Where Exams Make Sense
Premium academic resources for international exam students
www.examslogic.com
Higher Level Revision Notes
Leaving Cert Higher Level Biology
Chapter 3: Enzymes
Action, factors, investigations
Subtopics Covered
- Learning Outcomes
- Big Picture: Why Enzymes Matter
- Key Definitions
- Enzyme Action: Lock-and-Key and Induced Fit
- Activation Energy
- Factors Affecting Enzyme Activity
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Define enzyme, substrate, active site and product.
- Explain the lock-and-key model and induced-fit idea at Higher Level.
- Describe how enzymes lower activation energy.
- Explain the effects of temperature, pH, enzyme concentration and substrate concentration.
- Interpret enzyme activity graphs accurately.
- Describe enzyme immobilisation and its uses.
- Design and evaluate enzyme investigations using fair-test principles.
HL focus: Do not only memorise graphs. You must explain the molecular reason behind each graph shape.
2. Big Picture: Why Enzymes Matter
Speed
Enzymes make biochemical reactions fast enough to support life at body temperature.
Specificity
Each enzyme usually acts on a specific substrate because the active site has a complementary shape.
Control
Cells regulate metabolism by controlling enzyme production, conditions and substrate availability.
Biology link: Digestion, respiration, photosynthesis, DNA copying and immunity all depend on enzymes.
3. Key Definitions
| Term | Definition | Exam wording |
|---|---|---|
| Enzyme | A biological catalyst made mainly of protein. | Speeds up a chemical reaction and is not used up. |
| Substrate | The substance on which an enzyme acts. | The molecule that fits into the active site. |
| Active site | The region of the enzyme where the substrate binds. | Has a specific 3D shape. |
| Product | The molecule or molecules formed after the reaction. | Released from the active site. |
| Denaturation | Change in enzyme shape so it no longer functions properly. | Often caused by high temperature or extreme pH. |
Exam Tip: Always include the words biological catalyst when defining enzyme.
4. Enzyme Action: Lock-and-Key and Induced Fit
Lock-and-Key Model
The active site has a fixed shape that is complementary to one substrate.
Induced Fit
The active site changes slightly as the substrate binds, helping the reaction occur more easily.
Examiner trap: Do not say the enzyme is consumed. It is reused after the products leave.
5. Activation Energy
Reactions need energy to begin. This is called activation energy. Enzymes lower the activation energy, so more reactions occur per second.
6. Factors Affecting Enzyme Activity
| Factor | Effect | Reason | Graph shape |
|---|---|---|---|
| Temperature | Activity increases to optimum, then falls quickly. | More collisions at first; high temperature denatures active site. | Rises then steep drop. |
| pH | Each enzyme has an optimum pH. | Extreme pH changes bonds and active-site shape. | Peak at optimum pH. |
| Substrate concentration | Rate rises then levels off. | All active sites become occupied. | Plateau. |
| Enzyme concentration | Rate increases if substrate is available. | More active sites available. | Usually proportional at first. |
Common mistake: Students write enzymes are killed by heat. Enzymes are not alive; they are denatured.
7. Enzyme Experiments
Investigation: Effect of Temperature on Catalase
Catalase breaks down hydrogen peroxide into water and oxygen. The oxygen produced can be measured as bubbles, foam height or gas volume.
Set water baths→Add catalase→Add H₂O₂→Measure oxygen→Compare rates
| Variable type | Example |
|---|---|
| Independent variable | Temperature. |
| Dependent variable | Rate of oxygen production. |
| Controlled variables | pH, enzyme volume, substrate concentration, reaction time. |
| Control | Boiled enzyme or no enzyme sample. |
8. Enzyme Immobilisation
Immobilisation means fixing enzymes to an inert material such as alginate beads so they can be reused.
Advantages
- Enzyme can be reused.
- Product is easier to separate.
- Process can be continuous.
- Enzyme is often more stable.
Uses
- Lactose-free milk production.
- Food processing.
- Industrial biotechnology.
- Biosensors.
HL note: Immobilised enzymes are not dissolved in the product, so purification is easier.
9. Interactive Simulator: Enzyme Condition Detective
Choose the Best Explanation
Scenario: Enzyme activity rises from 10°C to 37°C.
Feedback
Ready
Use molecular reasoning, not memorised phrases only.
