of the transcriptional output is non- mRNA 3-4% tRNA 10-20% rRNA 80%
protein coding in eukaryotes. So, they are
called non-coding RNA.

Types of RNA
 There are three main types of RNA.

Messenger RNA (mRNA)

 Messenger RNA carries the genetic information from DNA in nucleus to ribosomes in the cytoplasm,
where amino acids are arranged according to the information in mRNA to form specific protein
molecule. This process is called translation.
 This type of RNA consists of a single strand of variable length.
 Its length depends upon the size of the gene as well as the protein for which it is taking the message.
 For example, for a protein molecule of 1,000 amino acids, mRNA will have the length of 3,000
nucleotides.
 Because every three nucleotides in mRNA encode a specific amino acid, such triplets of nucleotides
along the length of mRNA are called codons of genetic codes.
 mRNA is about 3 to 4% (BTB=3-5%) of the total RNA in the cell.

Ribosomal RNA (rRNA)

 It is the major portion of RNA in the cell, and may be up to 80% of the total RNA.
 It is strongly associated with the ribosomal protein where 40 to 50% of it is present.
 It is synthesized by the genes on DNA of several chromosomes found within the region of nucleus
called nuclear organizer.
 On the surface of the ribosome the mRNA and tRNA molecules interact to translate the information
from genes into a specific protein.
 rRNA acts as a machinery for the synthesis of proteins.
 rRNA is the catalytic component of the ribosome.
 rRNA has the largest size among RNA.
 The base sequence of rRNA of all organisms is simar therefore, there is only one type of rRNA.

Transfer RNA (tRNA)

 It comprises about 10 to 20% (BTB = 15%) of the
cellular RNA.
 Transfer RNA molecules are smallest among RNA,
each with a chain length of 75 to 90 (BTB = 80)
nucleotides.
 A tRNA is a single stranded molecule but it shows a
duplex appearance at its some regions where
complementary bases are bonded to one another.
 It shows a flat cloverleaf shape in two dimensional
views.
 Its 5' end always terminates in Guanine base while the
3' end is always terminated with base sequence of
CCA.(mnemonic:Come CAt = CCA)
 Amino acid is attached to tRNA at 3 end.
 It transfers amino acid molecules to the ribosomes
where peptide chains are being synthesized to form Cloverleaf Model of tRNA.
proteins.
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 There is one specific tRNA for cach amino acid. So, the cell will have at least 20 kinds of tRNA
molecules.
 Sixty tRNA have been identified. However, human cells contain about 45 different kinds of tRNA.
 tRNA has three loops:
1. Middle loop
The middle loop in all the tRNA is composed of 7 bases, the middle three of which form the anticodon;
it is complementary to specific codon of mRNA.
2. D-loop
D loop recognizes the activation enzyme.
3. Theta loop
Theta loop recognizes the specific place on the ribosome for binding during protein synthesis.
Mnemonic: MA = Middle Anticodon, AD = Activation of enzyme D-loop, TB = Theeta Binds ribosomes

Conjugated molecules
 Two different molecules, belonging to different categories, usually combine together to form
conjugated molecules.
Glycoproteins (Carbohydrate + Protein)
 Carbohydrates combine covalently with proteins to form glycoprotein.
 Most of the cellullar secretions are glycoprotein in nature.
(mnemonic: SP = Secretion Proteins (glyco), not glycolipids)
 They function as hormones, transport proteins, structured proteins and receptors.
 The blood group antigens contain glycoproteins, which also play an important role in blood grouping.
 They are also present in egg albumin.
 Glycoproteins are integral components of plasma membrane.
 Glycoprotein and glycolipids have a structural role in the extracellular matrix of animals and bacterial
cell wall.
Glycolipids (Carbohydrate + Lipid)
 These are lipids attached with a carbohydrate through glycosidic bond.
 Glycolipids are complex lipids containing one or more simple sugars in connection with long fatty
acids or alcohol.
 Glycolipids are present in white matter of brain and myelin sheath of nerve fibres and chloroplast
membrane.
 They are also integral structural components of plasma membrane.
Lipoproteins (Lipid+ Protein)
 Lipoprotein formed by combination of lipids and proteins.
 It is basic structural framework of all types of membranes in the cells.
 They occur in milk, blood, cell nucleus, egg yolk membrane and chloroplasts of plants.
Nucleoproteins (Nucleic acid + Protein)
 Nucleic acids have special affinity for basic proteins. They are combined together to form
nucleoproteins.
 They are present in ribosomes.
 The nucleohistones (histone protein+nucleic acid) are present in chromosomes.
 They play an important role in regulation of gene expression.
 These are slightly acidic and soluble in water.

