Download Carbohydrates

LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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CARBOHYDRATE
Carbohydrates are compounds of carbon, hydrogen and oxygen in the ratio of 1:2:1.
They are also called hydrate of carbon because in them hydrogen and oxygen occur in
the ratio 2: 1 similar to that found in water. They are also called saccharide because
they are sugars or are polymers of sugars or monosaccharides examples glucose,
fructose, starch, glycogen, sugar and cellulose.
Molecular Structure of Carbohydrate:
Carbohydrates are represented by a general formula Cn (H 2 O)n, where n represents
number of carbon atoms. Chemically carbohydrates are aldehyde or ketone derivatives
of alcohols with more than one –OH group. Carbohydrates with aldehyde group are
called aldoses and those with ketone group are called ketoses.
Classification:
In living organism carbohydrates are classified into following three forms:
1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides
1) Monosaccharides: Simple sugar with one saccharide molecule. They are
represented by general formula Cn H2n On. The number of carbon atoms in
monosaccharides varies from 3 to 7. The carbon atoms form an unbranched straight
chain, joined together by single covalent bonds. A monosaccharide molecule has
two functional group 1) a carbonyl (- C = O) group and 2) a hydroxyl (-OH) group. On
the basis of number of carbon atoms monosaccharides are classified into following
forms.
a) Trioses: Smallest carbohydrate molecules with three carbon atoms (C3H6 O3).
They are formed only during metabolic breakdown of hexose. They do not occur
in nature. Examples are glyceraldehyde and dihydroxyacetone.
b) Tetroses: With four carbon atoms (C4 H8 O4). Examples are erythrose and
threose.
c) Pentoses: With five carbon atoms (C5 H10 O5). Examples are ribose, deoxyribose,
ribulose, xylulose, arabinose.
d) Hexoses: with six carbon atoms (C6 H12 O6). Examples are glucose or dextrose
(grape sugar), fructose or levulose (fruit sugar) and galactose and mannose. All
of them are represented by the same formula C 6 H12 O6. Glucose, fructose and
mannose differ only in their first and second carbon atoms. The carbonyl group in
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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glucose is attached to the first carbon atom and forms aldehyde group. But in
fructose it is found with second carbon and keto group.
e) Heptoses: With seven carbon atoms (C7 H14 O7)
2) Oligosaccharides: These are formed with 2 to 10 molecules of monosaccharide
units joined by glucosidic bond. Based on the number of monosaccharide unite,
oligosaccharides may be of following types.
a) Disaccharides: These are formed by the condensation of two molecules of
same or different monosaccharide with the loss of one molecule of water. Their
empirical formula is C12H22 O11. Examples are Sucrose (cane sugar found in
plants), Lactose (milk sugar), Maltose (malt sugar produced by hydrolysis of
starch and glycogen) and Cellobiose (obtained from cellulose).
b) Trisaccharides: This type of oligosaccharides produces three molecules of
monosaccharides. Examples are
i. Mannotriose: Formed of 2 molecule of galactose and 1 molecule of
glucose.
ii. Robinose: Formed by condensation of one molecule of galactose and two
molecules of rhamnose.
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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iii. Raffinose: Formed of one molecule of glucose, one molecule of galactose
and one molecule of fructose.
iv. Gentianose: Formed of one molecule of fructose and two molecules of
glucose. Formed from the gentian roots.
v. Melibiose: give rise to one molecule of fructose and two molecules of
glucose.
c) Tetrasaccharides: These are formed of four molecules of monosaccharides.
Only two tetrasaccharides are known.
i. Stachyose: It is composed of D-glucose, fructose and two molecules of
galactose. Found in haricot beans.
ii. Scorodose: It is found in the bulbs of garlic and onion.
3) Polysaccharides: These are complex sugar formed of 10 to 100 or thousand of
monosaccharide molecules joined covalently by glucosidic bond. They do not have
sweet taste. These are represented by general formula (C5 H10 O5)n. Example
cellulose, starch and glycogen.
Biological Significance of Carbohydrate:
1. Major source of energy: Carbohydrates are main source of energy for living cell.
They are used as respiratory fuel. In the presence of oxygen these decompose into
CO2 and water, and liberate energy to be utilized by body cells.
2. Structural components: In plant cells carbohydrates (cellulose) constitute the
structural frame work. In animal cells these form protective coat on the surface of
cells.
