This free quiz has 12 real AQA A-level Biology exam questions on proteins (24 marks, about 30 minutes). It marks itself: tick-box and table questions are marked for you, and for written answers you check your work against the real AQA mark scheme.
What the quiz covers (AQA 3.1.4.1)
the bonds in secondary, tertiary and quaternary structure
primary structure and peptide bonds
identifying amino acids from their R groups
the biuret test for protein
why a change in amino acid sequence can stop a protein binding
a calculation on the structure of haemoglobin
how a quaternary protein is formed from its monomers (5 marks)
How to use it
Write each answer in full before you open the mark scheme. Name bonds precisely, saying which groups they form between, and say tertiary structure rather than “shape”.
Where students lose marks (from AQA examiners’ reports and mark schemes)
Each point below is followed by wording that gained marks.
Amino acid structure. When drawing an amino acid, students often bonded the hydrogen from the amine or carboxyl group straight to the central carbon.
✔ Gains credit: a central carbon bonded to an H atom, an R group, an amine (NH₂) group and a carboxyl (COOH) group.
Building the primary structure. Some called the joining reaction hydrolysis. Many described a “chain” of amino acids without saying it is a sequence, or confused it with DNA and wrote about an “amino acid base sequence”.
✔ Gains credit: a condensation reaction between the amine group of one amino acid and the carboxyl group of another forms a peptide bond; the primary structure is the sequence of amino acids joined by peptide bonds.
Tertiary structure bonds. Students often listed the levels of structure without naming the bonds, or said something “changes the shape” without saying how.
✔ Gains credit: a different sequence of amino acids means ionic, hydrogen and disulfide bonds between R groups form in different places, giving a different tertiary structure.
Binding sites, not active sites. Many wrote about an “active site” or “substrate” for proteins such as hormones, receptors and toxins, which lost the mark. “3D shape” wasn't credited instead of tertiary structure.
✔ Gains credit: the protein has a specific tertiary structure that is complementary to the molecule it binds to, at a binding site.
What quaternary means. A sizeable number of students think quaternary structure means four polypeptides, or wrote about several “tertiary structures” joining.
✔ Gains credit: a quaternary structure contains more than one polypeptide chain, held together by bonds between the polypeptides, such as disulfide bridges.
Biuret test. Saying that biuret solution is heated cost the mark, and misspellings such as “burette” were rejected.
✔ Gains credit: add biuret solution (sodium hydroxide and copper sulfate); a purple (lilac, violet or mauve) colour shows protein is present.
Keep practising
The other AQA 3.1 self-marking quizzes: Carbohydrates · Lipids · Enzymes · Nucleic acids · ATP, water and ions
The AQA topic index
weekly AQA group classes: Year 12 on Tuesdays at 5.15pm, Year 13 on Wednesdays or Thursdays at 6.30pm
Questions © AQA, from past A-level and AS Biology papers, with their mark schemes and examiners’ reports.