Classification of carbohydrates

Physical PropertiesCARBOHYDRATES

POLYSACCHARIDES‘SUGARS’Small moleculesSweetReadily soluble in waterCrystallineSuffix –ose used in naminge.g. glucose, sucrose

MacromoleculesNot sweet Insoluble or slightly soluble in waterNon-crystalline

Synthesis of carbohydrates‘SUGARS’ POLYSACCHARIDES

MONOSACCHARIDES‘simple sugars’

DISACCHARIDESMade by joining

two monosaccharides

Made by joining many

monosaccharides

Diagrammatic representation of structure

Molecules is sometimes branchedGlycosidic bonds

General formula:

(CH2O)n

where n = 3-9

Contain the elements C, H & O

POLYSACCHARIDES

MONOSACCHARIDES

DISACCHARIDES C12H22O11

(two hexoses)

Cx(H2O)y

Monosaccharides are classified on the number of C atoms

1. Trioses 3C atoms glyceraldehyde

(C3H6O3)

2. Pentoses 5C atoms ribose (C5H10O5) deoxyribose (C5H10O4)

3. Hexoses 6C atoms glucose, fructose, galactose (C6H12O6)

Deoxyribose has ONE oxygen atom less than ribose

Chief functions of monosaccharides

Trioses C3H6O3

e.g. glyceraldehydeis an intermediate in:respirationphotosynthesis other branches of carbohydrate

metabolism

Glyceraldehyde glycerol triglyceride (lipid)

Pentoses C5H10O5

e.g. ribose, deoxyribose, ribulose

• Synthesis of nucleic acids;

Ribose in RNA

Deoxyribose in DNA

Synthesis of some coenzymes e.g. NAD, NADP, and prosthetic groups e.g. FAD (all have ribose)

NADPPentoses C5H10O5

[NAD – nicotinamide adenine dinucleotide; NADP – nicotinamide adenine dinucleotide phosphate; FAD – flavin adenine dinucleotide]

FAD

Coenzyme binds loosely to the enzyme.Prosthetic group binds tightly to the enzyme.

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Apoenzyme Holoenzyme

*Vitamins and their roles as coenzymes• vitamin is an organic compound required as a

nutrient in tiny amounts and cannot be synthesised in sufficient quantities by an organism

• in animals some types of coenzymes are often produced from vitamins in the diet

• NAD is made from vitamin B complex • FAD is made from the vitamin riboflavin (B2)

Question: [SEP, 2010]Living organisms require a range of vitamins in their diet in order to ensure adequate functioning of body systems. 1 .What is a vitamin? (2)

A vitamin is an organic compound required as a nutrient in tiny amounts and cannot be synthesised in sufficient quantities by an organism.

2. Humans and goats both require ascorbic acid. This chemical is considered a vitamin for humans (Vitamin C) but is not considered a vitamin for goats. Suggest a reason for this. (2)Goats are able to manufacture vitamin C while humans do not.

3. Folic acid (Vitamin B9) is an important coenzyme in

the human body. What is a coenzyme? (2)A coenzyme is a non-protein component that binds loosely to the enzyme for its efficient functioning.

Synthesis of AMP, ADP and ATP (all have ribose)

Pentoses C5H10O5

AMP; adenosine monophosphateADP; adenosine diphosphateATP; adenosine triphosphate

Pentoses C5H10O5

Ribulose bisphosphate: is the CO2 acceptor in photosynthesis has ribulose

Hexoses C6H12O6

e.g. glucose, fructose, galactose

• Sources of energy in respiration glucose is the most common:

respiratory substrate monosaccharide

Eat me. I’m sweet.

Fructose in the nectar of flowers.

Galactose in the cells of red algae.

Hexoses C6H12O6

needed to synthesise:disaccharides polysaccharides – especially glucose is particularly

important

Hexoses C6H12O6

Monosaccharide Monosaccharide Disaccharide

Polysaccharide

:needed to synthesise disaccharides

fructoseglucose

glucose glucose

glucose galactose

Hexoses C6H12O6

Functional Group in MonosaccharidesCarbonyl group: either as part of:1. an aldehyde group [an aldose or aldo sugar]

2. a keto group [ketose or keto sugar]

Glucose

keto group

Isomers are two different compounds having the same molecular formula

Isomerism

a characteristic of monosaccharides

isomers arise due to one or more asymmetric carbon atoms

Two types of isomerism:-

Structural – the same chemical groups being

bonded to different carbon atoms

Stereoisomerism – the same chemical groups are bonded

to the same carbon atoms but in different orientations

Optical isomerism :- a type of stereoisomerism- a feature of:

monosaccharides amino acids any compound whose structures are mirror images

Stereoisomers of Glyceraldehyde

A substance is called D or L isomer depending on the direction in which it rotates the plane of polarization Right = dextro-rotatory (D)

Left = laevo-rotatory (L)

D-isomer

L-isomer

Pentoses & hexoses : - several asymmetric

carbon atoms per molecule

What is the system used to name D and L forms?

