Free Sample | Enzymes

📋 Before We Start

💡 Suppose you met a question like this in the exam...

(a) Define the terms: (i) catalyst (ii) enzyme. [3]

(b) Explain enzyme action using the lock and key hypothesis. [4]

(c) A student investigates the effect of temperature on enzyme activity. Describe and explain the results expected between 0 °C and 80 °C. [5]

(d) Explain what happens to enzyme activity when pH falls far below the optimum. [3]

(e) State ONE use of enzymes in the food industry and ONE use in biological washing powders. [2]

💡 Study all sections carefully. Every part of this question is fully answered in the Model Answers section.

📖 Key Definitions

CATALYST — syllabus definition

A catalyst is a substance that speeds up a chemical reaction and is not changed by the reaction.

ENZYME — syllabus definition

Enzymes are proteins that function as biological catalysts.

Key Properties of Enzymes — Learn All Six

Property	Explanation
Biological catalysts	Speed up chemical reactions in living organisms without being used up
Proteins	Made of amino acids; have a specific 3D shape
Specific	Each enzyme acts on only one substrate — the substrate fits the active site like a key in a lock
Reusable	The enzyme is released unchanged after the reaction — it can be used again
Affected by temperature	Activity increases with temperature up to the optimum, then falls sharply when denatured
Affected by pH	Each enzyme has an optimum pH; activity decreases above or below this

⚠️ Common mistake: Students say enzymes are "destroyed" or "used up." Enzymes are reusable — they are not consumed. However, they can be denatured (permanently changed shape) by high temperature or extreme pH. "Destroyed" is imprecise — always use "denatured."

Quick check: A student says "amylase breaks down all food molecules." Why is this incorrect?

Show Answer

Enzymes are specific — each enzyme has an active site with a unique shape.
Amylase only catalyses the breakdown of starch (a carbohydrate) — its active site is complementary to starch molecules only.
Amylase cannot break down proteins or fats — their molecules do not fit the active site of amylase.

🔑 The Lock and Key Hypothesis

The lock and key hypothesis explains why enzymes are specific. The enzyme is the lock; the substrate is the key.

Step 1: Substrate approaches
enzyme active site

→

Step 2: Substrate fits active site
— enzyme-substrate complex forms

→

Step 3: Reaction occurs; products
released; enzyme reusable

Explaining the Lock and Key — Step by Step

The enzyme has a region called the active site — a specifically shaped area on its surface.
The substrate molecule has a complementary shape to the active site — it fits precisely, like a key into a lock.
The substrate binds to the active site, forming an enzyme-substrate complex.
The enzyme catalyses the reaction — the substrate is converted into products.
The products are released from the active site.
The enzyme is unchanged — its active site is free to bind another substrate molecule.

📌 Specificity explained: Only a substrate with the correct shape can fit the active site. A different substrate molecule has a different shape → does NOT fit → the enzyme cannot catalyse that reaction. This is why each enzyme is specific to ONE substrate.

Q: Explain why amylase cannot digest protein, using the lock and key hypothesis.

Show Answer

The active site of amylase has a specific shape that is complementary to starch molecules.
Protein molecules have a different shape from starch — they do not fit into the active site of amylase.
An enzyme-substrate complex cannot form between amylase and protein.
Therefore amylase cannot catalyse the breakdown of protein.

🌡️ Effect of Temperature on Enzyme Activity

What Happens at Each Temperature Range

Temperature	Effect on activity	Reason
Low (0–10 °C)	Activity very low / almost zero	Molecules have little kinetic energy → substrate and enzyme collide infrequently → few enzyme-substrate complexes form per second
Rising (10–37 °C)	Activity increases	More kinetic energy → more frequent collisions between enzyme and substrate → more enzyme-substrate complexes per second
Optimum (~37 °C for most human enzymes)	Activity at maximum	Best balance of collision frequency and enzyme shape
Above optimum (e.g. 60 °C+)	Activity rapidly decreases → zero	High temperature causes the enzyme's active site to change shape (denature) → substrate can no longer fit → no enzyme-substrate complexes form

⚠️ The word "denatured" is essential. Do not say the enzyme is "killed" or "destroyed." Say: "the active site changes shape" and "the enzyme is denatured." Denaturation is permanent at very high temperatures.

