Introduction
to carbohydrates
Contents
– Introduction to carbohydrates
– Classification of carbohydrates
Objective
• At the end of this lecture, student
will be able to
– Explain carbohydrates
– Classify carbohydrates
– Discuss the major pathways of carbohydrate metabolism
– Outline the functions of
carbohydrate
Introduction
to carbohydrates
• Carbohydrates
means hydrates of carbon
• Defined
as polyhydroxy aldehydes or ketones or compounds which produce them on
hydrolysis
• Soluble
in water and sweet in taste
• These
are most abundant organic molecules in nature
• Primarily
composed of C, H, & O
Function
• Most abundant dietary source of
energy (4 Cal/g)
• Precursors for many organic
compounds (fats, amino acids)
• Participate in the structure of cell
membrane and cellular functions such as cell growth, adhesion and fertilization
• Structural components of many
organisms
Fiber
(cellulose) of plants
Cell wall of
microorganisms
• Serve as the storage form of energy
(glycogen) to meet the immediate energy demands of the body
Classification of carbohydrates
Based on the number of sugar units – classified into 3 types
1.
Monosaccharides
• These are simplest group of
carbohydrates and are often referred as simple sugars
• General formula Cn(H20)n,
& cannot be further hydrolysed
• Based on the functional group &
number of carbon atoms, they are furthers classified into two types
a. Aldoses: here the functional group is aldehyde e.g.
glyceraldehyde, glucose, erythrose etc
b. Ketoses: here the functional group is keto e.g. fructose
Based on
the number of carbon atoms;
• Trioses (3C)
• Tetroses (4C)
• Pentoses (5C)
• Hexoses (6C)
• Heptoses (7C)
2. Oligosaccharides
• Contain 2-10 monosaccharide
molecules which are liberated on hydrolysis
• Based on the number of
monosaccharide units, further subdivided to
a. Disaccharides: sucrose, Lactose, maltose
b. Trisaccharides: Raffinose
c. Tetrasaccharides: Stachyose
3. Polysaccharides
• Polymers of monosaccharide units
with high molecular weight
• Usually tasteless( non-sugars) and
form colloids with water
• Polysaccharides are of two types
a.
Homopolysaccharides: They contain
single type of monosaccharide units e.g. Starch, Inulin, Glycogen, Dextrin.
b.
Heteropolysaccharides: They possess two or more different type of
monosaccharide units e.g. Heparin
• Carbohydrates are the major source
of energy for the living cells
• They are the 1st cellular
constituents synthesized by green plants during photosynthesis from CO2 &
H2O on absorption of light
• Glucose is the central molecule in
carbohydrate metabolism since all the major pathways of carbohydrate metabolism
are connected with it
• Fasting blood glucose level in normal
individuals is 70-100 mg/dl
• Liver plays a key role in monitoring
and stabilizing blood glucose levels
Major pathways of carbohydrate
metabolism
1. Glycolysis: oxidation of glucose to pyruvate and lactate
2. Citric acid cycle: oxidation of acetyl CoA to CO2. it is the
final common oxidative pathway for carbohydrates, fats or amino acids, through
acetyl CoA
3. Gluconeogenesis: synthesis of glucose from non-carbohydrate
precursors (e.g. amino acids, glycerol etc.)
4. Glycogenesis: formation of glycogen from glucose
5. Glycogenolysis: breakdown of glycogen to glucose
6. Hexose monophosphate shunt: This pathway is an alternative to
glycolysis and TCA cycle for the oxidation of glucose (directly to carbon
dioxide and water)
7. Uronic acid pathway: Glucose is converted to glucuronic
acid, pentoses and in some
animals to ascorbic acid (not in man) this pathway is also an alternative oxidative
pathway for glucose
8. Galactose metabolism: Pathways concerned with the
conversion of galactose to glucose and the synthesis of lactose
9. Fructose metabolism: Oxidation of fructose to pyruvate
and the relation between fructose and glucose metabolism
10. Amino sugar and mucopolysaccharide
metabolism: The
synthesis of amino sugars and other sugars for the formation of
mucopolysaccharides and glycoproteins
Summary
• Carbohydrates
means hydrates of carbon
• Primarily
composed of C, H, & O
• Based
on the number of sugar units, they are classified into 3 types; monosaccharide,
oligosaccharide and polysaccharides
• Based on the number of carbon atoms,
they are classified as trioses, tetroses, pentoses, hexoses & heptoses
• Fasting blood glucose level in
normal individuals is 70-100 mg/dl
• Major pathways of carbohydrate
metabolism are glycolysis, TCA, gluconeogenesis, glycogenesis, glycogenolysis,
HMP shunt, uronic acid pathway, galactose metabolism, Fructose metabolism &
amino sugar and mucopolysaccharide metabolism