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DSE Biology | Enzymes and Factors Practice Paper 3 (Graphs and Comparing Inhibition)
DSE Biology | Enzymes and Factors Practice Paper 3 (Graphs and Comparing Inhibition)
One Michaelis-Menten equation used to compare competitive and non-competitive inhibition, then to recover Vmax and Km from data.
Attempt first, mark afterwards Allow about thirty-five minutes; write v = Vmax x S / (Km + S) first, and ask whether Vmax has changed before naming the inhibition type.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 items |
| Suggested time | About 35 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: DSE Biology | Enzymes and Affecting Factors |
| Level | S4 (Secondary 4) Biology |
02 Questions (10 items)
- FoundationWhat is the main difference between induced fit and the lock-and-key model?
- Foundation0.4 mg of enzyme makes 12 units per minute and 0.8 mg makes 24. What does 0.2 mg give, and what relationship is this?
- Foundation72 micromol of product forms in 90 seconds. What is the average rate in micromol per second, and how much in 5 minutes at that rate?
- FoundationAn enzyme lowers activation energy from 58 to 22 kJ/mol. Give the fall in kJ/mol and as a percentage.
- IntermediateA competitive inhibitor raises Km from 6 to 24 while Vmax stays at 90. At substrate 15, what is the rate before and after, and the percentage fall?
- IntermediateA non-competitive inhibitor cuts Vmax from 90 to 45 with Km still 6. At substrate 15, what is the rate before and after, and how does it differ from competitive inhibition?
- IntermediateThe rate is 40 at substrate 10 and 60 at substrate 30. Find Vmax and Km.
- ChallengeAn enzyme has a half-life of 5 minutes at 60°C. What activity remains after 15 minutes?
- ChallengeEnzyme A has a specific activity of 2.5 micromol/min/mg and B has 4.0. How many times more efficient is B, and how much B is needed for 60 micromol/min?
- Error redoError drill: a student writes that non-competitive inhibition can be overcome simply by adding more substrate. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | The active site adjusts its shape slightly on binding, then catalyses | Lock-and-key assumes a rigid shape; induced fit says the site moulds itself around the substrate, which explains a slightly broader specificity. |
| 2 | 6 units; directly proportional to enzyme amount | 12 over 0.4 = 30 units per mg, so 0.2 mg gives 6; with saturating substrate the rate scales linearly with enzyme. |
| 3 | 0.8 micromol per second; 240 micromol in 300 seconds | 72 over 90 = 0.8, and 0.8 times 300 = 240; watch the unit conversion rather than plugging minutes into a per-second rate. |
| 4 | 36.0 kJ/mol, which is 62.1% | 58 minus 22 = 36.0, and 36.0 over 58 = 62.1%. |
| 5 | 64.3 down to 34.6, a fall of 46.2% | 90×15 over 21 = 64.3 and 90×15 over 39 = 34.6, a fall of 46.2% that extra substrate can partly overcome. |
| 6 | 64.3 to 32.1 (a 50.0% fall); non-competitive inhibition lowers Vmax, so extra substrate does not help | 45×15 over 21 = 32.1, exactly half; only half the enzyme molecules still work, so the saturated ceiling halves. |
| 7 | Vmax = 80.0, Km = 10.0 | Dividing the two Michaelis-Menten equations gives Km = 10.0, and substituting back gives Vmax = 80.0. |
| 8 | 12.5% | 15 over 5 = 3 half-lives, and 0.5 cubed is 0.125, so 12.5% remains. |
| 9 | B is 1.6 times as efficient; 15.0 mg of B | 4.0 over 2.5 = 1.6, and 60 over 4.0 = 15.0 mg, where A would need 24 mg. |
| 10 | No; non-competitive inhibition lowers Vmax, so extra substrate only approaches a lower ceiling | The inhibitor binds away from the active site and alters the enzyme conformation; Km may be unchanged but fewer enzyme molecules work, so the saturated rate must fall. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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