At a glance
- Cambridge section
- 0610 topic 5.1 Enzymes (Core 1 to 5, Supplement 6 to 9)
- Edexcel section
- 4BI1 statements 2.10 to 2.14B
- Paper 2 only (Edexcel)
- 2.14B practical on pH and enzyme activity
- Key words
- active site, complementary, enzyme-substrate complex, denatured, optimum
- Rate from time
- rate = 1 / time taken
Key ideas you must be able to write
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Enzymes are proteins that act as biological catalysts and are involved in all metabolic reactions. Without them, reactions in cells would be far too slow at body temperature to sustain life; this is the Cambridge Core statement 5.1.3 and the reason examiners give for why enzymes matter.
The active site is a region of the enzyme with a specific shape. The substrate has a complementary shape, so it fits into the active site and forms an enzyme-substrate complex. The reaction happens, the products leave, and the enzyme is unchanged and can be used again. Because only one substrate shape fits, enzymes are specific. Cambridge Core students describe this; Extended (Supplement) students must explain it using the terms active site, enzyme-substrate complex, substrate and product.
Edexcel 4BI1 statements 2.10 to 2.13 cover the same ideas: enzymes as biological catalysts, the effect of temperature including changes to the shape of the active site, and pH altering the active site. The temperature practical (2.12) can appear on either paper, while the pH practical (2.14B) is examined only in Paper 2.
Temperature and pH: what to write at each part of the graph
| Part of the curve | What happens | Mark-scheme explanation |
|---|---|---|
| Low temperature, rising | Rate increases | Enzyme and substrate molecules gain kinetic energy, move faster and have more frequent effective collisions, so more enzyme-substrate complexes form |
| Optimum temperature | Fastest rate | Most frequent effective collisions while the active site keeps its shape; about 37 °C for many human enzymes |
| Above the optimum | Rate falls quickly | Bonds holding the enzyme's shape break, the active site changes shape, the substrate no longer fits, the enzyme is denatured |
| Away from optimum pH (either side) | Rate falls | The active site changes shape so the substrate no longer fits; extreme pH denatures the enzyme |
| Back to low temperature after cooling | Enzyme works again slowly | Cold only slows enzymes; it does not denature them, which is why food keeps in a fridge |
Optimum pH depends on the enzyme. Pepsin in the stomach works best in acid conditions; most other human digestive enzymes work best near neutral or slightly alkaline.
Worked example: turning a practical result into a rate
Practical: amylase is mixed with starch at different temperatures. Every 30 seconds a drop of the mixture is added to iodine solution. The time is recorded when the iodine no longer turns blue-black, showing the starch has been digested. At 20 °C this takes 240 s; at 40 °C it takes 60 s.
Rate = 1 / time. At 20 °C, rate = 1 / 240 = 0.0042 per second (to 2 significant figures). At 40 °C, rate = 1 / 60 = 0.017 per second. The rate at 40 °C is 240 / 60 = 4 times faster.
Evaluation marks: because samples are taken only every 30 seconds, the end point is uncertain by up to 30 s, so a shorter sampling interval would improve accuracy. Control variables are the volume and concentration of amylase and of starch, and the pH (use a buffer). Repeat each temperature and calculate a mean to check repeatability.
Common mistakes that cost marks
- Writing that enzymes are "killed". Enzymes are not alive; they are denatured.
- Saying high temperature denatures the substrate, or that the enzyme "stops fitting". It is the active site that changes shape, so the substrate no longer fits.
- Saying low temperature denatures enzymes. It only reduces kinetic energy and collisions.
- Describing the substrate shape as "the same as" the active site. Use complementary.
- Leaving out "effective" or "successful" collisions in Extended answers.
- Stating one optimum pH for all enzymes.
- Forgetting a buffer as the way to control pH in the temperature practical.
Exam technique for enzyme questions
Graph questions usually ask you to describe, then explain. Describe means quote what the graph shows with numbers: "the rate increases from 0.2 to 1.0 units between 10 °C and 40 °C, then falls to 0 by 60 °C". Explain means give the reason in terms of kinetic energy, collisions, active site shape and denaturation. Mixing the two, or explaining when only a description is asked, wastes time and rarely gains marks.
For longer planning questions (in Edexcel written papers and Cambridge Papers 5 and 6), structure the answer as: independent variable with at least five values, dependent variable and how it is measured, three control variables with how they are controlled, repeats, and a safety point such as eye protection with iodine. Examiners reward named apparatus (water bath, thermometer, stopclock, spotting tile) over general wording.
Enzyme ideas also return in digestion, respiration, germination and biotechnology questions, so use the same vocabulary everywhere.
How one-to-one lessons help with this topic
Most students understand the lock-and-key picture but lose marks on wording. A tutor works on exactly that: the student explains a temperature graph aloud, the tutor marks it against real mark points (kinetic energy, collisions, active site shape, denatured), and the student rewrites until every point is there. Lessons also cover planning answers for the practicals using the school's board, which is where many students drop four or five marks.
Self-check
- Define catalyst and enzyme in one sentence each.
- Explain specificity using active site and complementary shape.
- Sketch an enzyme activity graph against temperature and label the optimum.
- Explain each section of the graph in mark-scheme language.
- Explain why pH changes affect enzyme activity.
- Plan the amylase and iodine practical with variables, repeats and safety.
- Calculate rate from a time using rate = 1 / time.
Common questions
Do Core students need to explain enzymes using collisions?
In Cambridge 0610, explaining temperature effects using kinetic energy and frequency of effective collisions is a Supplement statement. Core students describe the optimum temperature and denaturation. Edexcel statement 2.11 asks how temperature affects enzyme function, including the shape of the active site; adding kinetic energy and collisions for the rising part of the curve makes the answer complete.
What is the lock and key model?
It is the idea that the substrate fits the active site like a key fits a lock, because the shapes are complementary. It explains why each enzyme catalyses only one reaction.
Is the induced fit model needed for IGCSE?
Neither syllabus names induced fit. The complementary shape of the active site and the enzyme-substrate complex are what the mark schemes reward.
How much do LiveTutor biology lessons cost?
$15 a lesson for every subject and level, booked as a weekly plan of 1 to 5 lessons billed monthly. Lessons are 60 minutes, one to one and online. The first lesson is a free trial.
Which enzyme practical should I learn?
Learn one temperature practical (often amylase and starch with iodine) and one pH practical. Edexcel lists both; the pH practical is Paper 2 only. Cambridge asks you to investigate and describe the effects of temperature and pH.
Sources
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