Carbohydrates

(‘Hydrates of carbon') or saccharides (Greek meaning "sugar")

CarbohydrateS

Polyhydric alcohols having potentially active aldehyde and ketone groups

Carbohydrates

Carbohydrates

Monosaccharides

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

Ketoses (e.g., fructose) have a keto 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-fructose

A chain-form monosaccharide that has a carbonyl group (C=O) on an end carbon forming an aldehyde group (-CHO) is classified as an aldose. When the carbonyl group is on an inner atom forming a ketone, it is classified as a ketose.

The basic carbohydrate unit is called Monosaccharide

The general formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater

Include large group of compounds commonly known as starches or sugars which are widely distributed in plants and animals

Carbohydrates

Carbohydrates or hydrates of carbon

Empirical formula :

Cn(H2O)n

2 to 1 ratio of hydrogen to oxygen

Biological Significance1. Carbohydrates are the main source of

energy in the body.

• When oxidized in the body, they liberate CO2, water and energy.

Biological Significance1.Carbohydrates are the main source of

energy in the body.

• Supply the major portion of energy required by living cells.

• Brains cells and RBCs are almost wholly dependent on carbohydrates as energy source.

Biological Significance

2. Certain products of carbohydrate metabolism act as catalysts to promote oxidation of foodstuffs.

Biological Significance

3. Certain carbohydrates are the starting materials for the biological synthesis of fatty acids and amino acids.

Biological Significance

4. Carbohydrates save proteins from being used for the production of energy.

Sources Foods that are high in

carbohydrates include breads, pastas, beans, potatoes, bran, rice and cereals.

Occurrence

• Carbohydrates are widely distributed in plant and animal tissues.

Occurrence

• In plants, they are produced by photosynthesis and include the starches of the plant framework and plant cells.

Occurrence• Glycogen is the storage form of

carbohydrates in animals found abundantly in the liver and muscles

• Sucrose – cane sugar; also present in the nectar of flowers and in fruits

Occurrence

• Glucosides which are derivatives of carbohydrates are found in certain plants

• Glucose is the sugar in the blood and other body fluids

• Lactose – milk

General Properties

Physical Properties:Forms: The monosaccharides and disaccharides

are white crystalline substances.

Starches are amorphous powder.

Complex cellulose is fibrous

General Properties

Solubility: Monosaccharides and disaccharides

readily dissolve in water.

Higher carbohydrates like starch dissolve only slightly.

Cellulose is practically insoluble.

General properties

Relative Sweetness:fructose Lactose invert sugar xyloseSucrose starchglucosegalactoseMaltose

Carbohydrates with lower molecular weight have sweet taste.

Other Properties Many saccharide structures differ only

in the orientation of the hydroxyl groups (-OH).

This slight structural difference makes a big difference in the biochemical properties, organoleptic properties (e.g., taste), and in the physical properties such as melting point and Specific Rotation (how polarized light is distorted).

Other Properties Stereoisomerism : Same structural formula but

differ in spatial configuration With respect to the Penultimate Carbon atom

glucose has 2 stereoisomers D-glucose and L-glucose

L-GlucoseD-Glucose

D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose.

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

Optical Activity : Rotation of Plane polarized

light and (+)glucose or (–)glucose respectively

D-glucose L-glucose

Epimers

Structures that have opposite configurations of a hydroxyl group at only one position (C2 or C4)

such as glucose and mannose; glucose and galactose

D-glucose

Chemical Properties

Osazone formation with phenylhydrazine

Benedict’s test Oxidation Reduction to Alcohols, etc.

Structural Representation of Carbohydrates Open Chain Structure / Linear Structure/ Fischer Structure

Hemi-acetal Structure

Haworth Structure

Open Chain Structure

Long Straight chain form of carbohydrates

Haworth StructurePresence of a Pyranose Ring

Pentoses and hexoses can cyclize 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 pyranose ring, named after pyran.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

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)

Fructose forms either 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

Hemi-acetal Structure The 1st Carbon in glucose

condenses with the –OH group of the 5th Carbon to form a ring

Classification :1) The most useful Classification

Scheme divides the carbohydrates into groups according to the number of individual simple sugar units

1)Monosaccharide 2)Disaccharides 3)Polysaccharides

2) Number of Carbons 3) Functional Groups

MONOSACCHARIDEMonosaccharides are the simplest

carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. The general chemical formula of an unmodified monosaccharide is (C•H2O)n, where n is any number of three or greater.

