Carbohydrates are an important class of biological molecules that serve structural and fuel roles. This unit describes the structures, properties and functions of carbohydrates including monosaccharides like glucose and fructose, disaccharides like sucrose and maltose, and polysaccharides like starch, glycogen, cellulose, and glycosaminoglycans. Key topics covered include carbohydrate classification, monosaccharide structures and isomerism, glycosidic linkages in complex carbohydrates, and the roles of important polysaccharides in energy storage, structure, and extracellular matrices.

1. UNIT 7
CARBOHYDRATES
Chapter 11
Carbohydrates are important fuel
molecules, but they also offer protection
against high-impact forces & determine
the human ABO blood types

2. Student Learning Outcomes
p This unit is designed to allow you to:
n Describe the structure, properties, functions,
classification & nomenclature of carbohydrates.
n List main mono- & disaccharides & their main reactions.
n Describe the main polysaccharides in animals & plants.
n Differentiate the glyco-conjugates: glycoproteins,
proteoglycans, glycosaminoglycans & glycolipids.
n Identify the main dietary carbohydrates, their digestion,
enzymes & organs involved & absorption & transport.
n Describe intolerances & their causes & effects.
n Outline an overview of glucose metabolism.

3. Carbohydrates
p Most abundant organic molecules in nature.
p Carbohydrates are polar but uncharged molecules in
nature, except for GAG (glucosaminoglycans).
p Have several functions:
n Provide all cells with significant fraction of energy
p All human cells can oxidize glucose for energy
n Membrane components (about 10% by weight)
n Essential components of cell communication
n Structural components of cell walls of bacteria &
plants & exoskeleton of many insects
n Components of several coenzymes & nucleic acids
p Ribose (ATP & RNA) & deoxyribose (DNA)

4. Carbohydrates
p Classified based on the number of carbons they
contain:
n Monosaccharides, are the smallest units & can be
represented by the empirical formula (CH2O)n, where n ≥ 3
(n is usually 5 or 6 but can be up to 9).
n Oligosaccharides, are compounds of 2 to about 20
monosaccharide residues & disaccharides are the most
common among them.
n Polysaccharides, are polymers of many monosaccharides
(usually more than 20). A polymer of identical sugars is
called a homoglycan (or homopolysaccharide) & of different
sugars is called heteroglycan (or heteropolysaccharide).
2galactose
1,95
1Maltose
alactose
3Sucrose
1Starch
2glycogen
can
be
represented
as H2O
33,56 jigging t't't't't't 3cellulose
molecules OH
OH
OH atolteamons

5. Carbohydrates
p Glycoconjugates, such as proteoglycans,
glycoproteins & glycolipids, are carbohydrate
derivatives formed by covalently linking a
polysaccharide to a protein, peptide chain or lipid.
p They are also classified based on the type of
carbonyl group (functional group) they have:
n Aldoses, the carbonyl carbon is C1
n Ketoses, The carbonyl carbon is C2.
p Chemically, monossacharides are polyhydroxy
aldehydes or ketones. Physically, they are water-
soluble, white crystalline solids with a sweet taste.

6. Carbohydrates
p Most monosaccharides are chiral compounds
n They can form D & L stereoisomers
p Natural polysaccharides contain D-isomers
p Monosaccharides can exist in solution in an open or
closed form (cyclic).
p The most oxidized carbon of a monosaccharide, the
functional carbon is called the anomeric carbon.
p Humans can synthesize glycogen, lactose, glucose
& glycoproteins but most carbohydrates in the
foods we ingest are from plants.
glycine

7. Carbohydrates
p Carbohydrates are polar but uncharged molecules
in nature, except for GAG (glucosaminoglycans).
p Carbohydrates can be linked to proteins through
Asn (N-linked) or through Ser or Thr (O-linked).
p Proteoglycans, made of polysaccharide & protein,
are important component of cartilage.
p Mucins are glycoprotein components of mucus.
p Humans can synthesize glycogen, lactose & glucose
but most carbohydrates in the foods we ingest are
from plants.

