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Ordinary Level Revision Notes
Leaving Cert Ordinary Level Biology
Chapter 7: Cell Metabolism and Enzymes
Enzyme action + applications
Subtopics Covered
- Learning Outcomes
- Big Picture: Cell Metabolism
- Key Definitions
- Enzymes as Catalysts
- Active Site and Enzyme Specificity
- Induced Fit Model
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
By the end of this chapter, students should be able to:
- Define metabolism as all the chemical reactions taking place in living cells.
- Explain that enzymes are biological catalysts made by living cells.
- Describe the active site and explain enzyme specificity.
- Use the induced fit model to explain how an enzyme works.
- Explain how temperature, pH and substrate concentration affect enzyme activity.
- Describe denaturation and explain why it prevents enzymes from working properly.
- Explain immobilised enzymes and give examples of their use in industry.
- Understand the basic idea of a bioreactor in bioprocessing.
- Interpret enzyme graphs and answer Ordinary Level exam-style questions.
Ordinary Level focus: You do not need complex protein chemistry. Focus on definitions, active site, specificity, factors affecting enzyme activity, denaturation and practical/industrial uses.
2. Big Picture: Cell Metabolism
Metabolism means all the chemical reactions that occur in living cells. These reactions keep cells alive, allow growth, release energy, build new materials and break down waste substances.
Build
Some reactions join small molecules together to make larger useful molecules. These are building reactions.
Break
Some reactions break large molecules into smaller molecules, for example digestion and respiration steps.
Control
Enzymes control these reactions by speeding them up at body or cell temperature.
Biology link: Without enzymes, many chemical reactions in cells would be too slow to support life.
3. Key Definitions
| Term | Meaning | Exam detail |
|---|---|---|
| Metabolism | All the chemical reactions in a living cell or organism. | Includes reactions that build up and break down molecules. |
| Enzyme | A biological catalyst made by living cells. | Most enzymes are proteins. |
| Catalyst | A substance that speeds up a chemical reaction and is not used up in the reaction. | Enzymes can be used again and again. |
| Substrate | The substance on which an enzyme acts. | Example: starch is a substrate for amylase. |
| Active site | The part of the enzyme where the substrate attaches. | Its shape is important for enzyme specificity. |
| Product | The substance formed after the enzyme-controlled reaction. | Products leave the active site. |
| Denaturation | A change in enzyme shape that stops the enzyme working properly. | Usually caused by high temperature or unsuitable pH. |
Exam Tip 1: Learn the enzyme definition exactly: “a biological catalyst”. Those two words are gold in exam answers.
4. Enzymes as Catalysts
An enzyme speeds up a reaction without being used up. After the reaction, the enzyme is free to work again on another substrate molecule.
Substrate enters active site→Enzyme-substrate complex forms→Reaction happens→Products leave→Enzyme reused
Examiner Secret 1: The enzyme is not changed permanently and is not used up. This is why a small amount of enzyme can catalyse many reactions.
5. Active Site and Enzyme Specificity
The active site is a specially shaped region on the enzyme. The substrate attaches to the active site. Different enzymes have different active site shapes, so each enzyme usually works on one specific substrate or one type of reaction.
Specificity
Enzymes are specific because the substrate must fit the active site. If the substrate does not fit, the reaction will not happen.
Examples
- Amylase acts on starch.
- Catalase acts on hydrogen peroxide.
- Lipase acts on fats/lipids.
- Protease acts on proteins.
Common Mistake 1: Saying every enzyme can act on every food molecule. Correct: enzymes are specific because of the shape of the active site.
6. Induced Fit Model
The induced fit model says that the active site is not completely rigid. When the correct substrate enters, the enzyme changes shape slightly so the substrate fits more closely. This helps the reaction happen.
| Stage | What happens |
|---|---|
| 1. Substrate approaches | The correct substrate moves towards the active site. |
| 2. Temporary fit | The substrate begins to attach to the enzyme. |
| 3. Slight shape change | The enzyme changes shape slightly around the substrate. |
| 4. Products form | The reaction occurs and products are released. |
| 5. Enzyme reused | The enzyme returns to its normal shape and can work again. |
Examiner Trap 1: Do not write that the enzyme is permanently changed. In induced fit, the shape change is temporary.
7. Factor 1: Temperature
Temperature affects enzyme activity because it changes how fast enzyme and substrate molecules move.
| Temperature condition | Effect on enzyme activity | Reason |
|---|---|---|
| Low temperature | Slow reaction rate. | Molecules move slowly and collide less often. |
| Increasing temperature | Reaction rate increases. | Molecules move faster and collide more often. |
| Optimum temperature | Enzyme works at its fastest rate. | Best temperature for active site function. |
| Too high temperature | Reaction rate falls quickly or stops. | Enzyme denatures; active site changes shape. |
Exam Tip 2: In graph questions, the enzyme activity rises to an optimum and then drops sharply when the enzyme denatures.
