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2.1.32 · Biological Molecules

Testing for Non-Reducing Sugars — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 6 parts Lesson 5 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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2.1.3 Testing for Non-Reducing Sugars — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Test for non-reducing sugars by acid hydrolysis followed by Benedict's test.Classification — based on the ability to donate electrons; a reducing sugar is oxidised as it does so.Re…

  • Reducing products
  • First test
  • Semi-quantitative
  • Neutralisation
  • Glycosidic bond
  • Reducing sugars — galactose, glucose, fructose and maltose

Part 1 of 4Reducing and non-reducing sugars

Reducing and non-reducing sugars
Classification
  • Classification based on the ability to donate electrons; a reducing sugar is oxidised as it does so.
Reducing sugars
  • Reducing sugars galactose, glucose, fructose and maltose.
Non-reducing sugar
  • Non-reducing sugar sucrose is the only one required.

Part 2 of 4Method

1
Do a Benedict's test; a negative result shows no reducing sugar is present.
2
Add dilute hydrochloric acid to a fresh sample and heat in a boiled water bath.
3
Neutralise with sodium hydrogencarbonate, using red litmus paper to check.
4
Add a little more sodium hydrogencarbonate so the solution is slightly alkaline.
5
Add Benedict's reagent and heat in a boiled water bath.
6
An orange-red precipitate shows a non-reducing sugar was present.

Part 3 of 4Explanation

1Acid hydrolysis
  • Acid hydrolysis the acid hydrolyses the glycosidic bonds in carbohydrate molecules.
2Reducing products
  • Reducing products the monosaccharides formed have an aldehyde or ketone group that donates electrons to copper(II) sulfate.
3Example
  • Example sucrose gives glucose and fructose.
4First test
  • First test it proves that any later positive result is due to a non-reducing sugar being broken down.

Part 4 of 4More from the specification

More from the specification
Non-Reducing Sugar Hydrolysis and Benedict's Test
  • When testing biological solutions for non-reducing sugars like sucrose, the sample is first boiled with dilute hydrochloric acid to hydrolyse glycosidic bonds, neutralized with sodium hydrogencarbonate,
  • and reheated with Benedict's reagent to produce a characteristic colour change from blue to brick-red proportional to sugar concentration.
Serial Dilutions and Colorimetric Calibration
  • To determine the unknown concentration of reducing sugars or starch, a set of serial dilutions is carried out from a stock standard,
  • and light absorbance is measured using a colorimeter to plot a standard calibration curve.
Biochemical Tests for Starch and Lipids
  • Standard biological tests carried out on tissues include the iodine test for starch and the emulsion test for lipids,
    • (producing a blue-black colour change)
  • where dissolving a sample in ethanol and adding water forms a cloudy white emulsion if it contains lipids.
Non-Reducing Sugar Hydrolysis and Benedict's Test
  • When testing biological solutions for non-reducing sugars like sucrose, the sample is first boiled with dilute hydrochloric acid to hydrolyse glycosidic bonds, neutralized with sodium hydrogencarbonate,
  • and reheated with Benedict's reagent to produce a characteristic colour change from blue to brick-red proportional to sugar concentration.
Serial Dilutions and Colorimetric Calibration
  • To determine the unknown concentration of reducing sugars or starch, a set of serial dilutions is carried out from a stock standard,
  • and light absorbance is measured using a colorimeter to plot a standard calibration curve.
Biochemical Tests for Starch and Lipids
  • Standard biological tests carried out on tissues include the iodine test for starch and the emulsion test for lipids,
    • (producing a blue-black colour change)
  • where dissolving a sample in ethanol and adding water forms a cloudy white emulsion if it contains lipids.
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
Serial dilution
  • A stepwise series of dilutions that each use the same dilution factor.
Stock solution
  • The starting concentrated solution used for dilutions.
Starch
  • An insoluble polysaccharide of glucose used by plants for energy storage.
Lipids
  • Concentrated energy, insulation, cell membranes.
Semi-quantitative
  • Giving an approximate estimate of quantity.
Iodine
  • Element needed to make thyroxine.

Key terms

TermMeaning
Reducing sugarA sugar that can donate electrons to copper(II) ions.
Non-reducing sugarA sugar that cannot reduce Benedict's reagent until hydrolysed.
HydrolysisBreaking glycosidic bonds with water, helped by acid.
NeutralisationAdjusting the pH to 7 with an alkali such as sodium hydrogencarbonate.
SucroseA non-reducing disaccharide of glucose and fructose.
Glycosidic bondThe bond between monosaccharides.

Avoid these mistakes

1
Skipping the first Benedict's test — the final result would be inconclusive.
2
Not neutralising the acid — Benedict's reagent needs slightly alkaline conditions.
3
Saying sucrose is a reducing sugar — it is non-reducing until hydrolysed.
4
Giving a negative first test as the final answer — the sample could still contain a non-reducing sugar.

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