introduction to carbohydrates

CARBOHYDRATES

BIOMOLECULES OF LIFE

Carbohydrates:

Life’s

Sweet Molecules

Carbohydrates

Carbohydrates are

• a major source of energy from our diet.

• composed of the elements C, H and O.

• also called saccharides, which means “sugars.”

Carbohydrates

Carbohydrates

• are produced by photosynthesis in plants.

• such as glucose are synthesized in plants from CO2, H2O, and energy from the sun.

• are oxidized in living cells to produce CO2, H2O, and energy.

• Carbohydrates are sugars and provide energy

when consumed.

• Our bodies break down carbohydrates to extract

energy. Carbon dioxide and water are released

in the process.

• Glucose is the primary carbohydrate our bodies

use to produce energy.

• Carbohydrates are classified as biomolecules.

• Simple carbohydrates are referred to as

simple sugars and are often sweet to the taste.

• Consumption of more sugar than is needed for

energy results in conversion of these sugars to

fat.

• Complex carbohydrates include starches and

the plant and wood fibers known as cellulose.

Introduction to Carbohydrates, Continued

• Carbohydrates are found on the surface of cells

where they act as “road signs” allowing

molecules to distinguish one cell from another.

• ABO blood markers found on red blood cells

are made up of carbohydrates. They allow us to

distinguish our body’s blood type from a foreign

blood type.

• Carbohydrates in our body prevent blood clots.

They are also found in our genetic material.

• Carbohydrates also can combine with lipids to form glycolipids

OR

• With proteins to form glycoproteins.

Examples of isomers:

1. Glucose

2. Fructose

3. Galactose

4. Mannose

Same chemical formula C6 H12 O6

EPIMERS

• EPIMERS are sugars that differ in configuration at ONLY 1 POSITION.

• Examples of epimers :

– D-glucose & D-galactose (epimeric at C4)

– D-glucose & D-mannose (epimeric at C2)

– D-idose & L-glucose (epimeric at C5)

ENANTIOMERS

Non-Superimposable COMPLETE mirror image (differ in configuration at EVERY CHIRAL CENTER.

The two members of the pair are designated as D and L forms.

In D form the OH group on the asymmetric carbon is on the right.

In L form the OH group is on the left side.

D-glucose and L-glucose are enantiomers:

Classes of Carbohydrates

• Monosaccharides are the simplest

carbohydrates. They cannot be broken down to

smaller carbohydrates.

• Disaccharides consist of two monosaccharide

units joined together; they can be split into two

monosaccharides. Sucrose, table sugar, can be

broken down into glucose and fructose.

• Oligosaccharides contain anywhere from three

to nine monosaccharide units. ABO blood

groups are oligosaccharides.

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Classes of Carbohydrates, Continued

Polysaccharides are large molecules containing

10 or more monosaccharide units. Carbohydrate

units are connected in one continuous chain or

the chain can be branched.

Chapter 5 21 © 2011 Pearson Education, Inc.

Monosaccharides, Continued

Some Important Monosaccharides

• Glucose is the most abundant monosaccharide

found in nature.

• Glucose is also known as dextrose, blood sugar,

and grape sugar.

• Glucose is broken down in cells to produce

energy.

Monosaccharides, Continued

• Diabetics have difficulty getting glucose in their

cells, which is why they must monitor their blood

glucose levels regularly.

• Glucose is one of the monosaccharides of

sucrose (table sugar) and lactose (milk sugar)

as well as the polysaccharides glycogen, starch,

and cellulose.

Monosaccharides, Continued

• Galactose is found combined with glucose in the

disaccharide lactose, which is present in milk

and other dairy products.

• A single chiral center (carbon 4) in galactose is

arranged opposite that of glucose, which makes

it a diastereomer of glucose.

• Diastereomers that differ by one chiral center

are called epimers.

cyclization

• Less then 1%of CHO exist in an open chain form.

• Predominantly found in ring form.

• involving reaction of C-5 OH group with the C-1 aldehyde group or C-2 of keto group.

• Six membered ring structures are called Pyranoses .

• five membered ring structures are called

Furanoses .

Monosaccharides, Continued

• Mannose, a monosaccharide, is found in some

fruits and vegetables.

• Cranberries contain high amounts of mannose,

which has been shown to be effective in urinary

tract infections.

• Mannose is an epimer of glucose.

Monosaccharides, Continued

• Fructose, a ketose, is commonly referred to as

fruit sugar or levulose.

• Fructose is combined with glucose to give

sucrose, or table sugar.

