Form 4 · Common mistakes

Nutrition and Human Digestive System, common mistakes

The mistakes SPM students make on Nutrition and Human Digestive System, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Believing the duodenum and ileum have the same main function in digestion.	The two regions of the small intestine are often treated as interchangeable in revision notes.	The duodenum is where bile and pancreatic juice are added and where most chemical digestion is completed; the ileum, lined with millions of villi, is where most absorption of digested products takes place.
Naming amylase as the enzyme that digests protein.	Students confuse enzyme names that sound similar or forget which suffix matches which substrate.	Amylase digests starch into maltose; protein is digested by protease enzymes such as pepsin in the stomach and trypsin in the duodenum.
Assuming dietary fibre is digested and absorbed in the same way as starch.	Both are carbohydrates, so students assume both are broken down by the same enzymes.	Human digestive enzymes cannot break down cellulose, the main component of fibre, so it passes through the small intestine largely unchanged and adds bulk to the contents of the large intestine.
Stating that pepsin works best in an alkaline environment.	Students generalise that all enzymes prefer neutral or alkaline conditions.	Pepsin is a stomach enzyme that works best in the strongly acidic environment created by hydrochloric acid in gastric juice, at a pH of around 2.
Writing that bile is produced by the gall bladder.	Because bile is released from the gall bladder during a meal, its point of production is often confused with its storage site.	Bile is produced continuously by the liver and only stored and concentrated in the gall bladder before being released into the duodenum through the bile duct.
Assuming all vitamins can be stored in the body for long periods.	Students treat 'vitamin' as a single category with one storage pattern.	Fat-soluble vitamins such as A and D can be stored in the liver and fatty tissue, but water-soluble vitamins such as B and C are not stored in large amounts and must be obtained from the diet regularly.
Assuming every person needs the same daily energy intake regardless of lifestyle.	Balanced diet questions are sometimes answered with a single fixed number rather than a comparison.	Energy requirement depends on factors such as age, gender, body size and activity level, so an active teenager and a sedentary older adult need different daily energy intakes even on the same balanced diet.
Describing the stomach as only mechanically churning food, with no chemical digestion occurring there.	The stomach's muscular churning is more visually memorable than its chemical role.	The stomach both churns food mechanically to mix it with gastric juice and chemically digests protein through the enzyme pepsin, working together in the same organ.
Stating that mineral ions such as calcium and iron provide the body with energy.	Minerals are grouped with other diet components in revision lists, which can blur their distinct roles.	Mineral ions provide no energy; instead they act as structural components, such as calcium in bones, or as parts of molecules such as iron in haemoglobin.
Treating the appendix as an organ with a major role in human digestion.	The appendix is drawn near the caecum on diagrams and can look like part of the main digestive pathway.	In humans, the appendix is a small vestigial structure with only a minor immune role and does not carry out significant digestion, unlike the much larger caecum found in herbivorous animals that digest plant fibre.

How to avoid these mistakes

Build a table of enzyme, site, substrate and product for the main enzymes.
Label a villus and link each feature to efficient absorption.
Learn the components of a balanced diet and one deficiency each.
Memorise the reagent, colour change and nutrient detected for each food test, and practise matching a described result to the correct nutrient.
Learn one specific eating habit for each health issue, obesity, diabetes, gastric ulcer and constipation, rather than a general warning about unhealthy food.
Compare defaecation and excretion side by side, noting the source of the waste material and the organ that removes it.
Trace one mouthful of food from the mouth to the anus, naming the organ, the enzyme (if any) and the product formed at each stage.
Learn one deficiency disease for each key vitamin and mineral, together with the food source that would correct it.

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.