Edexcel A Level (IAL) Biology -2.8 Core Practical 4: Investigating the Rate of Enzyme Reactions- Study Notes- New Syllabus

CORE PRACTICAL 4 – Factors Affecting Enzyme Activity

🌱 Introduction

Enzymes are biological catalysts that speed up reactions without being used up.
Their activity depends on:

• Temperature
• pH
• Enzyme concentration
• Substrate concentration

This experiment studies how these factors affect the initial rate of enzyme-catalysed reactions.

🎯 Aim

To investigate how temperature, pH, enzyme concentration, and substrate concentration affect the initial rate of a reaction catalysed by an enzyme.

🔬 Materials

• Enzyme solution (e.g., catalase from potato, liver, or yeast)
• Substrate solution (e.g., hydrogen peroxide for catalase)
• Test tubes / reaction vessels
• Water bath (for temperature control)
• pH buffers of different values
• Pipettes / droppers
• Stopwatch
• Measuring cylinder / gas syringe / beaker

⚗️ Principle

Enzymes bind substrate at the active site → form enzyme-substrate complex → product is released.
Initial rate is measured before substrate depletion or product inhibition occurs.

Factors affecting rate:

• Temperature: affects kinetic energy & enzyme shape
• pH: affects active site structure
• Enzyme concentration: more active sites → faster reaction
• Substrate concentration: more substrate → faster reaction until enzyme saturation

⚡ Procedure Overview

• Temperature effect: Keep enzyme & substrate constant. Incubate at different temperatures (0°C, 20°C, 37°C, 50°C, 70°C). Measure initial rate (e.g., O₂ released).
• pH effect: Use buffers at different pH values. Keep enzyme & substrate same. Measure initial rate.
• Enzyme concentration effect: Keep substrate constant. Use increasing enzyme concentrations. Measure initial rate.
• Substrate concentration effect: Keep enzyme constant. Vary substrate concentration. Measure initial rate and observe saturation.

📊 Observations

🔍 Explanation

• Temperature: low → slow reaction; high → denaturation
• pH: affects charges on amino acids → active site shape changes
• Enzyme concentration: rate increases with enzyme if substrate enough
• Substrate concentration: rate increases with substrate until enzyme saturated

📈 Graphical Trends

• Temperature & pH: bell-shaped curve (optimal peak)
• Enzyme conc.: linear increase until substrate limiting
• Substrate conc.: hyperbolic curve → plateaus at saturation

⚠️ Precautions

• Use same enzyme & substrate source.
• Mix gently, avoid bubbles.
• Measure initial rate quickly.
• Control temperature & pH carefully.

🧠 Quick Recap 
Factor & Effect:
Temperature: Low → slow, High → denature → optimal ~37°C (human enzymes)
pH: Extremes → denature → specific to enzyme (e.g., catalase pH 7)
Enzyme conc.: Rate ↑ with conc. → until substrate limiting
Substrate conc.: Rate ↑ with conc. → plateaus at saturation
Initial rate: measured immediately after reaction starts
Enzymes = specific catalysts influenced by environmental factors