10. Interactive Simulator: Variable Sorter
Identify the Variable
Investigation: Effect of pH on amylase activity.
The pH buffer used in each test tube is the...
Feedback
Ready
Think: what is changed, what is measured, and what is kept the same?
11. High-Value Exam Guidance
Exam Tip 1: In graph questions, describe both the trend and the biological reason.
Exam Tip 2: Use the phrase active site changes shape when explaining denaturation.
Exam Tip 3: In practical questions, name the independent, dependent and controlled variables clearly.
Examiner Trap 1: Enzymes are not living organisms; they cannot be killed.
Examiner Trap 2: Optimum temperature is not always 37°C. It depends on the enzyme and organism.
Examiner Trap 3: A plateau in substrate graphs does not mean the enzyme stopped working; active sites are saturated.
Common Mistake 1: Saying pH changes temperature. pH affects charge and bonding in the enzyme.
Common Mistake 2: Forgetting to mention oxygen in catalase experiments.
Examiner Secret 1: Higher Level answers often require the link: active-site shape → substrate fit → rate of reaction.
Examiner Secret 2: Immobilised enzyme questions often ask for advantages, not the full method.
12. MCQs with Instant Answers
1. An enzyme is best described as:
Answer: B. Enzymes are biological catalysts.
2. What happens during denaturation?
Answer: B. The active site changes shape and no longer fits the substrate.
3. Why does enzyme activity increase as temperature rises below the optimum?
Answer: A. More kinetic energy increases successful collisions.
4. In an experiment testing pH, pH is the:
Answer: B. pH is deliberately changed.
13. Structured Questions
- Define enzyme and substrate. [4]
- Explain how enzymes lower activation energy. [3]
- Describe the effect of temperature on enzyme activity. [6]
- Explain what happens to enzyme activity as substrate concentration increases. [5]
- Design an investigation to test the effect of pH on enzyme activity. [9]
- Give three advantages of immobilised enzymes. [3]
14. Mark Scheme
Q1. Enzyme: biological catalyst [1], made mainly of protein [1]. Substrate: substance acted on by enzyme [1], binds to active site [1].
Q2. Enzymes provide an alternative pathway [1], lower activation energy [1], so more substrate molecules react per second [1].
Q3. Low temperature gives low kinetic energy [1]; increasing temperature increases collisions [1]; rate rises to optimum [1]; above optimum bonds are disrupted [1]; active site changes shape [1]; substrate no longer fits / rate decreases [1].
Q4. Rate increases at first [1]; more enzyme-substrate complexes form [1]; eventually active sites become occupied [1]; enzyme concentration becomes limiting [1]; graph reaches plateau [1].
Q5. Change pH using buffers [1]; keep enzyme/substrate concentration constant [2]; keep temperature constant [1]; measure rate using product formed or substrate disappearance [2]; repeat and average [1]; include control [1]; draw conclusion from evidence [1].
Q6. Reusable [1]; product easier to separate [1]; continuous process possible [1]; more stable [1]. Max 3.
Q2. Enzymes provide an alternative pathway [1], lower activation energy [1], so more substrate molecules react per second [1].
Q3. Low temperature gives low kinetic energy [1]; increasing temperature increases collisions [1]; rate rises to optimum [1]; above optimum bonds are disrupted [1]; active site changes shape [1]; substrate no longer fits / rate decreases [1].
Q4. Rate increases at first [1]; more enzyme-substrate complexes form [1]; eventually active sites become occupied [1]; enzyme concentration becomes limiting [1]; graph reaches plateau [1].
Q5. Change pH using buffers [1]; keep enzyme/substrate concentration constant [2]; keep temperature constant [1]; measure rate using product formed or substrate disappearance [2]; repeat and average [1]; include control [1]; draw conclusion from evidence [1].
Q6. Reusable [1]; product easier to separate [1]; continuous process possible [1]; more stable [1]. Max 3.
15. Mastery Checklist
- I can define enzyme, substrate, active site and denaturation.
- I can explain enzyme specificity using active-site shape.
- I can explain activation energy.
- I can interpret temperature, pH and concentration graphs.
- I can design fair enzyme experiments.
- I can explain enzyme immobilisation and its advantages.
- I can answer Higher Level enzyme questions using molecular reasoning.