Edexcel iGCSE Biology-2.13 changes in pH and enzyme function- Study Notes- New Syllabus

Effect of pH on Enzyme Function

📝 Introduction

Enzymes have a specific 3D shape with an active site.
pH (acidity/alkalinity) can affect the bonds in enzymes.
Wrong pH = changes the shape of the active site → substrate no longer fits.

🌟 Optimum pH

• Every enzyme has an optimum pH → works best.
• For most human enzymes → around pH 7 (neutral).
• Special enzymes:
  • Pepsin (in stomach) → works best at pH 2 (acidic).
  • Amylase (in saliva) → optimum at pH 7.

🔼 Below or Above Optimum pH

• The enzyme structure is disturbed.
• Bonds holding the active site break or change.
• Active site shape changes → enzyme is denatured.
• Reaction slows or stops completely.

📊 Graph (in words)

• X-axis = pH, Y-axis = Rate of reaction.
• Curve is a bell shape: rises to a peak at optimum, falls on both sides.
• Unlike temperature (where drop is sharp), with pH the drop is often symmetrical.

🧪 Example

• Pepsin: optimum at pH 2 → digests protein in acidic stomach.
• Trypsin: optimum at pH 8 → digests protein in alkaline small intestine.

📊 Summary Table – pH Effect

⚡ Quick Recap
Enzymes work best at their optimum pH.
Wrong pH → active site shape changes → enzyme denatured.
Human enzymes mostly optimum at pH 7, but some (pepsin, trypsin) adapted to extremes.
Graph = bell curve.