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Introducing carbohydratesCarbohydrates are a group of substances used as both energy sources and structural materials in organisms.

All carbohydrates contain carbon, hydrogen and oxygen, with the general formula: Cx(H2O)y.

There are three main groups of carbohydrates:

monosaccharides – these are simple sugars, with the general formula (CH20)n, where n can be 3–7

disaccharides – these are ‘double sugars’, formed from two monosaccharides

polysaccharides – these are large molecules formed from many monosaccharides.

Glucose

The structure of glucose can be represented in different ways:

Glucose is an abundant and very important monosaccharide. It contains six carbon atoms so it is a hexose sugar. Its general formula is C6H12O6.

Glucose is the major energy source for most cells. It is highly soluble and is the main form in which carbohydrates are transported around the body of animals.

straight chain ring ring (simplified)

Remembering how to draw α-glucose

Remember:

LOHexagonWings

Alpha and beta glucoseGlucose exists in different forms called structural isomers. Two common isomers are alpha glucose and beta glucose.

alphaglucose

betaglucose

The only difference between these two isomers is the position of the H and -OH group attached to carbon 1. In α-glucose, H is above the carbon and in β-glucose it is below the carbon.

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This minor structural difference has a major effect on the biological roles of α and β glucose.

Two forms of glucose

Fructose and galactose

Galactose is not as soluble as glucose and has an important role in the production of glycolipids and glycoproteins.

Two other important hexose monosaccharides are fructose and galactose.

fructose galactose

Fructose is very soluble and is the main sugar in fruits and nectar. It is sweeter than glucose.

The formation of disaccharides

Maltose, sucrose and lactoseMaltose (malt sugar) is formed from two glucose molecules joined by an alpha 1–4 glycosidic bond.

Sucrose (table sugar) is formed from glucose and fructose joined by an alpha 1–4 glycosidic bond.

Lactose (milk sugar) is formed from galactose and glucose joined by a beta 1–4 glycosidic bond.

Condensation and hydrolysis

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What are polysaccharides?

Polysaccharides are polymers containing many monosaccharides linked by glycosidic bonds. Like disaccharides, polysaccharides are formed by condensation reactions.

The major polysaccharides are starch and cellulose in plants, and glycogen in animals.

Polysaccharides are mainly used as an energy store and as structural components of cells.

Starch

Found in many parts of the plant as small grains.Large amounts in seeds and storage organsMajor energy source in most diets

Alpha glucose + alpha glucose + alpha glucose + alpha glucose + alpha glucose + alpha glucose + alpha glucose ............................. etc. = starch

1000s of glucose molecules bonded together by glycosidic bonds in condensation reactions makes starch

Starch: Amylose and Amylopectin

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The glucose molecules are joined by 1,4 glycosidic bonds.

Amylose is alpha glucose, forming straight chains.

The chains fold naturally into a helical shape.

Starch: Amylose and Amylopectin

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The branches are made with 1,6 glycosidic bonds.

Amylopectin is also alpha glucose, forming branched chains.

Starch: Amylose and Amylopectin

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Starch is a mixture of amylose and amylopectin; it's insoluble and is used as a storage molecule for plants.

Starch: Amylopectin

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Starch is a mixture of amylose and amylopectin; it's insoluble and is used as a storage molecule for plants.

The structure of starch

StarchMain role of starch is energy storage, something

it’s especially suited for because:

It is insolubleIt is compactWhen hydrolysed it forms α-glucoseStored as granules

What are the advantages of these properties?

Subunit structure of glycogen

VERY similar to amylopectin – but shorter and more highly branched

GlycogenStarch is never found in animal cells; instead you find glycogen

(energy storage)Similar to amylopectin in starch, except shorter chain and it’s

more highly branched• Stored as small granules mainly in the muscles and liver. • Consists of 1-4 glycosidic linked chains of alpha glucose

which are shorter and more branching• Not soluble in water• Readily hydrolysed

What are the advantages of these properties?

β-glucose polymers

β-glucose molecules bond together through condensation reaction to form long chain

Unlike α-glucose, forms long straight chains due to orientation of glycosidic bonds

Straight chains contain up to 10 000 β-glucose molecules

Cellulose chains found only in plants

Orientation of β-glucose and chain formation

Made of β-glucose rather than α-glucose like in starch and glycogen.

‘flip-flop’ arrangement of glucose

CelluloseForms straight unbranched chains rather than a

coiled chain like starch or a branched chain like glycogen.

Chains run parallel to each other allowing hydrogen bonds to form cross-links between adjacent chains (microfibrils)

Provides strength to cellulose cell walls Individual bonds are weak but large numbers

adds to considerable strength.

Cellulose chains, microfibrils and macrofibril (fibre)

Task

Construct a table to compare starch, glycogen and cellulose. Think about their structures, properties and functions.

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Starch, glycogen and cellulose

Aims:How are α-glucose monomers arranged to form the polymers of starch and glycogen?How are β-glucose monomer arranged to form the polymer cellulose?How do the molecular structures of starch, glycogen and cellulose relate to their functions?

Plenary: Compare and contrast starch, glycogen and cellulose