Biology • Year 11 • Module 2 • Lesson 11
The Mammalian Digestive System
Apply physical-vs-chemical digestion and enzyme-substrate specificity to real data, pH graphs, and patient scenarios.
1. Interpret enzyme activity vs pH data
The table below shows the relative activity (%) of three digestive enzymes at different pH values. Use the data to answer the questions. 8 marks
| pH | Salivary amylase activity (%) | Pepsin activity (%) | Pancreatic lipase activity (%) |
|---|---|---|---|
| 1 | 5 | 90 | 0 |
| 2 | 10 | 100 | 0 |
| 4 | 40 | 60 | 5 |
| 6 | 85 | 10 | 30 |
| 7 | 100 | 2 | 70 |
| 8 | 60 | 0 | 100 |
| 9 | 20 | 0 | 65 |
| 10 | 5 | 0 | 20 |
1.1 State the pH optimum of each enzyme and explain why each optimum matches the pH of the region where the enzyme is active. 3 marks
1.2 Describe what happens to pepsin activity when chyme moves from the stomach (pH ~2) into the duodenum (pH ~7.5). Using the data and your knowledge of enzyme structure, explain why this change occurs. 3 marks
1.3 A patient takes antacid medication that raises stomach pH from ~2 to ~5. Using the data, predict and explain the effect on protein digestion in the stomach. 2 marks
2. Trace the digestion of a meal
A student eats a ham and cheese sandwich with a glass of milk. The meal contains: bread (starch), ham (protein), cheese (protein and fat), and milk (lactose, protein, fat). 12 marks (3 per macromolecule trace)
For each macromolecule, trace its complete digestion from the mouth to absorbable products. Name: (a) each physical digestion step and location; (b) the enzyme(s), their location, and pH; (c) the final absorbable products.
2.1 Starch (from bread)
2.2 Protein (from ham and cheese)
2.3 Fat (from cheese)
2.4 Lactose (from milk)
3. Apply to a patient scenario, lactose intolerance
A 22-year-old patient presents to their GP with abdominal bloating, gas, and diarrhoea that reliably occur 30–60 minutes after consuming dairy products. Blood tests, endoscopy, and intestinal biopsy are all normal. Genetic testing reveals a mutation that prevents production of the enzyme lactase in the small intestine. 7 marks
3.1 Identify the enzyme that is absent in this patient and state its normal substrate and product(s). 2 marks
3.2 Explain, using enzyme-substrate specificity, why the patient can still digest all other nutrients in dairy foods (protein and fat from cheese; starch if any) normally. 3 marks
3.3 Explain why undigested lactose causes the patient's symptoms. In your answer, refer to the large intestine and bacterial fermentation. 2 marks
4. Interpret a surface area graph
The graph below models the relationship between particle size and total surface area for a fixed volume of food. Use it to answer the questions. 5 marks
Modelled data for a fixed volume of food. Exact values illustrative only.
4.1 Describe the relationship shown in the graph between particle size and total surface area. 1 mark
4.2 Explain, using the graph data, how mastication (chewing) increases the rate of chemical digestion by salivary amylase in the mouth. 2 marks
4.3 Explain why bile emulsification of fats also increases the rate of lipase action, using the same surface-area principle. 2 marks
Q1.1, pH optima and organ pH matching
Salivary amylase: pH optimum ~7 (100% activity at pH 7). Matches the mouth where pH is approximately 6.5–7.0, neutral/slightly acidic. Pepsin: pH optimum ~2 (100% activity at pH 2). Matches the stomach where HCl maintains pH of ~1.5–3.5. Pancreatic lipase: pH optimum ~8 (100% activity at pH 8). Matches the small intestine where bicarbonate from the pancreas raises pH to ~7.0–8.5. (1 mark per enzyme, state optimum and match to organ pH.)
Q1.2, Pepsin denaturation in the duodenum (3 marks)
Pepsin activity falls from 100% at pH 2 to 0% at pH 8, the data shows 2% at pH 7 and 0% at pH 8 or above [1]. This occurs because the active site of pepsin is determined by the precise folding (tertiary structure) of the enzyme, which is maintained by interactions (e.g. hydrogen bonds, ionic bonds) that are stable only at very low pH [1]. At pH 7.5, these interactions are disrupted, the active site changes shape (denatures) and pepsin can no longer bind its polypeptide substrate. This denaturation is irreversible [1].
