Secondary 3 Biology Practice Paper 5

TuitionGoWhere Practice Paper - Biology Secondary 3 (Answers)

Version: 5
Topic: Cells & Biomolecules
Total Marks: 40

Section A: Multiple Choice (8 marks)

1. B) Ribosome [1]
Teaching note: Ribosomes are the sites where proteins are made (protein synthesis). Mitochondria release energy, vacuoles store substances, nucleus controls the cell.

2. B) carbon [1]
Teaching note: Carbohydrates, fats, and proteins all contain carbon, hydrogen, and oxygen. Proteins also contain nitrogen; carbon is the common element in all three groups.

3. C) osmosis [1]
Teaching note: Osmosis is the movement of water from a dilute to a concentrated solution through a membrane. The cell loses water and shrinks (plasmolysis).

4. C) Starch test [1]
Teaching note: Iodine solution turns blue-black with starch. Benedict’s tests for reducing sugar, biuret for protein, ethanol for fat.

5. B) the substrate fitting into the active site of the enzyme [1]
Teaching note: Lock-and-key model: substrate fits exactly into the enzyme’s active site, like a key into a lock.

6. B) allow more space for haemoglobin [1]
Teaching note: No nucleus means more room for haemoglobin to carry oxygen.

7. C) Active transport of mineral ions into a root hair cell [1]
Teaching note: Active transport needs energy (ATP) to move ions against a gradient. Diffusion and osmosis are passive and need no energy.

8. C) glucose units [1]
Teaching note: Starch is a polysaccharide made of many glucose monomers joined together.

Section B: Structured Questions (16 marks)

9. Chloroplast [1]
Teaching note: Chloroplasts contain chlorophyll and are the site of photosynthesis in plant cells.

10. Controls what enters and leaves the cell (or maintains internal environment). [1]
Teaching note: The cell membrane is selectively permeable.

11. (a) Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. [2]
(b) Oxygen moving from the alveolus into the blood / carbon dioxide leaving a cell. [1]
Teaching note: Definition must include movement down a concentration gradient without energy.

12. [3]

• Water enters by osmosis because the cell sap is more concentrated than pure water. [1]
• The vacuole swells and pushes the membrane against the wall. [1]
• The cell becomes turgid (firm). [1]
  Teaching note: From the diagram, arrow shows water entering; the rigid wall prevents bursting.

13. (a) Reducing sugar. [1]
(b) Biuret test. [1]
Teaching note: Orange-red with Benedict’s after heating indicates reducing sugar. Biuret (blue to purple) confirms protein.

14. (a) pH 7. [1]
(b) At pH 11 the enzyme is denatured because the high pH changes the shape of the active site so the substrate no longer fits. [2]
Teaching note: Rate falls as enzyme structure is altered; 2 marks for linking pH to denaturation and loss of function.

15. (i) Osmosis needs no energy; active transport needs energy (ATP). [1]
(ii) Osmosis moves water down its gradient; active transport moves substances against their gradient. [1]
Teaching note: 1 mark each clear contrast.

Section C: Extended Response (16 marks)

16. [4]

• Long projection increases surface area for absorption. [1]
• Thin wall shortens diffusion distance. [1]
• Large vacuole for water uptake by osmosis. [1]
• Mitochondria provide energy for active transport of ions. [1]
  Teaching note: Adaptations must link structure to function for full marks.

17. [4]
Example: Nucleus – controls cell activities / contains DNA. [2]
Mitochondria – release energy by respiration. [2]
(Other valid pairs: ribosomes – protein synthesis; cell membrane – controls entry/exit.)
Teaching note: 2 marks per structure: name + function.

18. [4]

• 10–40°C: rate rises with temperature because molecules move faster and collide more often. [1]
• At 40°C: optimum temperature, fastest reaction. [1]
• 40–70°C: enzyme denatures, active site loses shape, rate falls. [1]
• At 70°C: little/no activity as enzyme fully denatured. [1]
  Teaching note: Use graph values; explain molecular basis of temperature effect.

19. [6 – 3 per cell]
Red blood cell:

• Biconcave shape increases surface area for oxygen exchange. [1]
• No nucleus gives space for haemoglobin. [1]
• No organelles allow easy passage in capillaries. [1]
  Palisade cell:
• Many chloroplasts for photosynthesis. [1]
• Tall column shape packs many cells at top of leaf. [1]
• Thin walls let gases diffuse quickly. [1]
  Teaching note: Must describe and explain adaptation for each cell.

20. (a) Rough endoplasmic reticulum (RER) / ribosome. [1]
(b) RER → Golgi body → secretory vesicle → cell membrane → out. [3: 1 each]
Teaching note: Amino acids are used at ribosomes on RER, modified in Golgi, packaged in vesicles and released.