GIANT MOLECULE

 

Week:                   ELEVEN                                 Date:                                                    

Period:                                 Duration: 1 HR 20 MIN.                                                  Average age of learners: 17YEARS

Subject:                               CHEMISTRY                                                                         Class: SS 3

Topic:                                    GIANT MOLECULES

Sub topic:

Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials: Rice ,beans, egg and yam

Entry behavior: The students are familiar with carbohydrate and protein foods.

Behavioural objective: At the end of the lesson the students should be able to:

1.       Define carbohydrate and their sources.

2.       Give example of monosaccharides, disaccharides, and polysaccharides.

3.       State the classification based on the functional group.

4.       Test for glucose and starch

5.       Give examples of protein.

CONTENT

GIANT MOLECULES

Carbohydrates are made up of carbon, hydrogen, and oxygen. They can also be defined as Polyhydroxy Aldehydes or Ketones.

Sources of Carbohydrates

1.       Simple sugars are found in the form of fructose in many fruits.

2.       Galactose is present in all dairy products.

3.       Lactose is abundantly found in milk and other dairy products.

4.        Maltose is present in cereal, beer, potatoes, processed cheese, pasta, etc.

5.       Sucrose is naturally obtained from sugar and honey containing small amounts of vitamins and minerals.

These simple sugars that consist of minerals and vitamins exist commonly in milk, fruits, and vegetables. Many refined and processed foods like white flour, white rice, and sugar lack important nutrients and hence, they are labeled “enriched.” It is quite healthy to use vitamins, carbohydrates and all other organic nutrients in their normal forms.

CARBOHYDRATES STRUCTURE

Generally, word saccharides are used for carbohydrates. Saccharides includes starch, sugars, cellulose etc. Formaldehyde is considered as simplest carbohydrate. They have general formula (CH₂O)_n where n is three or more. They have aldehyde or ketone with hydrogen atom and hydroxyl group.

Fig.1. Structure of Carbohydrates 

Classification of Carbohydrates / Types of Carbohydrates

1.       Classification on the basis of number of units of sugar

Saccharides can be monosaccharides, disaccharides, oligosaccharides and polysaccharides.

The simplest saccharide or unit of saccharides are known as Monosaccharides, such as Glucose. When two monosaccharides are joined by glycosidic bond, they form Disaccharides, such as Sucrose. Sucrose is composed of Glucose and Fructose.

When 2 to 10 units of monosaccharides are joined, they form Oligosaccharides, such as Raffinose. When more than 10 monosaccharides are joined, they form Polysachharides. Such as starch, cellulose etc. 

Classification of Monosaccharides on the Basis of Number of Carbons Present

If number of carbon atoms present is 6, then it is known as Hexose Sugar. For Example: Glucose.

Fig. 2. Structure of Glucose or Hexose Sugar

If number of carbon atoms present is 5, then it is known as Pentose Sugar. For Example: Ribose.

Fig. 3. Structure of Ribose

If number of carbon atoms present is 3, then it is known as Triose Sugar. For Example: Glyceraldehyde.

Fig. 4. Structure of Glyceraldehyde

1.       Classification on the Basis of Functional Group Present

I.                    Aldoses contain the aldehyde group. For Example: Glucose, Galactose, Ribose, Glyceraldehyde, etc.

II.                  Ketoses contain the ketone group. For Example: Fructose

III.                Reducing sugars contain a hemiacetal or hemiketal group. For Example: Glucose, Galactose, Fructose, Maltose, Lactose

IV.                Non-reducing contain no hemiacetal groups. For Example: Sucrose and all polysaccharides are non-reducing sugars. 

Monosaccharides

Glucose is the most important carbohydrate needed by living organisms. It easily crosses the cell membrane. Glucose on oxidation releases energy via process known as Respiration. Its chemical formula is C₆H₁₂O_6. The sugar that circulates in blood is glucose. The level of glucose determines whether the person is suffering from diabetes or not.

Galactose is made up of glucose and galactose. Lactose is a milk sugar. It is also a component of ABO blood groups in humans.

Fructose is a ketose sugar.  It is 5 carbon sugar. Fructose and glucose together forms a sucrose.

Ribose and deoxyribose sugars are found in nucleic acids. RNA contains ribose sugar whereas DNA contains deoxyribose sugar. In deoxyribose sugar, -OH is replaced by -H atom. 

Fig. 5. Cyclic Structure of Ribose and Deoxyribose

Disaccharides

Sucrose is the disaccharide found in plants. The end product of photosynthesis, is sucrose. It is a non-reducing sugar. The chemical formula of sucrose is C₁₂H₂₂O₁₁.

Lactose also known as milk sugar. It is commonly found in dairy products. It is a reducing sugar. It is an isomer of sucrose.

