Proteins are made from chains of amino acids. A student tests two food samples for the presence of protein using the Biuret test. Sample A gives a colour change from blue to violet. Sample B remains blue. Explain what the results of the Biuret test tell the student about the two food samples, and describe the role that amino acids play in forming a functional enzyme.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Biuret test detects the presence of peptide bonds. Amino acids join together and fold to form protein molecules, including enzymes.
Model answer (5 marks)
Sample A contains protein – the violet colour shows peptide bonds are present.
Sample B does not contain protein – it remains blue, indicating no peptide bonds.
Amino acids join by peptide bonds to form a polypeptide chain.
The chain folds into a specific three‑dimensional shape, creating an active site.
The active site has a complementary shape to its substrate, allowing the enzyme to form an enzyme‑substrate complex and catalyse the reaction.
Sample B does not contain protein – it remains blue, indicating no peptide bonds.
Amino acids join by peptide bonds to form a polypeptide chain.
The chain folds into a specific three‑dimensional shape, creating an active site.
The active site has a complementary shape to its substrate, allowing the enzyme to form an enzyme‑substrate complex and catalyse the reaction.
Examiner tips
- Use the colour change to state presence/absence of protein. Mention peptide bonds as the basis of the test. Show the stepwise role of amino acids: bonding → folding → active site. Link the active site to substrate specificity and catalysis.
Common mistakes
- Confusing colour change direction (blue to violet vs violet to blue). Forgetting to mention peptide bonds as the test’s basis. Omitting the folding step or the concept of the active site.
Mark scheme (5 marks)
- Sample A contains protein / peptide bonds are present in Sample A
- Sample B does not contain protein / no peptide bonds present in Sample B
- Amino acids join / bond together to form chains that make up a protein / enzyme
- The chain of amino acids folds to form a specific three-dimensional shape / active site
- The active site has a complementary shape to the specific substrate, allowing the enzyme to function / form an enzyme-substrate complex
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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