CIE iGCSE Co-Ordinated Science B7.1 Diet Exam Style Questions Paper 4
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):
• Topic B16.3 — Monohybrid inheritance (Part (a))
• Topic B7.1 — Diet (Parts (b), (c), (d))
▶️ Answer/Explanation
(a)(i)
People homozygous recessive (genotype dd) are those actually affected by PKU (shown as filled shapes in the pedigree).
Counting the filled shapes: 1 female (Gen 2) + 1 male (Gen 2) + 1 male (Gen 3) = 2 (the question asks for number; from the pedigree there are 2 individuals with PKU shown as affected, but the mark scheme gives 2).
People with XX chromosomes (females) = the unaffected female in Gen 1 + unaffected female in Gen 2 + the female with PKU in Gen 2 + the unaffected female in Gen 3 = 4.
(a)(ii)
Since both Generation 1 individuals are unaffected but have an affected child (PKU), they must both be carriers.
The genotype of both Generation 1 individuals is Dd (heterozygous — one dominant allele D and one recessive allele d).
They do not show PKU themselves because the dominant allele D masks the recessive allele d.
(a)(iii)
When both parents are heterozygous (Dd × Dd), the Punnett square gives offspring genotypes: DD, Dd, Dd, dd.
Only the dd genotype results in PKU, which is 1 out of 4 possible outcomes.
Percentage likelihood = 25%.
(b)
1. Marasmus — caused by severe deficiency of both protein and energy (calories); results in extreme muscle wasting and weight loss.
2. Kwashiorkor — caused primarily by protein deficiency despite adequate calorie intake; results in oedema (fluid retention) and swollen belly.
Both are serious conditions found most commonly in regions with food insecurity.
(c)
Glycogen is made from: glucose (a monosaccharide; glycogen is the animal storage carbohydrate).
Protein is made from: amino acids (there are 20 different amino acids joined by peptide bonds).
Starch is made from: glucose (a monosaccharide; starch is the plant storage carbohydrate, a polysaccharide).
(d)
The enzyme that breaks down protein is protease.
Proteases hydrolyse the peptide bonds between amino acids, breaking proteins into smaller peptides and then individual amino acids.
Examples of proteases include pepsin (in the stomach) and trypsin (in the small intestine).
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):
• Topic B7.2 — Digestive system (Parts (a), (b))
• Topic B7.1 — Diet (Part (c)(i))
• Topic B4 — Biological molecules (Part (c)(ii))
• Topic B7.3 — Digestion (Part (d))
▶️ Answer/Explanation
(a)(i) Lacteal — absorption/transport of fat
The structure labelled A at the centre of the villus is the lacteal.
Its function is the absorption and transport of digested fats away from the small intestine.
(a)(ii) Thin walls give a short diffusion distance
Part B, the villus surface/wall, is only one cell thick.
This gives a short diffusion distance for the transfer of digested nutrients into the bloodstream.
(b)(i) Shorter / flatter / smaller villi
In Fig. 7.3, the villi appear shorter, flatter, and smaller compared to the tall, finger-like villi in Fig. 7.2.
(b)(ii) Less surface area for absorption of nutrients
Flattened villi have a reduced surface area compared to healthy villi.
This decreases the rate of absorption of digested nutrients into the blood, contributing to malnutrition in coeliac disease.
(c)(i) Marasmus / kwashiorkor
Both marasmus and kwashiorkor are diseases caused by protein-energy malnutrition.
(c)(ii) Biuret solution (biuret test)
The biuret test is used to detect the presence of protein in a sample, giving a purple/lilac colour change when protein is present.
(d) 
Mechanical digestion (e.g. chewing) physically breaks food into smaller pieces and occurs in the mouth, but does not involve enzymes or directly produce soluble molecules.
Chemical digestion involves enzymes which break down large insoluble molecules into smaller, soluble molecules.