BIOMOLECULES IMPORTANT KEY POINTS - FOR RECAP

Carbohydrates are primarily produced by plants and form a very large group of naturally occurring
organic compounds

Most of them have a general formula, Cx (H2O)y , and were considered as hydrates of carbon from
where the name carbohydrate was derived. For example, the molecular formula of glucose
(C6H12O6 ) fits into this general formula, C6 (H2O)6 .

But all the compounds which fit into this formula may not be classified as carbohydrates. For
example acetic acid (CH3COOH) fits into this general formula, C2 (H2O)2 but is not a
carbohydrate. Similarly, rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition.

carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the

compounds which produce such units on hydrolysis.

Some of the carbohydrates, which are sweet in taste, are also called sugars.

Classification of Carbohydrates

(i) Monosaccharides: A carbohydrate that cannot be hydrolysed further to give simpler unit of
polyhydroxy aldehyde or ketone is called a monosaccharide. About 20 monosaccharides are known to
occur in nature. Some common examples are glucose, fructose, ribose, etc.

(ii) Oligosaccharides: Carbohydrates that yield two to ten 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.

For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule
of fructose whereas maltose gives two molecules of only glucose

(iii) Polysaccharides: Carbohydrates which yield a large number of monosaccharide units on

hydrolysis are called polysaccharides. Some common examples are starch, cellulose, glycogen, gums,
etc. Polysaccharides are not sweet in taste, hence they are also called non-sugars

The carbohydrates may also be classified as either reducing or nonreducing sugars. All those
carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing
sugars. All monosaccharides whether aldose or ketose are reducing sugars.

Glucose

Glucose is an aldohexose and is also known as dextrose.

Structure of Glucose

1. Its molecular formula was found to be C6H12O6 .

2. On prolonged heating with HI, it forms n-hexane, suggesting that all the six carbon atoms are
linked in a straight chain.

3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen
cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group (>C = O) in
glucose.

4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild
oxidising agent like bromine water. This indicates that the carbonyl group is present as an
aldehydic group.

5.Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the
presence of five –OH groups. Since it exists as a stable compound, five –OH groups should be
attached to different carbon atoms.

6.On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid,
saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose.

Glucose is correctly named as D(+)-glucose.

‘D’ before the name of glucose represents the configuration

whereas ‘(+)’ represents dextrorotatory nature of the molecule. They are also not related to letter
‘d’ and ‘l

‘D’ and ‘L’ have no relation with the optical activity of the compound. The letters ‘D’ or ‘L’ before the
name of any compound indicate the relative configuration of a particular stereoisomer of a
compound .

, the –OH group lies on right hand side in the structure. All those compounds which can be
chemically correlated to D (+) isomer

In L (–) isomer –OH group is on left hand side as you can see in the structure.

The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at
C1, called anomeric carbon Such isomers, i.e., α-form and β -form, are called anomers.

The six membered cyclic structure of glucose is called pyranose structure (aα– or β–), in analogy
with pyran.

Fructose

Fructose is an important ketohexose, It belongs to D-series and is a laevorotatory compound. It is

appropriately written as D-(–)-fructose.

Disaccharides
The two monosaccharides are joined together by an oxide linkage formed by the loss of a water
molecule. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic
linkage.

In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups

are bonded, these are non-reducing sugars, e.g., sucrose. On the other hand, sugars in which
these functional groups are free, are called reducing sugars, for example, maltose and lactose

Sucrose: One of the common disaccharides is sucrose which on hydrolysis gives equimolar
mixture of D-(+)-glucose and D-(-) fructose. These two monosaccharides are held together
by a glycosidic linkage between C1 of α -D-glucose and C2 of β -D-fructose. Since the
reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a
non reducing sugar.

Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory
fructose. Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose
(+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in
the sign of rotation, from dextro (+) to laevo (–) and the product is named as invert sugar.

Maltose: Another disaccharide, maltose is composed of two α-D-glucose units in which C1 of one
glucose (I) is linked to C4 of another glucose The free aldehyde group can be produced at C1 of
second glucose in solution and it shows reducing properties so it is a reducing sugar.

Lactose: It is more commonly known as milk sugar since this disaccharide is found in milk. It is
composed of β-D-galactose and β -D-glucose. The linkage is between C1 of galactose and C4 of
glucose. Free aldehyde group may be produced at C-1 of glucose unit, hence it is also a reducing
sugar

Polysaccharides

Starch: Starch is the main storage polysaccharide of plants,

is a polymer of α -glucose and consists of two components— Amylose and Amylopectin. Amylose is
water soluble component which constitutes about 15-20% of starch. Chemically amylose is a long
unbranched chain with 200-1000 a-D-(+)-glucose units held together by C1– C4 glycosidic linkage.

Amylopectin is insoluble in water and constitutes about 80- 85% of starch. It is a branched chain
polymer of α -D-glucose units in which chain is formed by C1–C4 glycosidic linkage whereas
branching occurs by C1–C6 glycosidic linkage.

