Carbohydrates can be classified as monosaccharides, disaccharides, or polysaccharides based on the number of sugar units present. Monosaccharides like glucose and fructose can form cyclic structures through ring-forming reactions. Disaccharides are formed through glycosidic bond formation between the hemiacetal or hemiketal group of one monosaccharide and the hydroxyl group of another. Examples of common disaccharides include maltose, lactose, and sucrose, which differ in their specific monosaccharide components and glycosidic linkage positions and orientations.

1. 1
Chapter 16
Carbohydrates

2. 2
1. Types of carbohydrates
Carbohydrates
consist of carbon, hydrogen, and oxygen only.
are made by plants during photosynthesis.
normally have the general formula (CH2O)n.
Carbohydrates are made up of saccharide (sugar) units.
Monosaccharides consist of a single sugar unit.
Disaccharides consist of two sugar units.
Oligosaccharides consist of 3-10 sugar units.
Polysaccharides consist of long chains, often branched, of sugar
units.

3. 3
1. Types of carbohydrates
What to know from this section:
Types of carbohydrates (16.13-14)
General formula for a simple sugar
Source of sugars (16.16)

4. 4
2. Monosaccharides
Monosaccharides have 3-7 carbons.
Aldoses have an aldehyde group.
Ketoses have a carbonyl group.

5. 5
2. Monosaccharides
Monosaccharides are further classified based on the
number of carbons they contain.
triose, tetrose, pentose, hexose, heptose
Examples
A four-carbon monosaccharide with an aldehyde group is an
aldotetrose.
A five-carbon monosaccharide with a ketone group is a
ketopentose.

6. 6
2. Monosaccharides
Once a monosaccharide has been named as an aldose or
a ketose, and the number of carbons has been
designated, there are still several different isomeric
forms for each.
Each specific monosaccharide has a unique name.
A prefix (D- or L-) is added to designate which of two possible
isomeric forms is being referred to.

7. 7
2. Monosaccharides
Examples

8. 8
2. Monosaccharides
Draw two possible monosaccharide structures for the molecular
formula C4H8O4.
pencast

9. 9
13.4. Reactions--addition
Addition of an alcohol to an aldehyde:
The product is called a hemiacetal (-OH and –OR
attached to the same carbon).
Hemiacetals are very reactive.
They react with an additional alcohol molecule, losing –OH and
adding another –OR.
H+

10. 10
13.4. Reactions--addition
The final product is an acetal (2 –OR groups attached to
one carbon).
hemiacetal acetal

11. 11
13.4. Reactions--addition
Ketones undergo analogous addition reactions with
alcohols.
The initial product is a reactive hemiketal (two –R groups, one –
OH, and one –OR).
An additional –OR group is added to the hemiketal to produce a
ketal.
hemiketal ketal

12. 12
13.4. Reactions--addition

13. 13
13.4. Reactions
Hemiacetal, acetal, hemiketal, or ketal?
pencast
pencast

14. 14
13.4. Reactions
Monosaccharide addition reactions
1
23
4
5
6
]
alcohol
aldehyde
D-glucose
]

15. 15
1
23
4
5
6
Hemiacetal:
one –H
one –OH
one –OR
one -R
13.4. Reactions
Monosaccharide addition reactions
The cyclic form is more stable than the linear form and no
further oxidation takes place in this case.

16. 16
2. Monosaccharides
What to know from this section
Definitions of aldose and ketose (16.23)
Simple sugar naming based on number of carbons (16.25-26)
Naming based on aldose/ketose and number of carbons (16.27-
28)
Identify hemiacetals and hemiketals (16.29)
Draw possible monosaccharide structures given molecular
formula (16.31-32)

17. 17
3. Stereoisomers and stereochemistry
Stereoisomers have
the same molecular formulas.
the same bonding of atoms to one another.
Stereoisomers differ in the spatial arrangement of the
atoms in the molecule.
Enantiomers are stereoisomers that are
nonsuperimposable mirror images of each other.
A molecule that can exist in enantiomeric forms is called a
chiral molecule.
Simple enantiomers [link—bromochloroiodomethane]

18. 18
3. Stereoisomers and stereochemistry
A carbon atom that has
four different groups
bonded to it is called a
chiral carbon atom.
Any molecule containing
a chiral carbon can exist
as a pair of enantiomers.
Larger biological
molecules often have
more than one chiral
carbon.
Glyceraldehyde enantiomers

19. 19
3. Stereoisomers and stereochemistry
Plane polarized string analogy for plane polarized light.
j
k
l
http://www.chemguide.co.uk/basicorg/isomerism/string4.GIF
demo

20. 20
3. Stereoisomers and stereochemistry
Enantiomers are also called optical isomers.
Enantiomers interact with plain polarized light.
They rotate the plane of the light in opposite directions.
This interaction with polarized light is called optical
activity.
Optical activity distinguishes the isomers.
It is measured in a device called a polarimeter.