Shortlisting
 Approximately 25 elements bio elements. Humans have 16 of them. Out of which 6 are major.
 These may be organic or inorganic.
 Proteins are the most abundant organic compounds while nucleic acids are the most essential
 about 20 percent water in bone cells and 85 percent (FTB = 85 to 90%) in brain cells.
 Hydrogen bonding is responsible for exceptional thermodynamic properties of water.
 The specific heat of vaporization of water is 574 kcal/kg.
 The specific heat capacity of water is 4.184 J/g°C or 1 cal/g°C.
 carbohydrates are defined as “polyhydroxy aldehydes or ketones, or complex substances which
on hydrolysis yield polyhydroxy aldehyde or ketone subunits.”
 They are classified into monosaccharides (having one monomer), oligosaccharides (having 3 to 7
monomers), polysaccharides (having 1000s of units).

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 Enantiomers: These are mirror images of each other, differing in configuration at every chiral center
(e.g., D-glucose and L-glucose).
 Diastereoisomers: Non-mirror image isomers differing in configuration at one or more (but not all)
chiral centers (e.g., D-glucose and D-mannose).
 Epimers: A subtype of diastereoisomers differing in configuration at only one specific chiral center
(e.g., D-glucose and D-galactose).
 Anomers: Isomers that differ at the newly formed chiral center after ring closure, typically at the C-1
carbon (e.g., α-D-glucose and β-D-glucose).
 Our blood normally contains 0.08% glucose.
 For the synthesis of 10g of glucose 717.6 kcal of solar energy is used.
 Sucrose: Glucose + Fructose, α-1,2 glyosidic linkage
 Lactose: Galactose + Glucose, β-1,4 glyosidic linkage
 Maltose: Glucose + Glucose, α-1,4 glyosidic linkage
Polysaccharides
 Starch: Glucose, α-1,4 and α-1,6 glyosidic linkages
 Cellulose: Glucose, β-1,4 glyosidic linkage
 Glycogen: Glucose, α-1,4 and α-1,6 glyosidic linkages
 Chitin: N-acetylglucosamine, β-1,4 glyosidic linkage
 About 170 types of amino acids have been found to occur in cells and tissues.
 Most of the proteins are however, made of 20 types of amino acids.
 Insulin is composed of 51 amino acids(49 peptide linkages) in two chains. One of the chains had 21
amino acids and the other had 30 amino acids
 Similarly, Haemoglobin is composed of 574 amino acids (570 peptide linkages) in four chains, two
alpha and two beta chains. Each alpha chain contains 141 amino acids, while each beta chain
contains 146 amino acids
 α-helix 3.6 amino acids in each turn.
 Primary structure: Comprises the sequence of amino acids linked by peptide bonds.
 Secondary structure: Includes α-helices and β-sheets formed through hydrogen bonds.
 Tertiary structure: Refers to three-dimensional folding driven by side chain interactions, involving
disulfide bonds, ionic bonds, hydrogen bonds, and hydrophobic interactions.
 Quaternary structure: Represents the assembly of multiple polypeptide chains, connected by the
same types of bonds as found in the tertiary structure — disulfide, ionic, hydrogen, and hydrophobic
interactions.
