1) The document discusses carbohydrate metabolism, including the classification, digestion, absorption and major metabolic pathways of carbohydrates in the body. 2) Key pathways include glycolysis, the citric acid cycle, gluconeogenesis, glycogenesis, glycogenolysis, and the hexose monophosphate shunt. These pathways break down and utilize carbohydrates for energy production and storage. 3) Tight regulation of blood glucose levels is achieved through hormones like insulin, glucagon, thyroid hormones and glucocorticoids that stimulate or inhibit glycogenolysis and gluconeogenesis.

1. GUANGDONG OCEAN UNIVERSITY
Carbohydrates Metabolism
(糖代谢---táng dài xiè)
Presenter: Muhammad Waseem (Ghani)
Class: Advanced Biochemistry

2. Contents
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3. Overview
CARBOHYDRATES:
Most abundant organic molecule on earth
Carbohydrates are defined as aldehyde or keto derivatives
of polyhydric alcohols
For example: Glycerol on oxidation is converted to
D-glyceraldehyde, which is a carbohydrate derived from the
trihydric alcohol (glycerol)
All carbohydrates have the general formula CnH2nOn
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4. CLASSIFICATION OF CARBOHYDRATE
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5. Importance and functions of Carbohydrates
Carbohydrates constitute the largest component of an average diet
The most important function of carbohydrates is to provide energy
Storage form of energy (starch and glycogen)
Energy production from carbohydrates will be 4 k calories/g (16 k
Joules/g).
Excess carbohydrate is converted to fat.
Some tissues, e.g. brain and erythrocytes, get energy almost
exclusively from glucose
If the conditions become anaerobic, only glucose can be used as a fuel
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6. Importance and functions of Carbohydrates
Ribose and deoxyribose are used to synthesize nucleotides and
nucleic acids
Some carbohydrates form the prosthetic group of hormones,
immunoglobulins, blood group substances etc.
Some carbohydrates act as structural constituents of tissues
Structural basis of many organisms. For example, cellulose of plants,
exoskeleton of insects etc.
Glycoproteins and glycolipids are components of cell membranes and
receptors.
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7. Over view of Carbohydrate Metabolism
The entire spectrum of chemical reactions, occurring in the living
system are referred as “Metabolism”
Types of metabolic pathways in cellular metabolism
1. Anabolic pathways: Protein synthesis.
2. Catabolic Pathways: Oxidative phosphorylation
3. Amphibolic pathways: Citric acid cycle.
• Cells break down excess carbohydrates first, then lipids, finally amino acids if
energy needs are not met by carbohydrates and fat
• Nutrients not used for energy are used to build up structure, are stored, or excreted
• 40% of the energy released in catabolism is captured in ATP, the rest is released as
heat
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8. Carbohydrate Metabolism
• Primarily glucose
Fructose and galactose enter the pathways at
various points
All cells can utilize glucose for energy
production
Glucose uptake from blood to cells usually
mediated by insulin and transporters
Liver is central site for carbohydrate
metabolism
Glucose uptake independent of insulin
The only exporter of glucose
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9. Digestion and Absorption of Carbohydrates
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10. Entry of Glucose into cells
• 1) Insulin-independent transport
system of glucose:
• Not dependent on hormone insulin.
This is operative
• in – hepatocytes, erythrocytes (GLUT-
1) and brain.
• 2) Insulin-dependent transport system:
Muscles and
• adipose tissue (GLUT-4)
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11. Metabolism
There are six major
metabolic pathways
of glucose:
1) Glycogenesis
2) Glycogenolysis
3) Gluconeogenesis
4) Hexose monophosphate
shunt
5) Glycolysis
6) Citric Acid Cycle
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12. Glycolysis
Glycolysis is defined as the sequence of
reactions converting glucose (or glycogen) to
pyruvate or lactate, with the production of ATP
1) Takes place in all cells of the body.
2) Enzymes present in “cytosomal fraction” of the cell.
3) Lactate – end product – anaerobic condition.
4) Pyruvate(finally oxidized to CO2 & H2O) – end product of aerobic
condition
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13. • A series of reactions in
the cytoplasm which
converts glucose (C6) to
two molecules of
pyruvate (a C3
carboxylate), and ATP
and NADH are
produced.
