Carbohydrates Biology PowerPoint PDF

Biology (16 - 18) Carbohydrates C O )y x (H (x H 2 2 O) C y © SSER Ltd.  The Carbohyd...

Biology (16 - 18) Carbohydrates C O )y x (H (x H 2 2 O) C y © SSER Ltd.  The Carbohydrates Carbohydrates are compounds of great importance in both the biological and commercial world. They are used as a source of energy in all organisms and as structural materials in membranes, cell walls and the exoskeletons of many arthropods. All carbohydrates contain the elements carbon (C), hydrogen (H) and oxygen (O) with the hydrogen and oxygen being present in a 2 : 1 ratio. The general formula of a carbohydrate is: Cx(H2O)y EXAMPLES C6H12O6 is the formula for glucose. C12H22O11 is the formula for sucrose.  The Classification Of Carbohydrates Carbohydrates are classified as either sugars or polysaccharides. Carbohydrates Sugars Polysaccharides Monosaccharides Monosaccharides Disaccharides Disaccharides Storage Storage Structural Structural Monosaccharides Disaccharides are Glycogen Cellulose and chitin are single sugar double sugar units and starch are important units that include: that include: are storage structural Glucose Maltose carbohydrates. carbohydrates. Fructose Sucrose Animal cells Cellulose forms the Galactose Lactose store glucose as fabric of many cells glycogen and walls and chitin is a plant cells store major component of glucose as starch. the exoskeletons of many arthropods. Glucose Maltose  Monosaccharides - Glucose Monosaccharides are single sugar units that form the building blocks for the larger carbohydrates. There are many different monosaccharides; they vary according to the number of carbon atoms that they possess and in the way the atoms are arranged in the molecules. Glucose, the main source of energy for most organisms, is a hexose sugar with six carbon atoms and the formula C6H12O6. Glucose exists in both straight chain and ring forms - rings form when glucose is dissolved in water (as in cytoplasm). chain structure ring structure  Glucose – Structure (1) This straight chain representation of the glucose molecule shows Aldehyde group how the carbon atoms The are numbered. The carbon carbon atom atom of of the the carbonyl carbonyl group group Glucose, in common with is is referred referred to to as as the the many other hexose sugars, anomeric anomeric carbon carbon has an aldehyde group atom atom andand for for glucose, glucose, as part of the structure. this this is is carbon carbon 1.1. The carbon atom that forms part of this aldehyde group is always carbon 1. The C = O carbonyl group has reducing properties such that all monosaccharides are reducing sugars. The remainder of the molecule is a series of bonded carbon atoms with attached hydrogen atoms and hydroxyl (OH) groups.  Glucose – Structure (2) In solution glucose exists in ring form. Glucose forms a six-membered ring when the hydroxyl group (OH) on carbon 5 (5C) adds to the aldehyde group on carbon 1 (1C).  Glucose – Structure (3) The ring structure of glucose is usually represented in Howarth projection…  Glucose - Molecular Models The relative positions of the atoms in molecules can be demonstrated through model building… Glucose Carbon Oxygen Hydrogen  Glucose – Isomers (1) Each hexose sugar exists in both alpha and beta forms. These isomers can be distinguished by the arrangement of the OH and H groups about the extreme right carbon atom in the ring (Carbon 1).  Glucose – Isomers (2) Simplified views of α-glucose and β-glucose are often used for representing these molecules.  Glycogen and starch are formed by the condensation of α-glucose.  Cellulose is formed by the condensation of β-glucose. Glucose Structure - Exercise Click here to view the exercise…  Disaccharides Disaccharides are sugars composed of two monosaccharides covalently bonded together by a glycosidic linkage. Maltose, also known as malt sugar, is formed from two glucose molecules. Lactose, or milk sugar, is a disaccharide formed when the monosaccharides glucose and galactose bond. Sucrose is common household sugar and is formed when the monosaccharides glucose and fructose bond. maltose = glucose + glucose lactose = glucose + galactose sucrose = glucose + fructose The Formation of Maltose (Condensation Reaction) Two a-glucose molecules undergo a condensation reaction and form an a-1,4-glycosidic bond between the two molecules. The resulting disaccharide sugar is maltose. condensation reaction Further condensation can + H2 O extend the length of the C1 C4 molecule until a large polymer such as amylose is formed. a-1,4-glycosidic bond Bonds formed between monosaccharide monomers are all glycosidic bonds.  