Cellulose: Cellulose occurs exclusively in plants and it is the most abundant organic substance in
plant kingdom. It is a predominant constituent of cell wall of plant cells. Cellulose is a straight chain
polysaccharide composed only of β -D-glucose units which are joined by glycosidic linkage between
C1 of one glucose unit and C4 of the next glucose unit.
(iii) Glycogen: The carbohydrates are stored in animal body as glycogen. It is also known as
animal starch because its structure is similar to amylopectin and is rather more highly branched. It
is present in liver, muscles and brain. When the body needs glucose, enzymes break the glycogen
down to glucose. Glycogen is also found in yeast and fungi

Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Cell
wall of bacteria and plants is made up of cellulose.

Proteins- All proteins are polymers of α -amino acids.

Amino acids contain amino (–NH2 ) and carboxyl (–COOH) functional groups

Only a-amino acids are obtained on hydrolysis of proteins.( α -amino acid (R = side chain))

Except glycine, all other naturally occurring α -amino acids are optically active, since the α -
carbon atom is asymmetric

. Most naturally occurring amino acids have L-configuration. L-Aminoacids are represented by
writing the –NH2 group on left hand side.

proteins are the polymers of a-amino acids and they are connected to each other by peptide bond
or peptide linkage. Chemically, peptide linkage is an amide formed between –COOH group and –NH2
group.

If a third amino acid combines to a dipeptide, the product is called a tripeptide. A tripeptide
contains three amino acids linked by two peptide linkages. - insulin which contains 51 amino acids.

(a) Fibrous proteins : When the polypeptide chains run parallel and are held together by hydrogen
and disulphide bonds, then fibre– like structure is formed. Such proteins are generally insoluble in
water. Some common examples are keratin (present in hair, wool, silk) and myosin (present in
muscles), etc.

(b) Globular proteins This structure results when the chains of polypeptides coil around to give a
spherical shape. These are usually soluble in water. Insulin and albumins are the common examples
of globular proteins.

(i) Primary structure of proteins: Each polypeptide in a protein has amino acids linked with each
other in a specific sequence and it is this sequence of amino acids that is said to be the primary
structure of that protein. Any change in this primary structure i.e., the sequence of amino acids
creates a different protein

(ii) Secondary structure of proteins: They are found to exist in two different types of structures
viz. alpha-helix and β -pleated sheet structure. These structures arise due to the regular folding of
the backbone of the polypeptide chain due to hydrogen bonding between C = O and –NH– groups of
the peptide bond.
alpha-Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen
bonds by twisting into a right handed screw (helix) with the –NH group of each amino acid residue
hydrogen bonded to the C = O of an adjacent turn of the helix

In b-pleated sheet structure all peptide chains are stretched out to nearly maximum extension and
then laid side by side which are held together by intermolecular hydrogen bonds. The structure
resembles the pleated folds of drapery and therefore is known as b-pleated sheet.

The main forces which stabilise the 2° and 3° structures of proteins are hydrogen bonds,
disulphide linkages, van der Waals and electrostatic forces of attraction.

Denaturation of Proteins

When a protein in its native form, is subjected to physical change like change in temperature or
chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold
and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein.
During denaturation secondary and tertiary structures are destroyed but primary structure
remains intact. The coagulation of egg white on boiling is a common example of denaturation.
Another example is curdling of milk which is caused due to the formation of lactic acid by the
bacteria present in milk.

Enzymes

This process involves a sequence of reactions and all these reactions occur in the body under very
mild conditions. This occurs with the help of certain biocatalysts called enzymes. Almost all the
enzymes are globular proteins.

activation energy for acid hydrolysis of sucrose is 6.22 kJ mol–1, while the activation energy
is only 2.15 kJ mol–1 when hydrolysed by the enzyme

Vitamins

Most of the vitamins cannot be synthesised in our body but plants can synthesise almost all of
them,

(i) Fat soluble vitamins: Vitamins which are soluble in fat and oils but insoluble in water are kept in
this group. These are vitamins A, D, E and K. They are stored in liver and adipose (fat storing)
tissues.

(ii) Water soluble vitamins: B group vitamins and vitamin C are soluble in water so they are grouped
together. Water soluble vitamins must be supplied regularly in diet because they are readily
excreted in urine and cannot be stored (except vitamin B12) in our body

Nucleic acids.
nucleus of a living cell is responsible for this transmission of inherent characters, also called
heredity. The particles in nucleus of the cell, responsible for heredity, are called chromosomes
which are made up of proteins and another type of biomolecules called nucleic acids. These are
mainly of two types, the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids
are long chain polymers of nucleotides, so they are also called polynucleotides.

Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen
containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is β -D-
2-deoxyribose whereas in RNA molecule, it is β -D-ribose.

DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also
contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).

A unit formed by the attachment of a base to 1¢ position of sugar is known as nucleoside.

Nucleotides are joined together by phosphodiester linkage between 5¢ and 3¢ carbon atoms of the
pentose sugar.

Two nucleic acid chains are wound about each other and held together by hydrogen bonds between
pairs of bases

Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with
guanine.