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 atoms1. Trioses [3C atoms]

glyceraldehyde(C3H6O3)

2. Pentoses [5C atoms]

RiboseC5H10O5

DeoxyriboseC5H10O4

Deoxyribose has ONE oxygen atom less than ribose

3. Hexoses [6C atoms] glucose, fructose, galactose (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

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

RIBOSE is needed in the synthesis of some coenzymes e.g.

NADnicotinamide adenine dinucleotide;

NADP

NADP nicotinamide adenine dinucleotide phosphate;

FAD flavin adenine dinucleotide

FAD is listed as a coenzyme in syllabus , however, in respiration it acts as a prosthetic group of flavoproteins

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

Apoenzyme

*Vitamins & their roles as coenzymes

VITAMIN: is an organic compound: required as a nutrient in tiny amounts cannot be synthesised in sufficient quantities by

an organism

In animals some types of coenzymes are often produced from vitamins in the diet e.g.:

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.

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

isomers are molecules that have the:

Isomerism

same chemical formula (the same kinds and numbers of atoms)

but the atoms are arranged differently

TWO kinds of isomers will be considered: structural isomers:

have different chemical properties, e.g. fructose is sweeter than glucose

optical isomers

Two types of isomerism:-

Structural – the same chemical groups being

bonded to different carbon atoms

Optical – the same chemical groups are bonded to the same carbon atoms but in

different orientations

Opticalisomer

Optical isomers:

• occur when a carbon atom has four different atoms or groups of atoms attached to it

• such a carbon atom is called asymmetric carbon

• an asymmetric carbon atom, allows two different ways of making the attachments, each the mirror image of the other

The molecules of all monosaccharides contain one or more asymmetric carbon atoms

Optical isomerism a feature of: monosaccharides amino acids any compound whose structures are

mirror images

Amino acids

Optical isomers are characterised by their effect on polarized light

• polarized light has a single plane, and optical isomers rotate this plane either to the:

Right = dextro-rotatory (D)Left = laevo-rotatory (L)

• enzymes in biological systems usually recognise only a single specific optical isomer

• living systems tend to produce only a single optical isomer of the two possible forms:

D-sugars L-amino acids

Open & Ring Structures

Note difference:

Glucose is much sweeter than galactose.

Ring Structures• the relatively long carbon chains of pentoses and

hexose sugars can bend, bringing the carbonyl group close enough to reduce one of the hydroxyl groups in the same molecule

Glucose forms six-sided rings where: carbon atom 1 is asymmetric, having four

different functional groups bonded to it

Note: carbonyl group is on carbon one

The additional asymmetric carbon atom enables - and - forms of D-glucose

to exist

OH on carbon one:

BELOW the ring ABOVE the ring

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)

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

forming starch and cellulose

Cellulose

Amylose (a form of starch)

Ring Structures of Fructose [not important]

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.

Naming the Bond or

1 4

1 4

Depends on glucose whose C1 is used.

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

A reducing sugar gives a brick-red

precipitate on heating with

Benedict’s solution.

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

The darker the colour, the higher the amount of reducing sugar present

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

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

Test for non-reducing sugars

1. Heat equal volumes of sucrose solution and dilute hydrochloric acid for 1 minute.

Reason: to break sucrose into monosaccharides – carbonyl groups are free to donate electrons

2. Add sodium hydrogen carbonate to neutralise the acid. (Check for neutrality using litmus.)

3. Perform Benedict’s test.

Polysaccharides• non-sugars, insoluble,

high molecular weight• Functions:1. food and energy

2. structural materials

5 ways in which Polysaccharides are convenient storage materials:

1. are insoluble

2. being large, they are not likely to move out of the cell

3. fold into compact shapes

4. are easily hydrolysed to sugars

amylase

hydrolysis

STARCH MALTOSE GLUCOSEmaltase

5. do not have an osmotic effect on the cell i.e. do not cause water to enter or leave the cell

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

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

Macrofibrils have tremendous tensile strength

Chains associate in groups to form

microfibrils

Microfibrils are arranged in larger bundles to form

macrofibrils

High tensile strength: 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

Two types of starch grains:-

AmylopectinAmylose

-1,4 linkage

-1,6 linkage

-glucose linkages in starch:

Linear molecule

Branched molecule

AMYLOSE AMYLOPECTIN

The starch-degrading enzymes cannot break the bond between two sugars with opposite orientations

Cellulose is chemically similar to amylose with one important difference:

Amylose

Cellulose

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

organisms lackWHICH ENZYME IS THIS?

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

• Cellulose is difficult to digestCows 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 is stored by:Animals chiefly in liver and muscles

Fungi

Chemically modified polysaccharides

• some carbohydrates are chemically modified by the addition of functional groups, such as:

phosphate amino groups

Heparin

Pectin

Modified Polysaccharides

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

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