5.carbohydrates

Structures and chemistry of
carbohydrates!!!!!!!!!!
Watch the video for more explanation  https://oke.io/xwwvq

Introduction
● Single most abundant class of organic molecules
found in nature.
● The name arises from the basic molecular formula
(CH2O)n where n=3 or more.
● Hence the name hydrates of carbon.
● They are also called saccharides.

FUNCTIONS OF CARBOHYDRATES
●As a major energy source for living
organisms (glucose is a principal energy source in animal and plants)
●As a means of transporting energy ( exp:
sucrose in plant tissues)
●As a structural material ( cellulose in plants, chitin in insects,
building blocks of nucleotides).
●As a precursor for other biomolecules
(purine, pyrimide)
●Watch the video for more explanation 
https://oke.io/xwwvq

FUNCTIONS OF CARBOHYDRATES cntd.....
● Non-digestible carbohydrates like cellulose, agar,
gum and pectin serve as dietary fibres.
● Serves as a storage form of energy e.g. Glycogen in
animals and starch in plants.
● Play a role in lubrication, cellular
intercommunication and immunity. Carbohydrates
e.g. Glycoprotein and glycolipid units on cell surfaces
are key participant in cell to cell recognition.
● Watch the video for more explanation 

cellulose, chitin,
starch, glycogen,
glucoaminoglycans
disaccharides
Glycoproteins
(bacterial cell
walls
CLASSIFICATION
Carbohydrate
Mono
saccharide
Oligo
saccharide
Poly
saccharide
Glyconoconjugate
s
Glucose, fructose
Ribose (aldopentose)
Deoxy ribose
glycoproteins
and
proteoglycans

monosaccharides
● Colorless, crystalline solids.
● Also called simple sugars.
● Soluble in water but not soluble in non polar
substances.
● Simplest form of carbohydrate

Classification of monosaccharides
● It is based on;
○ number of carbons they possess e.g. Trioses, tetroses,
pentoses.
○ The functional group present; that is,
❑ aldehyde(CHO)=aldoses
❑ Ketone(C=O) = ketoses.
Examples include trioses; glyceraldehydes and dihydroxyacetone
Tetroses; erythrose
Pentoses; ribose
Hexoses; glucose, fructose.

Monosacharides : Exp. aldoses & ketoses
Aldotetrose
Aldotriose
Aldopentoses
Ketotriose Ketotetrose Ketopentose Ketohexose
Aldohexose

STRUCTURE OF MONOSACCHARIDES
Physiologically and bio-medically, glucose is the most
important monosaccharide. the structure of glucose
can be represented in three ways;
1)The straight chain structural formula(Fischer’s
projection). The bond behind the mirror is
represented by the broken line or horizontally while
that in front of the mirror, is represented by the thick
line or as the vertical line.
2)Cyclic formula(ring structure or Haworth projection)
3)Boat and chair form
4)Watch the video for more explanation 

Structure of monosaccharides cntd.....

Structures of monosaccharides cntd....
● Isomerism: is the existence of molecules that have
the same number of the same kinds of atoms (and
hence the same molecular formula) but different
chemical and physical properties. There are
generally two types of isomerism exhibited by sugar
and they include;
○ Structural
○ Stereoisomerism
○ Watch the video for more explanation 

Structural isomerism
● Same molecular formula but with different
structures of atoms and bonds .i.e. There atoms are
linked in different sequences. This exists between
glucose, mannose, galactose and fructose in which
they have the chemical formula C6H12O6 but differ in
structure.
● Another examples is that of Aldose-Ketose
isomerism.

stereoisomerism
● Same molecular formula and unlike structural
isomerism, same structure but they differ in
configuration, that is the arrangement of their atoms
in space.
● Its a special type of isomerization
● This is made possible by the presence of a chiral
carbon.
● Chiral carbon is that which contains four different
chemical groups.

