B7: The Digestive System
Enzymes, digestion, bile, villi, and food tests
Key Definitions
Digestion — The breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood.
Mechanical digestion — Physical breakdown of food (e.g. chewing, churning in the stomach).
Chemical digestion — Breakdown of food using enzymes (biological catalysts).
Enzyme — A biological catalyst that speeds up chemical reactions without being used up; each enzyme acts on a specific substrate.
Absorption — The movement of small, digested molecules from the small intestine into the blood.
Assimilation — The use of absorbed molecules by body cells (e.g. glucose for respiration, amino acids for protein synthesis).
Egestion — The removal of undigested food (faeces) from the body via the rectum and anus (NOT excretion).
Mechanical digestion — Physical breakdown of food (e.g. chewing, churning in the stomach).
Chemical digestion — Breakdown of food using enzymes (biological catalysts).
Enzyme — A biological catalyst that speeds up chemical reactions without being used up; each enzyme acts on a specific substrate.
Absorption — The movement of small, digested molecules from the small intestine into the blood.
Assimilation — The use of absorbed molecules by body cells (e.g. glucose for respiration, amino acids for protein synthesis).
Egestion — The removal of undigested food (faeces) from the body via the rectum and anus (NOT excretion).
The Alimentary Canal
| Organ | Function | Key Processes |
|---|---|---|
| Mouth | Begin mechanical and chemical digestion | Teeth chew food (mechanical); saliva contains amylase (starch → sugars); tongue mixes food with saliva |
| Oesophagus | Transport food to stomach | Peristalsis — waves of muscular contraction push food along |
| Stomach | Continue digestion; kill bacteria | Muscular walls churn food (mechanical); produces protease (protein → amino acids) and HCl (kills bacteria, optimum pH for protease ~pH 2) |
| Liver | Produce bile | Bile emulsifies fats (increases surface area for lipase); neutralises stomach acid (raises pH to ~pH 8 in small intestine) |
| Pancreas | Produce digestive enzymes | Produces amylase, protease, and lipase; releases these into the small intestine via the pancreatic duct |
| Small intestine | Digestion and absorption | Final digestion by enzymes; villi and microvilli increase SA for absorption of digested molecules into blood |
| Large intestine | Absorb water | Water absorbed from remaining undigested material, forming faeces |
| Rectum | Store faeces | Faeces stored before being expelled through the anus (egestion) |
Enzymes and Digestion
| Enzyme | Substrate | Product | Where produced | Where it works | Optimum pH |
|---|---|---|---|---|---|
| Amylase | Starch | Maltose (and then glucose) | Salivary glands; Pancreas | Mouth; Small intestine | ~pH 7–8 |
| Protease | Proteins | Amino acids | Stomach; Pancreas | Stomach; Small intestine | ~pH 2 (stomach); ~pH 8 (small intestine) |
| Lipase | Lipids (fats) | Fatty acids + Glycerol | Pancreas | Small intestine | ~pH 8 |
Exam tip: Amylase works in the mouth AND small intestine. Protease works in the stomach AND small intestine. Lipase works ONLY in the small intestine. The pancreas produces ALL THREE enzymes. The stomach does NOT produce amylase or lipase.
The Lock and Key Model
Lock and key model of enzyme action:
— Each enzyme has a specifically shaped active site that only fits one type of substrate (the "key").
— The substrate binds to the active site, forming an enzyme-substrate complex.
— The reaction takes place and the substrate is broken down into products.
— The products are released and the enzyme is free to catalyse another reaction (it is not used up).
Specificity: Because the active site has a unique shape, each enzyme can only catalyse ONE specific reaction. For example, amylase can only break down starch — it cannot break down protein or lipid.
— Each enzyme has a specifically shaped active site that only fits one type of substrate (the "key").
— The substrate binds to the active site, forming an enzyme-substrate complex.
— The reaction takes place and the substrate is broken down into products.
— The products are released and the enzyme is free to catalyse another reaction (it is not used up).
Specificity: Because the active site has a unique shape, each enzyme can only catalyse ONE specific reaction. For example, amylase can only break down starch — it cannot break down protein or lipid.