📌 Optimum temperature: Human body enzymes have an optimum of approximately 37 °C (body temperature). Some bacterial enzymes (e.g. used in biological washing powders) have optima up to 60 °C.

Q: Explain why enzyme activity drops to zero above 60 °C.

Show Answer

At temperatures above the optimum, the high heat energy causes the bonds holding the enzyme's structure together to break.
The active site changes shape — it is no longer complementary to the substrate.
The substrate cannot bind to the active site — no enzyme-substrate complex forms.
The enzyme is denatured — this change is permanent.
Therefore the reaction rate falls to zero.

🧪 Effect of pH on Enzyme Activity

pH measures how acidic or alkaline a solution is. Every enzyme has an optimum pH at which it works best. Above or below this, activity decreases.

How pH Affects Enzyme Activity

pH condition	Effect	Reason
At optimum pH	Maximum activity	Active site has the correct shape; substrate fits perfectly
Above or below optimum	Activity decreases	Excess H⁺ or OH^- ions alter the bonds in the enzyme's active site → active site shape changes → substrate fits less well
Extreme pH (very high or very low)	Activity falls to zero	Enzyme is denatured — active site permanently changed → substrate cannot bind

Optimum pH of Key Enzymes

Enzyme	Location	Substrate	Optimum pH
Salivary amylase	Mouth	Starch	~7 (neutral)
Pepsin	Stomach	Protein	~2 (very acidic)
Pancreatic amylase	Small intestine	Starch	~7–8 (slightly alkaline)
Lipase	Small intestine	Fats	~7–8
Catalase	Most cells	Hydrogen peroxide	~7

📌 Why is pepsin's optimum pH 2? The stomach secretes hydrochloric acid, making it very acidic (pH ~2). Pepsin has evolved to work best under these conditions. If placed in neutral pH, its active site shape changes and activity decreases.

Q: A student adds hydrochloric acid to a tube containing amylase and starch solution (normally at pH 7). Explain the effect on the rate of starch digestion.

Show Answer

Adding HCl lowers the pH well below the optimum pH (~7) for amylase.
The excess hydrogen ions alter the bonds in the enzyme's active site.
The active site changes shape — it is no longer complementary to starch.
Fewer enzyme-substrate complexes form → rate of starch digestion decreases.
At very low pH, the enzyme may be denatured and activity stops completely.

📈 Interactive Enzyme Graphs

Click each button to display the correct graph shape. Study how to describe and explain each curve.

How to Describe a Graph — Exam Technique

Describing the temperature graph (3 marks):

Show model description

From 0 °C to ~37 °C, the rate of reaction increases as temperature rises (1)
The rate reaches a maximum at approximately 37 °C — the optimum temperature (1)
Above 37 °C, the rate rapidly decreases and falls to zero by approximately 60 °C (1)

Examiner tip: always quote values from the graph. "Increases then decreases" alone scores 1/3.

Describing the substrate concentration graph (2 marks):

Show model description

At low substrate concentrations, the rate increases steeply as concentration increases — more substrate molecules available to occupy active sites (1)
At high concentrations, the rate levels off (plateaus) — all active sites are occupied; adding more substrate has no further effect — the enzyme is the limiting factor (1)

🏭 Uses of Enzymes

Enzymes in Germination of Seeds

Seeds store food as starch and other complex molecules.
During germination, enzymes (e.g. amylase) break down starch → maltose → glucose.
Glucose is used in respiration to provide energy for the growing seedling.
Protease enzymes break down stored proteins into amino acids for growth.