Stereoisomerism cntd....
• The number of possible isomers depend on the
number of chiral carbons. E.g. the simplest
carbohydrates that contains a chiral carbon is
glyceraldehyde.
• Mirror image stereoisomer are also called
enantiomers. This is designated as D or L series.
The D or L series depends on the orientation of the H
and OH groups around the asymmetric carbon.
• when the OH group is on the right of the CHO group,
then its designated D and vice versa.
• Watch the video for more explanation 

Types of stereoisomers
● D and L isomerism; depends on the configuration of
the chiral carbon farthest from the carbonyl group.
○ Mirror image stereoisomers are known as enantiomers.
○ Thus the D and L isomerism depends on the orientation of the
H and OH group around the asymmetric carbon adjacent to
the terminal primary alcohol carbon.
○ When the OH group on this carbon atom is on the right, it
belongs to the D-series, when its on the left, it is a member of
the L-series.

Sterioisomerism.
Almost all the properties of the two members of an
enantiometric pair are the same.

Sterioisomerism.
● Optical isomerism depends on the capacity of a
substance to rotate the plane polarized light passing
through it.
○ The enantiomeric member that rotates the plane of polarized
light in a clockwise direction is said to be dextrorotatory(D) or
(+) while anticlockwise is said to be levorotatory(l) or (-).

Epimers.
● Epimerism ;If two monosaccharides differ only at
one carbon atom, then they are known as epimers.
e.g. glucose and galactose are C4 epimers while
glucose and mannose are C2 epimers.

Cyclization/ anomerization of monosacharides
in aqueous solution, monosaccharides with five or more carbon atoms in the
backbone occur predominantly as cyclic (ring) structures in which the carbonyl group
has formed a covalent bond with the oxygen of a hydroxyl group along the chain.
The new chiral center in cyclic (c1) is called anomeric carbon

Pyranoses& Furanoses
Pyranoses: six-membered ring compounds ( resemble pyran )
Furanoses : fivemembered rings, (resemble furan)
The structure systematic names glucose & fructose become
An English chemist W.N.
Haworth gave a more accurate
picture of carbohydrate
structure. (Ref. P.205 of
textbook)
HAWORTH STRUCTURES

Cyclization continued.
● If the OH group is below the plane, then its
designated alpha and if the OH group is above the
plane, then its designated beta.
● It should also be noted that substituents drawn to
the right in the fisher’s projection are below the ring
in the Haworth’s projection.

FISHER AND HAWORTH FORMS OF SUGAR

SUMMARY OF SUGAR STRUCTURES
● ISOMERS- compounds that have the same chemical formula e.g.
fructose, glucose, mannose, and galactose are isomers of each other
having formula C6H12O6.
● EPIMERS- refer to sugars whose configuration differ around one
specific carbon atom e.g. glucose and galactose are C-4 epimers and
glucose and mannose are C-2 epimers.
● ENANTIOMERS- a special type of isomerism found in pairs of
structures that are mirror images of each other. The mirror images
are termed as enantiomers and the two members are designated as
D- and L- sugar. The vast majority of sugars in humans are D-sugars.
● CYCLIZATION OF SUGARS- most monosaccharides with 5 or more
carbon atoms are predominately found in a ring form, where the
aldehyde or ketone group has reacted with an alcoholic group on the
same sugar group to form a hemiacetal or hemiketal ring.
Pyranose ring- if the ring has 5 carbons and 1 oxygen.
Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen
Watch the video for more explanation 

Reactions of monosaccharides.
● Mutarotation
● Reduction
● Isomerization
● Esterification
● Oxidation
● Glycoside formation.

IMPORTANT REACTIONS IN MONOSACCHARIDES
Details
1. Mutarotation –
alpha and beta forms of sugars are readily interconverted
when dissolved in water.
2 Oxidation and reduction
monosaccharides can be oxidised by relatively mild
oxidising agents such as ferric (Fe3+) or cupric (Cu2+) ion.
Such monosaccharidfes are known as reducing sugars and
vice versa.
All aldehydes and ketones in straight chain form can react as
reducing sugars.
Hemiacetals amd hemiketal (ring forms) are all reducing
sugars as long as there is an OH group at the anomeric
carbon and hasnt overgone any further reactions.