The Hydrolysis of Maltose (Catalysed by Maltase) Enzymes such as a-1,4-glycosidic bond amylase and maltase can catalyse the hydrolysis of a-1,4-glycosidic bonds. C1 C4 hydrolysis hydrolysis + H2 O reaction Maltase reaction The addition of water breaks the glycosidic bond in the polysaccharides and disaccharides. Glucose molecules are easily absorbed and transported throughout an organism. Disaccharides - Exercise Click here to view the exercise… Cloze Word Exercise – Disaccharides Click here to view the exercise…  Testing For Reducing Sugars (1) All the monosaccharides and many of the disaccharides are reducing sugars. Benedict’s test is used to determine the reducing properties of the different sugars. Benedict’s solution is a turquoise/blue solution containing copper ions and sodium hydroxide; the copper ions exist as Cu2+ in this reagent. If a sugar is a reducing sugar then the Cu2+ ions are reduced to Cu+ which, in the presence of alkaline sodium hydroxide, form copper oxide. Copper oxide is insoluble and precipitates out of the solution as a brick-red precipitate. Testing For Reducing Sugars (2) A solution containing equal quantities of food solution and Benedict’s solution are placed in a boiling tube and heated in a water bath for several minutes. A change of colour through green, then yellow to ‘brick red’ indicates the presence of a reducing sugar such as glucose. The ‘brick red’ colour is the definitive result...  Testing For Reducing Sugars (3) When Benedict's test is performed with the disaccharides maltose and sucrose, the following result is obtained... Sucrose is a Maltose is a non-reducing sugar. reducing sugar. Sucrose Maltose Result Result  Test For Sucrose In order to determine if sucrose is present in a sample or solution then the following procedure is performed: The sample or solution under consideration is boiled in hydrochloric acid. Boiling in acid breaks glycosidic bonds – the glycosidic bond is hydrolysed. This procedure is called acid hydrolysis. The solution is then neutralised by adding drops of alkali while testing with pH paper. Benedict’s test is now performed on the resulting solution. If a brick-red precipitate forms then sucrose was present in the original solution. Acid hydrolysis breaks the glycosidic bonds in the sucrose molecules releasing free glucose and free fructose into the solution. Glucose and fructose are both monosaccharides and therefore reducing sugars. If no precipitate is obtained then sucrose was not present in the original sample. The need to neutralise the solution following acid hydrolysis is due to the fact that the Benedict’s test requires an alkaline medium.  Polysaccharides Polysaccharides are large polymers of the monosaccharides and due to their relatively large size they are either insoluble in water or form colloidal suspensions. The insoluble nature of the polysaccharides makes them suitable as food storage molecules (no osmotic effect) in both animals and plants or as structural materials in plants. The principal storage polysaccharides are starch and glycogen. Starch (found in plants) is a polymer of alpha glucose and is, in fact, a mixture of two different polysaccharides – amylose and amylopectin. Amylose – long unbranched chain of α-glucose units (20%-25% of starch by weight). Starch Amylopectin – highly branched polymer of α-glucose units (75%-80% of starch by weight).  Amylose - Structure Amylose is formed by a series of condensation reactions that bond alpha glucose molecules together into a long chain forming many a-1,4-glycosidic bonds. The amylose chain, once formed, coils into a helix. The amylose helix formed from covalently bonded α-glucose molecules.  Amylopectin – Structure Amylopectin consists of a straight chain of a-glucose units with branch points occurring at approximately every 24 to 30 glucose units along the straight chain. The branch points form when carbon 6 of a straight chain glucose molecule forms a glycosidic bond with carbon 1 of a glucose molecule positioned above the chain (a-1,6-glycosidic bond). a-1,6-glycosidic bond  Amylopectin + Amylose = Starch This highly branched amylopectin molecule is wrapped around the amylose to make up the final starch molecule. This large insoluble molecule, with branch points that allow for easy access for enzymes when breaking down the molecule, makes starch an ideal food storage compound.  Reaction Between Starch And Iodine Solution When iodine in potassium iodide solution is added to starch, the iodine molecules pack inside the amylose helix to give a blue-black colour. When iodine reacts with the starch in this piece of potato, the blue-black colour develops. Starch Grains in Plant Cells Starch Grains  Glycogen Glycogen is often referred to as ‘animal starch’ and has a similar same overall structure as amylopectin but more branching. In glycogen branches occur every 8 to 12 glucose units along the straight chain. More of these branch points form.  