carbohydrates
TRANSCRIPT
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Classification of carbohydrates
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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
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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
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General formula:
(CH2O)n
where n = 3-9
Contain the elements C, H & O
POLYSACCHARIDES
MONOSACCHARIDES
DISACCHARIDES C12H22O11
(two hexoses)
Cx(H2O)y
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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)
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Deoxyribose has ONE oxygen atom less than ribose
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Chief functions of monosaccharides
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Trioses C3H6O3
e.g. glyceraldehydeis an intermediate in:respirationphotosynthesis other branches of carbohydrate
metabolism
Glyceraldehyde glycerol triglyceride (lipid)
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Pentoses C5H10O5
e.g. ribose, deoxyribose, ribulose
• Synthesis of nucleic acids;
Ribose in RNA
Deoxyribose in DNA
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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
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Coenzyme binds loosely to the enzyme.Prosthetic group binds tightly to the enzyme.
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Apoenzyme Holoenzyme
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*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)
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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.
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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.
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Synthesis of AMP, ADP and ATP (all have ribose)
Pentoses C5H10O5
AMP; adenosine monophosphateADP; adenosine diphosphateATP; adenosine triphosphate
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Pentoses C5H10O5
Ribulose bisphosphate: is the CO2 acceptor in photosynthesis has ribulose
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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.
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Fructose in the nectar of flowers.
Galactose in the cells of red algae.
Hexoses C6H12O6
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needed to synthesise:disaccharides polysaccharides – especially glucose is particularly
important
Hexoses C6H12O6
Monosaccharide Monosaccharide Disaccharide
Polysaccharide
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:needed to synthesise disaccharides
fructoseglucose
glucose glucose
glucose galactose
Hexoses C6H12O6
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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
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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
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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
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Optical isomerism :- a type of stereoisomerism- a feature of:
monosaccharides amino acids any compound whose structures are mirror images
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Stereoisomers of Glyceraldehyde
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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)
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D-isomer
L-isomer
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Pentoses & hexoses : - several asymmetric
carbon atoms per molecule
What is the system used to name D and L forms?
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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
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D- and L-isomers of glucose: are mirror images
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Only D-isomers of sugars are commonly found in organisms
Cells can distinguish between two isomers – one form is active, the other is inactive
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Pentoses & Hexoses form :
Ring Structures
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Open-chain & Ring Structures
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Note difference:
The main difference between glucose and galactose is that glucose is much sweeter than galactose.
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Glucose forms a ring structure
Note: functional group is on carbon one
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Two types of ring structures for glucoseOH on carbon one:
BELOW the ring ABOVE the ring
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The existence of - and -isomers:- leads to greater chemical variety is of importance in for example in
forming starch and cellulose
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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)
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Ring Structures of Fructose
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DisaccharidesFormed by: condensation reactions between two
monosaccharidesName of bond: glycosidic
Hydrolysis results in: monosaccharides
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Naming the Bond or
1 4
1 4
Depends on glucose whose C1 is used.
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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
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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
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Disaccharide Monomers UsesLactose Glucose +
GalactosePresent in Milk
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Disaccharide Monomers UsesSucrose Glucose +
FructoseTable Sugar; most prevalent
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Disaccharide Monomers UsesMaltose Glucose +
GlucoseImportant in Brewing beer
Brown malt
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Formation of Maltose
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Formation of Cellobiose
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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)
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Reducing sugars
What is a reducing sugar?
some disaccharides (maltose and lactose)
all monosaccharides
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Test for Reducing SugarsHeat test solution with an equal amount of Benedict’s solution.
A brick-red or orange final colour
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What causes a change in colour?
Blue Brick red precipitate Cu2+ ions + electrons Cu+
What is the source of electrons?
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Why are maltose and lactose reducing sugars? They have a free carbonyl group which
donates electrons that reduce Cu2+ ions to Cu+
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No free carbonyl (functional) group is present to donate electrons.
Sucrose is a non reducing sugar. Why?
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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.
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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
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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.
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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]
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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
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Polysaccharides• non-sugars, insoluble, high
molecular weight• Functions:1. food and energy
2. structural materials
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PolysaccharidesConvenient storage materials because they:-• are insoluble• being large, they are not likely to move out of
the cell
Starch in potato cells.
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Convenient storage materials because they:-• fold into compact shapes
Which is the COMPACT molecule?
A
B
Starch molecule
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Convenient storage materials because they:-
• are easily hydrolysed to sugars
amylase
hydrolysis
STARCH MALTOSE GLUCOSEmaltase
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Polysaccharides produce monosaccharides by a Hydrolysis Reaction
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Monosaccharides Join by a Condensation Reaction
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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
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Polysaccharides can be:
Linear
Branched
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Examples of polysaccharides:
StarchGlycogenCellulose
HAVE ONE FEATURE IN COMMON:All composed of glucose
-glucose
- glucose
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Cellulose• consists of long chains of glucose
residues –(about 10 000) per chain
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Cellulose-OH groups projects from all directions to make H-bonds with nearby chains
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Macrofibrils have tremendous tensile strength e.g. cotton
Chains associate in groups to form
microfibrils
Microfibrils are arranged in larger bundles to form
macrofibrils
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i.e. can stretch without tearing
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• the arrangement in layers is cemented by other polysaccharides namely pectins and hemicellulose
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Starch•is a polymer of -glucose
• stored as starch grains in:
storage organsseeds
chloroplasts
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-glucose linkages in starch:
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two types of starch grains:-
AmylopectinAmylose
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Linear molecule
Branched molecule
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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
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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
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• Cellulose is difficult to digest– Cows have microbes in their stomachs to
facilitate this process
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Test for Starch
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Glycogen• is a polymer of -glucose• similar to amylopectin, but side branches:
- occur more frequently - are more branched
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Glycogen• stored by:
1) animals chiefly in liver and muscles2) fungi
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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.
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Modified Polysaccharides
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Functions of Modified Polysaccharides
Structural
Protective
Food storage
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Structural FunctionsPectins + Hemicelluloses - Cell wall matrix in plants
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Structural Functions
Murein – Bacterial cell walls
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Structural Functions
Chitin – Exoskeleton in arthropods and cell walls of some fungi
Fungal hyphae
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Structural FunctionsOuter coats of animal cells:Glycoprotein
Glycolipid
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Structural Functions
Lignin – Found in the walls of xylem cells
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Structural Functions
Agar - Cell walls of red algae
A thickener for soups, in jellies and ice creams.
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Protective Functions
Heparin - Anticoagulant in mammalian blood and connective tissue.
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Protective FunctionsGums- gums form gels or sticky
solutions - formed as a result of injury
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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)
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Food storageMannan, arabinan - in some plants
Hemicelluloses - some seeds e.g. dates
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Test Your Memory!!
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CELLULOSE
GLYCOGEN
AMYLOPECTIN
AMYLOSE
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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)
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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