Enzymes don't work in isolation. Temperature, pH, and substrate concentration all push enzyme activity up or down — sometimes with dramatic consequences. Understanding these factors is essential for explaining how cells control their chemistry.
⏱ 45 min3 dot points5 MC · 3 Short AnswerLesson 14 of 17
Think First
A student sets up three test tubes containing starch solution and amylase enzyme:
Tube A: 5°C (refrigerator temperature)
Tube B: 37°C (human body temperature)
Tube C: 80°C (hot water bath)
After 5 minutes, she adds iodine solution to each tube. Iodine turns blue-black in the presence of starch.
Predict the colour of each tube and explain your reasoning:
Come back to this at the end of the lesson.
Know
How temperature affects enzyme activity and causes denaturation
How pH changes alter enzyme shape and function
How substrate concentration affects reaction rate
The meaning of optimum, denaturation, and saturation
Understand
Why enzymes have specific optimal conditions
How temperature and pH affect the active site conformation
Why reaction rate plateaus at high substrate concentration
Can Do
Interpret graphs showing enzyme activity vs. temperature/pH/substrate
Explain denaturation in terms of tertiary structure
Design experiments to investigate enzyme factors
Core Content
Three Factors That Control Enzyme Activity
Every enzyme operates best within a specific range of conditions. Push too far outside this range, and the enzyme stops working — sometimes permanently. The three key environmental factors are:
🌡️ Temperature
Affects kinetic energy of molecules and enzyme shape
Key concept: Denaturation at high temps
⚗️ pH
Affects ionisation of amino acid side chains
Key concept: Each enzyme has optimal pH
📊 Substrate Concentration
Determines collision frequency with active sites
Key concept: Saturation at high [S]
Core principle: Enzymes are proteins with specific 3D shapes. Any factor that changes this shape — or the chemistry of the active site — will affect enzyme activity.
Effect of Temperature on Enzyme Activity
As temperature increases, molecules move faster. This means more frequent collisions between enzyme and substrate, so reaction rate increases — up to a point.
At the optimum temperature, the enzyme works at maximum efficiency. For most human enzymes, this is around 37°C (body temperature).
Beyond the optimum, the kinetic energy becomes too great. The bonds holding the enzyme's tertiary structure break, and the active site changes shape. This is denaturation — and it's usually permanent.
HSC Key Term — Denaturation
The irreversible loss of an enzyme's 3D structure, causing the active site to lose its specific shape. Once denatured, the enzyme cannot bind substrate and becomes non-functional. High temperature, extreme pH, and some chemicals can cause denaturation.
At very low temperatures, enzymes work slowly but are not denatured. The enzyme retains its shape — there's just less kinetic energy for collisions. This is why freezing preserves food: enzymes still exist, but they work too slowly to cause spoilage.
Effect of pH on Enzyme Activity
pH affects the ionisation state of amino acid side chains, especially those in the active site. Changing pH can alter the charges that hold the active site in its specific conformation — or change the charges involved in binding substrate.
Different enzymes have different pH optima based on where they work:
Pepsin (stomach): Optimum pH ~2 — works in acidic gastric juice
Amylase (saliva): Optimum pH ~7 — works in neutral mouth conditions
Trypsin (small intestine): Optimum pH ~8 — works in alkaline pancreatic fluid
Like temperature, extreme pH causes denaturation. The enzyme's tertiary structure unravels as ionic bonds between charged amino acids are disrupted.
Why this matters: Your stomach needs to be acidic for digestion, but the same acidity would destroy intestinal enzymes. This is why food moves from stomach (pH 2) to small intestine (pH 8) — each compartment provides the right conditions for its specific enzymes.
Effect of Substrate Concentration
At low substrate concentration, reaction rate increases linearly with [S] — more substrate means more collisions with available enzyme active sites.
