CARBOHYDRATES

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Types of Carbohydrates

• Monosacchrides

• Disaccharides

Contain 2 monosacchride units

• Polysacchrides

Contain many monosacchride units

MONOSACCHARIDESEmpirical formula:

CH2OMonosaccharides contain:a) C=O group andb) at least two OH- groups C5H10O5 pentoses (e.g. ridose) C6H10O6 hexoses (e.g. glucose)They have between 3 and 6 carbon atoms.They have many structural isomers.Several carbon atoms are chiral and give rise to

optical isomerism.Chain and ring structures are possible.

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Monosaccharides

• Three Carbons = Triose

• Four Carbons = Tetrose

• Five Carbons = Pentose

• Six Carbons = Hexose

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Monosaccharides

• Aldoses are monosacchrides with an aldehyde group and many hydroxyl (-OH) groups.

• Ketoses are monosacchrides with a ketone group and many hydroxyl (-OH) groups.

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D notation

C

C

CH2OH

OHH

OHH

CO

H

Right = D

D-GLUCOSE

The form of glucose that is found in nature is known as D-glucose.

Figure 1: Straight chain formula of D-glucose.

To form the cyclic structure, connect the first and the fifth carbon (condensation)

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Cyclic Structures

• Monosaccharides with 5-6 carbon atoms form cyclic structures

• The hydroxyl group on C-5 reacts with the aldehyde group or ketone group

o O

D-glucose can cyclize in twoways forming either furanose orpyranose structures

β

β

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D-Glucose

oCH2OH

OHOH

OH

OH

oCH2OH

OHOH

OH

OH

-D-Glucose -D-Glucose

Fructose ring structures

POLYSACCHARIDES

o Monosaccharides can undergo condensation reaction to form disaccharides and polysaccharides.

Monosaccharides Disaccharides Polysaccharides

Condensation Condensation

MALTOSE

= α-D glucose + α-D glucose

α-1,4-glycoside linkage

LACTOSE

= β-D galactose + β-D glucose

β-1,4-glycoside linkage

SUCROSE

= β-D fructose + α-D glucose

a-1,2-glycoside linkage

POLYSACCHARIDES

o Monosaccharides can undergo condensation reaction to form disaccharides and polysaccharides.

Monosaccharides Disaccharides Polysaccharides

Condensation Condensation

POLYSACCHARIDES

Starch

Amylose

Amylopectin

α-glucoseStraight chain

Branched

α-1,4 Water soluble

Water insoluble

Glycogen α-glucose More branched α-1,4+α-1,6 Water insoluble

Cellulose β-glucose Straight chain β-1,4+H-bonds Water insoluble

α-1,4+α-1,6

Hydrolyzed by cellulose > found only in plant eating mammals> cannot be digested by humans

Starch Amylose

α-glucose

Straight chain

α-1,4

Water soluble

Starch

Amylopectin

α-glucose

Branched chain

Water insoluble

α-1,4+α-1,6

Glycogen

α-glucose

Branched chain

Water insoluble

α-1,4+α-1,6

β-glucose

Straight chain

Water insoluble

β-1,4 + H-bonds

MAJOR FUNCTIONS OF POLYSACCHARIDES IN THE HUMAN BODY

Carbohydrates have many uses:

a)They provide energy: bread, corn, cereals, rice, potatoes, fruits.

b)The store energy: starch is stored in the liver of the animals in the form of glycogen. Glycogen is hydrolyzed to glucose.

c)They used as precursors for other important biological molecules (nucleic acids)

d)They are used as dietary fibres: cellulose, lignin, pectin in fruits, grains, vegetables

Dietary fibreWater insolubleCellulose, ligninWhole grain foods, brown rice, carrots, tomatoesAbsorb waterBulk moving food through digestive system

Water solublePectinOatmeal, beans, peas,lentils, most fruitsFermented by bacteria to carboxylic acidsStabilize blood glucose levels (prevent diabetes)Stimulate production of antibodies

Prevent:ConstipationIrritable bowelObesityHaemorrhoids