Why state the time interval?

Initial and mean rates differ (12 against 9 cm³/min here), so the interval must be named to score.

Why divide by 4, not 5?

Five readings create only four intervals; dividing by 5 underestimates the fall per reuse.

IB Biology: Enzymes and Reaction Kinetics (10 Challenge Items)

2026-11-08

Ten challenge items on experimental design and controlled variables, unit conversion, why Q10 fails near the optimum, initial versus mean rate, and the trend in immobilised enzyme activity.

Focus　State the definition and equation before substituting, and name the time interval for any rate.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	30
Difficulty mix	Foundation 4 · Intermediate 2 · Challenge 3 · Error redo 1
Chapter	Chapter: IB Biology: Enzymes and Reaction Kinetics
Level	IB Biology (HL)

02　Questions (10 items)

FoundationWhy is a buffer used in enzyme activity experiments?
FoundationWhat is the purpose of a control in an experiment?
FoundationA reaction rate is 0.5 cm³/s. Convert it to cm³/min.
FoundationWhy repeat enzyme experiments and average the results?
IntermediateQ10 is 1.8 and the rate at 25°C is 8 cm³/min. Predict the rate at 35°C.
IntermediateFrom 40°C to 50°C the rate rises only 1.2 times instead of the usual Q10 of about 2. Explain.
ChallengeActivity at the optimum is set at 100 and measures 45 at pH 6. Find the relative activity and the decrease.
ChallengeIn minute 1 the product rises from 0 to 12 cm³, and by another 6 cm³ in minute 2. Find the initial rate, the mean rate over two minutes, and why the rate falls.
ChallengeAn immobilised enzyme is reused 5 times with activities 96, 88, 80, 72 and 64 (%). Find the mean fall per reuse.
Error redoA student writes that above the optimum temperature only the activity falls and the structure is unchanged. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	The buffer holds pH constant so that substrate or product cannot shift pH and alter enzyme activity, keeping pH as a controlled variable.	If pH is uncontrolled, temperature and pH change together and the result cannot be attributed to one factor.
2	A control lacks the independent variable (no enzyme, or no inhibitor), showing that the observed change really comes from that variable.	Without a control, any change could be explained by temperature, time or impurities.
3	0.5 × 60 = 30 cm³/min.	Multiplying by 60 converts per second to per minute; mismatched units are the commonest error.
4	To reduce random error (reading, bubble size, pipetting) and make the results more reliable, so real differences can be judged.	Replicates reduce random error only; systematic error such as mis-calibration needs correction instead.
5	8 × 1.8 = 14.4 cm³/min, provided the enzyme is not denatured at that temperature.	First check the temperature is still below the optimum, or the Q10 prediction does not hold.
6	40°C is near or above the optimum, so some enzyme is already denatured; fewer active sites remain and the gain from heating is offset by denaturation.	Near the optimum, warming raises kinetic energy but also damages structure, so the net gain shrinks.
7	Relative activity = 45 ÷ 100 × 100% = 45%, a fall of 100 − 45 = 55 percentage points.	Relative activity and percentage decrease are different figures; answer the one actually asked.
8	Initial rate 12 cm³/min; mean (12 + 6) ÷ 2 = 9 cm³/min; the rate falls because substrate concentration drops, so collisions with active sites become rarer.	Initial and mean rates differ; always state which time interval is meant.
9	(96 − 64) ÷ 4 = 8, so activity falls 8 percentage points per reuse.	Five readings give only four intervals; dividing by 5 is the classic off-by-one error.
10	Heat denatures the enzyme: hydrogen and ionic bonds break, the tertiary structure and active site shape change, usually irreversibly. The structure does change.	Structural change is exactly why activity does not return on cooling; it must be distinguished from a mere drop in activity.

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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