8. Factor 2: pH
Each enzyme has an optimum pH at which it works best. If the pH is too acidic or too alkaline, the active site may change shape and the enzyme may denature.
pH Examples
| Enzyme | Typical best pH |
|---|---|
| Pepsin in stomach | Acidic pH |
| Amylase in mouth/small intestine | Near neutral pH |
| Many cell enzymes | Near neutral pH |
Important idea
pH affects the shape of the active site. If the active site shape changes, the substrate may no longer fit.
Common Mistake 2: Saying all enzymes work best at pH 7. Some do, but not all.
9. Factor 3: Substrate Concentration
If substrate concentration increases, enzyme activity usually increases at first because more substrate molecules collide with active sites. Eventually, all active sites may be occupied, so the reaction reaches a maximum rate.
Examiner Secret 2: A flat part of a substrate concentration graph means another factor is limiting the reaction, usually the amount of enzyme available.
10. Denaturation
Denaturation happens when an enzyme loses its correct shape. The active site changes shape, so the substrate can no longer fit properly. The enzyme then stops working or works much more slowly.
Causes
- High temperature.
- Very acidic pH.
- Very alkaline pH.
Effect
- Active site shape changes.
- Substrate cannot fit.
- Enzyme activity decreases or stops.
Examiner Trap 2: Do not say enzymes are “killed”. Enzymes are not living organisms. Say the enzyme is denatured.
11. Enzymes in Everyday Biology
| Enzyme | Substrate | Product / action | Where or why it matters |
|---|---|---|---|
| Amylase | Starch | Maltose / smaller sugars | Digestion in mouth and small intestine. |
| Protease | Protein | Amino acids / smaller peptides | Digestion of proteins. |
| Lipase | Lipids/fats | Fatty acids and glycerol | Fat digestion. |
| Catalase | Hydrogen peroxide | Water and oxygen | Protects cells from toxic hydrogen peroxide. |
Exam link: Enzyme questions often use digestion examples because they are easy to connect to substrate, product, optimum pH and temperature.
12. Bioprocessing and Immobilised Enzymes
Bioprocessing means using living cells or enzymes to make useful products. In industry, enzymes can be used in a controlled container called a bioreactor.
Immobilised enzymes are enzymes that are trapped or attached to an insoluble material. The substrate flows over them, products are formed, and the enzymes stay in place.
Enzyme prepared→Enzyme immobilised→Substrate added→Product formed→Enzyme reused
Exam Tip 3: The big advantage of immobilised enzymes is that they can be reused and easily separated from the product.
13. Preparing Immobilised Enzymes
One common school-level method is to trap enzymes in gel beads. The enzyme is mixed with sodium alginate and dropped into calcium chloride solution. Beads form and trap the enzyme inside.
| Step | What happens | Why it matters |
|---|---|---|
| 1. Mix enzyme with sodium alginate | The enzyme is evenly mixed into the gel-forming solution. | Allows enzyme to be trapped later. |
| 2. Drop mixture into calcium chloride | Small beads form. | The enzyme becomes immobilised inside beads. |
| 3. Wash beads | Extra chemicals are removed. | Prevents contamination of product. |
| 4. Add substrate | Substrate passes into beads. | Enzyme acts on substrate. |
| 5. Collect product | Product leaves beads and is collected. | Enzyme remains in beads and can be reused. |
14. Bioreactors
A bioreactor is a container used to carry out a biological process under controlled conditions. It may contain enzymes or microorganisms. In enzyme bioprocessing, a bioreactor can hold immobilised enzymes while substrate flows through.
| Condition controlled | Why it is controlled |
|---|---|
| Temperature | Keeps enzyme near optimum temperature. |
| pH | Prevents enzyme denaturation and maintains activity. |
| Substrate flow | Controls how much substrate reaches enzymes. |
| Sterility/cleanliness | Prevents unwanted contamination. |
15. Industrial Applications of Immobilised Enzymes
| Application | Enzyme idea | Why useful |
|---|---|---|
| Lactose-free milk | Lactase breaks lactose into simpler sugars. | Helps people who are lactose intolerant. |
| Glucose syrup production | Enzymes break starch into glucose. | Used in food industry. |
| Biological washing powders | Proteases/lipases/amylases break stains. | Remove protein, fat and starch stains at lower temperatures. |
| Food processing | Specific enzymes modify food components. | Improves texture, flavour or production speed. |
Exam Tip 4: If asked for an advantage of immobilised enzymes, give “reused”, “easier to separate from product”, or “continuous production”.
Examiner Trap 3: Do not say immobilised enzymes are dead or inactive. Immobilised means fixed/trapped in place, not destroyed.