3. Storage: These are stored in body for immediate source of energy. The stored
carbohydrate is starch in plants and glycogen in animals.
4. Role in Metabolism: Carbohydrate plays a key role in the metabolism of amino
acids and fatty acids.
5. Special Function:
i. Some glycoprotein acts as hormones.
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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ii. Glycoprotein on cell surface help in cell recognition and help in immune system
of the body.
iii. Heparin, a mucopolysaccharide acts as anticoagulant.
(Question 1: What do you know about carbohydrate metabolism?
(2011))
CARBOHYDRAE METABOLISM
Absorbed sugars by intestine are distributed to different body cells for oxidation and
release of energy. However excess of glucose in blood is metabolized in the liver cells
or muscles for storage as glycogen.
A. METABOLISM OF GLUCOSE IN LIVER CELLS:
1. Glycogenesis (Conversion of glycogen and storage in the liver):
 In liver cells glucose combines with phosphate to form glucose – 6phosphate in the presence of hexokinase.
 Glucose – 6- phosphate converted into glucose – 1- phosphate in the
presence of phosphoglucomutase.

Glucose – 1- phosphate reacts with uridine triphosphate (UTP) and
changes into Uridine diphosphate glucose (UDP-Glucose) under the
catalytic action of phosphorylase. UDP-Glucose converted into glycogen by
the enzyme glycogen synthetase. Glycogen is stored in liver cells until
needed.
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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2. Glycogenolysis: Whenever blood sugar level goes down, the glycogen from the
body is converted into glucose inside the liver. The process is known as
glycogenolysis and follows reverse chain of reactions of Glycogenesis. The
overall reaction for the breakdown of glycogen to glucose-1-phosphate is:
glycogen(n residues) + Pi
glycogen(n-1 residues) + glucose-1-phosphate
3. Lipogenesis (conversion to fat): It involves the transformation of excess of
glucose into fat. It occurs when the liver and muscles are incapable of storing
more glycogen. It largely occurs in liver. The glucose is probably metabolized to
a two carbon compound, which is then polymerized into a long fatty acid chain.
Glycerol on combining with the fatty acids forms the neutral fats and stored in the
adipose tissue.
4. Gluconeogenesis: it is the formation of glucose or glycogen from noncarbohydrate sources, such as glucogenic amino acids, lactate and glycerol is
called gluconeogenesis. Gluconeogenesis takes place only when carbohydrates
are not available in sufficient amount from the diet.
B. IN MUSCLE CELLS:
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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5. Conversion of Glucose into muscle Glycogen: The blood sugar on reaching
the muscle cells is converted into glucose-6-phosphate, which either changes into
muscle glycogen for storage in the muscles or may break into pyruvic acid and lactic
acid releasing energy to be utilized by the cells.
C. IN THE CELLS OF BODY:
6. Carbohydrate Catabolism in tissue Cells:

Glucose from the blood is distributed to the cells. The muscle cells being most
active utilize the maximum amount of glucose, next in the series are the cells of
nervous system and in the end are the cells of other body tissue.
 The breakdown of glucose releases energy which is trapped in ATP and is used
for various body activities. The process involves a series of reactions which are
broadly separated into the following steps:
(a) Glycolysis: Glucose undergoes anaerobic glycolysis inside the cell cytoplasm
and breaks down into pyruvic acid.
(b) Aerobic Respiration (Kreb’s Cycle): In presence of oxygen pyruvic acid enters
Kreb’s tricarboxylic acid cycle and breaks down into CO 2 and H2 O releasing 30
ATP molecules.
(c) Anaerobic Respiration: In the absence of oxygen pyruvic acid changes into
lactic acid releasing only 2 ATP.
(Question 2: What is Glycolysis? Explain various steps of this process. (2008,
2011))
GLYCOLYSIS: During glycolysis, one molecule of glucose (6 carbon compound) splits
into two molecules of pyruvic acid (3 carbon atom compound) and liberate energy in the
form of 2 ATP molecules. The various steps of glycolysis were worked out by two
German biochemists, Embden and Meyerhoef. After their names, the glycolysis cycle
is called Embden-Meyerhoef pathway. It is completed in following steps:
Step I. Phosphorylation: One phosphate group is transferred from ATP to the 6 th
carbon atom of glucose molecule to form glucose – 6- phosphate in the presence of
enzyme phosphohexokinase.