Enzymes in Biological Washing Powders

🧺 Protease

Breaks down protein-based stains (e.g. blood, egg, meat, sweat)
Digests proteins into soluble amino acids that wash away

🧺 Lipase

Breaks down fat/grease stains (e.g. oil, butter, cooking fat)
Digests fats into glycerol and fatty acids that wash away

📌 Advantage: Biological washing powders work at lower temperatures (30–40 °C) — saving energy and being gentler on fabrics. Their enzymes have optima around this range.

Enzymes in the Food Industry

Enzyme	Use	Process
Pectinase	Fruit juice production	Breaks down pectin in cell walls of fruit → releases more juice → clearer juice with better yield
Protease	Meat tenderising; baby food	Partially digests proteins → softer texture; easier digestion for babies
Amylase	Syrup / glucose production	Converts starch → glucose syrups for sweeteners and fermentation
Isomerase	Fructose syrup	Converts glucose → fructose (sweeter) for use in diet foods
Chymosin (Rennet)	Cheese making	Causes milk proteins to coagulate (curdle) → forms cheese

Q: Explain why pectinase is added to crushed fruit in juice production.

Show Answer

Pectin is a substance in plant cell walls that makes the juice thick and cloudy.
Pectinase breaks down pectin in the cell walls.
This releases more juice from the fruit cells → greater yield of juice.
The juice is also clearer (less cloudy) as pectin is broken down.

🦠 Penicillin Production & Microorganisms in Industry

The Role of Penicillium in Producing Penicillin

Penicillin is an antibiotic — a substance that kills or inhibits the growth of bacteria.
It is produced by the fungus Penicillium notatum (or Penicillium chrysogenum).
The fungus secretes penicillin naturally as a defence against competing bacteria.
Penicillin was discovered by Alexander Fleming in 1928 when he noticed that Penicillium mould inhibited bacterial growth on his plates.

Using Fermenters (Bioreactors) for Large-Scale Production

Fermenters are large industrial vessels used to grow microorganisms under carefully controlled conditions to produce useful substances (enzymes, antibiotics, etc.).

⚙️ Conditions Controlled in a Fermenter

Temperature — kept at optimum for enzyme activity; water jacket and cooling system used
pH — monitored and adjusted to maintain optimum
Oxygen supply — stirrers and air pumps ensure aerobic conditions
Nutrient supply — glucose, mineral salts, nitrogen source added
Sterility — vessels sterilised before use; prevents contaminating organisms

🏭 Products Made Using Fermenters

Penicillin — from Penicillium
Enzymes for washing powders — protease, lipase from bacteria
Insulin — from genetically engineered bacteria (Topic 17)
Yoghurt — from Lactobacillus bacteria (Topic 5)
Ethanol — from yeast fermentation
Single cell protein — Quorn from Fusarium fungus

Q: State TWO reasons why conditions in a fermenter must be carefully controlled during penicillin production.

Show Answer

To maintain the optimum temperature for enzyme activity of Penicillium — maximum rate of penicillin production (1)

To maintain sterile conditions — prevents contaminating bacteria or fungi from outcompeting Penicillium and reducing yield (1)

Also accept: controlling pH; ensuring adequate oxygen/nutrients for maximum growth

🔬 Practical: Investigating Effect of Temperature/pH on Enzyme Activity

Experiment 1: Effect of Temperature on Amylase Activity

Principle: Amylase breaks down starch. As starch disappears, iodine solution (which turns blue-black with starch) stays yellow-brown. The time taken for colour change = measure of enzyme activity.

Prepare water baths at different temperatures: 10, 20, 30, 37, 50, 70 °C.
Place tubes of starch solution and amylase solution separately in each water bath for 5 minutes to reach the set temperature.
Mix the starch and amylase solutions. Start a stopwatch.
Every 30 seconds, place a drop of the mixture onto a spotting tile containing iodine solution.
Record the time when iodine stays yellow-brown (starch fully digested).
Repeat at all temperatures. Calculate rate = 1 / time (faster = higher rate).

📌 Why use 1/time as the rate? A shorter time means faster digestion. Rate = 1/time gives a value that increases as enzyme activity increases — making graphs easier to interpret.