• Fructose is the sweetest monosaccharide and

is found in fruits, vegetables, and honey.

• Fructose is not an epimer of glucose, but it can

be broken down for energy in the body.

Oxidation and Reduction Reactions, Continued

Monosaccharides and Redox

• An aldehyde functional group can undergo

oxidation by gaining oxygen or it can undergo

reduction by gaining hydrogen.

• During oxidation, aldehydes form carboxylic

acids, and during reduction, they form alcohols.

• In monosaccharides, oxidation produces a sugar

acid, and reduction produces a sugar alcohol.

Oxidation and Reduction Reactions, Continued

• Benedict’s test is a useful test to determine the

presence of an oxidation reaction that occurs

with sugars.

• Aldose sugars are oxidized by Cu2+ ion, while

the Cu2+ ion is reduced to Cu+ ion.

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Oxidation and Reduction Reactions, Continued

The product of this reaction, copper(I) oxide

(Cu2O), is not soluble and forms a brick red

precipitate in solution.

Oxidation and Reduction Reactions, Continued

• Aldoses are easily oxidized. They serve as

reducing agents and are referred to as reducing

sugars.

• Fructose and other ketoses are also reducing

sugars, even though they do not contain an

aldehyde group.

• The oxidizing agents can cause a

rearrangement of the ketose to an aldose.

Oxidation and Reduction Reactions, Continued

• Benedict’s test can be used in urine dipsticks to

determine the level of glucose in urine. Excess

glucose in urine suggests high levels of glucose

in blood, which is an indicator of diabetes.

• Aldoses or ketoses can be reduced by hydrogen

under the correct conditions, producing sugar

alcohols.

• Sugar alcohols are produced commercially as

artificial sweeteners and found in sugar-free

foods.

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Oxidation and Reduction Reactions, Continued

• When glucose levels are high in the blood

stream, sorbitol can be produced by an enzyme

called aldose reductase.

• High levels of sorbitol can contribute to

cataracts, which is a clouding of the lens in the

eye.

• Cataracts are commonly seen in diabetics.

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Disaccharides

Condensation and Hydrolysis—Forming and Breaking Glycosidic Bonds

• The –OH group that is most reactive in a monosaccharide is the one on the anomeric carbon.

• When this hydroxyl group reacts with another hydroxyl group on another monosaccharide a glycosidic bondis formed.

Disaccharides, Continued

Formation of glycosides is an example of

another type of organic reaction. During this

reaction, a molecule of water is eliminated as

two molecules join.

Disaccharides, Continued

• Condensation reaction is a type of reaction

that occurs when two molecules are joined and

a water molecule is produced. This type of

reaction is referred to as a dehydration

reaction.

• Hydrolysis reaction is the reverse of a

condensation reaction. A larger molecule forms

two smaller molecules and water is consumed

as a reactant.

Disaccharides, Continued

Condensation reactions occur between different

types of functional groups that contain an –H in

a polar bond, like O–H or N–H, and an –OH

group that can be removed to form water.

Disaccharides, Continued

• In the case of maltose, the glycosidic bond is

specified as α(1→4) and is simply stated as

alpha-one-four.

• If the –OH group had been in the beta

configuration when the glycosidic bond was

formed, the bond would be in the β(1→4)

configuration. The molecule formed would be

named cellobiose and would have a different

two-dimensional and three-dimensional shape

than maltose.

Chapter 5 42 © 2011 Pearson Education, Inc.

Disaccharides, Continued

Chapter 5 43 © 2011 Pearson Education, Inc.

Disaccharides, Continued

Maltose

• Maltose is known as malt sugar.

• It is formed by the breakdown of starch.

• Malted barley, a key ingredient in beer,

contains high levels of maltose.

• During germination of barley seeds, the starch

goes through hydrolysis to form maltose. This

process is halted by drying and roasting

barley seeds prior to their germination.

• One of the anomeric carbons is free, so

maltose is a reducing sugar.

Chapter 5 44 © 2011 Pearson Education, Inc.

Disaccharides, Continued

Maltose, Continued

• The glycosidic bond is α(1→4).

Chapter 5 45 © 2011 Pearson Education, Inc.

Disaccharides, Continued

Lactose

• Lactose is known as milk sugar.

• It is found in milk and milk products.

• An intolerance to lactose can occur in people

who inherit or lose the ability to produce the

enzyme lactase that hydrolyzes lactose into

its monosaccharide units.

• The glycosidic bond is (1→4).

