Tests and Uses of Starch, Sugars and Proteins for SS 3

Class: SS 3
Term: First Term
Week: 11
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Topic: GIANT MOLECULES
Subject Matter: Test for starch and sugars; Uses of starch and glucose: proteins, sources, structure and properties, test for protein, and uses of proteins
Previous Lesson: Hydrolysis of Sucrose and Starch

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Explain the chemical tests for starch and reducing sugars.
• State the uses of starch and glucose.
• Identify common sources of proteins.
• Describe the basic structure and properties of proteins.
• Explain the chemical tests for proteins.
• List the uses of proteins.

Affective Domain

• Appreciate the importance of macromolecules in daily life and nutrition.
• Participate actively and safely in practical demonstrations.

Psychomotor Domain

• Perform simple laboratory tests to identify starch, reducing sugars, and proteins.
• Handle laboratory apparatus and reagents correctly and safely.

Reference Materials

The following resources were used in planning this lesson:

• 2025 Revised 9 Years Basic Education Curriculum
• Relevant State Unified Scheme of Work
• A suitable Chemistry textbook for SS 3
• The HeadTeacher Scheme of Work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Samples of food containing protein (e.g., beans, groundnuts, egg white, meat, fish)
• Starch solution
• Glucose solution
• Benedict’s solution
• Iodine solution
• Million’s reagent
• Concentrated HNO₃ (nitric acid)
• Biuret reagent (consisting of sodium hydroxide solution and copper (II) sulphate solution)
• Test tubes
• Test tube rack
• Droppers
• Beakers
• Source of heat (Bunsen burner or spirit lamp)
• Safety goggles

Rationale for the Lesson

This lesson is important as it introduces students to the practical identification of essential macromolecules found in food. Understanding the tests for starch, sugars, and proteins helps students appreciate the chemical composition of food and their nutritional value. It also develops practical laboratory skills in qualitative analysis.

Prerequisite/Previous Knowledge

Students should have prior knowledge of basic organic compounds, functional groups, and the concept of macromolecules.

Lesson Content/Board Summary

GIANT MOLECULES

Tests for Starch and Sugars

Giant molecules like starch, sugars, and proteins are essential biomolecules. They can be identified using specific chemical tests.

1. Test for Starch (Iodine Test):

   Starch is tested using iodine solution. A few drops of iodine solution are added to the sample. A positive test for starch is indicated by a blue-black colouration.

2. Test for Reducing Sugars (Benedict’s Test):

   Reducing sugars (e.g., glucose, fructose, maltose) are identified using Benedict’s solution. Add Benedict’s solution to the sample and heat gently in a water bath. A colour change from blue through green, yellow, orange, to brick-red precipitate indicates the presence of a reducing sugar. The intensity of the colour indicates the concentration.

Uses of Starch and Glucose

1. Uses of Starch:

   Starch is a major source of energy for living organisms. It is used in the food industry as a thickening agent, in paper manufacturing, and in the production of adhesives and textiles.

2. Uses of Glucose:

   Glucose is the primary source of energy for cellular respiration in living organisms. It is used in the confectionery industry, in the production of ethanol through fermentation, and in medicine as an energy supplement.

Proteins

Proteins are complex giant molecules made up of amino acids linked by peptide bonds. They are crucial for the structure and function of all living cells.

1. Sources of Proteins:

   Proteins are found in both animal and plant sources. Animal sources include meat, fish, eggs, milk, and cheese. Plant sources include beans, groundnuts, lentils, soy, and certain grains.

2. Structure of Proteins:

   Proteins have complex structures:

   1. Primary Structure: The specific sequence of amino acids in the polypeptide chain.
   2. Secondary Structure: Regular repeating structures formed by hydrogen bonds, such as alpha-helices and beta-pleated sheets.
   3. Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, resulting from interactions between side chains.
   4. Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) in a protein complex.
3. Properties of Proteins:

   Proteins exhibit several properties, including:

   1. Denaturation: The loss of a protein’s three-dimensional structure due to factors like heat, strong acids, bases, or heavy metals. This often results in loss of biological activity.
   2. Coagulation: The irreversible precipitation of proteins, often as a result of denaturation (e.g., cooking an egg).
   3. Amphoteric Nature: Proteins can act as both acids and bases due to the presence of amino and carboxyl groups in their amino acid residues.
4. Tests for Proteins:

   Proteins can be identified using various chemical tests:

   1. Biuret Test:

      Add sodium hydroxide solution to the sample, then add a few drops of dilute copper (II) sulphate solution. Shake gently. A violet or purple colour indicates the presence of peptide bonds, hence protein.

   2. Million’s Test:

      Add Million’s reagent (mercuric nitrate in nitric acid) to the sample and heat gently. A white precipitate forms, which turns brick-red upon heating, indicating the presence of proteins containing tyrosine residues.

   3. Xanthoproteic Test:

      Add concentrated nitric acid to the protein sample and heat gently. A yellow colour develops. On cooling, add an excess of ammonium hydroxide or sodium hydroxide solution, and the yellow colour changes to orange. This test is positive for proteins containing aromatic amino acids (e.g., tyrosine, tryptophan, phenylalanine).

5. Uses of Proteins:

   Proteins perform numerous vital functions in the body:

   1. They are essential for growth and repair of body tissues.
   2. They form enzymes, which catalyze biochemical reactions.
   3. They form hormones, which regulate body processes.
   4. They are components of antibodies, providing immunity.
   5. They transport substances (e.g., haemoglobin transports oxygen).
   6. They provide structural support (e.g., collagen in skin and bones).
   7. They can serve as an energy source when carbohydrates and fats are insufficient.