Denaturation
Denaturation — When an enzyme's active site changes shape so the substrate can no longer fit. The enzyme can no longer catalyse the reaction.
Causes of denaturation:
— High temperature — Vibrations break the bonds holding the enzyme's shape; the active site deforms. This is permanent (irreversible).
— Extreme pH — Affects the charges on the enzyme's amino acids, breaking bonds and changing the active site shape.
Important: Denaturation is NOT the same as a low reaction rate at low temperatures. At low temperatures, the enzyme still works — just slowly (less kinetic energy). Denaturation permanently destroys the enzyme's function.
Causes of denaturation:
— High temperature — Vibrations break the bonds holding the enzyme's shape; the active site deforms. This is permanent (irreversible).
— Extreme pH — Affects the charges on the enzyme's amino acids, breaking bonds and changing the active site shape.
Important: Denaturation is NOT the same as a low reaction rate at low temperatures. At low temperatures, the enzyme still works — just slowly (less kinetic energy). Denaturation permanently destroys the enzyme's function.
Exam tip: If asked "what happens to an enzyme at high temperature?", say "the active site changes shape and the enzyme is denatured, so the substrate no longer fits". Do NOT say "the enzyme is killed" — enzymes are not alive.
Example 1: How Temperature Affects Enzyme Activity
At low temperatures (e.g. 0°C), enzyme activity is very low because molecules have little kinetic energy and move slowly — fewer enzyme-substrate collisions. As temperature increases, activity increases because molecules move faster and collide more often. At the optimum temperature (around 37°C for human enzymes), the rate is at its maximum. Above the optimum, the enzyme denatures — the active site changes shape permanently and the rate drops sharply.
Example 2: How pH Affects Protease in the Stomach
Stomach protease (pepsin) has an optimum pH of about 2 because the stomach produces hydrochloric acid, creating very acidic conditions. If the pH rises above or falls below 2, the protease works less efficiently. At pH 7 or above, the enzyme becomes denatured because the change in pH alters the charges and bonds that maintain the active site shape. This is why bile is needed to neutralise acid before small intestine enzymes can work.
Bile
Bile is produced in the liver, stored in the gall bladder, and released into the small intestine.
Two functions of bile:
1. Neutralises stomach acid — Bile is alkaline (pH 8–9); it raises the pH from ~2 in the stomach to ~8 in the small intestine, creating the optimum pH for pancreatic enzymes (amylase, lipase, and small intestine protease).
2. Emulsifies fats — Bile breaks large fat droplets into tiny droplets (much smaller), increasing their surface area. This means lipase enzyme has a much larger surface area to act on, so fat digestion is faster.
Bile is NOT an enzyme — it does not chemically break down fats; it only physically breaks them into smaller droplets (emulsification). Lipase is the enzyme that actually digests lipids.
Two functions of bile:
1. Neutralises stomach acid — Bile is alkaline (pH 8–9); it raises the pH from ~2 in the stomach to ~8 in the small intestine, creating the optimum pH for pancreatic enzymes (amylase, lipase, and small intestine protease).
2. Emulsifies fats — Bile breaks large fat droplets into tiny droplets (much smaller), increasing their surface area. This means lipase enzyme has a much larger surface area to act on, so fat digestion is faster.
Bile is NOT an enzyme — it does not chemically break down fats; it only physically breaks them into smaller droplets (emulsification). Lipase is the enzyme that actually digests lipids.
Example 3: Why Bile Is Needed Before Fat Digestion
Without bile, large fat droplets have a small surface area for lipase to act on, so fat digestion is slow. Bile emulsifies the fat — breaking large droplets into many tiny droplets — greatly increasing the surface area. Lipase can now act on a much larger area, speeding up digestion of lipids into fatty acids and glycerol. Bile also neutralises the acidic chyme from the stomach, providing the alkaline pH needed for lipase to work at its optimum rate.
Villi and Microvilli
Villi — Finger-like projections that line the inner surface of the small intestine, increasing the surface area for absorption.
