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Carbohydrates (sugars or saccharides) Tutorial

Key Concepts

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Examples of Carbohydrates

Carbohydrate Name Formula Source
(simple sugars)
glucose C6H12O6 fruits
fructose C6H12O6 fruits, honey
galactose C6H12O6 not naturally occurring

(double sugars)
sucrose (glucose + fructose) C12H22O11 sugar cane, sugar beet
lactose (glucose + galactose) C12H22O11 milk
maltose (glucose + glucose) C12H22O11 germinating grain

Polysaccharides starch -(C6H10O5)-n energy store in plants
glycogen -(C6H10O5)-n energy store in animals
(liver and muscles)
cellulose -(C6H10O5)-n plant fibre

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Structure of Monosaccharides

Glucose, fructose and galactose are isomers, they have the same chemical formula (C6H12O6) but different structures.

Glucose, fructose and galactose are all examples of hexoses, simple sugars containing 6 carbon atoms.

Name glucose galactose fructose
Molecular Formula C6H12O6 C6H12O6 C6H12O6

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Structure of Disaccharides

Sucrose, lactose and maltose are isomers, they have the same chemical formula, C12H22O11, but different structures.

An ether, or glycosidic, link joins 2 monosaccharides to form a disaccharide.

When two monosaccharides react in a condensation reaction the products are a disaccharide and a molecule of water.

(Molecular Formula)
Formed from Structure
glucose + fructose
→ sucrose + H2O

→ lactose + H2O

→ maltose + H2O

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Structure of Polysaccharides

Polysaccharides are polymers of monosaccharides.

Starch, glycogen and cellulose are all polymers of glucose but they differ from each other in the number of glucose units in a chain and in the amount of branching and the position of the glycosidic or ether link.

  Starch Glycogen Cellulose
Monomer glucose glucose glucose
Structure 2 components:
(a) amylopectin: 3000 - 6000 glucose units (branched, open structure)
(b) amylose: 60 - 300 glucose units (unbranched linear structure)
more branched than amylopectin with an open structure Relatively unbranched linear polymer of ~3000 glucose units.
Chains linked by hydrogen bonds.
Solubility in water insoluble in cold water
soluble in hot water
(forms a colloid)
soluble in cold water
(forms a colloid)
Found in plants animals plants

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