 Sickle cell Hb shows only one difference from normal Hb i.e., glutamic acid is replaced by valine at
position number six in both β-chains.
 The most abundant protein:
1. Earth = rubisco
2. Human body = collagen type (I)
3. Blood = albumin
 Lipids classes:
 Acylglycerols: Glycerol + fatty acids; triacylglycerols are most abundant.
 Waxes: Long-chain esters; melting point is high, e.g., beeswax.
 Phospholipids: Amphiphilic lipids; form cell membranes.
 Glycolipids: Lipid + carbohydrate; present in membranes.
 Terpenoids: Isoprene polymers; C5H8 units, e.g., vitamin A.
 Steroids: Four-ring structure; includes cholesterol, hormones. 17 C
 Prostaglandins: Arachidonic acid derivatives; 20-carbon structure.
 Nucleic acids were first isolated (in 1869) by a Swiss physician, Fredrick miescher from the nuclei of
pus cells (white blood cells) and sperm of salmon fish.
 Nucleotide: Consists of a phosphate group, a sugar, and a nitrogenous base.
 Nucleoside: Contains only a sugar and a nitrogenous base, without the phosphate group.
 In polynucleotides there is phosphodiester linkage(C-O-P-O-C) between consecutive nucleotides.
 Glycolipids: Lipid + carbohydrate; function in cell recognition and signaling.
 Lipoproteins: Lipid + protein; transport lipids like cholesterol and triglycerides in blood.
 Glycoproteins: Protein + carbohydrate; involved in immune response and intercellular
communication.
 Nucleoproteins: Protein + nucleic acid; key roles in genetic material organization (e.g., chromatin)
and protein synthesis (e.g., ribosomes).
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Past MDCAT paper + Most repeated MCQs
Introduction to biological molecules and importance of water
1. Of the total weight of a bacterial cell, carbohydrates constitute only:
A. 1% B. 2% C. 3% D. 4%
2. 18% of the total weight of a mammalian cell is the:
A. Water B. Proteins C. Carbohydrates D. Lipids
3. The amount of heat required to raise the temperature of 01g of water by 01 degree:
A. Specific Heat Capacity C. Electronegativity
B. Heat of vaporization D. Electron affinity
4. Name the process that involves the breakdown of large molecules into smaller ones utilizing
water molecules:
A. Hydrolysis B. Reduction C. Oxidation D. Condensation
5. All of the following are formed through condensation reactions except:
A. Glucose B. Amylopectin C. Insulin D. Triglyceride
6. Ester bond is present in all except:
A. Two nucleotides C. Glycerol and fatty acid
B. Glycerol and phosphoric acid D. Two amino acids
7. All are uses of water by living organisms except:
A. as lubricant & for protection C. used in metabolism
B. as cooling agent & thermo-stabilizer D. source of energy
8. Value of heat capacity of water is:
A. 1.0 cal/g B. 10 cal/g C. 100 kcal/kg D. 574 kcal/g
9. What will happen if water content of protoplasm is reduced as low as 10 percent?
A. Cell will die C. No effect
B. Metabolism will pace up D. Metabolism will slow down
10. Temperature of water rises and falls more slowly as compared to other liquids due to its
______ property.
A. Polar nature B. Heat of vaporization C. Specific Heat D. Ionization

