Lesson 3.1.4.1.3

3.1.4.1.3 Bonds in protein structure and the biuret test: AQA Biology, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.4.1.3, Bonds in protein structure and the biuret test: 20 multiple choice questions for the AQA Biology (7402), Unit 1: Biological molecules, written with Revision Ninja.

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The 20 questions

  1. What is the reagent used in the biuret test for proteins?

    • Alkaline copper(II) sulfate solution, made with sodium hydroxide and dilute copper(II) sulfate
    • Iodine dissolved in potassium iodide solution, which turns blue-black in the presence of starch molecules
    • Ethanol followed by water, which forms a cloudy white emulsion when lipid molecules are present
    • Benedict's solution that is heated in a boiling water bath to reveal the presence of reducing sugars

  2. What is a positive result in the biuret test?

    • A brick-red precipitate
    • A purple or violet colour
    • A blue-black colour
    • A cloudy white emulsion

  3. What does the biuret test detect?

    • Ester bonds
    • Phosphodiester bonds
    • Peptide bonds
    • Glycosidic bonds

  4. What colour is a negative result in the biuret test?

    • Blue
    • Brick-red
    • Blue-black
    • Purple

  5. Which bond is the weakest of those that stabilise protein tertiary structure?

    • Hydrogen bond
    • Peptide bond
    • Disulfide bridge
    • Ionic bond

  6. Between which groups do ionic bonds form in a protein?

    • Two glucose units that lie on the same polypeptide chain and are linked by a glycosidic bond
    • Two peptide bonds that share a single carbon atom in the backbone of the polypeptide chain
    • Oppositely charged R groups, such as an amino group and a carboxyl group
    • Two identical amino groups that lie on different polypeptide chains within the same protein molecule

  7. Which amino acid contributes the sulfur atoms that form disulfide bridges?

    • Alanine
    • Glycine
    • Cysteine
    • Serine

  8. A solution of a protein gives a purple colour with the biuret reagent. What can be concluded?

    • Starch is present, because purple is the iodine colour for starch
    • Glucose is present in high concentration in the solution
    • Peptide bonds are present, so protein is present in the solution
    • Lipid is present, because the biuret test detects triglycerides

  9. A dipeptide solution is tested with the biuret reagent. What is the likely result, and why?

    • Blue, because a dipeptide has only one peptide bond and the test needs at least two
    • Purple, because a dipeptide has one peptide bond that reacts strongly
    • Brick-red, because the dipeptide contains a reducing sugar
    • Blue-black, because the dipeptide contains starch

  10. A glucose solution is tested with the biuret reagent and stays blue. Why?

    • Glucose is a non-reducing sugar, which stops the copper from reacting
    • Glucose contains a high proportion of starch, which blocks the biuret reaction
    • Glucose contains no peptide bonds, so the biuret test is negative
    • The biuret reagent only detects fats, so glucose is never detected

  11. A colorimeter reading of biuret-treated protein standards is 0.8 at 2 mg/cm3 and 1.6 at 4 mg/cm3. An unknown gives 1.2. What is the protein concentration?

    • 6 mg/cm3
    • 2.4 mg/cm3
    • 2 mg/cm3
    • 3 mg/cm3

  12. A pentapeptide (five amino acids) is tested with the biuret reagent. What is the expected result?

    • Purple, because it contains four peptide bonds, which is more than the minimum needed
    • Brick-red, because the pentapeptide is a reducing sugar that reduces the copper ions present
    • Blue, because a pentapeptide of this kind contains only one peptide bond available to react
    • Blue-black, because the pentapeptide contains starch granules that react with the iodine

  13. Which bond type is involved in hydrophobic interactions within protein tertiary structure?

    • Ionic bonds formed between two oppositely charged sugar units attached to the protein surface
    • Glycosidic bonds formed between two amino acid side chains that are close together in space
    • Non-polar R groups clustering away from water, held by weak interactions
    • Covalent bonds formed between two phosphate groups that lie on the surface of the protein

  14. A student says the biuret test proves that a sample contains only protein. Which evaluation is correct?

    • Correct, because the biuret test is specific to protein and does not react with any other substance at all with no exceptions
    • Incorrect, because the test shows peptide bonds are present but cannot show the sample is free of other substances
    • Incorrect, because the biuret test cannot detect protein at all, and it only detects simple sugars
    • Correct, because a purple colour rules out any carbohydrate being present anywhere in the sample

  15. Why do free amino acids give a negative biuret result even though they contain amine and carboxyl groups?

    • They are hydrolysed by the biuret reagent, which destroys the copper complex as it forms
    • They contain no amine groups, so the biuret reagent has nothing to react with in the solution
    • They have no peptide bonds, because the amine and carboxyl groups have not been linked by condensation
    • They are too small to bind copper ions, so the characteristic purple colour does not develop

  16. Why does the biuret test require alkaline conditions?

    • The alkaline conditions ensure that the protein is fully hydrolysed into free amino acids before the test
    • The alkaline conditions allow copper(II) ions to form a coloured complex with the nitrogen atoms of peptide bonds
    • The alkaline conditions break the peptide bonds in the protein, releasing free nitrogen atoms into solution with no exceptions
    • The alkaline conditions convert the copper into a gas that then reacts with the protein in the sample

  17. Heating a protein breaks its weak bonds but not its peptide bonds. Explain why the primary structure is unaffected.

    • Peptide bonds are glycosidic in nature, so heat cannot break them under any of the normal conditions
    • Peptide bonds are hydrogen bonds, which are strong enough to resist heating without being broken at all
    • Peptide bonds are ionic bonds, and heat only affects covalent bonds, so the peptide backbone is left intact
    • Peptide bonds are strong covalent bonds, whereas heat disrupts the weaker hydrogen and ionic interactions

  18. A protein sample is diluted so the biuret reaction gives a pale lilac colour. What does this indicate?

    • A higher concentration of protein than a purple sample
    • A lower concentration of peptide bonds than a strong purple sample
    • That the biuret reagent has expired and cannot react
    • The presence of starch in the sample instead of protein

  19. Explain why ionic bonds in a protein can be broken by a change in pH but peptide bonds cannot be broken in the same way.

    • Peptide bonds are charged and so are neutralised by a change in pH, forming free amino acids in solution
    • Ionic bonds are covalent and so are broken by pH, while peptide bonds are weak ionic bonds in the chain
    • Ionic bonds depend on the charge of R groups, which pH alters, while peptide bonds are strong covalent links needing hydrolysis
    • Ionic bonds involve phosphate groups, which pH converts into peptide bonds that strengthen the protein

  20. A student tests a sample with biuret reagent and gets a purple colour. The student concludes that the protein has a quaternary structure. Which evaluation is correct?

    • Unsupported, because the biuret test detects peptide bonds and cannot reveal whether the protein has quaternary structure
    • Unsupported, because the biuret test cannot detect any peptide bonds at all, so it gives no result
    • Supported, because the biuret test measures the number of protein subunits in the sample directly
    • Supported, because a purple colour appears only in proteins that have a quaternary structure of subunits

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