A Level Biology The Biochemical Tests | The ‘food’ Tests

The Biochemical Food Tests identify 3 of the 4 main biologically important chemical compounds (Carbohydrates, Proteins, Lipids). For the Benedict's test, iodine test and biuret test, take a small amount of the substance, mix and shake it in water in a test tube. If the sample is a piece of food, then grind it with some water in a pestle and mortar to break up the cells and release the cell contents. Many of these compounds are insoluble, but the tests work just as well on a fine suspension. For the emulsion test remember that lipids do not dissolve in water, but do dissolve in ethanol. This characteristic is used in the emulsion test . So, do not start by dissolving the sample in water, but instead gently 'shake' some of the test sample with ethanol. If there are lipids dissolved in the ethanol, they will precipitate in the water, forming a cloudy white emulsion. The emulsion test can be improved by adding the dye Sudan III, which stains lipids red.

To test for the presence of carbohydrates you'll carry out either the Benedict's test for reducing or non-reducing sugars or the iodine test if you are investigating a test sample for the presence of starch.

To investigate if a sample contains lipids you'll carry out the emulsion test and to test for the presence of Protein you'll carry out the Biuret test.

All of these tests are explained in detail in each video below, and each of the biochemical tests has an accompanying knowledge check PDF with questions to test your understanding of these commonly examined procedures. 

A Level Biology: The Benedict's Test for

Reducing and Non-Reducing sugars

 

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson "The Benedict's Test for Reducing and Non-Reducing sugarswe'll begin by finding out what makes a Reducing sugar a Reducing sugar. Next Non-Reducing sugars will be discussed before moving on to the Benedict's Test and How Benedict's Reagent works. You'll have to be able to describe how to carry out the Benedict's test for Reducing sugars and explain the expected results - so thats covered before moving on to describe how to carry out the Benedict's Test for Non-Reducing Sugars.

 

When you’re confident you know how to describe the Benedict's test procedure for both Reducing and Non-Reducing sugars and explain the expected results it's time for you to complete the accompanying “Benedict's Test for Reducing and Non-Reducing sugars” knowledge check questions. You’ll be able to check your answers with mine written in the back of the work book. No vague mark schemes here though, you’ll see exactly how you need be write your answers so you gain maximum marks in the exams!

A-Level biology Benedict's Test for Reducing and Non-Reducing sugars Revision Notes with Knowledge Check Questions and Answers pdf
A-Level biology Benedict's Test for Reducing and Non-Reducing sugars Revision Notes Poster A3 PDF.

A Level Biology: The Benedict's Test for Reducing and Non-Reducing sugars

Sugars can be classified as either Reducing or Non-Reducing. Monosaccharides and some disaccharides are reducing sugars – A sugar with a “free” Aldehyde [CO] or Ketone group [CHO]. These functional groups allow the sugar to donate electrons – making that sugar the “reductant” i.e. the “Reducing Sugar”. Thus, it is the reduction of Copper II Sulphate that gives the colour change when testing for the presence of reducing sugars in a test sample.

 

The reducing sugars you must know are:

  • Glucose

  • Fructose

  • Lactose

 

The only non-reducing sugar you must know is Sucrose.

 

You could be asked to Describe, Explain, Analyse and Evaluate the procedure testing for Reducing and / or Non-Reducing Sugars.

 

The Benedict’s test for reducing sugars: -

  1. Heat the test sample with Benedict’s Reagent.

  2. Observe the colour change.

  3. A brick red precipitate indicates the presence of a reducing sugar.

 

The Benedict’s test for non-reducing sugars: -

 

  1. Heat the test sample with dilute hydrochloric acid.

  2. Neutralise the test sample by adding sodium hydrocarbonate.

  3. Heat the test sample with Benedict’s Reagent.

  4. Observe the colour change.

  5. A brick red precipitate indicates the presence of a reducing sugar.

A Level Biology: The Iodine test for Starch

 

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson "The Iodine test for starch" You will learn the procedure for testing foods for the presence of starch. You must be able to describe the Iodine test and explain the results. A colour change (Purple [blue-black] colour) indicates a positive result, staining with iodine in potassium iodide solution... Remember starch is a compact coiled polymer of a-glucose.

When you’re confident you can describe the Iodine test procedure and explain the results it's time for you to complete the accompanying Iodine test for starch” knowledge check questions. You’ll be able to check your answers with mine written in the back of the work book and you can see exactly how you should write your answers in a way that gains maximum marks in the exams!

A-Level biology the iodine test for starch Revision Notes with Knowledge Check Questions and Answers pdf
A-Level biology the iodine test for starch Revision Notes Poster A3 PDF.

A-Level Biology "The Iodine Test for Starch"

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 a-1,4 glycosidic bonds and a-1,6 glycosidic bonds) making it more readily hydrolysed “releasing those a-glucose molecules”. Whereas Amylose is tightly coiled and compact resulting in a much slower release of the a-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.

A Level Biology: Lipids - The Emulsion Test

 

Check Where this Lesson fits into your Exam Specification!