The Killiani Synthesis

Tetroses

D-Erythrose D-Threose

Pentoses

The ring form of ribose is a component of ribonucleic acid (RNA).Deoxyribose, which is missing an oxygen at position 2, is a component of deoxyribonucleic acid (DNA). In nucleic acids, the hydroxyl group attached to carbon number 1 is replaced with nucleotide bases.

Hexoses

Hexoses, such as the ones illustrated here, have the molecular formula C6H12O6.

German chemist Emil Fischer (1852-1919) identified the stereoisomers for these aldohexoses in 1894. He received the 1902 Nobel Prize for chemistry for his work.

Heptoses

Sedoheptulose has the same structure as fructose, but it has one extra carbon.

Disaccharides contain two sugar units.

In almost all cases one of the

sugars is glucose, with the other sugar being galactose, fructose, or another glucose.

Common disaccharides are maltose, lactose, and sucrose.

Disaccharide descriptions and components

TrisaccharidesRaffinose, also called melitose,

is a trisaccharide that is widely found in legumes and cruciferous vegetables, including beans, peas, cabbage, brussels sprouts, and broccoli. It consists of galactose connected to sucrose via a 1α→6 glycosidic linkage.

Trisaccharides

Humans cannot digest saccharides with this linkage and the saccharides are fermented in the large intestine by gas-producing bacteria.

Polysaccharides

contain many sugar units in long polymer chains of many repeating units. The most common sugar unit is glucose. Common poly saccharides are starch, glycogen, and cellulose.

Classification

1. Starch Group*starch, glycogen, inulin, dextrin

2. Cellulose Groupa. celluloseb. hemicellulose

Pentosans – gum arabic Hexosans – agar-agar Hexopentosans - pectins

According to nature of units Homopolysaccharides

only one kind of monosaccharide unit Heteropolysaccharides

mixtures of monosaccharides + derived products

According on the type of linkages

Linear polysaccharides only one kind of glycosidic linkage

Branched polysaccharides two or more kinds of linkages

Starch Starch is the major form of stored

carbohydrate in plants. Starch is composed of a mixture of two substances: amylose, an essentially linear polysaccharide, and amylopectin, a highly branched polysaccharide.

polymers of α-D-Glucose. Natural starches contain 10-20% amylose and 80-90% amylopectin.

Amylose molecules -consist typically of 200 to 20,000 glucose units which form a helix as a result of the bond angles between the glucose units.

Amylopectin -differs from amylose in being highly branched.

Short side chains of about 30 glucose units are attached with 1α→6 linkages approximately every twenty to thirty glucose units along the chain.

Amylopectin molecules may contain up to two million glucose units.

Inulin Inulins are present in many

vegetables and fruits, including onions, leeks, garlic, bananas, asparagus, chicory, and Jerusalem artichokes. Inulins are polymers consisting of fructose units that typically have a terminal glucose.

Glycogen

Glucose is stored as glycogen in animal tissues by the process of glycogenesis.

When glucose cannot be stored as glycogen or used immediately for energy, it is converted to fat.

Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin.

But glycogen has more (16) branches.

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

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

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

Cellulosepolymer of β-D-Glucose, which in

contrast to starch, is oriented with -CH2OH groups alternating above and below the plane of the cellulose molecule thus producing long, unbranched chains.

Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with (14) linkages.Every other glucose is flipped over, due to linkages.

c e l lu lo s e

H O

OH

H

OHH

OH

CH 2 OH

HO

H

OHH

OH

CH 2 OH

HO

H H O

O H

OHH

OH

CH 2 OH

HH O

H

OHH

OH

CH 2 OH

H

H

OHH O

O H

OHH

OH

CH 2 OH

HO

H H H H

1

6

5

4

3

1

2

CelluloseCellulose is the major structural

material of plants. Wood is largely cellulose, and cotton is almost pure cellulose. Cellulose can be hydrolyzed to its constituent glucose units by microorganisms that inhabit the digestive tract of termites and ruminants.

Cellulose may be modified in the laboratory by treating it with nitric acid (HNO3) to replace all the hydroxyl groups with nitrate groups (-ONO2) to produce cellulose nitrate (nitrocellulose or guncotton) which is an explosive component of smokeless powder.