Adaptations of villi for efficient absorption:
— Large surface area — Thousands of villi per square millimetre; each villus is covered in microvilli (even smaller projections) on its epithelial cells, further increasing SA.
— Single layer of epithelial cells — Very thin walls provide a short diffusion distance from the gut lumen to the blood.
— Dense network of blood capillaries — Carries absorbed glucose and amino acids away quickly, maintaining a steep concentration gradient for continued diffusion.
— Lacteal — A small lymph vessel inside each villus that absorbs fatty acids and glycerol (products of fat digestion) — these are too large to enter blood capillaries directly.
Adaptations of villi for efficient absorption:
— Large surface area — Thousands of villi per square millimetre; each villus is covered in microvilli (even smaller projections) on its epithelial cells, further increasing SA.
— Single layer of epithelial cells — Very thin walls provide a short diffusion distance from the gut lumen to the blood.
— Dense network of blood capillaries — Carries absorbed glucose and amino acids away quickly, maintaining a steep concentration gradient for continued diffusion.
— Lacteal — A small lymph vessel inside each villus that absorbs fatty acids and glycerol (products of fat digestion) — these are too large to enter blood capillaries directly.
Example 4: How Villi Maintain a Concentration Gradient
As glucose is absorbed into the blood capillaries of a villus, the blood carries it away in the bloodstream. This keeps the glucose concentration in the blood low compared to the gut lumen, maintaining a steep concentration gradient. This ensures glucose continues to diffuse from the gut (high concentration) into the blood (low concentration) rapidly. Good blood supply is essential for this process.
Food Tests Required Practical
| Food Test | Reagent | Positive Result | Tests For |
|---|---|---|---|
| Biuret test | Biuret reagent (copper sulfate + sodium hydroxide) | Purple / violet colour | Protein |
| Iodine test | Iodine solution | Blue-black colour | Starch |
| Benedict's test | Benedict's reagent (heat in water bath) | Orange / brick-red precipitate (positive); blue (negative); green (low); yellow (moderate) | Reducing sugars (glucose) |
| Sudan III test | Sudan III stain | Red-stained oil layer floats on top of water | Lipids (fats) |
Example 5: Interpreting Food Test Results
A food sample is tested: Biuret test turns purple, Iodine test stays orange-brown, Benedict's test turns brick-red, Sudan III shows no red layer. Conclusion: the sample contains protein and reducing sugars, but no starch and no lipids.
Example 6: Benedict's Test and Concentration
The colour of the Benedict's test result indicates the concentration of reducing sugar: blue = none, green = low, yellow = moderate, orange = high, brick-red = very high. This is a semi-quantitative test — it gives an indication of how much sugar is present, not just whether it is present.
Exam tip: Benedict's test requires HEATING (in a water bath, not directly over a Bunsen burner — it is a safety hazard). The Biuret test does NOT require heating. For the Sudan III test, the sample must be mixed with water and shaken — lipids will separate and the red-stained layer floats on top.
Practice Questions
1. Foundation Name the enzyme that digests starch and state where it is produced and what it produces.
Amylase digests starch into maltose (and then glucose). It is produced in the salivary glands and the pancreas. It works in the mouth and the small intestine.
2. Foundation Describe two functions of bile in digestion.
1) Bile neutralises stomach acid, raising the pH to alkaline conditions (~pH 8) which is the optimum pH for enzymes in the small intestine (amylase, lipase, protease). 2) Bile emulsifies fats — breaking large fat droplets into tiny droplets, increasing the surface area for lipase to act on, speeding up fat digestion.
3. Higher Explain how the lock and key model accounts for the specificity of enzymes.
Each enzyme has an active site with a unique shape that is complementary to only one specific substrate. Only the correct substrate can fit into the active site, like a key fitting into a lock. When the substrate binds, an enzyme-substrate complex forms and the reaction occurs. If the substrate does not match the active site shape, no reaction can take place. This is why amylase can only break down starch and cannot digest proteins or lipids.