16. Advantages and Disadvantages of Immobilised Enzymes
| Advantages | Disadvantages |
|---|---|
| Enzymes can be reused. | Initial preparation may be expensive. |
| Product is easier to separate from enzyme. | Enzyme activity may be reduced if substrate cannot reach active sites easily. |
| Continuous production is possible. | Beads/support material may need replacing. |
| Process can be controlled in a bioreactor. | Conditions still need careful control. |
17. Interactive Simulator: Enzyme Detective
Choose the Correct Answer
Clue: This part of the enzyme has a shape that fits the substrate.
Feedback
Ready
Use the clue to identify the enzyme term.
18. Interactive Simulator: Enzyme Conditions
Predict Enzyme Activity
Result
Choose conditions
Remember: high temperature or unsuitable pH can denature enzymes.
19. Interactive Simulator: Bioprocessing Flow Builder
Build the Correct Sequence
Current stage: Start with enzyme solution.
Feedback
Start: Enzyme solution
Select the next correct step in preparing and using immobilised enzymes.
Progress: 0/5
20. High-Value Exam Guidance
Exam Tip 1: Define enzymes as biological catalysts made by living cells.
Exam Tip 2: Use active site language when explaining specificity.
Exam Tip 3: When describing temperature effects, include optimum temperature and denaturation.
Exam Tip 4: In immobilised enzyme questions, always mention reuse and easy separation.
Examiner Trap 1: Enzymes speed up reactions but are not used up.
Examiner Trap 2: High temperature denatures enzymes; low temperature usually slows them down but does not normally denature them.
Examiner Trap 3: pH does not just “slow” enzymes; unsuitable pH can change the active site shape.
Common Mistake 1: Calling the active site the “active side”. Use the correct term: active site.
Common Mistake 2: Forgetting that enzymes are reusable.
Examiner Secret 1: Graph questions are common. Always describe the trend before explaining it.
Examiner Secret 2: The phrase “active site changes shape” is the key mark in denaturation answers.
Examiner Secret 3: For bioprocessing, marks often come from simple practical points: beads, substrate, product, reuse.
21. MCQs with Instant Answers
1. An enzyme is best described as:
Answer: B. An enzyme is a biological catalyst.
2. The active site is:
Answer: A. The active site is where the substrate attaches.
3. Why are enzymes specific?
Answer: B. Enzyme specificity depends on active site shape.
4. Denaturation means:
Answer: C. Denaturation changes the active site shape.
5. At very high temperature, most enzymes:
Answer: B. High temperature can denature enzymes.
6. Immobilised enzymes are useful because they:
Answer: B. Immobilised enzymes can be reused and separated easily.
7. A bioreactor is used to:
Answer: A. A bioreactor controls biological processes such as enzyme reactions.
8. If substrate concentration increases at first, reaction rate usually:
Answer: A. More substrate causes more enzyme-substrate collisions at first.
22. Structured Exam Questions
- Define the term enzyme. [2]
- Explain what is meant by an enzyme active site. [2]
- Explain why enzymes are specific. [3]
- Describe the induced fit model of enzyme action. [5]
- Describe the effect of temperature on enzyme activity. [6]
- Describe the effect of pH on enzyme activity. [4]
- Explain what happens to enzyme activity as substrate concentration increases. [4]
- Define denaturation and explain its effect on enzyme activity. [4]
- Describe how immobilised enzyme beads can be prepared. [5]
- Give two advantages of using immobilised enzymes in industry. [4]
- Explain the role of a bioreactor in bioprocessing. [4]
23. Detailed Mark Scheme
Q1. Biological catalyst [1] made by living cells / speeds up reactions without being used up [1].
Q2. Region/part of enzyme [1] where substrate attaches / where reaction occurs [1].
Q3. Enzymes have active sites [1] with specific shapes [1]. Only a substrate with a complementary shape can fit [1].
Q4. Substrate approaches active site [1]. Substrate attaches to enzyme [1]. Active site changes shape slightly [1]. Enzyme-substrate complex forms / reaction occurs [1]. Products leave and enzyme can be reused [1].
Q5. Low temperature gives slow reaction [1] because molecules move slowly [1]. Increasing temperature increases rate [1] due to more collisions [1]. Enzyme reaches optimum temperature [1]. Very high temperature denatures enzyme / active site changes shape so rate falls [1].
Q6. Each enzyme has an optimum pH [1]. Away from optimum, activity decreases [1]. Extreme pH changes active site shape [1] causing denaturation / substrate no longer fits [1].
Q7. Increasing substrate concentration increases reaction rate at first [1] because more enzyme-substrate collisions occur [1]. Eventually rate levels off [1] because all active sites are occupied / enzyme concentration becomes limiting [1].
Q8. Denaturation is a change in enzyme shape [1]. Active site changes shape [1]. Substrate cannot fit properly [1]. Enzyme activity decreases or stops [1].