21. 21
Polarimeter
Compounds that rotate light in a clockwise direction are
dextrorotatory, designated by (+) before the angle.
Compounds that rotate light in a counterclockwise direction
are levorotatory, designated by (-) before the angle.
3. Stereoisomers and stereochemistry
Home-made polarimeter

22. 22
3. Stereoisomers and stereochemistry
Fischer projections are two-dimensional
drawings that represent a three-dimensional
molecule with one or more chiral carbons.
The intersection of two lines represents a chiral
carbon.
Horizontal lines represent bonds projecting outward.
Vertical lines represent bonds projecting backward.

23. 23
3. Stereoisomers and stereochemistry
Bromochlorofluoromethane

24. 24
3. Stereoisomers and stereochemistry
Conventions for drawing monosaccharides as Fischer
projections :
The most oxidized carbon is closest to the top.
The carbons are numbered from the top.
The chiral carbon with the highest number determines the D or L
designation.
If the OH is to the right, the sugar is D.
If the OH is to the left, the sugar is L.
Most common sugars are in the D form.

25. 25
3. Stereoisomers and stereochemistry
Determine whether each of the following
monosaccharides is D- or L–.
pencast

26. 26
3. Stereoisomers and stereochemistry
What to know from this section:
Definitions (bold-face terms)
Explanation of plane polarized light, and relationship to
stereoisomers (16.37-38)
How to identify chiral carbons in molecules (16.45-46)
How to interpret Fischer projections
How to identify D- and L- sugars (16.44)
How to draw the mirror image of a molecule (16.42)

27. 27
4. Monosaccharides: glucose
Glucose is an aldohexose (C6H12O6).
D- form
k
j
l
n
m
o
most oxidized carbon

28. 28
4. Monosaccharides: glucose
Under physiological conditions, glucose exists almost
entirely in a cyclic hemiacetal form.
The C-5 hydroxyl reacts with the C-1 aldehyde group.
C-1 becomes a chiral carbon.

29. 29
4. Monosaccharides: glucose
When the ring
forms, the –OH on
C-1 can be below (α-
) or above (β-) the
ring.
Isomers that differ in
the arrangement of
bonds around a
hemiacetal carbon
are called anomers.

30. 30
4. Monosaccharides: glucose
The ring forms are represented as Haworth projections on
the previous slide.
Groups on the left of the Fischer projection are above the ring.
Groups on the right of the Fischer projection are below the ring.
For the cyclic forms of D-sugars, the -CH2OH group is always up.
If the –OH on C-1 is cis to the -CH2OH group, it is a β-D-sugar.
If the –OH on C-1 is trans to the -CH2OH group, it is an α-D-sugar.

31. 31
4. Monosaccharides: glucose
Fischer and Haworth projections for D-glucose:

32. 32
4. Monosaccharides: fructose
Fructose, the sweetest of all sugars, is a ketohexose.
Cyclization of D-fructose produces a hemiketal.

33. 33
4. Monosaccharides: reducing sugars
Benedict’s test* is used to distinguish between reducing
and non-reducing sugars.
A reducing sugar can be oxidized.
The substance reduced is Cu+2.
+ Cu+2  + Cu2O
*Benedict’s reagent = a basic buffer solution plus Cu+2 ions

34. 34
4. Monosaccharides: reducing sugars
In general, Benedict’s reagent is used to distinguish
between aldehydes (e.g., aldoses) and ketones.
Ketoses, however, can convert to aldoses in basic
solution.
D-fructose enediol D-glucose

35. 35
4. Monosaccharides
What to know from this section:
Be able to relate the open chain form of a monosaccharide to
its cyclic form.
Understand the relationship between Fischer projections and
Haworth projections.
Understand the ring-forming reactions that yield hemiacetals or
hemiketals.
Identify α and β anomers.
Know the composition of Benedict’s reagent.
Understand what characteristics a monosaccharide needs to be
a reducing sugar (react with Benedict’s reagent).
Identify the enediol reaction that converts a ketose to an
aldose.

36. 36
5. Disaccharides
In biological systems, monosaccharides exist in the cyclic
form, as hemiacetals or hemiketals.
When a hemiacetal reacts with an alcohol, the product is an
acetal.
When a hemiketal reacts with an alcohol, the product is a ketal.