Carbohydrates
11. A biochemical test used for detection of reducing sugars is:
A. Biuret test B. Benedict test C. Spot test D. Iodine test
12. All of the following elements are found in all carbohydrates except:
A. Carbon B. Hydrogen C. Nitrogen D. Oxygen
13. Ratio of hydrogen and oxygen in carbohydrates is:
A. 1:1 B. 2:2 C. 1:2 D. 2:1
14. General formula of carbohydrates is:
A. Cn(H2O)n B. (CH2O)n C. Cx(H2O)y D. Cn(H2O)n-1
15. These are primary products of photosynthesis:
A. Carbohydrates B. Proteins C. Lipids D. Nucleic acids
16. It is not true for carbohydrates:
A. Carbohydrates are used in synthesis of lipids
B. Carbohydrates are less soluble than fats in water
C. Carbohydrates form a component of nucleic acids
D. Carbohydrates are utilized in the synthesis of amino acids
17. It is not a carbohydrate:
A. Starch B. Glycogen C. Chitin D. Cutin
18. Most of the energy from glucose is released by breakdown of:
A. C—C B. C—N C. C—H D. C—OH
19. Cn(H2O)n is not applicable on:
A. Glucose B. Fructose C. Deoxyribose D. Ribose
20. The reducing sugars are so called because they can ____ electron/s:
A. Donate B. Gain C. Share D. Excite
21. All of the following are polymers of glucose except:
A. Glycogen B. Cellulose C. Amylase D. Amylopectin
22. Glycosidic bond cannot be found in:
A. Glucose B. Cellulose C. Glycogen D. Lactose
23. The covalent bond between two monosaccharides is called:
A. Peptide Linkage B. Glucosidic linkage C. Glycosidic linkage D. Ester linkage

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24. Most familiar disaccharide is:
A. Maltose B. Lactose C. Sucrose D. Cellobiose
25. An important sugar occurring only in animals is:
A. Glucose B. Lactose C. Sucrose D. Fructose
26. Hydrolysis of which of the following would yield only glucose?
A. Lactose B. Maltose C. Cellulose D. Cellulase
27. Structural polysaccharides include:
A. Cellulose, Hemicellulose, Chitin C. Cellulose, Starch, Glycogen
B. Cellulose, Starch, Chitin D. Cellulose, Glycogen, Chitin
28. It is an example of polysaccharide that is soluble in hot water:
A. Amylose B. Glycogen C. Cellulose D. Amylopectin
29. Most abundant carbohydrate in nature is:
A. Starch B. Glycogen C. Hemicellulose D. Cellulose
30. Type of glycosidic linkage present in cellulose is:
A. α-1, 4 B. β-1, 4 C. α-1, 4 and α-1, 6 D. β-1, 4 and β-1, 6

Proteins
31. Primary structure of a protein molecule does not comprise:
A. Number of amino acids C. Size of protein molecule
B. Sequence of amino acids D. Shape of protein molecule
32. All proteins in living organism always show following structural levels:
A. Primary & secondary structure C. Tertiary & quaternary structure
B. Secondary & tertiary structure D. Secondary & quaternary structure
33. In quaternary structure, polypeptide chains are aggregated and held together by all of the
following except:
A. Hydrophobic interactions B. Glycosidic linkages C. Hydrogen bond D. Ionic bond
34. It is an example of globular protein:
A. Myosin B. Fibrin C. Fibrinogen D. Keratin
35. An amino acid molecule has the following structure... Which two of the groups combine to
form a peptide link?

A. 1 and 2 B. 1 and 3 C. 2 and 3 D. 3 and 4

36. The sequence of amino acids in a protein is determined by sequence of ____ in ____:
A. Bases, tRNA B. Nucleotides, tRNA C. Nucleotides, DNA D. Nucleotides, mRNA
37. Bond that is most sensitive to rise in temperature is:
A. Peptide bond B. Ionic bond C. Disulphide bond D. Hydrogen bond
38. Number of carbon atoms in simplest amino acid are:
A. 2 B. 3 C. 4 D. 5
39. Which amino acid is essential for formation of disulphide linkages in proteins?
A. Glycine B. Alanine C. Serine D. Cysteine
40. β-Pleated sheet is an example of:
A. Primary structure B. Secondary structure C. Tertiary structure D. Quaternary structure

