Food, Digestion and Respiration
Balanced Diet
A balanced diet provides the correct amounts of:
- Carbohydrates — energy (bread, pasta, rice)
- Proteins — growth and repair (meat, fish, legumes); made of amino acids
- Lipids (fats/oils) — energy store, insulation, cell membranes
- Vitamins — e.g. vitamin C (prevents scurvy), vitamin D (bone health)
- Minerals — e.g. calcium (bones/teeth), iron (haemoglobin)
- Fibre — aids digestion / peristalsis; prevents constipation
- Water — all metabolic reactions occur in solution; blood plasma
Malnutrition can result from too little (undernutrition → starvation, kwashiorkor) or too much (overnutrition → obesity, type 2 diabetes, cardiovascular disease).
Enzymes
Enzymes are biological catalysts — they speed up metabolic reactions without being used up. Key features:
- Protein molecules; have a specific active site shape.
- Each enzyme is specific to one substrate (lock-and-key model).
- Affected by temperature: rate increases up to optimum (~37°C in humans); above optimum, enzyme denatures (active site changes shape permanently).
- Affected by pH: each enzyme has an optimum pH (amylase ~7; pepsin ~2).
Digestive enzymes:
| Enzyme | Produced by | Substrate → Product | Optimum pH |
|---|---|---|---|
| Amylase | Salivary glands, pancreas | Starch → maltose | ~7 (neutral) |
| Protease (pepsin) | Stomach | Proteins → amino acids | ~2 (acidic) |
| Protease (trypsin) | Pancreas | Proteins → amino acids | ~8 (alkaline) |
| Lipase | Pancreas | Lipids → fatty acids + glycerol | ~8 |
Required Practical: investigate effect of temperature or pH on amylase activity using starch and iodine solution (blue-black → colourless when starch is digested).
Digestive System
Food travels: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
- Mouth: teeth break food (mechanical); salivary amylase begins starch digestion; bolus formed.
- Oesophagus: peristalsis moves food (waves of muscle contraction).
- Stomach: churns food; secretes HCl (kills bacteria, pH for pepsin); protease digests proteins.
- Small intestine: bile from liver emulsifies fats (increases surface area); pancreatic enzymes digest all food groups; villi and microvilli increase surface area for absorption.
- Large intestine: absorbs water; compacts undigested material → faeces.
Bile: made in liver, stored in gall bladder, released into small intestine. Emulsifies fat (does not digest it — not an enzyme); neutralises stomach acid.
Villi: finger-like projections in small intestine wall. Adaptations: large surface area, good blood supply (capillaries), thin epithelium → efficient absorption of glucose and amino acids into blood.
Aerobic Respiration
Occurs in mitochondria. Requires oxygen.
Word equation: glucose + oxygen → carbon dioxide + water (+ ATP energy released) Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
ATP (adenosine triphosphate) is the energy currency of the cell — used for muscle contraction, active transport, protein synthesis, maintaining body temperature.
Anaerobic Respiration
Occurs without oxygen. Less ATP produced than aerobic respiration.
In animals (and humans): glucose → lactic acid Results in oxygen debt (must repay O₂ to oxidise lactic acid back to glucose or CO₂/H₂O).
In yeast (and plants): glucose → ethanol + carbon dioxide This is fermentation — used in bread-making and brewing.
⚠Common mistakes
- Bile is NOT an enzyme — it emulsifies fat but does not chemically digest it.
- Denaturation is permanent — the enzyme cannot recover when temperature drops.
- Anaerobic respiration still starts with glucose — it does not use oxygen at any stage.
- Lactic acid (not lactate) causes muscle fatigue in humans; yeast produces ethanol not lactic acid.
AI-generated · claude-opus-4-7 · v3-wjec-biology