8. Representative Monosaccharides
does
not
haveastereoisomer

9. Dihydroxyacetone
p Dihydroxyacetone is the only
monosaccharide that does NOT have chiral
carbons, therefore does not have
stereoisomers.
n Similar to glycine in this regard

10. Common Carbohydrates
c aldose
c ketose

11. Enantiomers of Glyceraldehyde

12. Important Monosaccharides
Aldoses Ketoses
Gyceraldehyde (C3-molecule) Dihydroxyacetone (C3)
Erythrose (C4) Ribulose (C5)
Ribose (C5) Xylulose (C5)
Deoxyribose (C5) Fructose (C6)
Glucose (C6) Sedoheptulose (C7)
Galactose (C6)
Mannose (C6)
Fucose (C6) (methyl group at C-6)
more
common
cannot
berepresented
as Chao nble
it'snotatypicalcarbohydrate

13. Nomenclature of Monosaccharides
p Monosaccharides with 3 carbons are called
trioses:
n Aldotrioses such as glyceraldehyde
n Ketotrioses such as dihydroxyacetone
p Tetroses have 4 carbons
p Pentoses have 5 carbons
p Hexoses have 6 carbons
p Heptoses have 7 carbons

14. Epimers

15. Epimers
p Glucose & galactose are C-4 epimers
p Glucose & mannose are C-2 epimers
p Epimers are stereoisomers differing only in
the orientation of a hydroxyl group
n See previous graph & compare glucose with
mannose & glucose with galactose
n Galactose & mannose are just formula isomers
p They can be represented by the same formula, C6H12O6

16. Formation of Glucopyranose

17. Formation of Fructopyranose

18. Pyran & Furan Rings

19. Ring Structures of Fructose

20. Fructose Structures
p Fructose forms both pyranose & furanose
rings.
p The pyranose form predominates in fructose
free in solution & the furanose form
predominates in many fructose derivatives.
p β-D-Fructopyranose, found in honey is one of
the sweetest chemicals known.

21. Ring Forms of Common Sugars
sweet
tastes
are
mono
d saccharides

22. Glucose is a Reducing Sugar

23. Glucose One-C Oxidation
p Carbons 1 & 6 of glucose can be fully oxidized
to their carboxylate forms.
p Full oxidation of carbon 1 generates gluconic
acid
n gluconate
p Full oxidation of carbon 6 generates
glucoronic acid
n glucoronate

24. Reducing Sugars
p Sugars that react with solutions of cupric ion (Cu2+)
(Fehling’s solution) are called reducing sugars; those
that do not are called non-reducing sugars.
p Reducing sugars can often nonspecifically react with
other molecules, such as hemoglobin.
p Glycosylated hemoglobin (HbA1c) levels are useful in
assessing the effectiveness of treatments for
diabetes.
p In non-diabetics, HbA1c is 4-5.6%, levels of 5.7-6.4%
increase the risk of diabetics & 6.5 % or higher means
that you have diabetes.

25. Modified Monosaccharides

26. Sugar Phosphates

27. Sugar Phosphates
p Phosphorylated sugars are key intermediates in
energy generation & biosynthesis.
p Phosphorylation makes sugars anionic; the negative
charge prevents these from spontaneously leaving
the cell:
n There are not carriers for sugar phosphates in plasma
membranes of cells, but they are present in internal cell
membranes
p Phosphorylation also generates reactive
intermediates that will more readily form linkages
with other molecules.

28. Abbreviations in Carbohydrates

29. Maltose, a Disaccharide

30. Linkages in Carbohydrates
p Disaccharides & more complex carbohydrates
are made by covalently attaching
monosaccharides via glycosidic linkages.
p Chemically, all glycosidic linkages are ether
linkages:
n Ether linkages, R-O-R

31. Common Disaccharides

32. Common Disaccharides
p Lactose
n Animal carbohydrate found in milk & its food products
n Made of galactose & glucose with β-1,4 glycosidic linkage
n Only dietary source of galactose
p Sucrose
n Plant carbohydrate found in grains, fruits, veggies, nuts &
beans
n Made of glucose & fructose with α-1,2 glycosidic linkage
p Maltose
n Plant carbohydrate & main product of partial starch
degradation
n Made of glucose & glucose with α-1,4 glycosidic linkage