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Practical Activity, Question and Answer, Explanation, Guided Practice.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Review/Engage

Teacher’s Activity: The teacher asks students to recall what they know about large organic molecules found in food and introduces the concept of giant molecules (macromolecules) like starch, sugars, and proteins.

Pupils’ Activity: Pupils respond to questions and listen attentively to the introduction.

Learning Point: Macromolecules introduction

Step 2: Test for Starch and Sugars

Time: 7 minutes

Teaching Skill: Demonstration/Explanation

Teacher’s Activity: The teacher demonstrates the iodine test for starch and the Benedict’s test for reducing sugars, explaining the reagents, procedure, and expected colour changes for positive and negative results. The teacher ensures safety measures are observed.

Pupils’ Activity: Pupils observe the demonstrations, ask questions, and note the procedures and observations.

Learning Point: Starch and sugar tests

Step 3: Uses of Starch and Glucose

Time: 5 minutes

Teaching Skill: Explanation/Discussion

Teacher’s Activity: The teacher explains the various uses of starch (energy storage, industrial applications) and glucose (primary energy source, fermentation) in living organisms and industries.

Pupils’ Activity: Pupils listen, contribute to the discussion, and note down the uses.

Learning Point: Starch and glucose uses

Step 4: Proteins: Sources and Structure

Time: 7 minutes

Teaching Skill: Explanation/Listing

Teacher’s Activity: The teacher guides students to list common sources of protein (animal and plant). The teacher then explains the basic structure of proteins, emphasizing amino acids and peptide bonds, and briefly introduces primary, secondary, tertiary, and quaternary structures.

Pupils’ Activity: Pupils list protein sources and listen to the explanation of protein structure.

Learning Point: Protein sources and structure

Step 5: Properties of Proteins

Time: 5 minutes

Teaching Skill: Explanation

Teacher’s Activity: The teacher explains key properties of proteins such as denaturation and coagulation, providing simple examples like cooking an egg.

Pupils’ Activity: Pupils listen and ask questions for clarification on protein properties.

Learning Point: Protein properties

Step 6: Tests for Proteins

Time: 7 minutes

Teaching Skill: Demonstration/Explanation

Teacher’s Activity: The teacher demonstrates the Biuret test, Million’s test, and Xanthoproteic test for proteins, explaining the reagents, procedures, and expected colour changes. The teacher emphasizes the importance of safety when using concentrated acids and heat.

Pupils’ Activity: Pupils observe the demonstrations, noting the procedures, observations, and safety precautions.

Learning Point: Protein identification tests

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Describe the test for starch and state the expected observation.
2. What is the reagent used to test for reducing sugars, and what colour change indicates a positive result?
3. Mention three animal and two plant sources of protein.
4. Briefly explain what happens during protein denaturation.
5. Outline the procedure for the Biuret test for protein.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding giant molecules

Step 8: Note-Taking

Time: 4 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on tests and uses of starch, sugars, and proteins into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Board summary recording

Step 9: Conclusion

Time: 2 minutes

Teaching Skill: Consolidation

Teacher’s Activity: The teacher briefly summarizes the key tests and uses of starch, sugars, and proteins, reiterating their importance in biology and everyday life.

Pupils’ Activity: Pupils listen and ask any final questions.

Learning Point: Lesson recap

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook.

1. Differentiate between the iodine test and the Benedict’s test, including the substances they detect and the expected results.
2. Explain how heat affects the structure of a protein, using the term “denaturation.”
3. List five important functions of proteins in the human body.
4. Suggest an experiment to test for the presence of starch, reducing sugar, and protein in a sample of milk.

Lesson Keywords

• Starch – A complex carbohydrate that serves as energy storage in plants.
• Glucose – A simple sugar (monosaccharide) that is a primary energy source.
• Protein – A complex macromolecule made of amino acids, essential for structure and function.
• Benedict’s test – A chemical test for reducing sugars.
• Iodine test – A chemical test for starch.
• Biuret test – A chemical test for proteins (specifically peptide bonds).
• Million’s test – A chemical test for proteins containing tyrosine residues.
• Xanthoproteic test – A chemical test for proteins containing aromatic amino acids.
• Amino acids – The building blocks of proteins.
• Peptide bond – The chemical bond linking amino acids in a protein.
• Denaturation – The process by which proteins lose their native structure.

Differentiation

For learners who need additional support, provide simplified diagrams of protein structure and a step-by-step checklist for each chemical test. Faster learners can research specific examples of enzymes or hormones and their protein nature, or investigate other tests for carbohydrates and proteins.

Suggested Lesson Videos

For further understanding, students can search YouTube for:

• “Tests for Starch Sugars Proteins SS3 Chemistry”
• “Biuret test for protein explanation”
• “Benedict’s test for reducing sugars”

Teacher Guide for Using This Lesson Plan

Before the lesson, ensure all instructional materials, especially the chemical reagents (iodine, Benedict’s, Biuret, Million’s, HNO₃), starch solution, glucose solution, and protein samples, are readily available and properly labelled. Set up the demonstration area with a heat source and safety goggles. Begin by reviewing students’ prior knowledge of organic molecules. Guide students through each chemical test, emphasizing the procedure, expected observations, and the underlying chemical principles. For protein structure, use simple analogies to explain the different levels. During practical demonstrations, strictly enforce safety rules, particularly when using concentrated acids and heating substances. Allow students to observe closely and ask questions. Encourage active participation in listing sources and uses. The Board Summary should be copied after the main teaching points have been covered and understood, typically in Step 8. Check for understanding throughout the lesson by asking targeted questions and observing student engagement. Provide extra guidance for students struggling with the concepts or practical steps, and challenge advanced learners with more complex questions or research tasks.