In this A-Level Biology Lesson “Lipids: The Emulsion Test for the presence of Lipids” following the learning outcomes we’ll go through each of the steps involved in this common food test. You’ll need to be able to describe how and why each step in the emulsion test is done and how to interpret the results

 

When you’re happy you can write out a suitable method for the emulsion test and know what to expect you’re ready to download the knowledge check PDF and test your knowledge. When you’ve answered the questions, check your answers with mine which are written out in full in the back of the work book. Here you can compare your answers to mine and you’ll see exactly how you should write answers in a way that gains maximum marks in the exams!

A-Level biology Lipids: The Emulsion test for lipids Notes with Knowledge Check Questions and Answers pdf
A-Level biology Lipids: The Emulsion test for lipids Revision Notes Poster A3 PDF.

A-Level Biology "Lipids: The Emulsion Test"

Lipids (fats and oils) are insoluble in water, but they do readily dissolve in ethanol. This property of dissolving fats in alcohol is the basis of the emulsion test.

 

If your testing a food sample for the presence of lipid follow these steps.

 

  1. Crush / grind the test sample and add to a test tube with ethanol

  2. Gently mix the sample with the ethanol, ensure all lipid is dissolved in the ethanol.

  3. Decant the dissolved liquid into another test tube containing water.

  4. Observe the results.

 

A positive result will show a milky / white emulsion. 

 

In preparation for exam style questions on this topic, ensure you know why each step is performed. Understand also that a Bunsen burn is NOT needed for this procedure.

A Level Biology: Proteins - The Biuret Test for Proteins

 

Check Where this Lesson fits into your Exam Specification!

In this A-Level Biology Lesson “A-Level biology The Biuret Test for Proteins" You'll learn about how the biuret test for proteins is carried out (and that the biuret reagent "identifies" the peptide bonds joining polypeptides together - a reminder that long chain polypeptides have many peptide bonds! Then we'll go through each of the 4 steps of biuret test. Once we have those all important steps boxed off we'll go through how to describe each step when it comes to those exams.

 

Once again like all the other food tests (Benedict's test, Iodine test and Emulsion test) being able to describe this test (and know the expected results) is a frequently encountered question in A-level biology. So, when you're confident you know all the steps involved and how and why this test is carried out, you’ll be ready to download the knowledge check PDF and test your knowledge. When you’ve answered all the questions, compare your answers to the ones I’ve written - you’ll see exactly how you should write answers in a way that gains maximum marks in the exams.

A Level Biology: Proteins - Biuret test for Proteins Revision Notes with Knowledge Check Questions and Answers pdf
A-Level biology: Proteins - Biuret test for Proteins Revision Notes Poster A3 PDF

Check Your Exam Specification

 

AQA A Level Biology Specification Reference: - 3.1 Biological molecules: Students could use, and interpret the results of qualitative tests - The Benedict's Test for reducing and non-reducing sugars. Students could use, and interpret the results of, qualitative tests - iodine/potassium iodide for starch. Lipids. Students could use, and interpret the results of, the emulsion test for lipids. The biuret test for proteins. Students could use, and interpret the results of, a biuret test for proteins.

 

CIE A Level Biology Specification Reference: - 2 Biological Molecules: Testing for biological molecules. a) carry out tests… The Benedict's Test for reducing and non-reducing sugars. The iodine in potassium iodide solution test for starch. The emulsion test for lipids. The biuret test for proteins to identify the contents of solutions.

 

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

 

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

 

OCR A Level Biology (Biology A) Specification Reference: - 2.1.2 Biological molecules (q) how to carry out and interpret the results of The Benedict's Test for reducing and non-reducing sugars. how to carry out and interpret the results of the iodine test for starch. How to carry out and interpret the emulsion test for lipids. How to carry out and interpret the results of the biuret test for proteins.

 

OCR A Level Biology (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange. Use of qualitative reagents to identify biological molecules. The Benedict's Test for reducing and non-reducing sugars. Test for the identification and measurement of starch. To include the qualitative test for starch using iodine. How to carry out and interpret the emulsion test for lipids. The methodology and interpretation of the results of the Biuret test.

 

★ WJEC A Level Biology Specification Reference: - Core Concepts 1. Chemical elements are joined together to form biological compounds - SPECIFIED PRACTICAL WORK - Food tests to include: The Benedict's test for reducing and non-reducing sugars; Iodine-potassium iodide test for starch; the emulsion test for fats and oils; the biuret test for protein.

★ BTEC Level 3 Nationals in Applied Science. Unit 10: Biological Molecules and Metabolic Pathways: A: Understand the structure and function of biological molecules and their importance in maintaining biochemical processes.  A2 Carbohydrates: Structure and features: contain carbon (C), hydrogen and oxygen atoms. Monosaccharides, e.g. α and β glucose, galactose, fructose, ribose and deoxyribose. Disaccharides, e.g. lactose, maltose and sucrose. Polysaccharides, e.g. amylose, amylopectin, cellulose. Use of iodine and Benedict's solution as tests for presence of carbohydrates.

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