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Carbohydrates Carbohydrates Presentation Transcript

  • Classification of carbohydrates
  • Physical Properties CARBOHYDRATES ‘SUGARS’ Small molecules Sweet Readily soluble in water Crystalline Suffix –ose used in naming e.g. glucose, sucrose POLYSACCHARIDES Macromolecules Not sweet Insoluble or slightly soluble in water Non-crystalline
  • Synthesis of carbohydrates ‘SUGARS’ MONOSACCHARIDES ‘simple sugars’ POLYSACCHARIDES DISACCHARIDES Made by joining two monosaccharides Glycosidic bonds Made by joining many monosaccharides Molecules is sometimes branched Diagrammatic representation of structure
  • General formula: MONOSACCHARIDES (CH2O)n where n = 3-9 DISACCHARIDES C12H22O11 (two hexoses) POLYSACCHARIDES Cx(H2O)y Contain the elements C, H & O
  • 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. glyceraldehyde is an intermediate in: respiration photosynthesis 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
  • Pentoses C5H10O5 NADP Synthesis of some coenzymes e.g. NAD, NADP, and prosthetic groups e.g. FAD (all have ribose) [NAD – nicotinamide adenine dinucleotide; NADP – nicotinamide adenine dinucleotide phosphate; FAD – flavin adenine dinucleotide] FAD
  • Page 95 Apoenzyme Holoenzyme Coenzyme binds loosely to the enzyme. Prosthetic group binds tightly to the enzyme.
  • *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.
  • Pentoses C5H10O5 Synthesis of AMP, ADP and ATP (all have ribose) AMP; adenosine monophosphate ADP; adenosine diphosphate ATP; adenosine triphosphate
  • Pentoses C5H10O5 Ribulose bisphosphate:  is the CO2 acceptor in photosynthesis  has ribulose
  • Hexoses C6H12O6 Eat me. I’m sweet. e.g. glucose, fructose, galactose • Sources of energy in respiration  glucose is the most common:  respiratory substrate  monosaccharide
  • Hexoses C6H12O6 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 Monosaccharide Monosaccharide Polysaccharide Disaccharide
  • Hexoses C6H12O6 :needed to synthesise disaccharides glucose fructose glucose glucose glucose galactose
  • Functional Group in Monosaccharides Carbonyl group: either as part of: 1. an aldehyde group [an aldose or aldo sugar] 2. a keto group [ketose or keto sugar] keto group Glucose
  • Isomerism  Isomers are two different compounds having the same molecular formula  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 glucose OH 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
  • Disaccharides Formed by:  condensation reactions between two monosaccharides Name of bond: glycosidic Hydrolysis results in: monosaccharides
  • Naming the Bond  or  1 4   1  4  Depends on glucose whose C1 is used.
  • Forming Disaccharides Glucose CH2OH CH2OH CH2OH C H C O C H H C C H OH OH H C OH OH OH O H OH H C C C H H C OH H OH Maltose CH2OH H C OH C CH2OH O H OH H C C H H C H C OH OOH H C O H OH H C C C H OH H H OH Glycosidic Bond OH This is a CONDENSATION reaction, where a water molecule is lost.
  • Write the formula of a disaccharide formed from two glucose molecules. C6H12O6 + C6H12O6 C12H24O12 C6H12O6 + C6H12O6 BUT a water molecule must be removed: C12H22O11 + H2O
  • Disaccharide Monomers Uses Lactose Glucose + Present in Galactose Milk
  • Disaccharide Sucrose Monomers Glucose + Fructose Uses Table Sugar; most prevalent
  • Disaccharide Maltose Brown malt Monomers Glucose + Glucose Uses Important in Brewing beer
  • 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 all monosaccharides some disaccharides (maltose and lactose) What is a reducing sugar?
  • Test for Reducing Sugars Heat test solution with an equal amount of Benedict’s solution. A brick-red or orange final colour
  • What causes a change in colour? Blue Cu2+ ions + electrons Brick red precipitate 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+
  • Sucrose is a non reducing sugar. Why? No free carbonyl (functional) group is present to donate electrons.
  • Question: Benedict’s test was performed on two carbohydrates: A and B. Which one is sucrose? Explain your answer. B. The solution containing sucrose remains blue because sucrose is a nonreducing sugar. A B
  • 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.
  • JUNE 2009 [end-of-year] 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.
  • 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
  • Polysaccharides Convenient 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 A B Starch molecule Which is the COMPACT molecule?
  • Convenient storage materials because they:• are easily hydrolysed to sugars STARCH amylase MALTOSE hydrolysis maltase GLUCOSE
  • 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: Starch Glycogen Cellulose -glucose - glucose HAVE ONE FEATURE IN COMMON: All composed of 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
  • Microfibrils are arranged in larger bundles to form macrofibrils Chains associate in groups to form microfibrils Macrofibrils have tremendous tensile strength e.g. cotton
  • 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: chloroplasts storage organs seeds
  • -glucose linkages in starch:
  • two types of starch grains:- Amylose Amylopectin
  • 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 muscles 2) fungi
  • Forming polysaccharides Glucose CH2OH H C OH C CH2OH O H OH H C C H OH H H C OH H C O OH H C CH2OH O H OH H C C H OH H H C OH H C O OH H C CH2OH O H OH H C C H OH H H C OH H C O OH H C O H OH H C C C H OH H In this example, 3 condensation reactions have produced 3 water molecules to produce the polysaccharide. A HYDROLYSIS reaction (addition of water) reverses the reaction and splits the polysaccharide releasing 3 monosaccharide molecules. OH
  • Modified Polysaccharides
  • Functions of Modified Polysaccharides Structural Protective Food storage
  • Structural Functions Pectins + 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 Functions Outer 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 Functions Gums - gums form gels or sticky solutions - formed as a result of injury
  • Protective Functions Mucilages - 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 storage Mannan, arabinan - in some plants Hemicelluloses - some seeds e.g. dates
  • Test Your Memory!!
  • 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]