Lipids
41. Chemical compounds that are defined on base of physical properties are:
A. Carbohydrates B. Proteins C. Lipids D. Nucleic acids
42. Lipids store energy due to higher proportion of:
A. C—C bonds B. C—O bonds C. C—H bonds D. C—N bonds
43. This is an example of conjugated molecule of lipid:
A. Acylglycerol B. Phospholipid C. Glycoprotein D. Glycolipid
44. Most important components of triglycerides are:
A. Glycerols B. Fatty acids C. Ketones D. Isoprenoid
45. Functional group of fatty acid is:
A. –CH₃ B. –OH C. –CHO D. –COOH
46. Which characteristics are applicable to all fats and oils?
FATS OILS

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I) Saturated fat acids Unsaturated fat acids
II) Solid at room temperature Liquid at room temperature
III) Can be crystalized Cannot be crystallized
IV) Obtained from animals Obtained from plants

A. I & II B. I & III C. I, II & III D. I, II, III & IV

47. Saturated fatty acids have C-C double bonds:
A. 0 B. 2 C. 1 D. 3
48. All of the following nitrogenous bases are found in phospholipids except:
A. Choline B. Cytosine C. Ethanolamine D. Serine
49. Fatty acids are found in all of the following except:
A. Acylglycerols B. Phospholipids C. Terpenoids D. Waxes
50. Formation through condensation of which of the following is not associated with release of
water:
A. Amylose B. mRNA C. Fibrin D. Carotenoid

Nucleic acids and conjugated molecules

51. Identify the monocyclic structure:
A. Uracil B. Guanine C. Adenine D. None
52. Which of the following represents high energy bonds in ATP?
A. Ribose – Adenine C. Phosphate – Adenine
B. Ribose – Phosphate D. Phosphate – Phosphate

53. In a typical nucleotide the nitrogenous base is attached to ______ carbon of pentose sugar:
A. C-1 B. C-5 C. C-3 D. C-6
54. Phosphodiester bond is formed between:
A. Two phosphate groups C. One phosphate & two hydroxyl groups
B. Two phosphates & one hydroxyl group D. Two phosphate & two hydroxyl groups
55. All of the following are examples of dinucleotides except:
A. ADP B. NAD C. NADP D. FAD
56. It is the major proportion of RNA in the cell:
A. mRNA B. tRNA C. rRNA D. rDNA
57. Messenger RNA carries the genetic information from:
A. Nucleus to nucleolus B. Nucleolus to ribosome C. DNA to tRNA D. DNA to ribosome
58. Which one is correct about the following diagram?
A. It is a nucleotide C. It is used to form DNA
B. It contains pyrimidine nitrogen D. It is used to form RNA
59. Conjugated molecules are of ______ significance:
A. Structural significance only C. Structural and functional significance
B. Functional significance only D. Has no structural and functional role
60. Which of the following conjugated molecule is incorrectly matched?
A. Lipo-proteins ------- cell membrane B. Glyco-proteins ------- cell surface antigens/ receptors
C. Glyco-lipids ------- cell wall D. Nucleic acids ------- chromosomes

