A Level Biology Lesson 6:

The Iodine Test for Starch.

Task 1. Check Where this Lesson fits into your Exam Specification!

Task 2. Watch the Revision Notes Lesson in Full.

00:00 Intro Screen / Learning Outcomes

00:19 Introduction

01:05 To test foods for the presence of starch...

01:39 A colour change indicates a positive result

01:50 staining with iodine in potassium iodide solution...

02:05 Remember starch is a compact coiled polymer of alpha-glucose.

Task 3. 

Download, Print out & complete the accompanying Interactive Worksheet.

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A-Level Biology "The Iodine Test for Starch" Lesson 6. Summary.

Starch is a complex polysaccharide composed of Amylose and Amylopectin. The combination of these two polysaccharides is the reason starchy foods are a great source of energy.

 

Remember, Amylopectin is branched (having both α-1,4 glycosidic bonds and α-1,6  glycosidic bonds) making is more readily hydrolysed – releasing those α-glucose molecules. Whereas Amylose is tightly coiled and compact resulting in a much slower release of the α-glucose molecules.

 

The Iodine Test.

To test for the presence of Starch in a sample…

  1. Add 10 drops of Iodine in Potassium Iodide Solution to the test sample.

  2. Observe the results: Positive results will show a Purple [blue-black] colour.

Check Your Spec!

★ AQA Specification Reference: - 3.1 Biological molecules: 3.1.2 Carbohydrates. Students could use, and interpret the results of, qualitative tests - iodine/potassium iodide for starch.

 

★ CIE Specification Reference: - 2 Biological Molecules: Testing for biological molecules. a) carry out tests… the iodine in potassium iodide solution test for starch.

 

★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Appendix 5c: Use of apparatus and Techniques. 6. use qualitative reagents to identify biological molecules

 

★ Edexcel (Biology B) Specification Reference: - Appendix 5c: Use of apparatus and Techniques. 6. use qualitative reagents to identify biological molecules

 

★ OCR (Biology A) Specification Reference: - 2.1.2 Biological molecules (q) how to carry out and interpret the results of the iodine test for starch.

 

★ OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange. Use of qualitative reagents to identify biological molecules. (ii) a test for the identification and measurement of starch. To include the qualitative test for starch using iodine.

 

★ WJEC Specification Reference: - Core Concepts 1. Chemical elements are joined together to form biological compounds - SPECIFIED PRACTICAL WORK - Food tests to include: iodine-potassium iodide test for starch.

 

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