B5: The Digestive System

Q: A student tests a food sample with iodine solution and it turns blue-black. They then do the Biuret test and it stays blue. What nutrients are present and absent?

The blue-black result with iodine shows that starch is present . The blue result (no colour change) with the Biuret test shows that protein is absent . A positive Biuret result would be purple/violet.

FoundationHigher AQAEdexcelOCRCCEA

The human digestive system and enzyme action

Exam Board:

📋 Key Concepts

Digestion breaks down large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream. Mechanical digestion physically breaks food apart; chemical digestion uses enzymes to break molecules apart.

Key Terms

Alimentary canal - the long tube from mouth to anus through which food passes
Mechanical digestion - physical breakdown (e.g. chewing, churning)
Chemical digestion - breakdown using enzymes
Amylase - enzyme that breaks down starch into sugars
Protease - enzyme that breaks down protein into amino acids
Lipase - enzyme that breaks down lipids into fatty acids and glycerol
Bile - substance produced by the liver that emulsifies fats
Denaturation - when an enzyme's active site changes shape due to high temperature or extreme pH

📝 The Alimentary Canal

The alimentary canal is the long tube running from the mouth to the anus. Food is digested and absorbed as it passes through each section.

Organ	Function	Key Digestive Process
Mouth	Breaks food down mechanically and chemically	Teeth chew food; salivary amylase begins starch digestion
Oesophagus	Carries food from mouth to stomach	Peristalsis (muscle contractions) pushes food along
Stomach	Churns and digests food	Protease digests protein; HCl kills bacteria and provides optimum pH
Small intestine	Digests food and absorbs nutrients	All three enzymes act here; villi absorb small soluble molecules into blood
Large intestine	Absorbs water	Water is absorbed from undigested food, forming faeces
Rectum	Stores faeces	Faeces stored until they leave the body via the anus

Example 1

Question: Describe the role of the mouth in digestion.

Answer: In the mouth, teeth carry out mechanical digestion by chewing and breaking food into smaller pieces (increasing surface area). Salivary glands release saliva containing amylase, which begins chemical digestion by breaking down starch into maltose (a sugar). The tongue mixes food with saliva and shapes it into a bolus for swallowing.

📝 Enzymes and Digestion

Enzymes are biological catalysts that speed up the breakdown of large food molecules into smaller ones. Each enzyme works on a specific substrate. Digestive enzymes are produced by glands in the mouth, stomach, pancreas and small intestine.

Enzyme	Substrate	Product	Where Produced	Where It Acts
Amylase	Starch	Sugars (maltose)	Salivary glands, pancreas	Mouth, small intestine
Protease	Protein	Amino acids	Stomach, pancreas	Stomach, small intestine
Lipase	Lipids (fats)	Fatty acids and glycerol	Pancreas	Small intestine

Digestion equations:
Starch → (amylase) → Sugars (maltose)
Protein → (protease) → Amino acids
Lipids → (lipase) → Fatty acids + Glycerol

The pancreas produces all three digestive enzymes (amylase, protease and lipase) and releases them into the small intestine. It is both an enzyme producer and a hormone producer (insulin and glucagon).

Example 2

Question: Explain why amylase cannot break down protein.

Answer: Each enzyme has a specific active site that is complementary in shape to only one substrate. Amylase's active site is complementary to starch, not protein. The protein molecule does not fit into amylase's active site, so it cannot be broken down. Only protease can break down protein because its active site is the correct shape.

Example 3

Question: A person's pancreas stops producing lipase. Explain what effect this would have on their digestion.

Answer: Without lipase, lipids (fats) cannot be broken down into fatty acids and glycerol in the small intestine. The large, insoluble lipid molecules cannot be absorbed into the blood, so the person would be unable to get energy and nutrients from fats. Undigested fat would pass through the digestive system and appear in the faeces.

📝 The Lock and Key Model

The lock and key model explains enzyme specificity. The enzyme's active site has a specific shape (like a lock), and only the substrate with the complementary shape (like a key) can fit into it. When the substrate binds, the enzyme catalyses the reaction and releases the products.

Lock and key model:
Enzyme (lock) + Substrate (key) → Enzyme-substrate complex → Enzyme + Products
The enzyme is NOT used up in the reaction - it can be reused.

Denaturation

If the temperature is too high, or the pH is too extreme, the active site changes shape
The substrate can no longer fit into the active site
The enzyme is said to be denatured
Denaturation is permanent - the enzyme cannot return to its original shape

Denaturation vs breakdown: Denaturation changes the shape of the active site so the enzyme no longer works. It is NOT the same as the enzyme being broken down or digested. The enzyme molecule still exists, but its active site has the wrong shape.

