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THE NATURE OF 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 The formula for glucose is C6H12O6 The formula for sucrose is C12H22O11

THE CLASSIFICATION OF CARBOHYDRATES Carbohydrates are classified as either sugars or polysaccharides Sugars Polysaccharides Monosaccharides Disaccharides Storage Structural Monosaccharides are single sugar units that include: Glucose Fructose Galactose Disaccharides are double sugar units that include: Sucrose Maltose Lactose Glycogen and Starch are storage carbohydrates; animal cells store glucose as glycogen and plant cells store glucose as starch Cellulose and Chitin are important structural carbohydrates; cellulose forms the fabric of many cells walls and chitin is a major component of the exoskeletons of many arthropods Glucose

MONOSACCHARIDES 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 form with rings forming when glucose is dissolved in water chain structure ring structure

GLUCOSE This straight chain Aldehyde group This straight chain representation of the glucose molecule shows how the carbon atoms are numbered The carbon atom of the carbonyl group is referred to as the anomeric carbon atom and for glucose, this is carbon 1 Glucose, in common with many other hexose sugars, has an aldehyde group as part of the structure 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 In solution glucose exists in ring form Glucose forms a six-membered ring when the hydroxyl group (OH) on carbon 5 adds to the aldehyde group on carbon 1

GLUCOSE The ring structure of glucose is usually represented in Howarth projection

MOLECULAR MODELS The relative positions of the atoms in molecules can be demonstrated through model building Glucose Carbon Oxygen Hydrogen

Each hexose sugar exists in both alpha and beta forms ISOMERS 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 Simplified views of a and b glucose are often used for representing these molecules

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 Maltose forms when two alpha glucose molecules undergo a condensation reaction and form a glycosidic bond between the two molecules - H2O condensation reaction C1 C4 1 4a glycosidic bond Maltose is a disaccharide, formed when two alpha Glucose molecules are covalently bonded together

REDUCING SUGARS 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

REDUCING SUGARS Sucrose Result Maltose Sucrose is a non-reducing sugar When Benedict's test is performed with the disaccharides maltose and sucrose, the following result is obtained Sucrose Result Maltose Sucrose is a non-reducing sugar Maltose is a reducing sugar

The anomeric carbon atom for glucose is carbon 1 REDUCING SUGARS Why is sucrose a non-reducing sugar? Sugars reduce Benedict's solution when the anomeric carbon atom is made 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

The anomeric carbon atom for fructose is carbon 2 REDUCING SUGARS Why is sucrose a non-reducing sugar? Glucose Fructose Sugars reduce Benedict's solution when the anomeric carbon atom is made available to reduce the copper ions in the solution The anomeric carbon atom for fructose is carbon 2 Ketone group Fructose is a ketose sugar and bonds to glucose to form sucrose

Why is sucrose a non-reducing sugar? 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

Sucrose is a non-reducing sugar Sucrose is synthesised when glucose forms a glycosidic bond with fructose The anomeric carbon atom for fructose is carbon 2 glucose fructose glycosidic bond The anomeric carbon atom for glucose 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

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 for at least fifteen minutes 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

Starch POLYSACCHARIDES Polysaccharides are large polymers of the monosaccharides Unlike monosaccharides and disaccharides, polysaccharides are either insoluble or form colloidal suspensions The principal storage polysaccharides are starch and glycogen Starch is a polymer of alpha glucose and is, in fact, a mixture of two different polysaccharides – amylose and amylopectin Starch Amylose – long unbranched chain of glucose units Amylopectin – highly branched polymer of glucose units

The amylose chain, once formed, coils into a helix AMYLOSE STRUCTURE Amylose is formed by a series of condensation reactions that bond alpha glucose molecules together into a long chain forming many glycosidic bonds The amylose chain, once formed, coils into a helix

formed from covalently bonded a glucose molecules The amylose helix formed from covalently bonded a glucose molecules

AMYLOPECTIN STRUCTURE Amylopectin consists of a straight chain of alpha glucose units with branch points occurring at approximately every twelfth glucose unit along the straight chain The branch points form when carbon 6, of a glucose molecule in the straight chain, forms a glycosidic bond with carbon 1 of a glucose molecule positioned above the chain 1 6a glycosidic bond

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

Glycogen is often referred to as animal starch Glycogen has the same overall structure as amylopectin but there is significantly more branching in this molecule More of these branch points form

Glucose is stored as glycogen in large amounts in both the liver and skeletal muscles

STRUCTURAL POLYSACCHARIDES 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 1 4b glycosidic bond The orientation of the beta glucose units places many hydroxyl (OH) groups on each side of the molecule Many parallel chains of beta glucose units form and each chain forms hydrogen bonds between the OH groups of adjacent chains

STRUCTURAL POLYSACCHARIDES 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 beta glucose The rigidity and strength of plant cell walls is a consequence of the incorporation of cellulose into their structure

STRUCTURAL POLYSACCHARIDES Chitin Chitin is a polysaccharide forming the exoskeleton of insects and crustaceans and the fabric of fungal cell walls; it is a linear polymer of the sugar derivative called N-acetylglucosamine N-acetylglucosamine

Chitin forms the exoskeleton of insects and crustaceans

Chitin forms the fabric of fungal cell walls

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