Introduction to Carbohydrates

Carbohydrates

By Kathryn Boddy and

WEIRD FACT​:

The brain is the only carbohydrate-dependent organ in the body. The brain exclusively uses glucose, a basic simple sugar carbohydrate, to function.​

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Formed by the condensation reaction of two monosaccharides.​

​Disaccharides

The monomer from which larger carbohydrates are made​ - common monosaccharides are; glucose, galactose and fructose.​

Monosaccharides​

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? + ? = Maltose

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Fructose + ? = ?

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Glucose + ? = Lactose

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​Types of Glucose

Draw

Draw a-glucose.

Draw

Draw b-glucose.

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B-Glucose​

​A-Glucose

Polysaccharides

​Examples: Starch, Glycogen and Cellulose.

​Glycogen

Function - Energy store in animal cells.

Structure - made up of a-glucose, with ​C1-4 and C1-6 glycosidic bonds, highly branched

Structure related to function

• Branched - rapidly hydrolysed to release glucose for respiration

• Insoluble in water - water potential of cell not affected​

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​Starch

Function - Energy store in plant cells.

Structure - Made up of a-glucose, C1-4 and C1-6 glycosidic bonding, branched​

Structure related to function

• Helical - compact for storage in cells

• Large molecule - can't ​diffuse out of cell

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​Cellulose

Function - Provides strength and structural support to plant cell walls.

Structure - Made from b-glucose, straight and unbranched chains, ​C1-4 glycosidic bonding.

Structure related to function

• ​Every 2nd b-glucose is inverted - long, straight unbranched chains

• Many hydrogen bonds​ - form microfibrils and fibres for strength and support.

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Open Ended

Hydrogen bonds are important in cellulose molecules. Explain why.

​The many hydrogen bonds bind cellulose molecules together to form microfibrils, which provide strength and structural support.

​Example Answer

Biochemical Tests

​Reducing Sugars:

• All monosa​ccharides

• Some disaccharides such as malt​ose and lactose

​Non-Reducing Sugars:

• No monosa​ccharides

• Some disaccharides such as sucrose.

​Iodine test for Starch:

1) Add iodine (or iodine dissolved in potassium iodide) to solution and shake.

2) Blue-black colour = Starch present​

Open Ended

Describe the biochemical test for non-reducing sugars.

1) Add a few drops of dilute HCl*

2) Heat in boiling water bath.

3) Neutralise with sodium bicarbonate.

4) Add benedicts regent and heat again.

5) Positive = green/yellow/orange/red precipitate​

Benedicts test for non-reducing sugars​

1) Add benedict's regent (blue) to sample.

2) Heat in boiling water bath.

3) Positive = green/yellow/orange/red precipitate.​

Benedicts test for reducing sugars​

Biochemical test: reducing and non-reducing sugars.

​*You add HCl to hydrolyse sugar into its constituent reducing sugars.