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6.1.5Rates of Reaction and Energy Changes

Enzymes

10 min read 8 parts Lesson 6 of 10 Unit quiz

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Section 1 of 8Enzymes

1
Enzymes are needed because reactions would be too slow at body temperature to sustain life.
2
The substrate collides with and binds to the active site; the reaction occurs and products are released; the enzyme is unchanged.
3Supplement
Supplement specificity arises because the active site shape is complementary to only one substrate.
4Supplement
Supplement an enzyme–substrate complex forms before products are made.
5
As temperature rises towards the optimum, molecules have more kinetic energy, so more frequent effective collisions and more enzyme–substrate complexes form.
6
Above the optimum, bonds holding the enzyme shape break, the active site changes shape, and the enzyme is denatured — the rate falls.
7
At low temperatures enzymes are inactive but not denatured.
8
Each enzyme has an optimum pH; pH changes alter the active site shape, reducing the fit and eventually denaturing it.
9
Enzymes are used in digestion (amylase, protease, lipase) and in industry (e.g. biological washing powders).
10
In investigations, change one independent variable and keep others constant; repeat for reliability and calculate means.

Section 2 of 8Process & sequence

iodine concentration time data
iodine concentration time data
    • Set up water baths at a range of temperatures (e.g. 10, 20, 30, 40, 50, 60 °C).
    • Put starch solution and amylase solution in separate tubes in each bath for 5 minutes to reach temperature.
    • Place one drop of iodine in each well of a spotting tile.
    • Mix the amylase with the starch, start a stopwatch and every 30 s transfer a drop of the mixture to a fresh iodine well.
    • Record the time when the iodine stays orange-brown (starch fully digested).
    • Calculate rate = 1 ÷ time and plot rate against temperature.
    • Control variablesvolume and concentration of starch and amylase, pH (use a buffer).

    Lock-and-key action of an enzymewrite these steps in this order; each step is a marking point.

    1. The substrate and enzyme move randomly and collide.
    2. The substrate fits into the complementary active site.
    3. An enzyme–substrate complex forms.
    4. The substrate is converted into product(s).
    5. The products leave the active site; the enzyme is unchanged and can be reused.

Section 3 of 8Compare

Effects of temperature and pH
ConditionEffect on rateExplanation
Low temperatureSlowLow kinetic energy; few collisions (enzyme not denatured)
Rising to optimumIncreasesMore kinetic energy, more frequent effective collisions, more E–S complexes
Above optimumFalls rapidly to zeroActive site changes shape — denatured; substrate no longer fits
pH away from optimumFallsActive site shape changes; denaturation at extreme pH
Examples of digestive enzymes
EnzymeSubstrateProduct(s)
AmylaseStarchSimple reducing sugars (maltose)
Protease (e.g. pepsin, trypsin)ProteinAmino acids
LipaseFats and oilsFatty acids and glycerol
CatalaseHydrogen peroxideWater and oxygen

Section 4 of 8Worked examples

Worked examples
Starch was fully digested by amylase in 40 s at 30 °C and 25 s at 40 °C. Calculate the rate at each temperature.
  • Rate = 1/time. 30 °C: 1/40 = 0.025 s−1. 40 °C: 1/25 = 0.040 s−1. Rate is higher at 40 °C, nearer the optimum.
Catalase produced 36 cm3 of oxygen in 3 minutes. Calculate the mean rate.
  • Rate = volume ÷ time = 36 ÷ 3 = 12 cm3/min.
Explain why the rate at 70 °C is zero.
  • At 70 °C, above the optimum, the enzyme is denatured: the active site has permanently changed shape,
    • so the substrate is no longer complementary, cannot bind, and no enzyme–substrate complexes form.

Section 5 of 8Keep going

  1. Enzymes IGCSE Biology topic: what to do next

    These Enzymes IGCSE Biology topic notes covered the key ideas, worked examples and exam technique for this topic. Check your Enzymes IGCSE Biology topic understanding with the practice questions below, then reread any Enzymes IGCSE Biology topic point you were unsure about before moving on.

    Enzymes IGCSE Biology topic: revision checklist

    • Revise the main Enzymes IGCSE Biology topic definitions until you can write them from memory.
    • Practise Enzymes IGCSE Biology topic exam questions under timed conditions.

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Catalyst · Enzyme · Substrate · Product · Active site · Enzyme–substrate complex (Supplement) · Specificity · Optimum temperature · Optimum pH · Denaturation (Supplement) · Kinetic energy (Supplement)
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.

Words to avoid

1
the enzyme is killedthe enzyme is denatured (enzymes are not alive).
2
the active site is the same shape as the substratethe active site has a shape complementary to the substrate.
3
enzymes are denatured at low temperatureat low temperature enzymes have less kinetic energy, fewer collisions; not denatured.
4
enzymes work best at 37 °C (for all)each enzyme has its own optimum temperature and pH.
5
more collisionsmore frequent effective collisions, forming more enzyme–substrate complexes.
6
the enzyme is used upenzymes are catalysts; they are not changed by the reaction.
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