Maltose is formed by joining of two glucose units via glycosidic bond. When amylase breaks down into starch, it forms maltose or malt sugar. It is also a reducing sugar.
 Polysaccharides

Plants store food in the form of starch. Starch is made up of amylose and amylopectin. Both amylose and amylopectin are made up of glucose. Amylose is an unbranched polymer of glucose whereas amylopectin is branched polymer of glucose. 

In animals, glucose is stored in the form of glycogen. It is also made up of amylose and amylopectin. But in glycogen, branching occurs at very short distances.

Cellulose is another polymer of glycose. It is a straight chain polymer of glucose. Plant cell wall is made up of cellulose. It provides strength and rigidity to the cell wall. Cellulose forms a tough structure due to strong hydrogen bonding between the chains of the polymer.

Human beings are unable to digest cellulose as they do not contain any enzyme required for cellulose digestion. Ruminants have special stomach for digestion of cellulose. 

Functions of Carbohydrates

1. It is the most common source of energy.

2. Cellulose is present in cell wall of plants that provides and rigidity to it.

3. Lactose sugar is found in milk.

4. Chitin is also another sugar found in exoskeleton of insects.

5. It is a dietary fiber.

  TEST FOR GLUCOSE

Add few drops of fehling’s solution to glucose solution in a test tube. A brick-red precipitate is obtained on boiling.

USES OF GLUCOSE

1.       Glucose is used in the manufacture of jam and sweets.

2.       It is used as an immediate source of energy for sick people and sportmen.

TEST FOR STARCH

Add few drops of iodine to some boiled starch. A dark-blue colouration, which disappears on heating and reappears on cooling result.

USES OF STARCH

1.       It is used mainly as food.

2.       It is used to produce ethanol and glucose.

3.       It is used as a stiffening agent.

PROTEINS

Proteins may be regarded as the products of successive condensation between carbonyl group of one amino acid and the amino group of the other amino acid e.g.

NH₂CH₂COOH + NH₂CH₂COOH                                  NH₂CH₂CONHCH₂.COOH + H₂O

Each end of the chain can now react with more amino acid and so a polymer (polyamide) will result. The amide linkages –CO-NH- are known as peptide linkages. Example of proteins with peptide link myoglobin (one polypeptide); haemoglobin, insulin, ribonuclease and collagen (more than one polypeptide).

Proteins are complex combinations of amino acid which are essential constituents of all living cells being responsible for growth and maintenance off all tissues.

Classification of protein in our food

I.                    First class proteins: it contain amino acids and are of animal origin e.g. fish, meat, egg, milk, cheese.

II.                  Second class protein: are mainly plants proteins which are found in vegetables such as peas, beans.

Properties of proteins

1.       Denaturation: when proteins are exposed to drastic conditions such as, high temperature (above 40^oC), extreme pH and chemical reagents, they tend to lose their biological activity and undergo changes in properties.

2.       Hydrolysis: proteins can be hydrolysed to give amino acid by boiling them with solutions of hydrochloric acid or sodium hydroxide. Hydrolysis can also be carried out by using suitable enzymes.

Test for Proteins.

Biuret test: add about 1cm³ of dilute caustic soda solution into a test-tube containing some dilute egg-white solution. Then add 1% copper(II) tetraoxosulphate(VI) solution drop by drop and shake thoroughly. The formation of a violet colouration indicates the presence of proteins.

Million’s test: add few drops of million’s reagent to some egg-white solution in a test-tube. The formation of a white precipitate which turns brick-red on heating indicates the presence of proteins.

Trioxonitrate(V) acid test: add few drops of concentrated hydrogen trioxonitrate (V) acid to 2cm³ of egg-white solution. The formation of an intense yellow colour indicates the presence of proteins.

PRESENTATION

I.                    The teacher explains meaning of carbohydrate.

II.                  The teacher explains different classification of carbohydrate.

III.                The students differentiate between an aldo- and keto- sugar.

IV.                The teacher leads the students on how to test for glucose and starch.

V.                  The teacher explains protein and their properties.

EVALUATION

 The teacher evaluates the lessons by asking the following questions:-

1.       Define carbohydrate and their sources.

2.       Give example of monosaccharides, disaccharides, and polysaccharides.

3.       State the classification based on the functional group.

4.       Test for glucose and starch

5.       Give examples of protein.

ASSIGNMENT

1a.           What is fermentation?.

 (b)         Write an equation for the fermentation of glucose.

(c)           What must be added to glucose solution to make it ferment.

 (d)         Explain why a tightly corked glass bottled filled to the brim with fresh palm wine shattered on standing.

2.            A protein is boiled for a long time with dilute HCl and a reaction occurred.

(a)          State the type of reaction that occurred

 (b)         Name the major product formed

(c)           Give the functional group present in (b) above.

d.            Mention the monomer of protein.