Substrate Concentration vs. Reaction Rate
Rate
│
│ ______ Plateau
│ ___╱ (Vmax)
│ ___╱ Enzyme
│ ___╱ saturated
│ ___╱
│ ___╱
│ ___╱
│ ___╱
│╱
└───────────────────────────────────────────→ [Substrate]
Low → → High
Initially linear → → Becomes constant
(many free active (all active sites
sites available) occupied; limiting
factor is enzyme)
However, once all active sites are occupied, adding more substrate cannot increase the rate further. The enzyme is working at Vmax (maximum velocity), and the reaction is limited by enzyme concentration, not substrate availability.
Key distinction: Temperature and pH affect enzyme shape and therefore function. Substrate concentration affects collision frequency but doesn't change the enzyme itself — it simply determines whether active sites are occupied.
Real World — Fever and Hypothermia
Your body maintains 37°C because that's the optimum for most human enzymes. During infection, fever raises body temperature to slow bacterial growth — but above 40°C, your own enzymes begin to denature. This is why high fevers are dangerous.
Conversely, hypothermia (low body temperature) slows all metabolic reactions. This can be exploited therapeutically: during some surgeries, patients are cooled to reduce metabolic demand and protect tissues from oxygen deprivation.
The HSC requires you to understand how to design valid experiments investigating enzyme activity. Here's a template for investigating any of the three factors:
Standard Experimental Design
Aim: To investigate the effect of [independent variable] on the activity of [enzyme name]
Keep substrate concentration constant (unless it's the IV)
Keep pH and temperature constant (unless one is the IV)
Use a buffer solution to maintain constant pH
Use a water bath to maintain constant temperature
Run a control tube with boiled enzyme (should show no activity)
Common methods for measuring enzyme activity:
Enzyme
Substrate
Measurement Method
Amylase
Starch
Iodine test (blue-black → yellow/brown as starch is digested)
Catalase
Hydrogen peroxide
Oxygen gas production (count bubbles or measure volume)
Protease
Protein (gelatin)
Clearing of cloudy suspension or digestion of photographic film
Safety note: Always wear safety glasses when working with enzymes. Some enzymes (like proteases) can irritate skin. Hydrogen peroxide is corrosive at high concentrations.
Key Definitions
Optimum temperature/pH: The condition at which enzyme activity is highest
Denaturation: Irreversible loss of 3D structure and active site shape
Saturation: When all active sites are occupied by substrate
Vmax: Maximum reaction rate when enzyme is saturated
Identify the optimum temperature for this enzyme. Explain your reasoning.
Explain why the activity at 10°C is low but not zero.
Explain what happens to the enzyme at 60°C and why this change is usually irreversible.
Sketch how you would expect the graph to look if the student repeated the experiment using pepsin from the stomach.
Write your responses here:
Activity 02
Design an Experiment — pH and Catalase
Practical investigation design
Catalase is an enzyme found in liver that breaks down hydrogen peroxide (H₂O₂) into water and oxygen gas. Design an experiment to investigate how pH affects catalase activity.
In your answer, include:
A clear statement of the independent, dependent, and at least three controlled variables
The pH values you would test and why
How you would measure the dependent variable
One safety consideration specific to this experiment
A brief description of the results you would expect and why
Write your experimental design here:
Assessment
Multiple Choice — 5 marks
1. Which of the following best explains why enzyme activity decreases at temperatures above the optimum?
A The substrate molecules move too slowly
B The enzyme's tertiary structure is disrupted
C The active site becomes permanently occupied
D The reaction becomes endothermic
2. An enzyme has an optimum pH of 2. Where in the human body is this enzyme most likely to be found?
A Stomach
B Mouth
C Small intestine
D Blood plasma
3. In an enzyme-catalysed reaction, what happens when substrate concentration increases beyond the point where all active sites are occupied?
A The enzyme denatures
B The reaction rate continues to increase
C The reaction rate remains constant
D The enzyme is permanently inhibited
4. Why can enzymes that have been kept at 0°C resume normal activity when warmed to 37°C, while enzymes kept at 80°C cannot?