33. Structures of Polysaccharides

34. Starch

35. Hepatic Glycogen

36. Starch, a Plant Polysaccharide
p A nutritional reservoir or energy storage form of
glucose in plants.
p Starch is made of two polymers:
n Amylose, which is linear (or unbranched)
n Amylopectin, a branched (or nonlinear) polysaccharide
p The proportion of these two polysaccharides
depends on the starch source.
p Typical starch is one that has about 20% amylose &
about 80% amylopectin
n Wheat, corn, rice & white potatoes have typical starch

37. Starch, a Plant Polysaccharide
p Amylose is about 20%
n Linear polysaccharide
p Only one type of glycosidic linkage, α-1,4
p Amylopectin is about 80%
n Branched polysaccharide
p Two types of linkages
p Most are α-1,4
p Few are α-1,6
§ Branches every 25 glucose residues on average (4% branching)

38. Starch Amylose

39. Branch Point of Amylopectin or Glycogen
(Branching point)

40. Glycogen, a Bacterial & Animal Polysaccharide
p The storage form of glucose in animals & bacteria is
glycogen.
p Glycogen is present in most human cells but only in
significant amounts in muscle, heart & liver.
p Glycogen is a single polysaccharide.
p It is like amylopectin but more branched
p Two types of linkages, most of them are α-1,4 but
branches (α-1,6) are also present
§ Branches every 10 glucose residues on average (10%
branching)

41. Effects of Branching in Polysaccharides
p Increase solubility
p Can be more rapidly synthesized
p Can be more rapidly degraded
p Can potentially elevate blood glucose more
rapidly
p Have better gelling properties
n Cooking
n Food properties

42. Cellulose, a Plant Polysaccharide
p Cellulose, the other major polysaccharide of glucose
found in plants, serves a structural rather than
nutritional role as an important component of the
plant cell wall.
p Cellulose is arguably the most abundant organic
molecule on the biosphere:
n ~1015 kg of cellulose is metabolized on Earth each year
p Cellulose is a linear polymer of glucose residues
joined by β-1,4 linkages, in contrast with the α-1,4
linkages seen in starch & amylopectin.

43. Glycosidic Bonds & Structure

44. Glycosidic Bonds & Structure
p The β-1,4 linkages allows cellulose to form
very long straight chains.
p Fibrils are formed by parallel chains that
interact with each another though H bonds.
p The straight chains formed by the β linkages
are optimal for the construction of fibers
having a high tensile strength.
p Cannot be digested by mammals but it is an
important constituent of dietary fiber.

45. Glycoproteins
p Carbohydrates can be linked to proteins to
form glycoproteins.
p There are three main classes of
glycoproteins:
n Glycoproteins
p Protein > Carbohydrate
n Proteoglycans
p Carbohydrate > Protein
n Mucins or mucoproteins
p Carbohydrate > Protein

46. Glycoproteins
p Glycoproteins typically have more protein
than carbohydrate by weight.
p Many glycoproteins are components of cell
membranes, where they take part in
processes such as cell adhesion, the binding
of sperm to eggs & transport of sugars.
p Many soluble proteins that are secreted are
also glycoproteins.

47. Glycoprotein Linkages
p Carbohydrate can be linked to proteins
through asparagine or through serine or
threonine:
n Asparagine: N-linked
n Serine or Threonine: O-linked

48. Glycoprotein Linkages
(or Thr)

49. N-Linked Oligosaccharides

50. Erythropoietin
p Erythropoietin (EPO) is a glycoprotein hormone in the blood
serum that has dramatically improved treatment for
anemia, particularly that induced by cancer chemotherapy.
p EPO is secreted by the kidneys & stimulates the production
of red blood cells.
p EPO has 165 AA & is N-glycosylated at 3 asparagine residues
& O-glycosylated on a serine residue.
p EPO is 40% carbohydrate by weight & glycosylation
enhances its stability in blood & its activity.
p r-EPO has greatly aided the treatment of anemia & HIV
patients.
p It is also used by some endurance athletes to cheat in sports.

