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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Each enzyme has an optimum pH at which it works fastest. Above or below this pH, the enzyme's active site changes shape and the enzyme denatures. Most human enzymes have an optimum pH around 7, but pepsin (stomach) works best at pH 2 and pancreatic lipase at pH 8–9.
Different parts of the digestive system have different pH levels, and each digestive enzyme is adapted to work best at the pH of its location. Understanding how pH affects enzyme activity is essential for digestion questions and practical enzyme experiments.
How pH Affects Enzyme Activity
pH measures the concentration of hydrogen ions (H⁺) in a solution. Changes in pH affect enzymes by:
Altering the charges on the amino acids in the active site.
Breaking ionic and hydrogen bonds that maintain the enzyme's 3D shape.
Causing the active site to change shape: the substrate can no longer fit.
The enzyme is denatured.
Small changes in pH away from the optimum reduce activity. Large changes cause irreversible denaturation.
Optimum pH of Key Digestive Enzymes
Enzyme
Optimum pH
Location
Substrate → Product
Pepsin
pH 2 (acidic)
Stomach
Proteins → shorter peptides
Salivary amylase
pH 7 (neutral)
Mouth
Starch → maltose
Pancreatic lipase
pH 8–9 (alkaline)
Small intestine
Lipids → glycerol + fatty acids
Trypsin
pH 8 (alkaline)
Small intestine
Proteins → amino acids
Why does the stomach produce HCl? Hydrochloric acid provides the acidic conditions (pH 2) needed for pepsin to work. It also kills many bacteria entering with food.
Why does the pancreas produce sodium bicarbonate? This neutralises the acid from the stomach, creating the alkaline conditions needed for pancreatic enzymes to work in the small intestine.
The pH-Activity Graph
The graph is bell-shaped (like the temperature graph) but can be symmetrical:
Rate increases as pH approaches the optimum.
Rate is maximum at the optimum pH.
Rate decreases on either side of the optimum as the active site begins to denature.
Comparing enzymes: If asked to sketch curves for pepsin and amylase on the same axes, pepsin's peak is at pH 2 and amylase's peak is at pH 7. They have different optimum pH values because they work in different parts of the body.
Try a focused question (3 marks)
Explain why pepsin works in the stomach but not in the small intestine. (3 marks)
Show the answer and marking guidance
Pepsin has an optimum pH of about 2 (acidic), which matches the stomach environment.
The small intestine has an alkaline pH (about 8) due to bile and pancreatic secretions.
At pH 8, pepsin is denatured: its active site changes shape and cannot bind to protein substrates.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying all enzymes have an optimum pH of 7: pepsin works best at pH 2.
Confusing the effect of pH with the effect of temperature: both cause denaturation but through different mechanisms.
Forgetting to explain WHY the active site changes: pH alters charges on amino acids, breaking bonds that maintain the 3D shape.