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Carbohydrates (sugars)

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.
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 hydroxy (alcohol, OH) functional group.
Disaccharides and polysaccharides contain both the hydroxy (alcohol, OH) functional group and an ether or glycosidic link (R-O-R') between 2 simple sugar molecules.
2 monosaccharides react in a condensation reaction producing a disaccharide and water.

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

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

Formula C₆H₁₂O₆ C₆H₁₂O₆ C₆H₁₂O₆

Structure

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 Formula Formed from Structure

sucrose C₁₂H₂₂O₁₁

glucose + fructose

+

---> sucrose + H₂O

lactose C₁₂H₂₂O₁₁

glucose + galactose

+

---> lactose + H₂O

maltose C₁₂H₂₂O₁₁
glucose + glucose

+

---> maltose + H₂O

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