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 trial class to drill these adaptation answers.
Key terms to connect with this topic
Use these definitions inside explanations and questions rather than memorising them as isolated sentences.
- Absorption: The movement of digested food molecules from the small intestine into the blood (or lymph in the case of fats). Small soluble molecules such as glucose and amino acids pass through the villus wall into blood capillaries by diffusion and active transport.
- Assimilation: The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.
- Balanced diet: A diet that contains all the necessary nutrients in the correct proportions for the body to function properly: carbohydrates, proteins, fats, vitamins, minerals, water and fibre (roughage). It provides enough energy but not too much.
- Bile: An alkaline fluid produced by the liver, stored in the gall bladder, and released into the duodenum. Bile emulsifies fats into smaller droplets to increase surface area for lipase action, and neutralises stomach acid to provide the alkaline pH needed by intestinal enzymes.
- Egestion: The passing out of undigested food (faeces) through the anus.
- Fibre: Indigestible plant material (mainly cellulose) in the diet that adds bulk to food in the intestine, stimulating peristalsis and helping move food through the digestive system. Also called roughage or dietary fibre.
- Ingestion: The taking of food into the body through the mouth. It is the first stage of the digestive process, before digestion, absorption, assimilation and egestion.
- Lacteal: A small lymph vessel found in the centre of each villus in the small intestine. Lacteals absorb fatty acids and glycerol (the products of fat digestion), which are then transported in the lymphatic system before entering the blood.
- Peristalsis: Waves of muscular contraction and relaxation that push food along the digestive tract. Circular muscles behind the food bolus contract while those ahead relax, squeezing the food forward through the oesophagus, stomach and intestines.
- Villus: A tiny finger-like projection on the inner lining of the small intestine that increases the surface area for absorption of digested food. Each villus has a thin epithelium, a dense network of blood capillaries and a central lacteal.
- Vitamin C: A water-soluble vitamin found in citrus fruits and vegetables that is needed for the formation and maintenance of connective tissue, healthy skin and blood vessels. Deficiency of vitamin C causes scurvy.
- Vitamin D: A fat-soluble vitamin needed for the absorption of calcium and phosphorus from the gut, essential for strong bones and teeth. Sources include oily fish, eggs and sunlight exposure on skin. Deficiency causes rickets in children.
Browse the full IGCSE Biology glossary when a related term needs checking.
Apply the topic in practical work
These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.
- Food Tests: Reagents, Methods and Colour Changes: Use Benedict’s plus heat for reducing sugar, iodine for starch, biuret for protein, ethanol then water for fat, and DCPIP decolourisation for vitamin C. State both the reagent and the positive colour change.