Optimum Conditions

Optimum temperature - the temperature at which an enzyme works best (around 37°C for human enzymes)
Optimum pH - the pH at which an enzyme works best (varies: stomach protease ~pH 2; pancreatic enzymes ~pH 7-8)

Enzyme	Optimum pH	Why?
Stomach protease	~pH 2 (acidic)	The stomach produces hydrochloric acid, creating acidic conditions
Salivary amylase	~pH 6-7 (near neutral)	The mouth has near-neutral conditions
Pancreatic enzymes	~pH 7-8 (slightly alkaline)	Bile neutralises acid and makes the small intestine slightly alkaline

Example 4

Question: Explain why stomach protease works well in the stomach but not in the small intestine.

Answer: Stomach protease has an optimum pH of around 2 (acidic), which matches the conditions in the stomach where hydrochloric acid is produced. The small intestine has a pH of around 7-8 (slightly alkaline) because bile neutralises the stomach acid. At this higher pH, the stomach protease is denatured - its active site changes shape so it can no longer bind to its substrate and catalyse the reaction.

📝 Bile

Bile is produced in the liver, stored in the gall bladder, and released into the small intestine. It has two important functions: emulsifying fats and neutralising stomach acid.

Two Functions of Bile

Emulsification: Bile breaks large drops of fat into smaller droplets (emulsification). This increases the surface area of the fat, making it easier for lipase to break it down. Note: emulsification is NOT digestion - it is physical breakdown, not chemical.
Neutralisation: Bile is alkaline, so it neutralises the hydrochloric acid from the stomach. This creates the slightly alkaline conditions that pancreatic enzymes need to work at their optimum pH.

Exam trap: Bile does NOT digest fat - it emulsifies it. Bile does NOT break chemical bonds. Only lipase chemically digests fat by breaking the bonds in lipid molecules.

Example 5

Question: Explain how bile helps lipase work more efficiently.

Answer: Bile emulsifies fats, breaking large fat drops into many smaller droplets. This increases the surface area of the fat, giving lipase more surface to work on and speeding up digestion. Bile also neutralises the acidic mixture from the stomach, creating the slightly alkaline pH (around 7-8) that lipase needs to work at its optimum rate.

📝 Absorption in the Small Intestine

The small intestine is where small soluble molecules are absorbed into the blood. The inner surface is covered with millions of finger-like projections called villi, which increase the surface area for absorption.

How Villi Increase Absorption

Large surface area - villi and microvilli (tiny projections on villi cells) greatly increase the surface area for absorption
Single layer of cells - the wall of each villus is only one cell thick, so there is a short diffusion path
Good blood supply - each villus contains a network of blood capillaries that carry away absorbed molecules, maintaining a steep concentration gradient
Lacteal - each villus contains a lymph vessel (lacteal) that absorbs fatty acids and glycerol

Example 6

Question: Explain how the structure of villi is adapted for efficient absorption. (4 marks)

Answer: Villi have a large surface area (increased further by microvilli) which provides more space for absorption. The wall of each villus is only one cell thick, creating a short diffusion path for molecules to pass into the blood. Each villus has a dense network of blood capillaries that maintain a steep concentration gradient by carrying absorbed molecules away. Each villus also has a lacteal to absorb the products of fat digestion.

📝 Food Tests Practical

You must know the four food tests for identifying the presence of different nutrients in food samples. These are required practicals for GCSE Combined Science.

Nutrient	Test	Method	Positive Result
Starch	Iodine test	Add iodine solution to the food sample	Blue-black colour
Reducing sugars	Benedict's test	Add Benedict's reagent, heat in a water bath at 75°C for 5 minutes	Blue → green → yellow → orange → brick red
Protein	Biuret test	Add Biuret reagent (sodium hydroxide and copper sulfate) to the food sample	Blue → purple/violet
Lipids	Sudan III test	Add Sudan III stain to the food sample and shake	Red-stained oil layer separates and floats to the top

Important: For Benedict's test, you must HEAT the sample. For Biuret test, you do NOT heat the sample. For the Sudan III test, lipids float because they are less dense than water.

Example 7

Question: A student tests a food sample with iodine solution and it turns blue-black. They then do the Biuret test and it stays blue. What nutrients are present and absent?

Answer: The blue-black result with iodine shows that starch is present. The blue result (no colour change) with the Biuret test shows that protein is absent. A positive Biuret result would be purple/violet.

❓ Practice Questions

Q1: Name the enzyme that breaks down starch and state where it is produced. (2 marks)

Q2: Describe how bile helps the digestion of fats. (3 marks)

Q3: Explain why an enzyme denatures at high temperature. (3 marks)

Q4: Describe how villi are adapted for efficient absorption in the small intestine. (4 marks)

Q5: A student tests a food sample with Benedict's reagent and heats it. The solution turns brick red. What does this result show? Name one other food test and describe how to carry it out. (4 marks)

Q6: Explain why stomach protease has a different optimum pH to pancreatic protease. (3 marks)