Step 2. Isomerization: In the presence of enzyme phosphohexose isomerase
glucose -6- phosphate changes into fructose – 6- phosphate.
`
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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Step 3. Phosphorylation: By addition of one more phosphate group fructose -6phosphate changes to fructose 1-6- diphosphate in the presence of enzyme
phosphofructokinase.
Step 4. Cleavage: In the presence of enzyme aldolase fructose 1-6- diphosphate splits
into two molecules of 3-phosphoglyceraldehyde (PGAL) or glyceraldehydes-3phosphate (3 carbon atom compound).
Step 5. Phosphorylation and oxidative dehydrogenation: Both the molecules of 3phosphoglyceraldehyde undergo Phosphorylation and oxidative dehydrogenation to
forms 1,3- diphosphoglyceric acid or 1,3- diphosphoglyceate. Each molecule of
PGAL releases 2H which are received by NAD forming NADH + H+.
Step 6. ATP Formation: 1,3-diphosphoglyceric acid releases one energy rich –PO4 and
changes into 3-phosphoglyceric acid (PGA) in the presence of enzyme
phosphoglycerylkinase. –PO4 binds to ADP to form ATP. Since two molecules of
PGAL are involved therefore 2 molecule of ATP are produced.
Step 7. Insomerization: 3-phosphoglyceric acid undergoes isomerization and changes
into 2-phosphoglyceric acid (2-PGA).
Step 8. Dehydration: 2-phosphoglyceric acid (2-PGA) loses one molecule of water and
changes into phosphoenolpyruvic acid.
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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Step 9. ATP formation: high energy –PO4 from phosphoenol pyruvic acid is transferred
to ADP which forms ATP and pyruvic acid in the presence of enzyme pyruvate
kinase.
End Products of Glycolysis:
1. From one molecule of glucose two molecules of pyruvic acid are formed.
2. 2 ATP molecules are used while 4 ATP molecules are formed. Therefore, there is
net gain of 2 ATP molecules.
3. 2H+ atoms are formed. These are accepted by NAD, which changes into NADH +
H+. Hydrogen ions are transferred to ETS.
ANAEROBIC RESPIRATION:
(Question 3: Describe Formation of Lactic Acid through Embden Meyerhoef
Pathway.)
The end products of Embden Meyerhoef Pathway are two molecules of pyruvic acid.
For aerobic respiration pyruvic acid enters mitochondria and then changes into acetyl
CoA. But in absence of oxygen or shortage of oxygen supply, the pyruvate generated
during glycolysis is reduced to lactic acid in animal cells and into ethyl alcohol in yeast
and other microorganisms. The reaction occurs in the cell cytoplasm.
Under anaerobic condition, pyruvic acid acts as hydrogen acceptor. It obtains hydrogen
ions from NADH and oxidizes it into NAD + and in turn is reduced to lactic acid. The
reaction is catalyzed by enzyme lactase dehydrogenase. This reaction does not
produce any hydrogen ions or ATP. During the conversion of glucose into lactic acid,
the net production of ATP is just two molecules.
CH3.CO.COOH
Pyruvic acid
Lactate dehydrogenase
NADH + H
NA D
CH3.CHOH. COOH
Lactic Acid
NAD + is essential for glycolysis, because it is an electron acceptor in the reaction
in which glyceraldehydes – 3- phosphate is oxidized. Therefore, its recovery from NADH
is necessary. Lactic acid is formed and accumulates in the erythrocytes, peripheral
nerves, retina, malignant tumors and in muscles during vigorous exercise.
In microorganism, yeast, certain green plants and some animals and also
germinating seeds during anaerobic respiration, pyruvic acid undergoes alcoholic
fermentation. It is decarboxylated to acetaldehyde and is then reduced to ethyl alcohol
or ethanol. During this process NADH is oxidized to NAD + and pyruvic acid is reduced
to acetaldehyde.
CH3.CO.COOH
Pyruvic acid
NADH + H
CH3CHO
Acetaldehyde
CH3CH2 OH
Ethanol
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
AEROBIC RESPIRATION (Fate of Pyruvic Acid):
(Question 4: Describe various steps of Kreb’s cycle.
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(2002, 2008, 2010))
The pyruvic acid produced during glycolysis in the cytosol enters mitochondrial
matrix. Here it is oxidized to acetyl CoA which then enters the Kreb’s cycle.
