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CIE iGCSE Co-Ordinated Science B4. Biological molecules Exam Style Questions Paper 4

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Question

(a) State the balanced symbol equation for photosynthesis.
(b) Photosynthesis is an enzyme-controlled reaction.
Fig. 4.1 is a graph showing the effect of temperature on the rate of photosynthesis.
Complete the sentences to describe and explain the graph in Fig. 4.1.
As the temperature increases, the rate of photosynthesis increases until it reaches the optimum temperature of ____ °C.
The rate increases because the particles gain more ____ energy causing more ____ collisions.
At 45°C the enzymes are ____. Photosynthesis stops because the ____ of the enzyme has changed shape so the substrate can no longer fit.
(c) Complete the sentence to describe the transfer of energy during photosynthesis.
During photosynthesis chlorophyll transfers ____ energy into ____ energy.
(d) Carbohydrates are produced during photosynthesis.
State the name of the carbohydrate in plants that is used for storage and for transport.
(e) Plant cells are adapted for their function.
State the name of the cells
  • in the leaf that are adapted for efficient photosynthesis
  • that are adapted for absorption of water from the soil.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B6.1 — Photosynthesis (Parts (a) and (c))
• Topic B5.1 — Enzymes (Part (b))
• Topic B4.1 — Biological molecules / Topic B8 — Transport in plants (Part (d))
• Topic B2.1 — Cell structure (Part (e))

▶️ Answer/Explanation

(a) \(6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2\)

Carbon dioxide and water are the reactants, combined using light energy absorbed by chlorophyll.
Glucose and oxygen are the products, and the equation must be balanced on both sides.

(b) 35; kinetic; successful/frequent; denatured; active site

The rate of photosynthesis is highest at the optimum temperature of \(35\,^{\circ}C\), as read from the peak of the graph.
Increasing temperature gives particles more kinetic energy, causing more frequent/successful collisions between enzyme and substrate.
At \(45\,^{\circ}C\) the enzymes are denatured, because the shape of the active site has changed permanently so the substrate can no longer bind.

(c) Light energy into chemical energy

Chlorophyll absorbs light energy from sunlight.
This light energy is converted into chemical energy, stored in the bonds of glucose molecules.

(d) Storage = starch; Transport = sucrose

Glucose made in photosynthesis is converted to starch for storage, since starch is insoluble and does not affect water potential.
Glucose is converted to sucrose for transport around the plant in the phloem, as sucrose is more soluble and stable for translocation.

(e) Palisade mesophyll cells; root hair cells

Palisade mesophyll cells are packed with chloroplasts and are positioned near the top of the leaf for efficient photosynthesis.
Root hair cells have a large surface area, adapting them for efficient absorption of water (and mineral ions) from the soil.

Question

(a) Fig. 7.1 shows the structure of a villus.
(i) State the name and function of the part labelled A in Fig. 7.1.
(ii) Explain how the structure of the part labelled B in Fig. 7.1 is adapted for its function.
(b) Coeliac disease results in damage to the small intestine when gluten is eaten.
Fig. 7.2 shows villi from a person without coeliac disease and Fig. 7.3 shows villi from a person with coeliac disease.
(i) Describe one way the shape of the villi in Fig. 7.3 are different from the villi in Fig. 7.2.
(ii) Explain the effect of this difference on villi function in a person with coeliac disease.
(c) Gluten is a type of protein.
(i) State the name of one disease caused by protein-energy malnutrition.
(ii) State the chemical test for protein.
(d) Tick (✓) the boxes to show the correct features of mechanical and chemical digestion.

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.

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