A At 0°C, the substrate is frozen but the enzyme is unchanged
B At 0°C, the enzyme's structure is preserved; at 80°C it is denatured
C At 80°C, all the substrate has been used up
D The active site changes shape reversibly at 80°C
5. Which of the following statements about enzyme denaturation is correct?
A Denaturation only affects the primary structure of enzymes
B Denaturation can be reversed by adding more substrate
C Denaturation only occurs at low temperatures
D Denaturation involves loss of tertiary structure and active site shape
Short Answer — 9 marks
1. Explain the effect of increasing temperature on enzyme activity from 0°C to 60°C. In your answer, refer to kinetic energy, collision theory, and denaturation. (3 marks)
1 mark for explaining increased activity below optimum; 1 mark for kinetic energy/collision theory; 1 mark for explaining denaturation above optimum
2. Compare and contrast the effect of substrate concentration versus temperature on enzyme-catalysed reactions. (3 marks)
1 mark for similarity (both affect rate); 1 mark for difference (substrate doesn't change enzyme, temperature does); 1 mark for explaining saturation vs. denaturation
3. A pharmaceutical company is developing a drug that must be stored for long periods. The active ingredient is a protein-based enzyme. Explain why the company might choose to store the drug at 4°C rather than at room temperature (25°C), and why storage at -20°C might be even better for long-term stability. (3 marks)
1 mark for explaining reduced activity at 4°C; 1 mark for explaining risk of denaturation at 25°C; 1 mark for explaining minimal molecular motion at -20°C
Answers
Q1 — B: Above the optimum temperature, the bonds holding the enzyme's tertiary structure break, causing denaturation and loss of active site shape.
Q2 — A: The stomach has a pH of approximately 2 due to hydrochloric acid. Enzymes like pepsin have evolved to work optimally in these acidic conditions.
Q3 — C: When all active sites are occupied (saturation), adding more substrate cannot increase the reaction rate. The enzyme is the limiting factor.
Q4 — B: At 0°C, the enzyme's 3D structure is intact — only molecular motion is reduced. At 80°C, the heat energy breaks bonds in the tertiary structure, causing irreversible denaturation.
Q5 — D: Denaturation specifically involves the loss of tertiary structure (and secondary structure), which includes the specific 3D shape of the active site required for substrate binding.
SA1 (3 marks): As temperature increases from 0°C to approximately 37°C (the optimum), enzyme activity increases because the kinetic energy of both enzyme and substrate molecules increases, leading to more frequent collisions and more successful substrate binding (collision theory). However, beyond the optimum temperature, the increased thermal energy disrupts the weak bonds (hydrogen bonds, ionic bonds) that maintain the enzyme's tertiary structure. This causes denaturation — the active site loses its specific shape, substrate can no longer bind, and enzyme activity decreases sharply.
SA2 (3 marks): Both increasing substrate concentration and increasing temperature (up to the optimum) increase the rate of enzyme-catalysed reactions. However, substrate concentration affects only the frequency of enzyme-substrate collisions and does not change the enzyme itself — once saturation is reached (Vmax), the rate plateaus. In contrast, temperature affects the enzyme's structure directly; above the optimum, the enzyme denatures and activity decreases permanently. Additionally, while increasing substrate concentration cannot damage the enzyme, excessive temperature causes irreversible loss of function.
SA3 (3 marks): At 4°C, the enzyme maintains its tertiary structure and would function normally if warmed, but its activity is significantly reduced due to lower kinetic energy — making it suitable for short-term storage. At 25°C (room temperature), the enzyme remains active and could gradually degrade over time due to ongoing catalytic activity and potential denaturation. Storage at -20°C is superior for long-term stability because the frozen state essentially stops all molecular motion, preventing both catalytic degradation and any gradual denaturation processes that might occur even at 4°C.
Revisit Your Thinking
Earlier you predicted the results of an amylase experiment at three temperatures. Compare your predictions with the correct answers below:
Tube
Temperature
Expected Result
Explanation
A
5°C
Blue-black (starch present)
Low kinetic energy — enzyme works very slowly. After only 5 minutes, most starch remains undigested.
B
37°C
Yellow/brown (little/no starch)
Optimum temperature for human amylase. Rapid digestion of starch occurs.
C
80°C
Blue-black (starch present)
Enzyme is denatured. The active site has lost its shape and cannot bind starch.
Were your predictions correct? Did you recognise that both 5°C and 80°C would show starch present, but for very different reasons?