51. Oligosaccharides of Erythropoietin

52. Proteoglycans
p In proteoglycans, the protein is attached to polysac-
charides called glycosaminoglycans (GAG).
p GAG is typically higher than protein by weight, as
much as 95% of the molecule by weight.
n GAGs are linear polysaccharides made of disaccharide units
p Proteoglycans function as lubricants, shock absorbers
& structural components in connective tissue & they
also mediate the adhesion of cells to the extracellular
matrix & bind factors for cell proliferation.
p Proteoglycans are acidic conjugated carbohydrates.

53. Glycosaminoglycans, GAG

54. Cartilage
p The proteoglycan aggrecan & collagen are key components
of cartilage.
p The triple helix of collagen provides structure & tensile
strength, whereas aggrecan serves as a shock absorber by
being highly hydrated.
p Aggrecan can cushion comprehensive forces because the
absorbed water enables it to spring back after having being
deformed.
p Osteoarthritis can result from the proteolytic degradation of
aggrecan & collagen in the cartilage.

55. Cartilage-cushioned!

56. Mucins & Mucus
p In mucins, the protein component is extensively glycosylated
to serine or threonine residues by N-acetylgalactosamine.
p Mucins can form large polymeric structures & are common in
mucus secretions, such as saliva where it functions as a
lubricant.
p Carbohydrate can account for as much as 80% of the
molecule by weight.
p Mucins adhere to epithelial cells & act as a protective
barrier; they also hydrate the underlining cells.
p They are overexpressed in bronchitis & cystic fibrosis & their
overexpression is characteristic of adrenocarcinomas —
cancers of the glandular cells of epithelial origin.

57. Structures & Roles of Polysaccharides
Polymer Type Repeating
Unit
Size
(# of units)
Role
Starch
Amylose
Amylopectin
Homo-
polysac-
charide
(α1→4)Glc, linear
(α1→4)Glc with
(α1→6)Glc branches
50-5,000
Up to 106
Energy storage in plants
Glycogen Homo- (α1→4)Glc with
(α1→6)Glc branches
Up to 50,000 Energy storage in animals &
bacteria
Cellulose Homo- (β1→4)Glc, linear Up to 15,000 Structural role: cell wall of
plants
GAG
Hyaluronate
Hetero-
polysac-
charide
Acidic
GlcA(β1→3)
GlcNAc(β1), linear
Up to 100,000
Structural role: extracellular
matrix & cartilage of
vertebrates

58. Dietary Carbohydrates
p Most dietary carbohydrates come from plants:
n Lactose from dairy is the main exception
p The main dietary carbohydrates are starch,
sucrose, lactose, glucose, fructose & cellulose
& other forms of fiber.
p The only dietary source of galactose is lactose.
p We process very little mannose during
digestion:
n Mannose is a main component of glycoproteins

59. Digestion of Dietary Carbohydrates
Enzyme Substrate Products Linkage
α-Amylase
(saliva, pancreas) Starch
Limit dextrins:
Maltose, isomaltose &
maltotriose
α-1,4
Maltase (SI) Maltose 2Glucoses α-1,4
Glucoamylase (SI) Maltotriose 3Glucoses α-1,4
Isomaltase (SI) Isomaltose 2Glucoses α-1,6
Sucrase (SI) Sucrose Glucose & fructose α-1,2
Lactase (SI)
(or β-Galactosidase)
Lactose Galactose & glucose β-1,4

60. Starch Digestion
p Four enzymes are needed to digest starch to glucose:
n α-amylase, maltase, isomaltase & glucoamylase
p Mouth salivary α-amylase starts the digestion, generating
mostly maltose residues & isomaltose & maltotriose residues
n Maltose, isomaltose & maltotriose are called dextrins because they are
residues made of dextrose, another name for glucose.
n The term limit dextrin is used to make the case for the inability of α-
amylase to breakdown dextrins.
n Pancreatic α-amylase completes the digestion of starch in the SI
p Maltase digests maltose into two glucose residues.
p Isomaltase digests isomaltose into two glucose residues.
p Glucoamylase digests maltotriose to 3 molecules of glucose