Answers and Explanations

Q Ans Explanation
Q.1 C Carbohydrate is 3% of the total Bacterial cell.
Q.2 B Protein is 18% of the total mammalian cell.
Q.3 A Heat capacity is the amount of energy to increase the temperature of that substance by
1°C.
Q.4 A Sucrose + H2O → Glucose + Fructose
Q.5 A Larger organic macromolecules are formed through condensation reactions. Glucose is
formed through photosynthesis.
Q.6 C Two amino acids are connected through peptide bond.
Q.7 D Mostly living organisms use glucose as major energy source.
Q.8 A To increase 1°C temperature of 1 gram of water we need 1 cal.
Q.9 A A 70–90% of body weight consists of water. Below 10% water in protoplasm, the
survival of cell becomes impossible.
Q.10 C Water has ability to absorb a lot of heat with a little change in its own temperature.
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Q.11 B Biuret test is to detect proteins.
Spot test is for fats.
Iodine test is for polysaccharides.
Q.12 C C, H and O are essential elements found in all carbohydrates while N is non-essential.
Q.13 D They have same ratio as in H2O (2:1).
Q.14 C Cn(H2O)n and (CH2)n are general formulae of monosaccharides.
Cn(H2O)n-1 is disaccharide.
Q.15 A All other organic compounds are synthesized from carbohydrates.
Q.16 B Fats are not soluble in water. They are hydrophobic compounds.
Q.17 D Cutin is part of the waxy plant cuticle.
Q.18 C C—H linkage is principal source of energy.
Q.19 C Formula of deoxyribose is C5H10O4.
Q.20 A Reducing sugars act as reducing agents and release electrons.
Q.21 C Amylase is enzyme and polymer of amino acids.
Q.22 A Monosaccharides being single sugars do not have glycosidic bond.
Q.23 A Glycosidic linkage is found between sugar units.
Peptide bond in amino acids in proteins.
Ester linkage in lipids and nucleic acids.
Q.24 C Sucrose is most common disaccharide.
Q.25 B Lactose is a milk sugar.
Q.26 B Lactose → Galactose + Glucose
Sucrose → Fructose + Glucose
Cellulase → Amino acids
Q.27 A Starch and glycogen are storage polysaccharides.
Q.28 A Amylose is soluble in hot water.
Q.29 D Cellulose is most abundant carbohydrate.
Q.30 B Cellulose is unbranched polymer of β-glucose.
Q.31 C Primary structure is sequence-based, not size-based.
Q.32 A Primary and secondary found in all proteins.
Q.33 B Glycosidic linkages are for carbohydrates, not proteins.
Q.34 C Fibroin is silk protein.
Fibrinogen is globular while fibrin is fibrous.
Q.35 B Peptide bond is formed between 1) NH2 and 3) COOH group.
Q.36 D DNA determines primary sequence of proteins.
Q.37 D Ionic bond is most sensitive to pH change.
Q.38 A Glycine has 2 carbon atoms.
Q.39 D Cysteine is sulfur-containing amino acid.
Q.40 B β-pleated sheet is secondary structure.
Q.41 C Lipids are defined on solubility.
Q.42 C C–H bond are potential source of energy and lipids have them in double the amount as
compared to carbohydrates.
Q.43 D Glycolipids are derivatives; others are simple.
Q.44 B Depends on fatty acid type.
Q.45 D –COOH is carboxylic group.
Q.46 B Saturated fatty acid Unsaturated fatty acid
No double bonds between carbon atoms Upto six double bonds
Straight chain Ringed / Branched
Solid at room temperature Liquid at room temperature
Fats Oils
Animals Plants
More useful for living things.
Q.47 A No double bond = saturated.
Q.48 B Cytosine is nucleic acid base.
Q.49 D Terpenoids have isoprenoid not fatty acids.
Q.50 D Carotenoids form without water release.
Q.51 A Adenine and guanine are bi-cyclic. Uracil is monocyclic.
Q.52 D Phosphodiester bond is a high energy bond.
Q.53 A Nitrogenous base is attached to C1.
Q.54 C Phosphodiester bond is formed between one phosphate and two hydroxyl groups.
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Q.55 A ADP is a mononucleotide, not a dinucleotide.
Q.56 C rRNA constitutes about 80% of total RNA found in any cell.
Q.57 D mRNA is formed from DNA through transcription and translated at ribosomes.
Q.58 D  It is a nucleoside as it lacks phosphate and contains ribose
 used to form RNA as it contains ribose sugar.
Q.59 C Conjugated molecules have both structural (plasma membrane) and functional
(regulation of gene expression) significance.
Q.60 C Cell wall composition
 Plants: Cellulose, Fungi: Chitin, Bacteria: peptidoglycan.

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