Experiment 2: Effect of pH on Catalase Activity

Principle: Catalase breaks down hydrogen peroxide → water + oxygen. The volume of oxygen gas produced measures activity.

Prepare buffer solutions of different pH (e.g. pH 4, 5, 6, 7, 8, 9, 10).
Add equal amounts of hydrogen peroxide and catalase (e.g. from potato/liver) to each buffer solution.
Collect oxygen gas in an inverted measuring cylinder over water.
Record volume of O₂ produced in 60 seconds at each pH.
Plot a graph of volume of O₂ vs pH — peak shows the optimum pH.

Q: In the amylase experiment, state TWO variables that must be controlled.

Show Answer

Concentration of starch solution (same for all temperatures) (1)

Concentration / volume of amylase solution (same for all temperatures) (1)

Also accept: pH of mixture; volume of iodine used; same spotting tile intervals

Q: A student finds that at 70 °C, the iodine solution stays blue-black even after 10 minutes. Explain this result.

Show Answer

At 70 °C, the temperature is well above the optimum for amylase (~37 °C).
The high temperature causes the active site of amylase to change shape (denature).
Starch molecules cannot bind to the denatured active site.
Starch remains undigested → iodine continues to turn blue-black indefinitely.

🧠 Memory Retention Questions

1. Define enzyme in one sentence.

Show Answer

Enzymes are proteins that function as biological catalysts — they speed up chemical reactions in living organisms without being used up.

2. What is an active site?

Show Answer

The active site is a specifically shaped region on the enzyme surface where the substrate binds. Its shape is complementary to the substrate molecule.

3. What is the optimum temperature for most human enzymes?

Show Answer

37 °C — the normal human body temperature. Enzyme activity is at its maximum at this temperature.

4. What happens to an enzyme at very high temperatures? Use the correct term.

Show Answer

The enzyme is denatured — the bonds holding its 3D structure together break, causing the active site to change shape permanently. The substrate can no longer bind.

5. What is the optimum pH of pepsin and where does it work?

Show Answer

Pepsin works in the stomach and has an optimum pH of approximately 2 (very acidic), matching the acidic conditions created by hydrochloric acid in the stomach.

6. Name the enzyme used in fruit juice production and state its function.

Show Answer

Pectinase — breaks down pectin in plant cell walls, releasing more juice and making the juice clearer.

7. Name the fungus that produces penicillin.

Show Answer

Penicillium (specifically Penicillium notatum or Penicillium chrysogenum).

8. Why do biological washing powders contain protease and lipase?

Show Answer

Protease digests protein stains (e.g. blood, egg, meat) into soluble amino acids.
Lipase digests fat/grease stains (e.g. oil, butter) into soluble glycerol and fatty acids.
Both allow stains to be washed away at lower temperatures, saving energy.

✅ Using the Notes to Answer the Opening Question

Full Model Answers with Mark Schemes

(a)(i) Define catalyst. (ii) Define enzyme. [3]

Show Model Answer

(i) A catalyst is a substance that speeds up a chemical reaction and is not changed by the reaction (1)

(ii) Enzymes are proteins that function as biological catalysts (1+1 — both "protein" AND "biological catalyst" needed)

Reject: "enzymes speed up reactions" alone — must state they are proteins (loses 1 mark)

(b) Explain enzyme action using the lock and key hypothesis. [4]

Show Model Answer

The enzyme has an active site with a specific shape (1)

The substrate has a complementary shape to the active site — fits like a key into a lock (1)

The substrate binds to the active site, forming an enzyme-substrate complex (1)

Products are released and the enzyme is unchanged / reusable — active site free for another substrate (1)

(c) Describe and explain results from 0–80 °C. [5]

Show Model Answer

From 0 °C to ~37 °C: rate of reaction increases as temperature rises (1)

Because molecules have more kinetic energy → more frequent enzyme-substrate collisions → more enzyme-substrate complexes per second (1)

