They are used as a source of energy in respiration.
They are important building blocks for large molecules.
Disaccharides – Structure, Properties, and Examples
Disaccharides are made up of two monosaccharides joined together by a condensation reaction.
The condensation reaction is the joining of two molecules with the formation of a new chemical bond and a water molecule is released when the bond is formed.
A glycosidic bond is formed between two monosaccharides. If carbon 1 on one monosaccharide joins to carbon 4 on another monosaccharide, it is called a 1,4-glycosidic bond.
Examples:Maltose is formed from two α-glucose molecules joined together by a glycosidic bond. Sucrose is formed from a condensation reaction between a glucose molecule and a fructose molecule. Lactose is formed from glucose and a galactose molecule.
Sucrose is a non-reducing sugar.
Disaccharides can be split apart into two monosaccharides by breaking the glycosidic bond by adding water molecules, which is known as hydrolysis reaction. The water provides a hydroxyl group (-OH) and hydrogen (-H), which helps the glycosidic bond to break.
Sucrose is the transport sugar and Lactose is the sugar found in milk which an important constituent of the diet of young mammals.
Polysaccharides – Structure, Properties, and Examples
Polysaccharides are polymers formed by combining many monosaccharide molecules (more than two) by condensation reactions.
Molecules with 3-10 sugar units are known as oligosaccharides while molecules containing 11 or more monosaccharides are true polysaccharides.
Polysaccharides do not taste sweet.
Because their molecules are so enormous, the majority of polysaccharides do not dissolve in water.
Polysaccharides made solely from one kind of monosaccharides are called homopolysaccharides (Starch) while those made of more than one monomer are called heteropolysaccharides (Hyaluronic acid).
Starch is made up of long chains of α-glucose (Amylose and Amylopectin). Glycogen is made of α-glucose linked together by glycosidic bonds. Cellulose is also made of many β-glucose molecules linked by glycosidic bonds between carbon 1 and carbon 4.
Starch is the main energy storage materials in plants. Glycogen is the main energy storage materials in animals. Cellulose is the major component of cell walls in plants.
The test for starch is called an Iodine test.
Ann Fullick, Jo Locke and Paul Bircher. 2015. A Level Biology for OCR- A. Oxford University Press. (Buy this book)