✅ Answers

Amylase. It is produced in the salivary glands and the pancreas (and acts in the mouth and small intestine).
Bile emulsifies fats by breaking large fat drops into smaller droplets, increasing the surface area for lipase to act on. Bile also neutralises stomach acid, creating the slightly alkaline conditions that pancreatic enzymes (including lipase) need to work at their optimum rate.
At high temperature, the bonds holding the enzyme's structure together break. This causes the active site to change shape so the substrate can no longer fit. The enzyme is denatured and can no longer catalyse the reaction. Denaturation is permanent.
Villi are finger-like projections that increase the surface area for absorption. Each villus has microvilli on the surface of its cells which further increase surface area. The wall of each villus is only one cell thick, creating a short diffusion path for molecules to enter the blood. Each villus has a good blood supply (capillary network) which maintains a steep concentration gradient by carrying away absorbed molecules.
The brick red result with Benedict's test shows that reducing sugars are present in a high concentration. Another food test is the iodine test for starch: add iodine solution to the food sample; a blue-black colour indicates starch is present.
Stomach protease works in the stomach, which contains hydrochloric acid creating very acidic conditions (pH ~2). Its active site is adapted to work at this low pH. Pancreatic protease works in the small intestine, where bile neutralises acid and creates slightly alkaline conditions (pH ~7-8). Its active site is adapted to work at this higher pH. Each enzyme has evolved to work at the pH of the region where it functions.

🎯 Exam Tips

Always say "breaks down starch into sugars" for amylase - don't say "breaks down carbohydrates" as this is too vague
Bile EMULSIFIES fat, it does NOT DIGEST fat - only lipase digests fat
When explaining denaturation, always mention the active site changing shape - do not say "the enzyme dies"
For food tests, learn both the method AND the positive result - you may be asked to describe the full procedure
Benedict's test requires HEATING - missing this loses marks
For villi questions, mention at least two adaptations: surface area, thin wall, blood supply, or lacteal
The pancreas produces ALL THREE enzymes - this is a common exam fact
Remember the difference: mechanical digestion (physical) vs chemical digestion (enzymes)

🔬 Required Practical

Food Tests (Biuret, Benedict's, Iodine, Ethanol Emulsion)

Nutrient	Test	Method	Positive Result
Starch	Iodine	Add iodine solution to sample	Blue-black
Reducing sugars	Benedict's	Add reagent, heat at 75 °C for 5 min	Blue → green → yellow → orange → brick red
Protein	Biuret	Add Biuret reagent (NaOH + CuSO₄) — do NOT heat	Blue → purple/violet
Lipids	Ethanol emulsion	Dissolve in ethanol, pour into water	Cloudy white emulsion

Safety: Wear eye protection. Benedict's test requires a hot water bath — never heat directly over a Bunsen burner.

🔢 Maths Skills

Mathematical Skills

Rate of digestion = mass of product formed ÷ time
Or: rate = volume of substrate broken down ÷ time

Maths Example

An enzyme breaks down 4.8 g of starch in 6 minutes. Calculate the rate of digestion.

Rate = 4.8 ÷ 6 = 0.8 g/min

⚠️ Common Misconceptions

Watch Out!

1. Wrong: Enzymes are used up in reactions Correct: Enzymes are catalysts — they are unchanged after the reaction and can be reused

2. Wrong: The stomach digests everything Correct: Different enzymes work in specific regions — stomach protease digests protein; lipase and amylase act mainly in the small intestine

✍️ 6-Mark Question

Extended Answer

6 marks: Explain how enzyme structure relates to function and how pH affects enzyme activity.

Enzymes have a specific active site shape that is complementary to their substrate (the lock and key model). This means each enzyme only catalyses one reaction — amylase only breaks down starch, protease only breaks down protein. The active site must match the substrate exactly for the enzyme-substrate complex to form. pH affects the shape of the active site. Each enzyme has an optimum pH at which it works best — for example, stomach protease works at pH 2 and pancreatic enzymes at pH 7–8. If the pH is too far from the optimum, the bonds holding the enzyme's shape break, the active site changes shape (denaturation), and the substrate can no longer fit. This is why bile neutralises stomach acid — to create the correct pH for pancreatic enzymes in the small intestine.

Mark scheme: 1 mark for active site shape; 1 mark for lock and key / complementarity; 1 mark for specificity; 1 mark for optimum pH; 1 mark for denaturation explanation; 1 mark for linking bile/pH to enzyme function

📊 AO3: Analyse & Evaluate

Analysis and Evaluation

The table shows the rate of an enzyme-catalysed reaction at different pH values:

pH	4	5	6	7	8	9	10
Rate (arbitrary units)	2	8	15	18	12	5	1

(a) What is the optimum pH? (b) Is this enzyme likely to be stomach protease or pancreatic protease? Explain your answer.

Answers: (a) pH 7 (highest rate of 18). (b) Pancreatic protease — the optimum pH is near neutral/alkaline, which matches conditions in the small intestine. Stomach protease has an optimum around pH 2.

📝 Exam Questions by Topic

AQA Past Papers - Topic Questions -- AQA
Edexcel Past Papers - Topic Questions -- Edexcel
OCR Past Papers - Topic Questions -- OCR
CCEA Past Papers - Topic Questions -- CCEA

🎬 Video Resources

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