Maximum rate at optimum temperature (~37 °C) (1)

Above 37 °C: rate rapidly decreases towards zero (1)

Because high temperature causes the active site to change shape (denature) → substrate cannot bind → no enzyme-substrate complexes form (1)

(d) Explain what happens when pH falls far below optimum. [3]

Show Model Answer

Excess hydrogen ions alter the bonds in the enzyme's structure (1)

The shape of the active site changes — it is no longer complementary to the substrate (1)

The enzyme may be denatured → substrate cannot bind → activity falls to zero (1)

(e) One use in food industry; one use in biological washing powders. [2]

Show Model Answer

Food industry: pectinase to increase juice yield / protease for meat tenderising / isomerase to convert glucose to fructose (any one) (1)

Biological washing powder: protease to digest protein stains / lipase to digest fat stains (any one) (1)

📋 Topic Test — Self-Marking (15 marks)

Answer all questions then click Submit Test for instant scored feedback.

📄 Mock Exam: Paper 2 Style (20 marks)

Time allowed: 30 minutes. Answer all questions. Check with mark schemes.

Question 1 (5 marks)
(a) Define the term enzyme. [2]
(b) Explain what is meant by the "specificity" of an enzyme, using the lock and key hypothesis. [3]

Question 2 (6 marks)
The graph below shows enzyme activity at different temperatures. Answer the questions.
(Imagine a bell-shaped curve peaking at 37 °C, falling to zero at ~60 °C)

(a) State the optimum temperature shown. [1]
(b) Explain the increase in activity from 10 °C to 37 °C. [2]
(c) Explain why activity drops to zero above 60 °C. [3]

Question 3 (5 marks)
Biological washing powders contain protease and lipase enzymes. Explain why each enzyme is useful, and state ONE advantage of using enzymes in washing powders compared to non-biological powders.

Question 4 (4 marks)
Penicillin is produced commercially using fermenters containing the fungus Penicillium.
(a) State TWO conditions that must be controlled in the fermenter. [2]
(b) Explain why sterile conditions are essential. [2]

🧪 Alternative to Practical Mock (Paper 3 Style)

Q1 — Planning (4 marks)
A student investigates the effect of pH on catalase activity using hydrogen peroxide and potato tissue.
(a) State the independent and dependent variables. [2]
(b) Describe how to measure the dependent variable. [1]
(c) State one controlled variable and explain why it must be controlled. [1]

Show Answer

(a) Independent: pH of buffer solution; Dependent: volume of oxygen produced (or rate of bubble production / time for reaction) (1+1)

(b) Collect O₂ gas in an inverted measuring cylinder over water; record volume produced in a set time (e.g. 60 seconds) (1)

(c) Temperature — must be controlled because temperature also affects enzyme activity; a change in temperature would make it impossible to tell if results are due to pH alone (1)

Q2 — Data table and rate calculation (3 marks)
The table shows time taken for starch to be digested at different temperatures.

Temperature (°C)	Time for digestion (s)	Rate (1/time) s^-¹
10	200
20	100
37	40
50	150
70	— (no digestion)

Q3 — Graph description (3 marks)
Using the data above, describe the relationship between temperature and rate of enzyme activity.

Show model answer

Rate increases from 10 °C to 37 °C — reaching a maximum at 37 °C (optimum temperature) (1)

Rate decreases above 37 °C — falling sharply between 37 °C and 50 °C (1)

At 70 °C, rate = 0 — enzyme is denatured / no digestion occurs (1)

Q4 — Identifying anomalies and improvements (2 marks)
In the experiment, a student did not allow the amylase and starch solutions to reach the water bath temperature before mixing. Explain the effect on results and suggest an improvement.

Show Answer

Effect: the actual temperature of the mixture would differ from the water bath temperature → results would be inaccurate / unreliable; true optimum may not be identified (1)

Improvement: allow both solutions to equilibrate (reach the water bath temperature) for at least 5 minutes before mixing (1)

K.H. Exam Prep