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Biochemistry – Carbohydrates Answers

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Carbohydrates
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There are three types:- Monosaccharides Disaccharides Polysaccharides
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Monosaccharides
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= 1 sugar unit
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Disaccharides
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= 2 sugar units
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Polysaccharides
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= many sugar units
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Monosaccharides
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These are sweet tasting and soluble
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Monosaccharides
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They have the general formula c6h12o6
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Glucose
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is one of the best know monosaccharides. It is a hexose and has the formula C6H12O6
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Glucose
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The atoms of carbon, hydrogen and oxygen in a monosaccharide form a ring, and can be arranged in different ways
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Glucose
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These differences are shown by letters in front of the name – for example & glucose.
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Glucose
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Molecules that have the same atoms, but in different arrangements are called isomers
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Glucose
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The differences are usually only small, but can give the molecules different properties
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Disaccharides
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Are formed of two monosaccharides, joined together with a glycosidic bond – this is a condensation reaction which means that one molecule of water is lost
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Disaccharides
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Glucose + glucose = maltose Glucose + galactose = lactose Glucose + fructose = sucrose
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Condensation reaction
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Here are two monosaccharides joining to make a disaccharide. The carbons on each ring are numbered, starting with the carbon after the oxygen. Here the 1st carbon on one ring is joining with the 4th carbon on another ring. So it is known as a 1,4 glycosidic bond
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disaccharide
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When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond into its original monosaccharides. This is called hydrolysis (breakdown by water). The breakdown is very slow unless it is catalysed by an appropriate enzyme
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Tests for reducing sugars
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All monosaccharides and some disaccharides are reducing sugars. This means they are capable of reducing Benedict’s solution. When a reducing sugar is heated with Benedict’s solution, the colour changes from blue to orange/red
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Test for non-reducing sugars
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Some disaccharides are non-reducing. That means they will have no effect on Benedict’s solution. There is no direct test for these sugars. We have to hydrolyse them (breaking the glycosidic bond) using dilute acid, and then we can use the benedict’s test
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Roles of monosaccharides and disaccharides
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Their main role is as a respiratory substrate – that is, animals and plants use them in their cells to release energy. The process that takes these sugars and releases energy is called respiration
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Polysaccharides
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The combining of many monosaccharides forms a polymer known as a polysaccharide
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Polysaccharides
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The monosaccharides are joined with glycosidic bonds. The resulting chains may vary in length, be branched or unbranched and be folded in different ways
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Polysaccharides
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These macromolecules are insoluble. There are 3 main types you have to know about – Starch, Glycogen and Cellulose
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Starch
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An insoluble store of glucose found in plants, and a major component of foods. Starch is a mixture of two substances – amylose and amylopectin
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Amylose
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-glucose 1,4 glycosidic bonds Spiral structure
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Amylopectin
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-glucose 1,4 and some 1,6 glycosidic bonds Branched structure
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Starch
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About 80% of starch is amylopectin and the remaining 20% is amylose
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Starch
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The main role of starch is for energy storage
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The main role of starch is for energy storage
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It is insoluble, so does not have any osmotic effect, i.e. does not draw water into the cells. Being insoluble, it does not easily diffuse out of cells. It is compact – a lot can be stored in a small place. When hydrolysed, it forms glucose, which is easily transported and readily used in respiration to provide energy
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Glycogen
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Glycogen is very similar in structure to amylopectin – made of many units of – glucose, joined in 1,4 and 1,6 glycosidic bonds
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Glycogen
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The main difference is that the chains in glycogen tend to be shorter and more branched than those of amylopectin
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Glycogen
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It is sometimes called ‘animal starch’ because it is the main unit of carbohydrate storage in animals
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Glycogen
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It readily hydrolyses to – glucose when needed
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Cellulose
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The big difference here is that cellulose is made up of monomers of -glucose rather than – glucose. The bonds are 1,4 glycosidic bonds
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Cellulose
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Rather than forming a coiled chain, cellulose has straight, unbranched chains that run parallel to one another
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Cellulose
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Hydrogen bonds form between these parallel chains, adding to the stability of the molecule
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Cellulose
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Chains can be bundled together giving great strength
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Cellulose
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Cellulose makes up 50% of a plant cell wall and its stability makes it hard to digest. Some animals have microorganisms in their gut that digest cellulose. Cotton and rayon fabrics are mainly cellulose. Paper is probably the best known and most widely used cellulose product