Balanced diet and deficiencies: the named answers
A balanced diet contains the right proportions of carbohydrates, proteins, fats, vitamins, mineral ions, fibre (roughage) and water. Examiners ask for the function and a deficiency disease for the key ones:
| Nutrient | Use | Deficiency |
|---|---|---|
| Vitamin C | Healthy skin/tissues | Scurvy |
| Vitamin D | Helps calcium uptake for bones | Rickets |
| Calcium | Strong bones and teeth | Weak bones / poor teeth |
| Iron | Making haemoglobin | Anaemia |
| Fibre/roughage | Helps movement of food / prevents constipation | Constipation |
State iron is needed to make haemoglobin, linking to transport in humans. A common slip is saying calcium 'is in bones' rather than its function (strengthening bones and teeth). Energy needs vary with age, activity and (for pregnancy) condition. A frequent application question.
The alimentary canal: order and roles
Know the path of food: mouth → oesophagus → stomach → small intestine (duodenum then ileum) → large intestine → rectum → anus, with the associated organs (salivary glands, pancreas, liver, gall bladder). Examiners test the function of each region:
- Mouth: mechanical digestion by teeth (increases surface area); salivary amylase begins starch digestion.
- Stomach: protease (pepsin) digests protein; hydrochloric acid gives the low pH for the enzyme and kills bacteria.
- Small intestine: completes digestion and absorbs nutrients.
- Large intestine: absorbs water; forms faeces.
Define ingestion, digestion, absorption, assimilation and egestion precisely. Examiners often ask you to distinguish absorption (nutrients into blood) from assimilation (using absorbed nutrients in cells) and egestion (removing undigested food), which students confuse with excretion.
Digestive enzymes: the table you must know
This is the highest-yield memorisation in the topic. State each enzyme, its substrate and its products:
| Enzyme | Substrate → Products |
|---|---|
| Amylase | Starch → maltose (then maltase → glucose) |
| Protease (e.g. pepsin) | Proteins → amino acids |
| Lipase | Fats → fatty acids + glycerol |
The product names must be exact: fats give fatty acids and glycerol (both needed), proteins give amino acids, and starch ultimately gives glucose. The denaturation and optimum-pH ideas from enzymes apply directly: pepsin works in the acidic stomach, while small-intestine enzymes work in alkaline conditions. A frequent error is writing 'sugar' instead of the named product. Drop the enzyme suffix carefully, 'protease' not 'proteinase' is the safest term, though both are accepted.
Bile: the two jobs examiners test (Extended)
(Extended only) Bile is made in the liver, stored in the gall bladder, and released into the small intestine. It has two roles that score separately:
- Neutralises the acidic mixture from the stomach, providing the alkaline pH that small-intestine enzymes need to work.
- Emulsifies fats: breaks large fat droplets into smaller droplets, increasing the surface area for lipase to act on.
The crucial distinction: bile is not an enzyme and does not chemically digest fat. Emulsification is physical. Students lose marks claiming bile 'digests fat'. The surface-area reasoning ('so lipase can work faster / digest fat more quickly') is the mark-scheme link examiners want for emulsification. Both functions together are worth two distinct marks, so give both.
Absorption and villi: adaptation marks
Absorption happens in the small intestine, whose inner surface is folded into villi (and microvilli). Each adaptation links to faster absorption:
- Large surface area (villi and microvilli). More area for absorption.
- Thin walls / one-cell-thick lining: short diffusion distance.
- Good (rich) blood supply: maintains a steep concentration gradient by carrying absorbed nutrients away.
- Lacteal: absorbs the products of fat digestion (fatty acids and glycerol).
Glucose and amino acids are absorbed into the blood capillaries; the absorption uses diffusion and, when concentrations are low, active transport (Extended). Connecting to movement in and out of cells. Examiners want each villus feature paired with its reason; a bare list of features without the 'so that' caps your marks. Book a free trial to drill these adaptation answers.