Q9. Mix enzyme with sodium alginate [1]. Drop mixture into calcium chloride [1]. Beads form [1]. Enzyme is trapped/immobilised inside beads [1]. Wash beads / add substrate to beads [1].
Q10. Enzymes can be reused [1]. Product is not contaminated with free enzyme / easier separation [1]. Continuous production possible [1]. Process can be controlled in a bioreactor [1]. Any two fully explained = 4.
Q11. Bioreactor is a controlled container/vessel [1]. Holds enzymes or microorganisms [1]. Controls conditions such as temperature or pH [1]. Used to produce useful products efficiently [1].
Q2. Region/part of enzyme [1] where substrate attaches / where reaction occurs [1].
Q3. Enzymes have active sites [1] with specific shapes [1]. Only a substrate with a complementary shape can fit [1].
Q4. Substrate approaches active site [1]. Substrate attaches to enzyme [1]. Active site changes shape slightly [1]. Enzyme-substrate complex forms / reaction occurs [1]. Products leave and enzyme can be reused [1].
Q5. Low temperature gives slow reaction [1] because molecules move slowly [1]. Increasing temperature increases rate [1] due to more collisions [1]. Enzyme reaches optimum temperature [1]. Very high temperature denatures enzyme / active site changes shape so rate falls [1].
Q6. Each enzyme has an optimum pH [1]. Away from optimum, activity decreases [1]. Extreme pH changes active site shape [1] causing denaturation / substrate no longer fits [1].
Q7. Increasing substrate concentration increases reaction rate at first [1] because more enzyme-substrate collisions occur [1]. Eventually rate levels off [1] because all active sites are occupied / enzyme concentration becomes limiting [1].
Q8. Denaturation is a change in enzyme shape [1]. Active site changes shape [1]. Substrate cannot fit properly [1]. Enzyme activity decreases or stops [1].
Q9. Mix enzyme with sodium alginate [1]. Drop mixture into calcium chloride [1]. Beads form [1]. Enzyme is trapped/immobilised inside beads [1]. Wash beads / add substrate to beads [1].
Q10. Enzymes can be reused [1]. Product is not contaminated with free enzyme / easier separation [1]. Continuous production possible [1]. Process can be controlled in a bioreactor [1]. Any two fully explained = 4.
Q11. Bioreactor is a controlled container/vessel [1]. Holds enzymes or microorganisms [1]. Controls conditions such as temperature or pH [1]. Used to produce useful products efficiently [1].
24. Exam-Style Graph Practice
Question A: Temperature graph
An enzyme reaction is tested at different temperatures. The rate rises from 10°C to 37°C, then falls sharply above 45°C.
- What is meant by optimum temperature? [2]
- Explain why the rate falls above 45°C. [3]
Answer A
Optimum temperature is the temperature at which the enzyme works fastest [1] / has maximum activity [1]. Above 45°C, the enzyme denatures [1], the active site changes shape [1], and the substrate can no longer fit properly [1].
Question B: Substrate graph
A reaction rate increases when substrate concentration is increased, but then the graph becomes flat.
- Explain why the rate increases at first. [2]
- Explain why the graph becomes flat. [2]
Answer B
The rate increases because there are more substrate molecules [1], causing more successful collisions with enzyme active sites [1]. The graph becomes flat because all active sites are occupied [1], so enzyme concentration becomes the limiting factor [1].
25. Premium Summary Table
| Topic | Must-know point | Best exam phrase |
|---|---|---|
| Enzyme definition | Enzymes speed up reactions and are not used up. | Biological catalyst. |
| Active site | Substrate attaches here. | Complementary shape. |
| Specificity | Each enzyme works with a specific substrate. | Only the correct substrate fits. |
| Induced fit | Active site changes shape slightly during binding. | Temporary shape change. |
| Temperature | Rate increases to optimum, then falls due to denaturation. | Active site changes shape. |
| pH | Each enzyme has an optimum pH. | Extreme pH can denature enzyme. |
| Substrate concentration | Rate increases then levels off. | All active sites occupied. |
| Immobilised enzymes | Enzymes fixed/trapped in place. | Can be reused and separated. |
| Bioreactor | Controlled container for biological production. | Controls temperature and pH. |
26. Mastery Checklist
- I can define metabolism.
- I can define enzyme as a biological catalyst.
- I can explain the role of the active site.
- I can explain enzyme specificity using shape.
- I can describe the induced fit model.
- I can describe how temperature affects enzyme activity.
- I can describe how pH affects enzyme activity.
- I can explain substrate concentration graphs.
- I can define denaturation and explain its effect.
- I can describe immobilised enzyme beads.
- I can give uses and advantages of immobilised enzymes.
- I can explain what a bioreactor does.
- I can answer Ordinary Level enzyme exam questions using correct terms.