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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.
Page 95
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