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MONOSACCHARIDESMONOSACCHARIDES Monosaccharides are carbohydrates that
cannot be hydrolyzed to obtain smaller molecules of carbohydrate.
White crystalline solids , very soluble in water, have sweet taste.
# Carbons Category Name Relevant examples
3 Triose Glyceraldehyde, Dihydroxyacetone
4 Tetrose Erythrose
5 Pentose Ribose, Ribulose, Xylulose
6 Hexose Glucose, Galactose, Mannose, Fructose
7 Heptose Sedoheptulose
9 Nonose Neuraminic acid, also called sialic acid
Hexoses Hexoses Three common sugars share the same molecular
formula: C6H12O6. Because of their six carbon
atoms, each is a hexose. hexose.
They are:
glucoseglucose, "blood sugarblood sugar", the immediate source of
energy for cellular respiration
galactosegalactose, a sugar in milk (and yogurt), and
fructose, a sugar found in honey.
D-GlucoseD-Glucose (Grape Sugar)(Grape Sugar)
Glucose is by far the most common carbohydrate
and classified as a monosaccharide,
an an aldosealdose, ,
a a hexosehexose,
and is a reducing sugar.
It is known as dextrosedextrose, because it is
dextrorotatorydextrorotatory
For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.
O H O H C C H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
OccurrenceOccurrence
Widely distributed in nature
Occurs in combination with other sugars forming disaccharides
SucroseSucrose MaltoseMaltose Lactose Lactose
Commercially obtained from starch
Glucose is also called blood sugar blood sugar
circulates in the blood at a concentration of 60-100 mg/dL 60-100 mg/dL of blood in the fasting condition
Normally trace amounts pass out in urine
Increased amounts in diabetes mellitus.
74% as sweet as sucrose74% as sweet as sucrose
ChemistryChemistry
Glucose can occur both in pyranose and furanose pyranose and furanose forms
O O
Pyranose furanose
PyranosePyranose form is more predominant due to stable ring
D-glucose: the chair conformer
OH
HH
H
HOH
H
O
OHOH
CH2OH
12
3
45
6
Four of the five bulky groups (OH and CH2OH on C 2,3,4,5)
on the ring are in the more stable equitorial positions!
FructoseFructose Fructose is classified as a:
monosaccharide,
the most important ketose sugar,
a hexose,
and is a reducing sugar.
Fructose, along with glucose are the
monosaccharides found in disaccharide,
sucrose.
Sweetest of all sugars.Sweetest of all sugars.
73% as sweet as sucrose73% as sweet as sucrose
Fermented by yeastFermented by yeast
Pyranose form levorotatoryPyranose form levorotatory
Furanose form dextrorotatoryFuranose form dextrorotatory
OccurrenceOccurrence
Occurs in plant kingdom as such OROR
in combination with glucose forming SucroseSucrose
Animal tissues also contain it in small amounts
Hydrolysis of cane sugar and of inulin.
Can be changed to glucose in the liver and so
used in the body.
Main source of energy for the spermatozoa
LevorotatoryLevorotatory (in sucrose dextrorotatorydextrorotatory)
Shows mutarotation
MutarotationMutarotation
Unlike the other stereoisomeric forms, α and β anomers spontaneously interconvert in solution.
This is called mutarotation.
Galactose
Galactose is classified as a:monosaccharide,monosaccharide, an aldosealdose, a hexosehexose, and is a reducing sugarreducing sugar.
Dextrorotatory shows mutarotationDextrorotatory shows mutarotation
Occurrence Occurrence
Galactose is more commonly found in the
disaccharide, lactose or milk sugar.
It is found in seed coat of legumesseed coat of legumes.
32% as sweet as sucrose32% as sweet as sucrose
C-4 epimer C-4 epimer of glucose
Galactose is part of nervous tissues as
galactolipidsgalactolipids
D-mannose
Present in the polysacchride component of
tuberculoproteintuberculoprotein
If ingested, it is absorbed and converted to glucose in
the body
C-2 epimer C-2 epimer of glucose Cannot be differentiated from glucose by
Ozasone testOzasone test
PentosesPentoses Occur widely in animal and plant
kingdom as component of polysacchride
Arabinose and xylose- glycoproteins
Ribose - RNA, ATP, GTP, NAD+,
Coenzymes
Deoxyribose - DNA
Properties Properties
May be aldoses or ketoses
Form ozasone crystals with phenylhadrazinephenylhadrazine
with acids converted to furfuralfurfural,
are strong reducing sugarsare strong reducing sugars
Not fermented by yeast
Deoxy sugars Ribose and its related compound,
deoxyribose, are the building blocks of the backbone chains in nucleic acids, better known as DNA and RNA.
Ribose is used in RNA and deoxyribose is used in DNA.
The presence or absence of the -OH group on
carbon (#2) is an important distinction between
ribose and deoxyribose.
Ribose has an alcohol at carbon # 2, while
deoxyribose does not have the alcohol group.
Deoxyribose give most of sugar reaction but do not form ozasonesDeoxyribose give most of sugar reaction but do not form ozasones
Disaccharides Three common disaccharides: Sucrose — common table sugar = glucose
+ fructose Lactose — major sugar in milk = glucose
+ galactose Maltose — product of starch digestion =
glucose + glucose
The resulting linkage between the sugars is called a glycosidic bond. The molecular formula of each of these disaccharides isC12H22O11 = 2 C6H12O6 − H2O
SucroseSucrose
Sucrose is made from glucose and fructose units
Sucrose or table sugar Cane, beet sugar, Pineapple Has no free anomeric carbon Is a non-reducing sugar No osazone crystals
H O H
H
OHH
OH
CH2OH
H
-D-Glucose
HO CH2OH
HHOH2C
OH H
H HOO
-D-Fructose
O
Dextrorotatory
on hydrolysis becomes levorotary
Invert sugarInvert sugar
Invert sugar 30% more sweet than sucroseInvert sugar 30% more sweet than sucrose
Honey is rich with invert sugar
In sucrase deficiency, malabsorption leads
to diarrhea and flatulence
MaltoseMaltose Glucose+ Glucose Formed in vivo by action of slivary slivary
amylase on starchamylase on starch In vitro formation by acidic hydrolysis of acidic hydrolysis of
starchstarch. Reducing sugar Has free anomeric carbon forms osazone crystals Shows mutarotation 32% as sweet as sucrose32% as sweet as sucrose Baby and invalid foods
LactoseLactose
Glucose + Galactose Milk sugar Reducing sugar In lactase deficiency, malabsorption leads
to diarrhea and flatulence Certain bacteria can ferment lactose to
lactic acid - souring of milk. ( lactobacillus bulgaricus)
16% as sweet as sucrose (least sweet sugar)16% as sweet as sucrose (least sweet sugar)
H O OH
HH
OHH
OH
CH2OH
H
-D-Glucose
HO O
H HH
OHH
OH
CH2OH
H
-D-Galactose
O