Types of Carbohydrates

Section 17.1

Four Types of Carbohydrates

Monosaccharides Contain a single sugar unit Examples: glucose and fructose

Disaccharides Contain two monosaccharides units joined

through bridging oxygen atoms AKA glycosidic bond

Examples: sucrose and lactose

Four Types, continued

Oligosaccharides Three to ten monosaccharide units joined by

glycosidic bonds

Polysaccharides Long, often highly branched, chains of

monosaccharides Examples: starch, glycogen, and cellulose

Monosaccharides

Composed of carbon, hydrogen and oxygen

Most follow the general formula (CH2O)n

All names end in –oseTwo types

Ketose – monosaccharide that contains a ketone carbonyl group

Aldose – monosaccharide that contains an aldehyde carbonyl group

Ketose OR Aldose?

CH

C OHH

C OHH

C OHH

C OHH

CH2OH

O

CH

C OHH

C HHO

C OHH

C OHH

CH2OH

O

CH2OH

C

C HHO

C OHH

C OHH

CH2OH

O

CH2OH

C

C HHO

C OHH

C OHH

CH2OH

O

Ketose OR Aldose

CH

C OHH

C OHH

C OHH

C OHH

CH2OH

O

CH

C OHH

C HHO

C OHH

C OHH

CH2OH

O

CH2OH

C

C HHO

C OHH

C OHH

CH2OH

O

CH2OH

C

C HHO

C OHH

C OHH

CH2OH

O

Another system – number of carbons

Another system of nomenclature tells us the number of carbon atoms in the main skeleton.

3 carbon triose4 carbon tetrose5 carbon pentose6 carbon hexoseEtc.

Triose, tetrose, pentose, etc?

CH

C OHH

CH2OH

O CH

C OHH

C OHH

C OHH

CH2OH

OCH

C OHH

C HHO

C OHH

C OHH

CH2OH

O

Answers

CH

C OHH

CH2OH

O CH

C OHH

C OHH

C OHH

CH2OH

OCH

C OHH

C HHO

C OHH

C OHH

CH2OH

O

Triose Pentose Hexose

Combining the two systems

CH

C OHH

CH2OH

OCH

C OHH

C HHO

C OHH

C OHH

CH2OH

OCH2OH

C

C HHO

C OHH

C OHH

CH2OH

O

AldoseTrioseAldotrioseD-Glyceraldehyde

AldoseHexoseAldohexoseD-Glucose

KetoseHexoseKetohexoseD-Fructose

Chiral Objects

Chiral compounds have the same number of atoms arranged differently in space.

A chiral carbon atom has four different groups attached.

Mirror Images

The three-dimensional structure of a chiral compound has a mirror image.

Your hands are chiral. Try to superimpose your thumbs, palms, back of hands, and little fingers. Is it possible? Why or why not?

Chiral or NOT?

Determine if there is a chiral carbon in each compound.

A B

C

Cl

CH3

CH2CH3

H C

Cl

CH3

H

H

Solution

A Yes, 4 different B No, the

groups are attached 2 H atoms

to the second C atom are identical

C

Cl

CH3

CH2CH3

H C

Cl

CH3

H

H

D and L Notation

D,L tells to which of the two chiral isomers we are referring.

If the –OH group on the next to the bottom carbon atom points to the right, the isomer is a D-isomer; if it points left, the isomer is L.

The D form is usually the isomer found in nature.

D notation

C

C

CH2OH

OHH

OHH

CO

H

Right = D

Glucose

C

C

CH2OH

H OHC

OHH C

HHO

H OH

C OH

D-Glucose

Homework

Complete problems 17.3 and 17.4 on page 472; 17.5 and 17.6 on page 474-5; 17.24, 17.25, and 17.26 on page 493

Importance of Glucose

Most important sugar in the human body.Glucose is broken down in glycolysis and

other pathways to release energy for body functions

The concentration of glucose in the blood is carefully controlled by insulin and glucagon.

Normal blood glucose levels are 100-120 mg/100mL

Importance of Glucose

Insulin stimulates the uptake of the excess glucose by most of the cells in the body.

1-2 hrs after eating the glucose levels return to normal.

If glucose concentrations drop too low, the individual feels lightheaded and shaky. When this happens, glucagon stimulates the liver to release glucose in the blood.

Diabetes / Hypoglycemia and Glucose

Type I diabetes or diabetes mellitus – caused by the inability to produce the hormone insulin; If untreated, end up with high blood sugar

Type II diabetes or adult-onset diabetes – caused by insulin resistance (body fails to properly use insulin) combined with a relative insulin deficiency; If untreated, end up with high blood sugar

Hypoglycemia – caused by the over excretion of insulin in response to a rise in blood sugar; If untreated, end up with low blood sugar

Structure of glucose

In reality, the open-chain form of glucose is present in very small concentrations in cells.

In most cases, the cyclic intramolecular hemiacetal is formed.

The hydroxyl group on C-5 reacts with the aldehyde group to form a hemiacetal.

Two isomers are formed because a new chiral carbon is created.

CH

OHH

HHO

OHH

OHH

CH2OH

O

O

CH2OH

HH

OHOH

H

H

OH

H

OH

O

CH2OH

HH

OHOH

H

H

OH

OH

H

D-glucose

α-D-glucose

β-D-glucose

1

2

3

4

5

6

1

1

2

2

3

3

4

4

5

5

6

6

Haworth Structure for D-Glucose

Write –OH groups on the right (C2, C4) down

Write –OH groups on the left (C3) upThe new –OH on C1 has two

possibilites: down for form, up for form

α-D-glucose

C

C OHH

C HHO

C OHH

C HH2CHO

H OH

O

CH2OH

HH

HOH

H

H

OH

H

OH

O

β-D-glucose

C

C OHH

C HHO

C OHH

C HH2CHO

HO H

O

O

CH2OH

HH

OHOH

H

H

OH

OH

H

You Try This One!

Write the cyclic form of -D-galactose

C

C

C

C

C

CH2OH

OH

OHH

HO H

HHO

OHH

Solution -D-galactose

o

OH

OH

CH2OH

OH

OH

C

C OHH

C HOH

C HOH

C HCH2HO

H OH

O

Benedict’s reagent

Benedict’s reagent – a basic buffer solution that contains Cu2+ ions

It readily oxidizes the aldehyde group of aldoses to form a brick red Cu2O precipitate.

It will also oxidize ketoses because of the –OH group on the carbon next to the carbonyl group.

Reducing Sugars

Reducing sugar – a sugar that can be oxidized by Benedict’s reagent

All monosaccharides and the common disaccharides (except sucrose) are reducing sugars.

Use of Benedict’s reagent

Benedict’s reagent was commonly used to qualitatively monitor excess glucose in the urine by diabetics to insure proper dosage of insulin.

These have been replaced by blood glucose tests that are more accurate.