37. 37
5. Disaccharides
A disaccharide is formed when the hemiacetal or
hemiketal group on one monosaccharide reacts with one
of the hydroxyl groups on another monosaccharide.
The acetal or ketal formed is called a glycoside.
The C-O-C bond is called a glycosidic bond.

38. 38
5. Disaccharides
Maltose is formed from α-D-glucose and a second D-
glucose (α- or β-).
In this example, carbon-1 on the α-D-glucose links to carbon-4
on the β-D-glucose.
The linking oxygen atom is α to (below) the left ring.
The connection is called an α(14) glycosidic linkage.

39. 39
5. Disaccharides
Maltose
The hemiacetal hydroxyl group on C-1 will react with Benedict’s
reagent, because the ring can open at that point to form an aldehyde
(reducing group).
The product is named β-maltose because of the position of this
hydroxyl group.

40. 40
5. Disaccharides
Lactose is formed from β-D-galactose and α- or β-D-glucose.
In this example, carbon-1 on the β-D-galactose links to carbon-4 on
the β-D-glucose.
The linking oxygen atom is β to (above) the galactose ring.
The connection is called a β(14) glycosidic linkage.

41. 41
5. Disaccharides
The first step in digestion of lactose is its hydrolysis to re-
form galactose and glucose.
Glucose is readily metabolized.
If the enzyme lactase is not present, lactose can’t be
hydrolyzed before it is eliminated.
This condition is called lactose intolerance.
It can be remedied by ingesting lactase when eating lactose-
containing foods.
An enzyme-catalyzed process makes galactose usable by
the body.
Galactosemia is a condition in which one or more of the
enzymes is missing.

42. 42
5. Disaccharides
Sucrose is formed from α-
D-glucose and β-D-
fructose.
Carbon-1 on the glucose
links to carbon-2 on the
fructose (a ketose).
The linking oxygen atom is α
to (below) the glucose ring
and β to (above) the
fructose ring..
The connection is called an
(α1β2) glycosidic linkage.

43. 43
5. Disaccharides
In sucrose, the anomeric
carbons of both glucose and
fructose are linked.
There is no hemiacetal or
hemiketal group that can be
oxidized.
Sucrose is not a reducing sugar
and will not react with Benedict’s
reagent.

44. 44
5. Disaccharides
What to know from this section:
Identify hemiacetal and hemiketal monosaccharides and the
acetal and ketal disaccharides that can be formed from them.
Identify glycosidic linkages of various types.
Know why maltose and lactose are reducing sugars, while
sucrose is not.
Understand the chemical origins of lactose intolerance and
galactosemia.

45. 45
6. Polysaccharides
Starch is a heterogeneous mixture of two polymers of
glucose.
Amylose (about 20% of plant starch) is a linear polymer of α-D-
glucose units connected by α(14) glycosidic bonds.
Amylose chains can contain up to 4,000 glucose units.
Amylose coils into a helix that repeats every six glucose units.

46. 46
6. Polysaccharides
Starch is a heterogeneous mixture of two polymers of
glucose.
Amylopectin consists of an amylose backbone with chains of [α(14)
glycoside-bonded] glucose units branching off the C-6 hydroxyl
groups by α(16) glycosidic bonds.
Each of the many branches contains 20-25 glucose units.

47. 47
6. Polysaccharides
Digestion of starch:
Two enzymes are produced in the pancreas and the salivary
glands.
α-Amylase cleaves the α(14) glycosidic bonds randomly along
the amylose chain to make shorter polysaccharides.
β-Amylase sequentially cleaves pairs of glucose units (the
disaccharide maltose) from the reducing ends of amylose
chains.

48. 48
6. Polysaccharides
Glycogen is the main glucose storage molecule.
Its structure is identical to amylopectin’s, except
the branches are shorter,
there are many more branches.
amylopectin
glycogen

49. 49
6. Polysaccharides
Cellulose is a straight-chain polymer of β-D-glucose units
linked by β(14) glycosidic bonds.
Typical cellulose molecules contain about 3,000 glucose units.
Cellulose is part of the structure of plant cell walls.
Humans (and all but a few animals) can’t digest cellulose because
they lack the enzyme cellulase that hydrolyzes the β(14) glycosidic
bond.

50. 50
6. Polysaccharides
What to know from this section:
For amylose, amylopectin, glycogen, and cellulose:
the arrangement of the glucose units
the type(s) of linkages
The functions of α-amylase and β-amylase
Why humans can’t digest cellulose