Partially nitrated cellulose, known as pyroxylin, is used in the manufacture of plastics, lacquers, and nail polish.

Hemicelluloseapplied to the polysaccharide

components of plant cell walls other than cellulose, or to polysaccharides in plant cell walls which are extractable by dilute alkaline solutions.

Hemicellulose

The chemical structure of hemicelluloses consists of long chains of a variety of pentoses, hexoses, and their corresponding uronic acids.

DextranDextran is a polysaccharide similar

to amylopectin, but the main chains are formed by 1α→6 glycosidic linkages and the side branches are attached by 1α→3 or 1α→4 linkages.

DextranDextran is an oral bacterial

product that adheres to the teeth, creating a film called plaque. It is also used commercially in confections, in lacquers, as food additives, and as plasma volume expanders.

Heteropolysaccharides- monosaccharides and derived products

1. neutral mucopolysaccharide those occurring in bacteria mucoids including immunological,

specific blood group substances

2. acid mucopolysaccharide cartilage tissues skin

Mucopolysaccharides- Glycosaminoglycans are linear polymers of repeating disaccharides.

The constituent monosaccharides tend to be modified, with acidic groups, amino groups, sulfated hydroxyl and amino groups, etc.

H O

H

H

O HH

O H

COO

H

H O

O H H

H

NH COCH 3H

CH 2O H

H

OO

D -g lucuronate

O

1

23

4

5

61

23

4

5

6

N -acetyl-D -g lucosam ine

hyaluronate

heteropolysaccharides

3. hyaluronic acid complex mucopolysaccharide main constituent of the ground

substance of connective tissues abundant in:

synovial fluid vitreous humour aqueous humour

heteropolysaccharides

in tissues, it serves as a cementing substance which allows the passage of metabolites

fragmented by an enzyme, hyaluronidase, also called the spreading factor

bacteria sperms poisonous secretions of reptiles and other

animals

Xylan - HOMOPOLYSACCHARIDE

Digestion of CarbohydratesPolysaccharides in the mouth are

broken through the introduction of amylase, a digestive enzyme in saliva.

The high acid content of the stomach inhibits amylase activity, so carbohydrate digestion is suspended in the stomach.

Digestion of CarbohydratesPancreatic amylase is secreted

by the pancreas into the duodenum and works with other enzymes to complete the breakdown of carbohydrate into a monosaccharide

Is then absorbed into the surrounding capillaries of the villi.

Nutrition

Carbohydrates are not essential nutrients

The body can obtain all its energy from protein and fats

Carbohydrates contain 3.75 Kilocalories

An essential diet contains between 40-65% of dietary energy from carbohydrates

Glycemic IndexA measure of the effects of

carbohydrates on blood glucose levels.

Carbohydrates that break down rapidly have a high GI; Those that break down slowly, releasing glucose gradually into the bloodstream, have a low GI

Glycemic IndexA lower glycemic response is

often thought to equate to a lower insulin demand, better long-term blood glucose control and a reduction in blood lipids.

Classification GI range Examples

Low GI >55 % most fruit and vegetables, grainy

breads, pulses

Moderate GI 55 – 69 % wheat bread, whole wheat products,

brown rice, orange

High GI 70 and above

corn flakes, baked potato, white bread

Structure of monosaccharides, disaccharides and polysaccharides

Formation of maltose and sucrose from their monosaccharides

Structure:Function of polysaccharides

Functions of CarbohydratesCarbohydrates spare protein from

being used up as an energy source. If there are not enough carbohydrates,

then large amounts of fat are used for energy. The body is not able to handle this large amount so quickly, so it accumulates ketone bodies

Functions of CarbohydratesCarbohydrate is necessary for the

regulation of nerve tissue and is the source of energy for the brain.

Some carbohydrates are high in fibre, which helps prevent constipation

Structural components

play a major role in promoting your health fitness.

Immediate respiratory substrates e.g. glucose

Energy stores e.g. glycogen in mammals starch in plants

Structural components e.g. cellulose in plant cell walls chitin in arthropod exoskeletonnpentose sugars - ribose and deoxyribose are components of RNA and DNA respectively.

Metabolites i.e. intermediates in biochemical pathways

Cell-to-cell attachment molecules e.g. combined with proteins to form glycoproteins or lipids to form glycolipids on plasma membrane

Transport e.g. sucrose in plant phloem tissue