Carbohydrates (sugars or saccharides) Chemistry Tutorial

Key Concepts

Carbohydrates are compounds of carbon, hydrogen and oxygen.

General formula of carbohydrates is C_x(H₂O)_y, or, C_xH_2yO_y.

Carbohydrates are also known as sugars or saccharides.

The most common simple carbohydrates have either:
⚛ 5 carbon atoms (pentoses)
OR
⚛ 6 carbon atoms (hexoses)

A monosaccharide is made up of one sugar (mono means 1).

A disaccharide is made up of two simple sugars chemically joined together (di means 2).

A polysaccharide is made up of many simple sugars chemically joined together (poly means many).

Monosaccharides contain the hydroxyl (alcohol, OH) functional group.

Disaccharides and polysaccharides contain both the hydroxyl (alcohol, OH) functional group and an ether or glycosidic link (R-O-R') between two simple sugar molecules.

Two monosaccharides react in a condensation reaction producing a disaccharide and water.

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Examples of Carbohydrates

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

Disaccharides (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
Structure

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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.

Name
(Molecular Formula)

Formed from

Structure

sucrose
(C₁₂H₂₂O₁₁)

glucose	+	fructose
	+

→ sucrose + H₂O

lactose
(C₁₂H₂₂O₁₁)

glucose	+	galactose
	+

→ lactose + H₂O

maltose
(C₁₂H₂₂O₁₁)

glucose	+	glucose
	+

→ 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)	insoluble
Found in	plants	animals	plants

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