The process includes following steps:
1. Oxidative Decarboxylation: on entering mitochondria each pyruvic acid molecule
releases one molecule of CO2 and changes into acetic acid. This is called
decarboxylation. Acetic acid combines with coenzyme A and forms acetyl coenzyme A.
During this process 2H are released which are accepted by NAD+ forming NADH+ + H+.
NADH transfers them to electron transport system (2H produce 3 ATP molecules).
2. Kreb’s Tricarboxylic Acid Cycle (TCA) or Kreb’s Cycle or Citric Acid Cycle
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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The Kreb’s cycle takes place inside the matrix of mitochondria. It is also called aerobic
respiration. The acetyl – coA enters the Kreb’s Cycle, undergoes a series of changes
and is oxidized to CO2 and H2 O liberating energy.
(i) Formation of Citric Acid: In presence of condensation enzyme Citrate Synthetase
the 2-carbon atom acetyl-CoA condenses with 4-carbon atom oxaloacetic acid
molecule forming a 6 – carbon atom Citric Acid and CoA is released.
(ii) Dehydration: Citric acid undergoes dehydration forming cisaconitic acid in
presence of enzyme aconitase.
(iii)Hydration-I: Cisaconitic acid changes into isocitric acid by the addition on one
molecule of water again in the presence of enzyme aconitase.
(iv)Dehydrogenation-I: In presence of enzyme isocitrate dehydrogenase isocitric
acid undergoes dehydrogenation to form oxalosuccinic acid. The pair of hydrogen
atoms (2H) released is accepted by NAD+ to form NADH +H+. It is transported to
electron Transport System (ETS).
(v) Decarboxylation-I: Oxalosuccinic acid (6C) undergoes decarboxylation to form αketoglutaric acid (5C) and one CO2 molecule is released. The reaction is known as
decarboxylation and is catalyzed by the enzyme carboxylase.
(vi)Dehydrogenation-II and Decarboxylation-II: The α-ketoglutaric acid undergoes
dehydrogenation and decarboxylation and combines with coenzyme-A to form
succinyl-CoA. The reaction is catalyzed by α-ketogluterate dehydrogenase
complex. The 2H atoms released are accepter by NAD which changes to NADH +
H+. It is carried to the ETS to release 3 ATP molecules.
(vii) Phosphorylation of ADP: Under the catalytic action of succinic thiokinase or
succinyl CoA synthetase succinyl CoA separates into succinic acid and CoA.
During this process GDP (guanosine diphosphate) is phosphorylated into GTP
(guanosine triphosphate). GTP transfers their energy rich-P group to ADP forming
ATP.
GDP+ GTP
IDP + - P
ITP
ITP/GTP + ADP
ATP + IDP/GDP
(viii) Dehydrogenation – III: Succinic acid undergoes dehydrogenation under the
catalytic action of succinic dehydrogenase and forms fumaric acid. The pair of
hydrogen atoms (2H) released is picked up by flavoprotein which contains FAD
(flavin adenine dinucleotide).
(ix) Hydration-IV: Fumaric acid is hydrated to form malic acid in presence of enzyme
Fumarase.
(x) Dehydrogenation –IV: Finally malic acid changes to oxaloacetic acid in presence
of enzyme malate dehydrogenase. The pair of hydrogen atoms is accepted by
NAD + and is passed on to the electron transport system (ETS) where it releases 3
ATP.
Oxaloacetic acid again combines with acetyl coenzyme A to enter Kreb’s cycle and
repeats the same chain of reactions.
During each citric acid cycle:
LECTURE NO TES BY: DR. B. C. JO SHI, SAPKM, KICHHA
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1. 2-carbon atoms are released as 2CO2 molecules as a result of decarboxylation.
2. One molecule of ATP is generated at the substrate level in each cycle.
3. Four dehydrogenation reactions occur in each cycle releasing four pair of
hydrogen atoms. Three pairs of H atoms are accepted by NAD to Form HADH+ +
H. One pair of hydrogen atoms is accepted by FAD to form FADH2. These
compounds transport hydrogen ions to ETS where these protons have following
fate:
a. 3X2H hydrogen atoms are accepted by NAD to form NADH ++ H. This
transports hydrogen atoms to ETS where these produce 6 X 3 = 18 ATP
molecules (2H= 3 ATP).
b. 1 X 2H hydrogen atoms are accepted by FAD forming FADH2. Along the
ETS these also produce 3 ATP molecules.