4. Higher Describe three adaptations of villi for efficient absorption of digested food.
1) Large surface area — thousands of villi and microvilli greatly increase the surface area for absorption. 2) Thin walls — single layer of epithelial cells provides a short diffusion distance from the gut into the blood. 3) Dense capillary network — carries absorbed molecules away quickly, maintaining a steep concentration gradient for continued diffusion. Also acceptable: lacteal absorbs fatty acids and glycerol.
5. Foundation A food sample turns blue-black with iodine solution and purple with Biuret reagent. Identify the nutrients present.
Blue-black with iodine indicates starch is present. Purple with Biuret indicates protein is present.
6. Higher Explain the difference between denaturation and a low reaction rate at low temperature.
At low temperature, the enzyme still has its correct shape but molecules have less kinetic energy so they move slowly and collide less often — the rate is low but the enzyme is not damaged. If the temperature returns to optimum, the rate increases again. Denaturation is caused by high temperature or extreme pH — the active site permanently changes shape so the substrate can no longer fit. Denaturation is irreversible; the enzyme cannot recover its function.
🔬 Required Practical: Food Tests
Food Tests (RP 3)
| Food Test | Reagent | Positive Result | Tests For |
|---|---|---|---|
| Biuret test | Biuret reagent (copper sulfate + sodium hydroxide) | Purple / violet colour | Protein |
| Benedict's test | Benedict's reagent (heat in water bath) | Orange / brick-red precipitate | Reducing sugars |
| Iodine test | Iodine solution | Blue-black colour | Starch |
| Ethanol emulsion test | Ethanol + water | Cloudy white emulsion | Lipids (fats) |
Safety: Benedict's test requires heating in a water bath (not a Bunsen burner directly). Wear eye protection. Biuret reagent contains sodium hydroxide — irritant, avoid skin contact.
🔢 Maths Skills
Mathematical Skills
Calculating rate of digestion: rate = amount of substrate broken down ÷ time. For example, if 12 g of starch is digested by amylase in 6 minutes, the rate = 12 ÷ 6 = 2 g/min. You may also need to calculate rate from graphs — find the gradient of the line (change in y ÷ change in x).
⚠️ Common Misconceptions
Watch Out!
Students often think enzymes are used up in reactions. Wrong: Enzymes are consumed during the reaction Correct: Enzymes are biological catalysts — they speed up reactions without being used up or changed
Students often think the stomach digests all food. Wrong: The stomach digests all types of food molecules Correct: Specific enzymes work in specific regions — the stomach mainly digests protein with protease; amylase and lipase work in the small intestine
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how enzyme structure relates to function and how temperature and pH affect activity.
Enzymes have a specifically shaped active site that is complementary to only one substrate (lock and key model). This means each enzyme catalyses only one reaction — amylase only breaks down starch. At low temperatures, enzyme activity is low because molecules have little kinetic energy and collide infrequently. As temperature increases, activity increases because molecules move faster and collide more often. At the optimum temperature (~37°C for human enzymes), the rate is maximum. Above the optimum, the active site changes shape permanently (denaturation) and the substrate can no longer fit, so the rate drops sharply. Each enzyme also has an optimum pH — for example, stomach protease works best at pH 2, while pancreatic enzymes work best at pH 8. Extreme pH changes the charges and bonds in the enzyme, altering the active site shape and reducing or stopping activity.
Mark scheme: 1 mark for describing the lock and key model and active site specificity; 1 mark for explaining the effect of low temperature (kinetic energy); 1 mark for stating the optimum temperature and peak activity; 1 mark for explaining denaturation at high temperature; 1 mark for explaining the effect of pH on active site shape with a named example; 1 mark for using correct scientific terminology (active site, denaturation, complementary, optimum)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Given a table of enzyme activity at different pH values, identify the optimum pH and explain your reasoning. For example: at pH 2, activity = 85%; pH 4, activity = 30%; pH 7, activity = 5%; pH 8, activity = 2%. The optimum pH is 2 because this gives the highest activity. The enzyme is likely stomach protease (pepsin), which works in the acidic conditions of the stomach. As pH increases, the active site shape changes, reducing the enzyme's ability to bind substrate.
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🎬 Video Resources
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