A balanced diet

A balanced diet contains the right amount of each food group. CCEA expects you to know seven components and what each is for:

• Carbohydrates — the body's main source of energy (e.g. bread, pasta, rice).
• Proteins — for growth and repair of cells and tissues (e.g. meat, fish, eggs).
• Fats / lipids — a concentrated energy store and insulation (e.g. butter, oils).
• Vitamins — needed in small amounts for health (e.g. vitamin C, vitamin D).
• Minerals — e.g. iron (for haemoglobin), calcium (for bones and teeth).
• Water — needed for reactions and to transport substances.
• Fibre (roughage) — not digested; keeps food moving through the gut and prevents constipation.

Watch out: fibre and water are part of a balanced diet even though they aren't broken down for energy. Your energy needs change with age, sex/gender and activity level (occupation).

Which food group?

Food tests — reagents & colour changes

CCEA expects you to memorise each test's reagent and its colour change for a positive result:

Extra: DCPIP is used to compare vitamin C content — vitamin C decolourises the blue DCPIP. For fats, the ethanol/emulsion test gives a cloudy white emulsion.

Naming the test

Which food test?

Tap a statement, then tap the test it belongs with.

Enzymes & denaturation

Enzymes are biological catalysts — they speed up reactions and are not used up. They are proteins. Digestive enzymes break large insoluble food molecules into small soluble ones that can be absorbed.

Denaturation: at high temperature (or the wrong pH) the enzyme's active site changes shape, so the substrate no longer fits and the reaction stops. This change is permanent — the enzyme cannot recover.

Too hot to work

Digestive enzymes & their products

Each food group is broken down by its own enzyme into small, soluble products:

• Carbohydrases (e.g. amylase) break starch/carbohydrates into sugars.
• Proteases break proteins into amino acids.
• Lipases break fats/lipids into fatty acids and glycerol.

Why break food down? Large food molecules are insoluble and too big to be absorbed. Enzymes turn them into small soluble molecules that can pass through the gut wall into the blood.

Enzyme & product

The alimentary canal

Food travels through the gut, with each part doing a specific job:

• Buccal cavity (mouth) — teeth grind food; amylase in saliva starts digesting starch.
• Stomach — makes gastric juice with protease and acid; digests protein; acid kills microbes.
• Liver — makes bile (stored in the gall bladder); bile neutralises acid and emulsifies fats.
• Pancreas & duodenal wall — release carbohydrases, proteases and lipases into the duodenum.
• Ileum (small intestine) — absorbs digested food molecules.
• Colon (large intestine) — absorbs water; rectum & anus store and remove faeces.

Bile: it is not an enzyme. It emulsifies fats — breaking them into small droplets to give a bigger surface area for lipase to work on.

Absorbing water

Villi — adapted for absorption

The lining of the ileum is folded into millions of finger-like villi. Their features make absorption fast and efficient:

• Finger-like shape + folds → very large surface area for absorption.
• Single layer of surface cells → short diffusion distance (thin, permeable membrane).
• Capillary network → good blood supply carries molecules away, keeping the gradient steep.
• Lacteal → absorbs the products of fat digestion.

Why finger-like?

Diet, health & BMI

An unbalanced diet harms health. Too much saturated fat/cholesterol and too much energy can lead to obesity, type 2 diabetes, coronary heart disease and high blood pressure. Too little fibre causes constipation and bowel problems.

Tip: square the height first, then divide the mass by it. Never divide by the height on its own.

Your turn — BMI

Your turn — energy per gram

Match each organ to its job

Tap an organ on the left, then its matching job on the right.

The big ideas to know

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