Of particular importance is the one furthest from the reducing group which is also the last-but-one carbon atom in

the carbon chain Reducing group

D- and L-isomers of glucose: are mirror images

Only D-isomers of sugars are commonly found in organisms

Cells can distinguish between two isomers – one form is active, the other is inactive

Pentoses & Hexoses form :

Ring Structures

Open-chain & Ring Structures

Note difference:

The main difference between glucose and galactose is that glucose is much sweeter than galactose.

Glucose forms a ring structure

Note: functional group is on carbon one

Two types of ring structures for glucoseOH on carbon one:

BELOW the ring ABOVE the ring

The existence of - and -isomers:- leads to greater chemical variety is of importance in for example in

forming starch and cellulose

Question: [SEP, 2008]

Glucose is one of the most important carbohydrates in the biosphere. Draw a

molecule of glucose, showing its ring structure, in the space provided below. (3)

Ring Structures of Fructose

DisaccharidesFormed by: condensation reactions between two

monosaccharidesName of bond: glycosidic

Hydrolysis results in: monosaccharides

Naming the Bond or

1 4

1 4

Depends on glucose whose C1 is used.

OOHOH

Forming Disaccharides

C C

C

C O

C

H

OH

OHOH

OH

HH

CH2OH

H

H

CH2OH

C C

C

C O

C

H

OH

OHOH

OH

HH

CH2OH

H

H

C C

C

C O

C

H

OH

OH

OH

HH

CH2OH

H

H

C C

C

C O

C

H

OH

OH

OH

HH

CH2OH

H

H O

H H

GlycosidicBond This is a CONDENSATION

reaction, where a water molecule is lost.

Glucose

Maltose

Write the formula of a disaccharide formed from two glucose molecules.

BUT a water molecule must be removed:

C6H12O6 +C6H12O6 C12H24O12

C6H12O6 + C6H12O6 C12H22O11 + H2O

Disaccharide Monomers UsesLactose Glucose +

GalactosePresent in Milk

Disaccharide Monomers UsesSucrose Glucose +

FructoseTable Sugar; most prevalent

Disaccharide Monomers UsesMaltose Glucose +

GlucoseImportant in Brewing beer

Brown malt

Formation of Maltose

Formation of Cellobiose

Question: [SEP, 2008]

1.1 What are carbohydrates? (2)

1.2Two glucose molecules combine to form maltose. Draw a molecule of maltose in the space provided below. (4)

1.3 What name is given to the bond between the two glucose molecules? (1)

Reducing sugars

What is a reducing sugar?

some disaccharides (maltose and lactose)

all monosaccharides

Test for Reducing SugarsHeat test solution with an equal amount of Benedict’s solution.

A brick-red or orange final colour

What causes a change in colour?

Blue Brick red precipitate Cu2+ ions + electrons Cu+

What is the source of electrons?

Why are maltose and lactose reducing sugars? They have a free carbonyl group which

donates electrons that reduce Cu2+ ions to Cu+

No free carbonyl (functional) group is present to donate electrons.

Sucrose is a non reducing sugar. Why?

Question:Benedict’s test was performed on two carbohydrates: A and B. Which one is sucrose? Explain your answer.

A B

B.The solution containing sucrose remains blue because sucrose is a non-reducing sugar.

Can sucrose ever give a brick red precipitate on heating with Benedict’s solution?

YES.

How and why?

If it is first hydrolysed into monosaccharides – carbonyl groups are free to donate electrons

Procedure :1. Place 2 cm3 of sucrose solution in a test

tube.2. Add an equal volume of dilute HCl solution.3. Place in a water bath for 1 minute.4. Add NaHCO3 to neutralise the acid.5. (Check for neutrality using litmus.)6. Perform Benedict’s test.

Mucus is produced by many organisms. It is a gel consisting of a tangled mesh of mucin molecules that holds water molecules. The diagram shows the structure of mucin.

JUNE 2009 [end-of-year]

The molecules that make up the side-chains attract water molecules.Suggest why. (2)Sugar molecules have oxygen-containing / OH groups;These form hydrogen bonds with water.

