AQA A-Level Biology Paper 1, June 2022: Question 5

✅ Correct Answer

More than one polypeptide (chain).

💡 Key Knowledge

• Proteins can consist of a single polypeptide (up to tertiary structure) or multiple polypeptide chains (quaternary).
• May include associated non-protein groups (prosthetic groups), though definition focuses on multiple chains.

❌ Common Errors

• Mentioning specific bonds like "disulfide bridges" or "hydrogen bonds" without stating multiple polypeptide chains.
• Confusing tertiary structure folding with multi-subunit assembly.

✅ Correct Answer

1. (Both) active sites have similar/identical tertiary structures (or identical amino acid sequences at the active site);
2. (So) form enzyme-substrate complexes with the same substrate.

🧠 Exam Technique

This is a 2-mark point requiring cause-and-effect. Mark point 1 explains why they bind the same substrate (active site structure), and Mark point 2 links this to function (forming enzyme-substrate complexes).

❌ Common Errors

Students often state that "the whole protein has the same shape." Examiners penalize this because the overall amino acid sequences are different—only the active site region needs to be conserved/complementary.

✅ Correct Answer

1. Same volume of (each) buffer / pH solution;
2. Same concentration / mass of substrate (at start);
3. Same concentration / mass of denatured enzyme (OR buffer and substrate and water / no enzyme);

💡 Key Knowledge

A control in an enzyme experiment proves that the decrease in substrate concentration is due to the active enzyme, not spontaneous breakdown or effects of other tube components like the buffer.

❌ Common Errors

Forgetting to account for the presence of protein mass in the active enzyme tubes. Using a denatured enzyme ensures the total protein concentration/viscosity remains consistent with the test tubes.

✅ Correct Answer (Choose any 3)

• Both / P and Q are active at pH 8.4;
• P is (equally/most) active at both pHs OR Q is less active than P at both pHs OR P not affected by pH change / Q is affected by pH change;
• Q is denatured / not active at pH 7.5 OR Q is less active than control at pH 7.5;
• Reaction occurs without enzyme(s) / reaction occurs in the control;
• All (reactions) reach same end point OR substrate is not used up.

🧠 Exam Technique

Do not "mark across all three conclusions" in a way that contradicts itself. Read the graphs carefully comparing the dashed line (P), grey line (Q), and solid line (Control) across both pH 8.4 and pH 7.5 grids.

❌ Common Errors

Using vague phrasing like "enzyme works better." Be precise: reference specific pH values, specific enzymes (P or Q), or comparison against the control line.