• Also called Embden-
Meyerhof pathway,
after the scientist who
elucidated the pathway
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14.  If there is adequate oxygen, an
aerobic pathway is followed
and pyruvate enters the Krebs
cycle.
 If there is insufficient oxygen
available, the anaerobic
pathway is continued and
pyruvate undergoes a series of
reactions to produce lactic acid.
 Lactic acid then is the end-
product of glycolysis, and if
there were not some
mechanism for its removal, it
would accumulate in the
muscle cells & cause muscle
“crumps”.
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15. The Citric Acid Cycle
• Citric acid cycle: A series of biochemical reactions in which the acetyl
portion of acetyl CoA is oxidized to carbon dioxide and ATP and the
reduced coenzymes FADH2 and NADH are produced
• Takes place in the mitochondria
• Also known as tricarboxylic acid cycle (TCA) or Krebs cycle:
• – Named after Hans Krebs who elucidated this pathway
• Two important types of reactions:
• – Reduction of NAD+ and FAD to produce NADH and FADH2
• – Decarboxylation of citric acid to produce carbon dioxide
• The citric acid cycle also produces 2 ATP by substrate level phosphorylation
from GTP
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The reactions of the cycle takes
place in the mitochondrial
matrix, except the succinate
dehydrogenase reaction that
involves FAD
The enzyme that catalyzes this
reaction is an integral part of
the inner mitochondrial
membrane
The “fuel “ for the cycle is
acetyl CoA, obtained from the
breakdown of carbohydrates,
fats, and proteins
– NADH acts as an inhibitor
– ADP as an activator

17. The Electron Transport Chain
 The electron transport chain (ETC) facilitates the passage of electrons
trapped in FADH2 and NADH during citric cycle
 ETC is a series of biochemical reactions in which intermediate carriers
(protein and non-protein) aid the transfer of electrons and hydrogen
ions from NADH and FADH2
 The ultimate receiver of electrons is molecular oxygen
 The electron transport (respiratory chain) gets its name from the fact
that electrons are transported to oxygen absorbed via respiration
 ETC is the sequence of reactions whereby the reduced forms of the
coenzymes are reoxidized, ultimately by O2
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18. The Electron Transport Chain
 The enzymes and electron carriers needed for the ETC are located along
inner mitochondrial membrane
 They are organized into four distinct protein complexes and two mobile
carriers
 The four protein complexes tightly bound to membrane:
 Complex 1: NADH-coenzyme Q reductase
 Complex II: Succinate-coenzyme Q reductase
 Complex III: Coenzyme Q - cytochrome C reductase
 Complex IV: Cytochrome C oxidase
 Two mobile electron carriers are:
 Coenzyme Q and cytochrome C
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Complex I: NADH-Coenzyme Q
Reductase:
Facilitates transfer of electrons from
NADH to coenzyme Q
Complex II: Succinate-Coenzyme Q
Reductase
Succinate is converted to fumarate by this
Complex
In the process it generates FADH2
CoQ is the final recipient of the
electrons from FADH2

20. • Complex III: Coenzyme Q –Cytochrome c
Reductase
• Several iron-sulfur proteins and cytochromes
are electron carriers in this complex
• Cytochrome is a heme iron protein in which
reversible oxidation of an iron atom occurs
• Complex IV: Coenzyme Q –Cytochrome c
Reductase
• The electrons flow from cyt c to cyt a to cyt a3
• In the final stage of electron transfer, the
electrons from cyt a3, and hydrogen ion (H+)
combine with oxygen (O2) to form water
• O2 + 4H+ + 4e- 2 H2O
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21. Summary of the flow of electrons through four
complexes of the electron transport chain
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23. Oxidative Phosphorylation
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24. Summary of the Common Metabolic Pathway
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25. ATP Production for the Complete Oxidation
of Glucose
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26. HEXOSE MONOPHOSPHATE SHUNT
Pentose Phosphate Pathway/Phosphogluconate Pathway
• This is an alternative pathway to glycolysis and TCA cycle for the
oxidation of glucose
• Anabolic in nature, since it is concerned with the
biosynthesis of NADPH and pentoses
Unique multifunctional pathway
• Enzymes located – cytosol
Tissues active – liver, adipose tissue, adrenal gland,
erythrocytes, testes and lactating mammary gland
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27. HMP
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28. Significance of HMP Shunt
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29. Glycogenesis and Glycogenolysis
Involved in the regulation of blood glucose
concentration
When the dietary intake of glucose exceeds immediate
needs, humans and other animals can convert the excess
to glycogen, which is stored in either the liver or muscle
tissue.