OH

Polysaccharides• non-sugars, insoluble, high

molecular weight• Functions:1. food and energy

2. structural materials

PolysaccharidesConvenient storage materials because they:-• are insoluble• being large, they are not likely to move out of

the cell

Starch in potato cells.

Convenient storage materials because they:-• fold into compact shapes

Which is the COMPACT molecule?

A

B

Starch molecule

Convenient storage materials because they:-

• are easily hydrolysed to sugars

amylase

hydrolysis

STARCH MALTOSE GLUCOSEmaltase

Polysaccharides produce monosaccharides by a Hydrolysis Reaction

Monosaccharides Join by a Condensation Reaction

Convenient storage materials because they:-• do not have an osmotic effect on the cell i.e.

do not cause water to enter or leave the cell

Polysaccharides can be:

Linear

Branched

Examples of polysaccharides:

StarchGlycogenCellulose

HAVE ONE FEATURE IN COMMON:All composed of glucose

-glucose

- glucose

Cellulose• consists of long chains of glucose

residues –(about 10 000) per chain

Cellulose-OH groups projects from all directions to make H-bonds with nearby chains

Macrofibrils have tremendous tensile strength e.g. cotton

Chains associate in groups to form

microfibrils

Microfibrils are arranged in larger bundles to form

macrofibrils

i.e. can stretch without tearing

• the arrangement in layers is cemented by other polysaccharides namely pectins and hemicellulose

Starch•is a polymer of -glucose

• stored as starch grains in:

storage organsseeds

chloroplasts

-glucose linkages in starch:

two types of starch grains:-

AmylopectinAmylose

Linear molecule

Branched molecule

Cellulose is chemically similar to amylose• with one important difference:

the starch-degrading enzymes cannot break the bond

between two sugars with opposite orientations

Amylose

Cellulose

The bond is NOT stronger, but its cleavage requires an enzyme which most organisms lack

WHICH ENZYME IS THIS?

• Cellulase breaks the β-glycosidic links that join the sugar units of cellulose

• Cellulose is difficult to digest– Cows have microbes in their stomachs to

facilitate this process

Test for Starch

Glycogen• is a polymer of -glucose• similar to amylopectin, but side branches:

- occur more frequently - are more branched

Glycogen• stored by:

1) animals chiefly in liver and muscles2) fungi

OOHOH

Forming polysaccharides

C C

C

C O

C

H

OH

OH

HH

CH2OH

H

H

C C

C

C O

C

H

OH

OH

OH

HH

CH2OH

H

H

O

H H

In this example, 3 condensation reactions have produced 3 water molecules to produce the polysaccharide.

O

H H

C C

C

C O

C

H

OH

OH

OH

HH

CH2OH

H

H

C C

C

C O

C

H

OH

OH

HH

CH2OH

H

H

OHOH OHOH

Glucose

O

O

H H

O

A HYDROLYSIS reaction (addition of water) reverses the reaction and splits the polysaccharide releasing 3 monosaccharide molecules.

Modified Polysaccharides

Functions of Modified Polysaccharides

Structural

Protective

Food storage

Structural FunctionsPectins + Hemicelluloses - Cell wall matrix in plants

Structural Functions

Murein – Bacterial cell walls

Structural Functions

Chitin – Exoskeleton in arthropods and cell walls of some fungi

Fungal hyphae

Structural FunctionsOuter coats of animal cells:Glycoprotein

Glycolipid

Structural Functions

Lignin – Found in the walls of xylem cells

Structural Functions

Agar - Cell walls of red algae

A thickener for soups, in jellies and ice creams.

Protective Functions

Heparin - Anticoagulant in mammalian blood and connective tissue.

Protective FunctionsGums- gums form gels or sticky

solutions - formed as a result of injury

Protective FunctionsMucilages - mucilages form looser gels or a slimy mass - may also retain water for drought resistance

Spirogyra

TS of stem of a cactus (Mucilage cell retains water)

Food storageMannan, arabinan - in some plants

Hemicelluloses - some seeds e.g. dates

Test Your Memory!!

CELLULOSE

GLYCOGEN

AMYLOPECTIN

AMYLOSE

Question: May, 2011 (End-of-Year Exam)

Use your knowledge to discuss the biological significance of the following:

a) H-bonds are present between water molecules.

b) Cellulose is a large linear molecule whilst both starch and glycogen molecules are large branched molecules. (5 marks each)

Discuss the importance of carbohydrates to the processes of life. [1993]

Discuss the role of carbohydrates in plant structure and function.[SEP, 2003]

Essay Titles