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Nutrition and Human Digestive System, worked answers

Fully worked answers for Nutrition and Human Digestive System, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Matching an enzyme to its substrate, site of action and product.
Explaining the villus adaptations for absorption.
A food test experiment on digestion (Paper 3).
Identifying the colour change and reagent used in a food test, then linking a positive result to the correct nutrient and the enzyme that produced it.
Explaining a health issue such as constipation, obesity or a gastric ulcer in terms of a specific eating habit or a failure in a named part of the digestive system.
Distinguishing defaecation, which removes undigested food, from excretion, which removes waste made inside the body's own cells.
Sequencing the organs of the alimentary canal correctly and stating the main nutrient digested, or reabsorbed, in the case of the large intestine, at each one.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

State the substrate and product of trypsin, and name the organ that secretes it into the duodenum.

[4]

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Trypsin acts on protein, breaking it down into shorter peptides. It is produced by the pancreas and released into the duodenum as part of pancreatic juice, where the alkaline environment neutralised by bile allows it to work effectively.

trypsinproteinpeptidespancreasduodenum

Explain two structural adaptations of the villus that make it efficient for absorption.

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The wall of the villus is only one cell thick, which shortens the diffusion distance between the gut lumen and the blood, allowing digested food to cross quickly. The villus is also covered in microvilli and occurs in huge numbers along the intestinal wall, which together give the small intestine a very large surface area for absorption to take place across.

one cell thickshort diffusion distancemicrovillilarge surface area

A food sample gives a positive result with Benedict's solution but a negative result with iodine solution. State the nutrient present and describe the colour change observed in the Benedict's test.

[5]

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The sample contains a reducing sugar but not starch. When Benedict's solution is added to the sample and the mixture is heated, the solution changes from blue to form a brick-red precipitate, confirming the presence of a reducing sugar; because the iodine test remained brown/orange rather than turning blue-black, starch is absent.

reducing sugarheatedbrick-red precipitateblue to brick-redstarch absent

Distinguish between defaecation and excretion, using one example of the material removed in each process.

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Defaecation removes undigested food material, such as fibre, together with dead cells and bacteria, through the anus as faeces; this material was never absorbed into the blood. Excretion removes waste substances that body cells actually produced during metabolism, such as urea from the breakdown of excess amino acids in the liver, mainly through the kidneys, lungs and skin. The two processes therefore remove different kinds of material through different organs.

defaecationundigested foodfibreanusexcretionureametabolic wastekidney

Explain, in terms of diet and digestive function, why a person who regularly eats a low-fibre diet is more likely to experience constipation.

[5]

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Fibre cannot be digested by human enzymes, so it adds bulk to the contents of the large intestine and stimulates the muscular contractions of peristalsis that push waste toward the rectum. A diet low in fibre produces less bulk, so peristalsis is weaker and waste moves through the colon more slowly; more water is reabsorbed from the smaller, slower-moving mass, making the faeces harder and more difficult to pass.

fibrebulkperistalsislarge intestinewater reabsorptionhard faeces

Bile is added to a fat sample before lipase is added, compared to a second sample in which lipase acts on the fat alone. Explain why the fat in the first sample is digested faster.

[6]

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Bile emulsifies the fat, breaking large fat droplets into smaller droplets. This greatly increases the total surface area of fat exposed to the surrounding liquid without changing the fat chemically, since bile itself is not an enzyme. Because lipase can only act at the surface of a fat droplet, the larger surface area created by emulsification allows lipase to break down the fat into fatty acids and glycerol more quickly in the first sample than in the second.

bileemulsificationsurface areanot an enzymelipasefatty acids and glycerol

Phrasing that earns marks

Digestive system: The alimentary canal from mouth to anus, plus glands such as the salivary glands, pancreas and liver.
Digestion: Large molecules are broken down by enzymes: carbohydrases to glucose, proteases to amino acids, lipases to fatty acids and glycerol.
Absorption: Digested food is absorbed into the blood mainly through the villi of the small intestine.
Villus adaptations: Thin walls, a large surface area, many capillaries and a lacteal make absorption efficient.
Assimilation: Absorbed nutrients are used by cells to build tissues and release energy.
Balanced diet: The right proportions of carbohydrates, proteins, lipids, vitamins, minerals, fibre and water for the person's needs.
Defaecation: Undigested material such as fibre, dead cells and bacteria moves from the small intestine into the large intestine. There, water and some mineral ions are reabsorbed back into the blood, and the remaining waste is compacted and expelled through the anus as faeces in a process called defaecation, distinct from excretion of metabolic waste.
Health issues from poor eating habits: Eating too much fatty or sugary food with too little exercise is linked to obesity and diabetes; eating irregularly or too quickly can lead to a gastric ulcer when stomach acid damages the stomach lining; and a diet low in fibre and fluids is linked to constipation.

Frequently asked questions

What is the difference between digestion, absorption and assimilation?

Digestion is breaking large food molecules into small soluble ones using enzymes. Absorption is taking those small molecules into the blood, mainly through the villi of the small intestine. Assimilation is when body cells use the absorbed nutrients to build tissues and release energy.

What does bile do if it is not an enzyme?

Bile is made by the liver and stored in the gall bladder. It emulsifies fats, breaking large fat droplets into many small ones, which greatly increases the surface area for the enzyme lipase to work on. It also neutralises the acidic food from the stomach.

Why is the villus good at absorbing food?

Each villus has a wall one cell thick for a short diffusion distance, a large surface area (helped by microvilli), a dense capillary network to carry away absorbed sugars and amino acids, and a lacteal to absorb fatty acids and glycerol. Millions of villi together make absorption very efficient.