61. Absorption of Dietary Carbohydrates
GLUT5
Fructose
(or galactose)
Fructose Fructose
Portal vein
Galactose
Glc
Galactose
Glc Glc
CO2 + H2O
CO2 + H2O

62. Carriers in Enterocytes
p Na-Glucose Symporter (SGS)
n Dietary glucose & galactose absorption (secondary active transport)
p GLUT5
n Dietary fructose absorption & release
p GLUT2
n Dietary glucose & galactose release
p GLUT1/GLUT3
n Basal glucose uptake
p Na-K ATPase (Primary active transport)
n Na/K cell equilibration
p Na-AA Symporters
n Dietary amino acid absorption
p Others

63. Transporters Types & Names
p There are 2 main classes of transporters:
n Carriers: for organic molecules such as glucose
n Channels: mostly for inorganic molecules such as sodium
p Passive carriers transport solutes across membranes
from high to low concentration so energy is not used.
p Carriers have different names:
n Transporters, carriers, translocases or permeases
p There are 2 main types of passive carriers:
n Uniporters, transport just one molecule
n Antiporters, transport 2 molecules in opposite direction

64. Transporters Types & Names
p Active carriers transport solutes across membranes
from low to high concentration so energy is always
needed to allow transport to take place.
p There are 2 types of active transporters:
n Primary active transporters: use the energy of ATP
hydrolysis to transport solutes across membranes against
concentration gradients.
p Such as Na/K pump (Na/K ATPase)
n Secondary active transporters: use the energy of ion
concentration gradients (such as Na+ & H+) to transport
solutes across membranes against concentration gradients.
p Such as Na/AA transporters & Na/Glc (or Gal) transporter

65. Ions & a Typical Mammalian Cell
Ion Intracellular
Concentration (mM)
Extracellular
Concentration (mM)
Na+ 5-15 145
K+ 140 5
Mg2+ 0.5 1-2
Ca2+ 1.0 x 10-4 1.2
H+ 1.58 x 10-4 (pH 6.8 ) 3.98 x 10-5 (pH 7.4)
Cl- 5-15 110
(Cytosolic) (Blood)
low
because
it's
a
and
messenger
activates
protein
kinases

66. Lactose Intolerance
p Lack or deficiency of lactase.
p >70% of world population is affected
n Lactase activity is reduced in many individuals to 5-10%
levels at birth after lactation
n N Europeans & a few tribes in Africa are the exception
p Some or most lactose remains indigested.
p Reaches colon & it is fermented by colonic bacteria.
p Fermentation produces lactic acid, other short-chain
acids & gases (H2, CH4)
n Gut distention & flatulence
n Diarrhea
4b
lo
it'sadisaccharide
that
we
can
tbreak
down
into
monosaccharides
to
digest

67. Sucrose Intolerance
p Lack or deficiency of sucrase.
p ~2% of Greenland Eskimos are affected.
p Some sucrose remains indigested.
p Reaches colon & it is fermented by colonic
bacteria.
p Fermentation produces short-chain fatty
acids & gases.

68. Blood Sugars
p Portal vein circulation delivers mostly dietary glucose
to the liver but also significant amounts of fructose &
galactose, depending on dietary habits.
p Very little or no dietary mannose reaches the liver.
p Liver readily releases glucose & fructose to the blood
for general circulation, but galactose or mannose
are not released under normal conditions.
p Glucose is metabolized by all peripheral cells.
p Fructose is metabolized by some peripheral cells,
such adipocytes & muscle, kidneys, retina & testis.

69. Major Pathways of Glucose Use
All human cells
All human cells
Mostly liver, muscle & heart cells
Some cells

70. Glucose Metabolism
p All human cells oxidize glucose to pyruvate in the
cytosol to generate energy.
p All human cells can also oxidize glucose to ribose 5-
phosphate for the synthesis of coenzymes,
nucleotides & nucleic acids & to generate NADPH for
biosynthesis & for the elimination of strong oxidants.
p Several human cells convert glucose to glycogen but
only liver, muscle & heart cells can store it in
significant amounts.