SS3 First Term Chemistry Lesson Note – Giant Molecules

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WEEK 5

TOPIC: GIANT MOLECULES

CONTENT:

1. Sugars (carbohydrate)
2. Hydrolysis of sucrose and starch
3. Proteins

PERIOD 1:  SUGARS

Carbohydrates are organic compounds of carbon, hydrogen and oxygen in which the ratio of atoms of hydrogen and oxygen is 2:1.

The general molecular formula of carbohydrate is C_x(H₂O)_y or C_xH_2yO_y

CLASSIFICATION                                  

The classification is based on the effect of acid hydrolysis of carbohydrates.

MONOSACCHARIDES: These are simple sugars. Examples are glucose and fructose. They cannot be split into simple molecules by acid hydrolysis.

Structure of glucose and sucrose

DISACCHARIDES: They are also sugars e.g. sucrose, maltose, and lactose. They can be split into two molecules of monosaccharide by acid hydrolysis.

POLYSACCAHRIDES: They are also known as complex sugars. They are no-crystalline, insoluble and tasteless substance e.g. starch and cellulose. They can be hydrolysed to give many molecules of the same or different types of monosaccharide. Other examples are glycogen (animal starch), inulin, dextrin.

SOURCES

Glucose can be found in grapes, honey, sap of plants and in the blood of animals.

Fructose is found together with glucose in sweet fruit juices and honey.

Sucrose or cane sugar is the common granulated sugar which we use to sweeten food.

It occurs naturally in many plants and fruits e.g. pineapple, carrots, sorghum and the sap of the sugar maple tree. The main sources are sugar cane and sugar beet.

Starch occurs naturally in most plants as a stored carbohydrate e.g., maize (corn), cassava, yam, cocoyam, potatoes, rice, wheat, barley and other cereals.

Starch is composed of thousands of repeating glucose units linked together to form a giant branched molecule.

Cellulose: it is composed of glucose units which are linked in a slightly different way from that in starch. It is found in cotton wool, hemp, jute, flax, sisal and straw

PERIOD 2: REDUCING AND NON-REDUCING SUGARS

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