• One of the anomeric carbons is free, so

lactose is a reducing sugar.

Disaccharides, Continued

Disaccharides, Continued

Sucrose

• Sucrose is known as table sugar.

• It is the most abundant disaccharide found in

nature.

• Sucrose is found in sugar cane and sugar

beets.

• The glycosidic bond is (1→2).

• Both anomeric carbons of the

monosaccharides in sucrose are bonded,

therefore, sucrose is not a reducing sugar. It

will not react with Benedict’s reagent.

Disaccharides, Continued

Polysaccharides

Polysaccharides

Polysaccharides are large molecules of monosaccharides that are connected to each other through their anomeric carbons. There are two types of polysaccharides:

1. Storage polysaccharides contain only -glucose units. Three important ones are starch, glycogen, and amylopectin.

2. Structural polysaccharides contain only -glucose units. Two important ones are cellulose and chitin. Chitin contains a modified -glucose unit.

Polysaccharides

• 2 types: – HOMOpolysaccharides (all 1 type of monomer),

e.g., glycogen, starch, cellulose, chitin

– HETEROpolysaccharides (different types of monomers), e.g., peptidoglycans, glycosaminoglycans

Chapter 5 51 © 2011 Pearson Education, Inc.

Polysaccharides, Continued

Storage Polysaccharides

Amylose and amylopectin—starch

• Starch is a mixture of amylose and amylopectin

and is found in plant foods.

• Amylose makes up 20% of plant starch and is

made up of 250–4000 D-glucose units bonded

α(1→4) in a continuous chain.

• Long chains of amylose tend to coil.

• Amylopectin makes up 80% of plant starch and

is made up of D-glucose units connected by

α(1→4) glycosidic bonds.

Chapter 5 52 © 2011 Pearson Education, Inc.

Polysaccharides, Continued

Amylose and amylopectin—starch

• About every 25 glucose units of amylopectin, a

branch of glucose units are connected to the

glucose by an α(1→6) glycosidic bond.

• During fruit ripening, starch undergoes

hydrolysis of the α(1→4) bonds to produce

glucose and maltose, which are sweet.

• When we consume starch, our digestive system

breaks it down into glucose units for use by our

bodies.

Chapter 5 53 © 2011 Pearson Education, Inc.

Polysaccharides, Continued

Polysaccharides, Continued

Glycogen

• Glycogen is a storage polysaccharide found in

animals.

• Glycogen is stored in the liver and muscles.

• Its structure is identical to amylopectin, except

that α(1→6) branching occurs about every

12 glucose units.

• When glucose is needed, glycogen is

hydrolyzed in the liver to glucose.

Glycogen

Glycogen

• is the polysaccharide that stores α-D-glucose in muscle.

• is similar to amylopectin, but is more highly branched.

Polysaccharides, Continued

Structural Polysaccharides

Cellulose

• Cellulose contains glucose units bonded

(1→4).

• This glycosidic bond configuration changes the

three-dimensional shape of cellulose compared

with that of amylose.

• The chain of glucose units is straight. This

allows chains to align next to each other to form

a strong rigid structure.

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Polysaccharides, Continued

Chapter 5 58 © 2011 Pearson Education, Inc.

Carbohydrates and Blood

ABO Blood Types

• ABO blood types refer to carbohydrates on red

blood cells.

• These chemical markers are oligosaccharides that

contain either three or four sugar units.

• Sugar units are D-galactose, L-fucose,

N-acetylglucosamine, and N-acetylgalactosamine.

Carbohydrates and Blood, Continued

Heparin

• Heparin is a medically important polysaccharide

because it prevents clotting in the bloodstream.

• It is a highly ionic polysaccharide of repeating

disaccharide units of an oxidized

monosaccharide and D-glucosamine. Heparin

also contains sulfate groups that are negatively

charged.

• It belongs to a group of polysaccharides called

glycosaminoglycans.

• Functions:– glucose storage (glycogen in animals & bacteria,

starch in plants)

– structure (cellulose, chitin, peptidoglycans, glycosaminoglycans

– information (cell surface oligo- and polysaccharides, on proteins/glycoproteins and on lipids/glycolipids)

• osmotic regulation

• Cellulose and chitin– Function: STRUCTURAL, rigidity important

– Cellulose:

• homopolymer, b(1-> 4) linked glucose residues

• cell walls of plants

– Chitin:

• homopolymer, b(1-> 4) linked N-acetylglucosamine residues

• hard exoskeletons (shells) of arthropods (e.g., insects, lobsters and crabs)