Reducing sugar continued.
● However if the hemiacetal undergoes further
reactions such as in the formation of glycosidic bond,
it becomes a different scenario.
● Not all disaccharides are reducing sugars.
● Reducing sugars have a free aldehyde group in solution
phase while a non reducing sugar does not bear a free
aldehyde group.
● E.g. In lactose as against sucrose.
● Reason will be further explained under
disaccharides.

Important reactions of monosaccharides.
3 REDUCTION
reduction of the aldehyde and ketone groups of
monosaccharides yield sugar alcohols (alditols)
Sugar alcohols e.g. sorbitol build up in the eye of
diabetic persons is implicated in cataract
formation.
4. ISOMERIZATION
Monosaccharides undergo several types of
isomerization e.g. D-glucose in alkaline solution
for several hours containn D-mannose and D-
fructose. The conversion of glucose to mannose is
termed epimerization.

IMPORTANT REACTIONS (Cont)
5 ESTERIFICATION
Free OH groups of carbohydrates react with acids to form
esters. This reaction can change the physical and chemical
propteries of sugar.
6. GLYCOSIDE FORMATION-
Hemiacetals and hemiketals reaction with alcohols to form
the corressponding aceta or ketal .On the contrary when a
cyclic hemiacetal or hemiketal form of monosaccharide
reacts with alcohol, the new linkage is called glycosidic
linkage and the compound glycoside.

Reactions of monosaccharides cntd..

Reaction of monosaccharides cntd...

Monosaccharide reactions cntd...

Reactions of monosaccharides cntd...
Reduction of glucose to sorbitol

DISACCHARIDES
● Oligosaccharides consist of a short chain of
monosaccharide units(2-10), joined together by a
characteristic bond called glycosidic bond.
● They are subdivided into different groups based on
the numbers of monosaccharide unit present.
● The simplest oligosaccharide unit is disaccharide
containing 2 monosaccharide units joined
covalently by an O-glycosidic bond. Most abundant
oligosaccharide in nature.
● Examples of disaccharides include;
❑ Maltose: glucose and glucose.
❑ lactose: glucose and galactose.
❑ Sucrose:glucose and fructose.

Glycosidic bond.
● Monomers of carbohydrates are linked together to
form disaccharides and oligosaccharides by
glycosidic bonds/ linkages.
● This is formed when a hydroxyl group of one sugar
reacts with the anomeric carbon of the other.
● Glycosidic bonds are named according to the
numbers of the connected carbons, and also with
regards to the position of the anomeric hydroxyl
group of the sugar involved in the bond.
● That is, if the anomeric OH is in the α configuration
then the linkage is an α-bond.

● Glycosidic bonds are readily hydrolyzed by acid but
resist cleavage by bases.
● Thus disaccharides can be hydrolysed to yield their
free monosaccharide units by boiling with dilute
acids.

Classification of oligosaccharides and their
examples
● Oligosaccharides with more than three subunits are
usually found bound as side chains in glycoproteins;
such as blood group antigens.
● Trisaccharides ; raffinose;
glucose+galactose+fructose.
● Tetrasaccharides; e.g. stachyose; 2 molecules of
galactose + glucose + fructose.
● Pentasaccharide ; e.g verbose; 3 molecules of
galactose + glucose + fructose.

Classification of oligosaccharides cntd...

polysaccharides
● They are polymers consisting of hundreds or
thousands of monosaccharide units.
● They are also called glycans or complex
carbohydrates.
● They have high molecular weight and are only
sparingly soluble in cold water but form colloidal
solutions when heated with water.
● They differ from each other in ;
● the identity of their recurring monosaccharide units.
● the type of bond linking the units.
● Degree of branching.

Classification of polysaccharides.

Heteropolysaccharides(heteroglycans)
● They contain two or more different types of
monosaccharides units or their derivatives. They are
closely related with proteins and lipids.
● Heteropolysaccharides present in human is
glycosaminoglycans, e.g. Heparin, chondritin
sulphate, hyaluronic acid, dermatan sulphate,
keratin sulphate and blood group polysaccharides.
● Plants heteroglycans are agar, gum, and pectins.
● Watch the video for more explanation  https://oke.io/xwwvq

homopolysaccharides
● They are made up of several units of one and the same
type of monosaccharide unit only.
● The most common homoglycans are ; starch, dextrins,
glycogen, and dietary fibre cellulose.
● Some homoglycans serve as a storage form of
monosaccharides used as fuel e.g. starch and glycogen.
● Some others serve as structural elements in plants e.g.
cellulose.

