A balanced diet

A balanced diet contains all the nutrients in the right amounts for good health. There are seven things you need:

• Carbohydrates — the main source of energy (e.g. bread, rice, pasta).
• Fats (lipids) — energy store and insulation (e.g. butter, oils).
• Proteins — for growth and repair of tissues (e.g. meat, fish, beans).
• Vitamins — e.g. vitamin C (fruit) and vitamin D (dairy, sunlight).
• Mineral ions — e.g. calcium (bones/teeth) and iron (haemoglobin).
• Fibre (roughage) — helps the gut move food along (peristalsis); prevents constipation.
• Water — needed for reactions and to transport substances.

Watch out: fibre is not digested and provides no nutrients — but it is still essential for a healthy gut.

The alimentary canal

Food travels through the alimentary canal — a long tube from mouth to anus — helped by glands like the salivary glands, pancreas and liver:

• Ingestion — taking food into the mouth.
• Digestion — breaking large insoluble molecules into small soluble ones.
• Absorption — soluble food passing into the blood (in the small intestine).
• Assimilation — using absorbed food in cells.
• Egestion — removing undigested food (faeces) at the anus.

Where does absorption happen?

Mechanical & chemical digestion

Digestion happens in two ways that work together:

• Mechanical (physical) digestion — food is physically broken up without changing the molecules. The teeth chew food and the stomach's muscular wall churns it. This gives a larger surface area for enzymes to act on.
• Chemical digestion — enzymes break the chemical bonds in large molecules, turning them into small soluble ones.

Mechanical or chemical?

Tap a statement, then tap the type of digestion it describes. (Some involve both.)

Digestive enzymes & bile

Each enzyme breaks down one type of large molecule into small, soluble products:

• Amylase (made in salivary glands & pancreas) → breaks starch into maltose (a sugar).
• Protease (e.g. pepsin in the stomach; also pancreas) → breaks protein into amino acids.
• Lipase (pancreas & small intestine) → breaks fats (lipids) into fatty acids and glycerol.

Bile is made by the liver, stored in the gall bladder and released into the small intestine. It does two jobs:

• Neutralises the acid from the stomach — makes conditions alkaline for the intestine's enzymes.
• Emulsifies fats — breaks large fat drops into tiny droplets, giving a larger surface area for lipase.

Key point: bile is not an enzyme. Emulsification is a physical change that speeds up the chemical digestion done by lipase.

Which enzyme?

Absorption in the villi

The small intestine is where soluble food is absorbed into the blood. Its inner wall is folded into millions of tiny finger-like villi (each villus is covered in even smaller microvilli). Villi are perfectly adapted:

• Large surface area — many villi & microvilli.
• Thin wall (one cell thick) — a short diffusion distance.
• Rich blood supply — capillaries keep the concentration gradient steep.
• Lacteal inside each villus — absorbs the products of fat digestion.

Your turn — surface area

Your turn — energy in food

Deficiency diseases

If a nutrient is missing from the diet you get a deficiency disease. Cambridge IGCSE names these key ones:

• Lack of vitamin C → scurvy (bleeding gums, poor wound healing).
• Lack of vitamin D or calcium → rickets (soft, weak, bent bones).
• Lack of iron → anaemia (too little haemoglobin, so tiredness).

Link it up: iron is needed to make haemoglobin, which carries oxygen in red blood cells — so an iron shortage means less oxygen delivered, causing tiredness.

Your turn — recommended intake

Match each nutrient to its role

Tap a description on the left, then its matching nutrient on the right.

What does bile do?

Why eat fibre?

The big ideas to know

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