Q1.3, Antacid effect on protein digestion (2 marks)
At pH 5, pepsin activity is approximately 0–5% (negligibly low based on the data, which shows 10% at pH 6 and 60% at pH 4) [1]. Raising stomach pH to ~5 would therefore severely reduce pepsin activity, meaning protein digestion in the stomach would be greatly impaired, proteins would remain largely as intact chains rather than being cleaved to polypeptides. Some protein digestion would still occur later in the small intestine via trypsin and chymotrypsin, which are not affected by antacid [1].
Q2.1, Starch trace (3 marks)
Physical digestion: Mastication (chewing) in the mouth cuts and grinds starch-containing food into smaller pieces, increasing surface area. Churning in the stomach further reduces particle size (though salivary amylase is denatured there). Chemical digestion: Salivary amylase (mouth, pH ~7) hydrolyses starch → maltose. Salivary amylase is denatured by stomach acid; pancreatic amylase (small intestine, pH ~7.5) continues starch → maltose. Maltase (small intestine brush border) then cleaves maltose → glucose + glucose. Final absorbable product: glucose (monosaccharide).
Q2.2, Protein trace (3 marks)
Physical digestion: Mastication in the mouth increases surface area. Churning in the stomach converts bolus to chyme. Chemical digestion: Pepsin (stomach, pH ~2, activated by HCl from pepsinogen) cleaves proteins → polypeptides. Trypsin and chymotrypsin (pancreatic, small intestine, pH ~8) continue hydrolysis: polypeptides → shorter peptides. Peptidases (intestinal brush border, small intestine) complete digestion: short peptides → amino acids. Final absorbable product: individual amino acids.
Q2.3, Fat trace (3 marks)
Physical digestion: Bile salts (produced by the liver, stored in the gall bladder, released into the duodenum) emulsify fat globules into tiny droplets, dramatically increasing surface area for lipase. This is physical digestion (no bonds broken). Chemical digestion: Pancreatic lipase (small intestine, pH ~7.5) hydrolyses triglycerides → fatty acids + glycerol. Final absorbable products: fatty acids and glycerol (and monoglycerides).
Q2.4, Lactose trace (2 marks)
Chemical digestion: Lactase (small intestine brush border, pH ~6) hydrolyses lactose → glucose + galactose. Final absorbable products: glucose and galactose (monosaccharides). No significant physical digestion step is specific to lactose beyond general churning.
Q3.1, Absent enzyme, substrate, product (2 marks)
The absent enzyme is lactase. Its normal substrate is lactose (a disaccharide found in milk and dairy products). Its products are the monosaccharides glucose and galactose. (1 mark for enzyme named; 1 mark for substrate and products correct.)
Q3.2, Why other nutrients digested normally (3 marks)
Each digestive enzyme has an active site with a shape complementary to one specific substrate, the lock-and-key model of enzyme-substrate specificity [1]. Lactase only binds to and acts on lactose; it has no role in fat, protein, or starch digestion [1]. The patient still produces all other enzymes normally, pepsin, pancreatic lipase, salivary amylase, trypsin, peptidases, maltase, etc., each of which acts on its own specific substrate. The absence of lactase creates no disruption to any other enzymatic pathway [1].
Q3.3, Why undigested lactose causes symptoms (2 marks)
Without lactase, lactose cannot be hydrolysed in the small intestine and passes unabsorbed into the large intestine [1]. There, resident bacteria ferment the lactose, producing gases (hydrogen, carbon dioxide, methane, causing bloating and flatulence) and organic acids, which also draw water into the bowel by osmosis, causing diarrhoea [1].
Q4.1, Relationship description
As particle diameter decreases (smaller particles), total surface area increases sharply (inverse/exponential relationship) for the same fixed volume of food. (1 mark)
Q4.2, Mastication and amylase rate (2 marks)
Mastication physically breaks a food piece into many smaller particles, increasing total surface area (e.g. from 6 cm² at 10 mm to >500 cm² at 0.1 mm per the graph) [1]. Salivary amylase can only act on the surface of starch particles that it directly contacts, a greater surface area means more enzyme-substrate collisions can occur simultaneously, increasing the rate of starch hydrolysis in the mouth [1].
Q4.3, Bile emulsification and lipase rate (2 marks)
Bile salts break large fat globules into millions of tiny droplets (emulsification), dramatically increasing the total surface area of fat exposed [1]. Pancreatic lipase, like all enzymes, can only act at the surface of its substrate, a greater fat surface area means lipase has many more active sites simultaneously accessible to triglyceride molecules, greatly increasing the rate of fat hydrolysis [1].