Glycogenesis is the pathway that converts glucose into
glycogen.
When there’s need for additional blood glucose,
glycogen is hydrolyzed and released into the
bloodstream.
Glycogenolysis is the pathway that hydrolyzes glycogen
to glucose.
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31. Gluconeogenesis
• Metabolic pathway by which glucose is synthesized from noncarbohydrate
sources:
– The process is not exact opposite of glycolysis
• Glycogen stores in muscle and liver tissue are depleted with in 12-18 hours
from fasting or in even less time from heavy work or strenuous physical
activity
• Without gluconeogenesis, the brain, which is dependent on glucose as a
fuel would have problems functioning if food intake were restricted for even
one day
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32. Importance of Gluconeogenesis
• Gluconeogenesis helps to maintain normal blood-glucose levels in times of
inadequate dietary carbohydrate intake
• About 90% of gluconeogenesis takes place in the liver
• Non-carbohydrate starting materials for gluconeogenesis:
– Pyruvate
– Lactate (from muscles and from red blood cells)
– Glycerol (from triacylglycerol hydrolysis)
– Certain amino acids (from dietary protein hydrolysis or from muscle protein
during starvation)
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33. Importance of Gluconeogenesis
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34. Gluconeogenesis
Cori cycle
• Lactate, formed from glucose under
anaerobic conditions in muscle cells
(glycolysis), is transferred to the
liver, where it is reconverted to
• Glucose (gluconeogenesis), which is
then transferred back to the muscle
cells.
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35. Relationships Among Four Common Metabolic Pathways That
Involve Glucose
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36. Regulation of Blood glucose
• Metabolic & hormonal mechanisms
• regulate blood glucose level
• Maintenance of stable levels of glucose in blood is by
Liver.
Extrahepatic tissues.
Hormones .
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37. Regulation of Blood glucose
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38. Blood Sugar and Its Regulation
Fate of Absorbed Glucose
1st Priority: glycogen storage
Stored in muscle and liver
2nd Priority: provide energy
Oxidized to ATP
3rd Priority: stored as fat
Only excess glucose
Stored as triglycerides in
adipose
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39. Regulation of Blood glucose
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40. Regulation of blood glucose levels
Insulin
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41. Role of glucagon
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42. Role of thyroid hormone
• It stimulates glycogenolysis & gluconeogenesis.
Hypothyroid
Fasting blood glucose
is lowered.
Patients have
decreased ability to
utilise glucose.
Patients are less
sensitive to insulin
than normal or
hyperthyroid
Hyperthyroid
Fasting blood
glucose is elevated
Patients utilise
glucose at normal
or increased rate
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43. Glucocorticoids
Glucocorticoids are antagonistic
to insulin
Inhibit the utilisation of glucose
in extrahepatic tissues
Increased gluconeogenesis
Epinephrine
Secreted by adrenal medulla
It stimulates glycogenolysis in
liver & muscle
It diminishes the release of
insulin from pancreas
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44. Other Hormones
Growth hormone:
Elevates blood glucose level & antagonizes action of insulin
Growth hormone is stimulated by hypoglycemia (decreases glucose
uptake in tissues)
Chronic administration of growth hormone leads to diabetes due to β-
cell exhaustion
Sex hormones
Estrogens cause increased liberation of insulin
Testosterone decrease blood sugar level
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45. Summary
All living organisms require energy for
their maintenance, growth, exercise
and reproduction
Most of this energy is provided by
carbohydrates
So carbohydrates are life line for living
organisms
That is why the carbohydrate
metabolism involve different complex
pathways for generation of energy
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46. References
• 1) Biochemistry – U.Satyanarayana-3rd Ed.
• 2) Textbook of Biochemistry- D.M.Vasudevan -14th Ed.
• 3) Textbook of Medical Biochemistry – M.N.Chattergy
• – 17th Ed.
• 4) Text book of Physiology –Ganong – 24th Ed
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