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2.1.12 · Biological Molecules

Biological Molecule Tests — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

2.1.1 Biological Molecule Tests — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the Benedict's, iodine, emulsion and biuret tests and state positive results.Purpose — quick laboratory tests show whether a sample contains carbohydrates, lipids or proteins.Qu…

  • Qualitative
  • Semi-quantitative
  • Negative result
  • Observation tip
  • Benedict's reagent
  • Iodine solution

Part 1 of 6Features of food tests

Features of food tests
Purpose
  • Purpose quick laboratory tests show whether a sample contains carbohydrates, lipids or proteins.
Qualitative
  • Qualitative they show presence but not an exact amount.

Part 2 of 6Benedict's test for reducing sugars

Benedict's test for reducing sugars
Add Benedict's reagent (blue, copper(II) sulfate) to the sample in a test tube.
Heat in a water bath of boiling water for a few minutes.
Observe a coloured precipitate if a reducing sugar is present.
Positive result
  • Positive result a colour change from blue to green, yellow, orange or brown/brick-red.
Chemistry
  • Chemistry copper(II) sulfate is reduced to insoluble copper(I) oxide.
Semi-quantitative
  • Semi-quantitative the colour indicates the concentration of reducing sugar.

Part 3 of 6Iodine test for starch

Method
  • Method add a few drops of orange-brown iodine in potassium iodide solution to the sample.
Reagent
  • Reagent iodine is dissolved in potassium iodide because it is insoluble in water.
Positive result
  • Positive result iodide ions interact with the centre of starch molecules to give a blue-black colour.
Use
  • Use it shows that starch has been digested when the blue-black colour disappears.

Part 4 of 6Emulsion test for lipids

Method
  • Method add ethanol to the sample, shake, then pour the mixture into water.
Reason
  • Reason lipids are non-polar and dissolve in ethanol but not in water.
Positive result
  • Positive result a milky white emulsion; more lipid gives a clearer milky colour.
Negative result
  • Negative result the solution stays clear.

Part 5 of 6Biuret test for proteins

Method
  • Method make the sample alkaline with sodium or potassium hydroxide, then add a few drops of copper(II) sulfate.
Positive result
  • Positive result blue turns lilac or purple.
Limit
  • Limit at least two peptide bonds are needed, so amino acids and dipeptides give a negative result.
Observation tip
  • Observation tip hold the tubes against a white tile because the change can be subtle.

Part 6 of 6More from the specification

1Benedict's Test for Reducing Sugars
  • Heating an unknown solution with Benedict's reagent causes reducing sugars like glucose to reduce copper(II) ions to copper(I) oxide,
  • producing a color change from blue to green, yellow, orange,
  • or brick-red.
2Testing for Non-Reducing Sugars
  • Non-reducing disaccharides like sucrose must first be hydrolysed by boiling with dilute hydrochloric acid, neutralised with sodium hydrogencarbonate,
  • and then re-tested with Benedict's reagent to produce a positive result.
3Serial Dilutions for Calibration Standards
  • A serial dilution is carried out by systematically diluting a stock glucose solution to prepare a known range of standard concentrations for testing.
4Colorimetric Determination of Unknown Concentrations
  • Passing light through reacted Benedict's solutions in a colorimeter provides absorbance or transmission values that can be plotted against standard dilutions to quantify unknown sugar concentrations.
5Benedict's Test for Reducing Sugars
  • Heating an unknown solution with Benedict's reagent causes reducing sugars like glucose to reduce copper(II) ions to copper(I) oxide,
  • producing a color change from blue to green, yellow, orange,
  • or brick-red.
6Testing for Non-Reducing Sugars
  • Non-reducing disaccharides like sucrose must first be hydrolysed by boiling with dilute hydrochloric acid, neutralised with sodium hydrogencarbonate,
  • and then re-tested with Benedict's reagent to produce a positive result.
7Serial Dilutions for Calibration Standards
  • A serial dilution is carried out by systematically diluting a stock glucose solution to prepare a known range of standard concentrations for testing.
8Colorimetric Determination of Unknown Concentrations
  • Passing light through reacted Benedict's solutions in a colorimeter provides absorbance or transmission values that can be plotted against standard dilutions to quantify unknown sugar concentrations.
9Glucose
  • A simple sugar; the unit that starch, glycogen and cellulose are built from.
10Non-reducing sugar
  • A sugar that cannot reduce Benedict's reagent until hydrolysed.
11Serial dilution
  • A stepwise series of dilutions that each use the same dilution factor.
12Stock solution
  • The starting concentrated solution used for dilutions.

Key terms

TermMeaning
Qualitative testA test showing presence but not amount.
Benedict's reagentAn alkaline copper(II) sulfate solution.
Iodine solutionIodine in potassium iodide used to test for starch.
EmulsionA suspension of tiny lipid droplets in water.
Biuret reagentAlkali with copper(II) sulfate used to test for protein.
PrecipitateAn insoluble solid formed in a solution.

Avoid these mistakes

1
Writing only the colour change — also give the reagent, the treatment such as heating, and the positive result.
2
Heating the iodine or biuret tests — they work at room temperature.
3
Saying the Benedict's test gives an exact concentration — it is semi-quantitative.
4
Expecting amino acids to give a positive biuret test — peptide bonds are needed.

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