Photosynthesis and its limiting factors
Photosynthesis makes glucose from carbon dioxide and water using light energy absorbed by chlorophyll, releasing oxygen. Balanced equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The rate is set by limiting factors — the one in shortest supply caps it: light intensity, carbon dioxide concentration and temperature. Plants also need mineral ions from the soil: nitrate to make amino acids and proteins, and magnesium to make chlorophyll.
Balanced diet, digestive enzymes and bile
A balanced diet supplies all food groups in proportion: carbohydrate (energy), protein (growth and repair), lipid (energy store, insulation), vitamins, minerals, water and fibre. Digestion breaks large insoluble molecules into small soluble ones using enzymes: amylase (starch → maltose), protease/pepsin (protein → amino acids), lipase (lipid → fatty acids + glycerol). Bile is alkaline (neutralises stomach acid) and emulsifies fats for lipase — it is not an enzyme.
The gut, peristalsis and absorption at the villi
Food passes along the alimentary canal: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus, moved by peristalsis (waves of muscular contraction of the gut wall). Absorption takes small soluble molecules through the small-intestine wall into the blood. Villi speed this up: a large surface area (villi plus microvilli), a thin one-cell-thick wall (short diffusion path), a rich capillary network (maintains the gradient) and a lacteal for fats.
Drawn from real examiner reports.
Get the glucose formula and balance right
In the balanced equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, two errors recur: the wrong glucose formula (writing C₆H₆O₆, or CO₂ for glucose) and forgetting to balance (dropping the 6s). Learn C₆H₁₂O₆ exactly. Also, a full definition names the reactants (carbon dioxide and water), light energy and chlorophyll — "plants make food from sunlight" is too vague.
Oesophagus not trachea; describe peristalsis
For food moving from mouth to stomach, name the oesophagus (the food tube) — not the trachea (the windpipe, which carries air). Name peristalsis AND describe it as waves of muscular contraction of the gut wall that push food along. Writing only "peristalsis", or "the muscles squeeze", without the idea of a travelling wave of contraction misses the mark.
Protein digestion: enzyme, site, product
A protein-digestion answer must name the enzyme (protease/pepsin), the site (stomach) and the product (amino acids). Errors: naming the mouth, naming amylase (a carbohydrase), or describing absorption instead of digestion. Keep digestion (breaking molecules down) separate from absorption (taking them into the blood).
Protein is for growth and repair
Match each food group to its job: protein is for growth and repair of tissues — not energy (that is carbohydrate) and not insulation (that is fat/lipid). A common error gives protein's role as energy or insulation. Carbohydrate is the main energy source; fat is an energy store and insulator.
Egestion is not excretion
Egestion removes undigested food (faeces) from the gut — it was never absorbed into cells, so it is not a metabolic waste. Excretion removes the waste products of metabolism (carbon dioxide, urea). Faeces is egested, not excreted, so do not describe passing faeces as excretion.
Bile is not an enzyme
Bile emulsifies fat physically — breaking large droplets into many small ones to give a larger surface area for lipase — and neutralises stomach acid (it is alkaline). It does NOT chemically digest fat; lipase does that. Writing that bile "digests fat" or calling it an enzyme loses the mark.
Calorimetry: use the mass of water
For energy content of food, energy = mass of water × 4.2 × temperature rise (4.2 J/g/°C is water's specific heat capacity). The classic error is using the food's mass instead. For energy per gram, divide by the food mass only at the end. Repeats improve reliability, not accuracy.
State a direction; name the new limiter
When describing photosynthesis data, give a direction ("as light intensity increases, the rate increases"), not "it affects the rate". When a graph levels off, explain that another factor — CO₂ concentration or temperature — has become limiting, so more light no longer helps.
Link each structure to its effect
For "how is the villus/leaf adapted" items, link each structure to its effect, not just name it: large surface area → more room for absorption; thin wall → short diffusion path; capillaries → maintain the concentration gradient. Naming features without the effect scores nothing.
Nutrition in 4SD0 covers two halves: how plants make their own food (photosynthesis) and how humans obtain and process food (diet, digestion and absorption).
Two-element, mark-scheme definitions:
Photosynthesis equations:
Limiting factors of photosynthesis (the one in shortest supply caps the rate): light intensity, carbon dioxide concentration, temperature.
Mineral ions a plant needs: nitrate (to make amino acids → proteins) and magnesium (to make chlorophyll). A shortage shows as poor growth (nitrate) or yellow leaves (magnesium).
Human digestive enzymes:
| Enzyme | Substrate → product | Main site |
|---|---|---|
| Carbohydrase (amylase) | starch → maltose / glucose | mouth (saliva), small intestine |
| Protease (e.g. pepsin) | protein → amino acids | stomach (pepsin), small intestine |
| Lipase | lipids/fats → fatty acids + glycerol | small intestine (pancreas) |
Food groups and their roles: carbohydrate (main energy source); protein (growth and repair); lipid/fat (energy store + insulation); vitamins; minerals; water; fibre (keeps food moving). Deficiencies: vitamin C → scurvy; vitamin D/calcium → rickets; iron → anaemia.
Define peristalsis.
The recommended daily amount of protein for an adult is 55 g. One chicken portion supplies 22 g of protein.
Calculate the percentage of the recommended daily protein supplied by the chicken portion. (2 marks)