Glycogen Storage Glucose is stored as glycogen in large amounts in both the liver and skeletal muscles. Glycogen is composed of shorter chains with more branch points than starch which makes it more readily hydrolysed to alpha glucose. This is important in animals as they have a much greater energy demand than plants.  Structural Polysaccharides - Cellulose Cellulose is one of the most important structural polysaccharides as it is the major component of plant cell walls. Cellulose is a polymer of beta glucose units where each glucose molecule is inverted with respect to its neighbour. b-1,4-Glycosidic bond The orientation of the beta glucose units places many hydroxyl (OH) groups on each side of the molecule. Many parallel chains of β-glucose units form and each chain forms hydrogen bonds between the OH groups of adjacent chains.  The Formation of b-1,4-Glycosidic Bonds (1) Two b-glucose molecules undergo a condensation reaction and form a b-1,4-glycosidic bond between the two molecules. The resulting disaccharide condensation reaction sugar is cellobiose. + H2 O Further condensation can extend the length of the molecule until the large C1 C4 polymer cellulose is formed. b-1,4-glycosidic bond Bonds formed between monosaccharide monomers are all glycosidic bonds.  The Formation of b-1,4-Glycosidic Bonds (2) Every other glucose unit within the cellulose is actually ‘flipped over’ because they present themselves at 180° rotation to each other. The multiple hydroxyl groups (OH) on the glucose condensation reaction molecules from one chain form hydrogen bonds with + H2 O oxygen atoms on the same or on a neighbouring chain. These hydrogen C1 C4 bonds hold the chains firmly together side-by-side and form cellulose microfibrils which have a b-1,4,-glycosidic bond high tensile strength.  Cellulose - Structure & Function (1) Cellulose is a straight chain polymer of at least 500 b-glucose units where every other glucose monomer is ‘flipped over’. The molecule does not coil or branch (unlike starch) and it has an extended and inflexible rod-like structure.  Cellulose - Structure & Function (2) The bundles of parallel chains, forming hydrogen bonds with each other, create a molecule that confers rigidity and strength to the structures of which they form a part. hydrogen bonds between parallel chains of β-glucose The rigidity and strength of plant cell walls is a consequence of the incorporation of cellulose into their structure. Extension  Reducing & Non-reducing Sugars (1) Why is sucrose a non-reducing sugar? Sugars reduce Benedict's solution if their anomeric carbon atom is available to reduce the copper ions in the solution. The anomeric carbon atom is the carbon of the carbonyl group present in the straight chain form of the sugar. The anomeric carbon atom for glucose is carbon 1.  Reducing & Non-reducing Sugars (2) Why is sucrose a non-reducing sugar? Glucose Sugars reduce Benedict's solution if their anomeric carbon atom is available to reduce the copper ions in the solution. The anomeric Fructose carbon atom for Ketone group fructose is carbon 2. Fructose is a ketose sugar and bonds to glucose to form sucrose.  Why Is Sucrose A Non-reducing Sugar? (1) This potential anomeric carbon atom is unavailable. This potential anomeric carbon atom is available to reduce Benedict’s reagent. When maltose is boiled with Benedict’s reagent, the region of the ring containing the anomeric carbon atom (carbon 1) may open exposing a carbonyl group capable of reducing Benedict's reagent. Only an anomeric carbon atom that is not involved in the formation of the glycosidic bond may be exposed. The one available anomeric carbon atom is sufficient for this molecule to reduce Benedict’s solution and thus maltose is a reducing sugar.  Why Is Sucrose A Non-reducing Sugar? (2) Sucrose is synthesised when glucose forms a glycosidic bond with fructose. The anomeric carbon atom for fructose is carbon 2. glucose glycosidic bond The anomeric carbon atom for glucose fructose is carbon 1. As both the anomeric carbon atoms are involved in forming the glycosidic bond when glucose and fructose join, there are no potentially free anomeric carbon atoms available to reduce Benedict’s solution. Sucrose is a non-reducing sugar.  Structural Polysaccharides – Chitin (1) Chitin is a polysaccharide forming the exoskeleton of insects and crustaceans and the fabric of fungal cell walls. Chitin is a linear polymer of the sugar derivative called N-acetylglucosamine. N-acetylglucosamine  Structural Polysaccharides – Chitin (2) Chitin forms the exoskeleton of insects and crustaceans. Structural Polysaccharides – Chitin (3) Chitin forms the fabric of fungal cell walls. End Show Copyright © SSER Ltd. and its licensors. All rights reserved. All graphics are for viewing purposes only.