Carbohydrates

Pensacola High School

IB Chemistry 3

Version 1.12

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Carbohydrates• Contain Carbon, Hydrogen and Oxygen• Can be characterized as

– Monosaccharides– Disaccharides– Polysaccharides

• Includes sugars, starches, cellulose,

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Carbohydrates• Carbohydrates are produced in green plants in the

presence of chlorophyll and sunlight in a process known as photosynthesis.

• They serve as food sources for living organisms and provide the structural support for plants.

• Many carbohydrates are large polymers composed of repeating units of simple sugars.

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Types of Carbohydrates

Monosaccharides - simple sugars with multiple -OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose.

Disaccharides - Two monosaccharides linked by a covalent bond.

Oligosaccharides - a few monosaccharides linked by covalent bonds

Polysaccharides - polymers consisting of chains of multiple monosaccharide or disaccharide units.

I (CH2O)n or H - C - OH

I

Carbohydrates have the following basic composition:

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fructose

Monosaccharides• Single (simple) sugars• Contain C, H, and O in a 1:2:1

ratio • Quick energy sources

Examples:Examples: Glucose C6H12O6

Fructose C6H12O6

Galactose C6H12O6

glucose

Carbohydrates

5Glucose Fructose

Monosaccharides• Empirical formula is CH2O

• Both open chain and ring structures are possible• Mulitple structural isomers are possible• Multiple chiral carbon atoms lead to optical isomers• Monosaccharides generally have between 3 and 6

carbon atoms• The most common monosaccharides are:

– Five carbons C5H10O5 - called pentoses

– Six carbons C6H12O6 - called hexoses

• Monosaccharide straight chains have at least one carbonyl group C=O.

• If the carbonyl group is at the end it is an aldose sugar. If it is within the chain it is a ketose sugar 6

Monosaccharides

Aldoses (e.g., glucose) have an aldehyde group at one end.

Ketoses (e.g., fructose) have a ketone group, usually at C2.

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose7

Optical Isomers: D and L Forms

D or dextrorotatory & L or levorotatory are designations for optical isomers that are based on the configuration about the single asymmetric C in glyceraldehyde.

The lower representations are Fischer Projections.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

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Sugar Nomenclature

For sugars with more than one chiral center, D and L refer to the asymmetric C farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

O H O H C C H – C – OH HO – C – H

HO – C – H H – C – OH

H – C – OH HO – C – H

H – C – OH HO – C – H

CH2OH CH2OH

D-glucose L-glucose

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D & L sugars are mirror images of one another.

They have the same name, e.g., D-glucose & L-glucose.

Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc.

The number of stereoisomers is 2n, where n is the number of asymmetric centers.

The 6-C aldoses have 4 asymmetric centers. Thus there are 16 possible stereoisomers (8 D-sugars and 8 L-sugars).

O H O H C C H – C – OH HO – C – H

HO – C – H H – C – OH

H – C – OH HO – C – H

H – C – OH HO – C – H

CH2OH CH2OH

D-glucose L-glucose

Steroisomers

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Ring StructuresPentoses and hexoses can form ring structures as the ketone or aldehyde reacts with a distal OH.

Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member ring known as a pyranose ring,

These representations of the cyclic sugars are called Haworth projections.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

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4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

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Fructose Ring Structures

a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or

a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

HOH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

Fructose may form either

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Monosaccharides

Some examplesof pyranose ring structures for hexose sugars. The ring is not actually planar but exists in boat and chair conformers

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Sugar Derivatives

An Amino sugar is a sugar in which an amino group substitutes for a hydroxyl. An example is glucosamine.

The amino group may be converted to an amide, as in N-acetylglucosamine.

H O

OH

H

OH

H

NH2H

OH

CH2OH

H

-D-glucosamine

H O

OH

H

OH

H

NH

OH

CH2OH

H

-D-N-acetylglucosamine

C CH3

O

H

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Anomers of Glucose

Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, & .

Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:

(OH below the ring) (OH above the ring).

H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

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4

5

6

1 1

6

5

4

3 2

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Glycosidic BondsThe anomeric hydroxyl groups of two sugars can join together, splitting out water to form a glycosidic bond. Two glucose molecules combine to form a disaccharide known as maltose.

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H H

•Double sugars•Good source of energy•Break down into simple sugars

Sucrose (glucose + fructose)Lactose (glucose + galactose)

Disaccharides

Other disaccharides include: -- Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose.

-- Because the configuration at the anomeric C of glucose is (O points down from ring), the linkage is (12). The full name of sucrose is -D-glucopyranosyl-(12)-D-fructopyranose.)

-- Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric OH of galactose. Its full name is -D-galactopyranosyl-(1 4)--D- glucopyranose

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H H

•Compare the structures of these three common disaccharides

Disaccharides

•Sucrose is an (1-4) link between D-Glucose and D-Fructose•Lactose is an (1-4) link between two D glucose

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Examples:Starch- (plants) found in leaves, tubers…Glycogen- (animals) found in the liver and

musclesCellulose- (plants) make up cell walls

•3 or more sugars linked together•Complex sugars•Important for energy storage

Starch

Polysaccharides

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• Plants store glucose as amylose or amylopectin. Both are glucose polymers collectively called starch.

Amylose is a glucose polymer with (14) linkages.

The end of the polysaccharide with an anomeric C1 that is not involved in a glycosidic bond is called the reducing end.

• Glucose storage in polymer form minimizes osmotic effects.

Polysaccharides - Starches

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Reducing end

Amylose

Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages. Branches are generally longer than those shown in the diagram above.

• The branches produce a compact structure & provide multiple chain ends at which enzyme activity can occur.

H O

OH

H

OHH

OH

CH2OH

HO H

H

OHH

OH

CH2OH

H

O

HH H O

OH

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

OH

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH2OH

HH H O

H

OHH

OH

CH2OH

HH

O1

OH

3

4

5

2

amylopectin

Amylopectin

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Amylopectin

• Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin found in plants

• Glycogen has more (16) branches than amylopectin

• The ability to rapidly mobilize glucose is more essential to animals than to plants.

• The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise.

H O

OH

H

OHH

OH

CH 2OH

HO H

H

OHH

OH

CH 2OH

H

O

HH H O

OH

OHH

OH

CH 2

HH H O

H

OHH

OH

CH 2OH

H

OH

HH O

OH

OHH

OH

CH 2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH 2OH

HH H O

H

OHH

OH

CH 2OH

HH

O1

OH

3

4

5

2

glycogen

Glycogen

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Glycogen

The essential difference between amylose starch and cellulose is in the glycosidic link between the saccharide units. Amylose has links. Cellulose has links.

Starch and Cellulose

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Cellulose

Amylose

• Cellulose is the major building component of plant cell walls

• Long chain of glucose molecules would be expected to be a great source of energy, but humans lack the necessary enzyme to digest cellulose

• The Endosymbiotic Protist in cow guts DOES have the enzyme

Cellulose

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Dietary Fiber• Dietary fiber is mainly plant material that is not

hydrolyzed by enzymes secreted by the human digestive tract but may be digested by microflora in the gut.

• Examples of dietary fiber include cellulose, hemicellulose, lignin and pectin.

• Dietary fiber may be helpful in the prevention of conditions such as diverticulosis, irritable bowel syndrome, constipation, obesity, Crohn’s disease, hemorrhoids and diabetes mellitus.

Carbohydrate Functions: Energy Sources

• During metabolism animals break down carbohydrates to carbon dioxide and water vapor.

• Monosaccharides and dissaccharides break down quickly and provide quick energy sources.

• Starches take longer to metabolize but the end products are the same.

• Human beings cannot break down cellulose, since we lack the appropriate enzyme to breakdown the 1-4 linkage

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Carbohydrate Functions: Storage

• The main storage polysaccharides are starches and glycogen. While plants use starch as their storage polysaccharides, animals use glycogen.

• When the body has a high glucose concentration, the pancreas releases insulin, which converts glucose into glycogen and stores it in the liver.

• When the glucose concentration is low, the hormone glucagon converts glycogen back into glucose.

• Glycogen is the primary energy reserve in human beings . Metabolism of glucose provides the energy necessary for our bodies to function and carry out daily activities.

• When it is broken down into glucose and oxidized, ultimately to CO2 and H2O, through cellular respiration, large amounts of energy are released.

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Carbohydrate Functions: Structure

• Cellulose is a major component of plant cell walls. It is a polymer of -D-glucose and forms a very strong fiber, which is excellent building material in plants.

• Cows and other ruminants have enzymes that break down cellulose. In humans it is primarily bulk or roughage.

• Chitin is a structural polysaccharide found in the exoskeletons of some insects.

• Chitin is a leather like structural substance that eventually hardens when it is shed.

• Chitin is often used in medicine for sutures because it is both strong and flexible, but it also decomposes over time.

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Carbohydrate Functions: Precursor Molecules

• Carbohydrates are precursors for the synthesis of certain biomolecules.

• Carbohydrates (ribose) form part of the skeletons of nucleic acids, DNA and RNA.

• The carbon skeletons of carbohydrates serve as raw material for the synthesis of other small organic molecules, such as amino acids and fatty acids.

• Disaccharides provide building material for structures that protect the cell or whole organism.

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The End

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