No. They are unchanged after the reaction and are reused; the substrate is what gets used up.

Must the rate carry a unit?

Yes. 6 cm³/min and 6 cm³ are different answers, and a missing unit usually loses the mark.

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

2026-11-08

Ten basic items on enzyme structure and active sites, lock-and-key versus induced fit, rate calculations, temperature and pH effects, and the classic misconception that enzymes are used up.

Focus　Separate structure from activity, and always divide product by time in rate items.

01　Paper details (check answers afterwards)

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

02　Questions (10 items)

FoundationWhat are enzymes made of, and what does the active site do?
FoundationHow do the lock-and-key and induced fit models differ?
FoundationAn enzyme produces 30 cm³ of gas in 5 minutes. Find the mean rate of reaction in cm³/min.
FoundationWhat is the optimum temperature for human enzymes, and why is activity almost zero above 60°C?
IntermediateSame enzyme: set A makes 45 cm³ in 10 minutes, set B makes 90 cm³ in 30 minutes. Which mean rate is higher?
IntermediatePepsin and trypsin have pH optima near 2 and 8. Why does activity fall away from the optimum?
IntermediateIf enzyme concentration doubles while substrate is plentiful, what happens to the rate, and why?
ChallengeAs substrate concentration keeps rising, the rate finally levels off. What is this rate called, and why does it plateau?
ChallengeRate without inhibitor is 40, and 16 with inhibitor. Find the percentage decrease in activity.
Error redoA student says enzymes are used up in the reaction, so more must be added each time. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Enzymes are mainly proteins (a few are RNA). The active site is complementary to the substrate, forming an enzyme-substrate complex and lowering the activation energy.	Enzymes do not change the direction of a reaction, only lower the activation energy so it proceeds faster at body temperature.
2	Lock-and-key assumes a rigid active site; induced fit says the active site adjusts slightly on binding so the fit is tighter.	Both models explain specificity; the induced fit model also explains why different substrates bind with different strength.
3	Rate = 30 cm³ ÷ 5 min = 6 cm³/min.	Rate is product divided by time, and the unit (cm³/min) must be stated with the answer.
4	About 37°C. High temperature denatures the enzyme: the active site loses its shape and cannot bind substrate, and denaturation is irreversible.	Cold slows an enzyme without destroying its structure, so activity returns on warming; heat denatures it irreversibly.
5	Set A: 45 ÷ 10 = 4.5 cm³/min; set B: 90 ÷ 30 = 3 cm³/min. Set A is higher.	Each total must be divided by its own time; comparing totals alone gives the wrong conclusion.
6	Too much acid or alkali alters the charges and hydrogen bonds of side chains at the active site, so the shape changes and the enzyme-substrate complex forms less easily.	Extreme pH can also denature the enzyme; each enzyme optimum reflects its working environment.
7	The rate roughly doubles: twice as many active sites are available, so more substrate is converted per unit time.	This only holds when substrate is plentiful; with limiting substrate, adding enzyme hardly helps.
8	It is Vmax, the maximum rate: every active site is occupied (saturated), so extra substrate has no free site to bind.	The plateau shows that enzyme amount, not substrate concentration, is now the limiting factor.
9	(40 − 16) ÷ 40 × 100% = 60%, so activity falls by 60%.	Find the decrease first, then divide by the original rate; never use the inhibited rate as the denominator.
10	Enzymes are catalysts: their shape and amount are unchanged after the reaction, so they are reused. It is the substrate that is used up.	This misconception makes students blame falling rate on used-up enzyme, when the real cause is falling substrate.

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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