Introduction to glycoconjugates
● They are compounds that result from covalent
linkages of carbohydrate molecules to both proteins
and lipids.
● They have a profound effects on the functions of
individual cells as well as cell-cell interactions of
multicellular organisms.
● Two classes of carbohydrate-protein conjugate:
glycoproteins and proteoglycans.
● The glycolipids (oligosaccharide-containing lipid
molecules) are found predominately on the outer
surface of plasma membrane.

Glycosylation.
● Glycosylation is the process or result of addition of
saccharides to proteins and lipids
● The process plays an important role in the synthesis of
membrane and secreted proteins
● Majority of proteins synthesized in the rough ER undergo
glycosylation
● It is an enzyme -directed site-specific process.
● Two types of glycosylation exist: N-linked glycosylation
to the amide nitrogen of asparagine side chains and O-
linked glycosylation to the hydroxyl group of serine and
threonine side chains.
● Glycosylation may play a role in cell-cell adhesion (a
mechanism employed by cells of the immune system), as
well.

Glycoproteins.
● Glycoproteins are proteins to which oligosaccharides are
covalently attached.
● They are carbohydrate-protein conjugates in which the
carbohydrate moieties are smaller and more structurally
diverse than the glycosaminoglycans of proteoglycans.
● Examples of glycoproteins include;

GLYCOPROTEIN FUNCTIONS
● Types of glycoproteins: asparagine-linked carbohydrate;
mucin-type cabohydrate.
● Examples: glycophorin (membrane protein, source – human
RBC, % carbohydrate - 60); potato lectin (lectin, carbohydrate
binding proteins, source – potato, % carbohydrate – 50).
● Functions: Many glycoproteins have structural functions:
constituent of the cell wall; form connective tissues such as
collagen; found in gastrointestinal mucus secretions; used as
protective agents and lubricants ;found abundantly in the
blood plasma.
● The human blood groups A, B, AB, and O depend on the
oligosaccharide part of the glycoprotein on the surface of
erythrocyte cells. The terminal monosaccharide of the
glycoprotein at the nonreducing end determines blood group.

5.4. PROTEOGLYCANS
● Proteoglycans represent a special class of glycoproteins
that are heavily glycosylated.
● They consist of a core protein with one or more covalently
attached glycosaminoglycan chain(s).
● These glycosaminoglycan (GAG) chains are long, linear
carbohydrate polymers that are negatively charged under
physiological conditions, due to the occurrence of sulphate
and uronic acid groups.
● Proteoglycans can be categorised depending upon the nature
of their glycosaminoglycan chains. These chains may be:
1. chondroitin sulfate and dematan sulfate
2. heparin and heparin sulfate
3. keratan sulfate

Glycolipids.
● Theses are lipids with a carbohydrate group attached
to them by glycosidic bond.
● They are commonly found in extracellular part of the
eukaryotic cell membrane.
● Examples include glycosphingolipids, cerebosides,
gangliosides.

Functions of glycolipids.
● Cell to cell communication, recognition and
interaction e.g. Glycosphingolipids in nerve cells are
responsible for nerve signalling.
● Enhance interaction between WBC and endothelial
cells of the blood vessels to initiate inflammatory
responses.
● It forms part of blood type/ blood group antigen.

Functions of glycolipids.
● E,g.
● Blood group A –Nacetylgalactoseamine is attached
to lipids on the RBC membrane.
● Group B – galactose is attached to lipids on the RBC
membrane.
● Blood group D - no extra sugar attached to the
oligosaccharide.
● AB- contains both A and B antigen.

END NOTES
● The destiny of a nation depends on the manner in
which it feeds itself.
● We eat to live, NOT, live to eat.
● Lower your carbohydrate consumption, but balance
it with the right amount of protein and fat.

5.carbohydrates

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