Secondary 3 Combined Science Life Sciences Quiz

Secondary 3 Combined Science Quiz - Life Sciences – ANSWER KEY

Total Marks: 40

Section A: Cell Biology & Movement of Substances (Questions 1–5)

1. (a) Plasmolysed / The cell membrane/cytoplasm has shrunk away from the cell wall. [1]

(b)

• The sugar solution has a lower water potential than the cell sap/vacuole [1]
• Water moves out of the cell by osmosis from a region of higher water potential to a region of lower water potential through a partially permeable membrane [1]

2.

• Function: Transport water and mineral salts from roots to other parts of the plant / Provide mechanical support [1]
• Adaptation: Lignified walls for strength / Hollow lumen for water transport / No cross-walls (continuous tubes) [1]

3. (a)

• The mass of the potato cube would increase [1]
• Water enters the potato cube by osmosis because the water potential of distilled water is higher than the water potential of the potato cells [1]

(b)

• The 1.0 M sucrose solution has a lower water potential than the potato cells [1]
• Water moves out of the potato cells by osmosis, causing the potato cube to decrease in mass [1]

4. (a) Red blood cell has no nucleus / is biconcave; Root hair cell has a long projection/root hair / has a nucleus [1] (Accept any one valid structural difference)

(b)

• The root hair cell has a long, narrow projection/root hair [1]
• This increases the surface area to volume ratio for efficient absorption of water and mineral salts from the soil [1]

5. Energy cannot be created or destroyed; it can only be converted from one form to another. The total energy in a closed system remains constant. [1]

Section B: Human Physiology – Digestion & Transport (Questions 6–10)

6. (a)

• In the stomach, proteins are partially digested by protease/pepsin into polypeptides [1]
• In the small intestine, polypeptides are further digested by protease/trypsin into amino acids, causing the amount of amino acids to increase [1]

(b) Amino acids are absorbed into the bloodstream through the walls of the small intestine [1]

7.

• Oxygenated blood returns from the lungs to the left atrium via the pulmonary veins [1]
• The left atrium contracts, increasing pressure and forcing blood into the left ventricle [1]
• The left ventricle contracts, increasing pressure [1]
• Blood is forced through the aortic valve/semilunar valve into the aorta [1]

8. (a)

• Left ventricle correctly labelled (thick-walled chamber at bottom left of diagram) [1]
• Aorta correctly labelled (large vessel emerging from top of left ventricle) [1]

(b)

• The left ventricle pumps blood to the entire body (systemic circulation) [1]
• It requires more muscular force/greater pressure to pump blood a longer distance compared to the right ventricle which only pumps blood to the lungs [1]

9. (a)

• The 'lock and key' hypothesis states that the enzyme (amylase) has an active site with a specific shape that is complementary to the substrate (starch) [1]
• At 40°C, the enzyme and substrate molecules have high kinetic energy, increasing the frequency of effective collisions [1]
• The substrate fits into the active site, forming an enzyme-substrate complex, leading to rapid digestion [1]

(b)

• At 60°C, the high temperature causes the enzyme to denature [1]
• The active site of the enzyme loses its specific shape and can no longer bind to the substrate, so no digestion occurs [1]

10. (Any two valid differences, 1 mark each)

• Arteries have thick, muscular, elastic walls; veins have thinner walls with less muscle and elastic tissue
• Arteries carry blood away from the heart; veins carry blood towards the heart
• Arteries have no valves (except pulmonary artery and aorta at base); veins have valves to prevent backflow
• Arteries carry oxygenated blood (except pulmonary artery); veins carry deoxygenated blood (except pulmonary vein)
• Arteries have a smaller lumen relative to wall thickness; veins have a larger lumen

Section C: Respiration & Gas Exchange (Questions 11–15)

11.

• Aerobic respiration word equation: Glucose + Oxygen → Carbon dioxide + Water (+ energy/ATP) [2]
• Anaerobic respiration word equation (in humans): Glucose → Lactic acid (+ energy/ATP) [2]
• Key differences:
  • Aerobic respiration requires oxygen; anaerobic respiration does not require oxygen [1]
  • Aerobic respiration releases more energy (38 ATP) than anaerobic respiration (2 ATP) [1]

12. (a) Alveoli / Alveolus [1]

(b) (Any two valid adaptations, 1 mark each)

• Thin walls (one cell thick) for short diffusion distance
• Large surface area for efficient gas exchange
• Rich supply of blood capillaries to maintain concentration gradient
• Moist surface for gases to dissolve

13. (a) Tidal volume = (value read from graph, typically ~0.5 dm³) [1]

(b)

• Number of breaths in time period shown on graph (e.g., 3 breaths in 15 seconds) [1]
• Breathing rate = (number of breaths ÷ time in seconds) × 60 = 12 breaths per minute [1]
• (Accept correct calculation based on graph data; award method mark even if final answer differs slightly)

14.

• During vigorous exercise, muscles contract more and require more energy [1]
• The rate of aerobic respiration increases, requiring more oxygen and producing more carbon dioxide [1]
• Faster and deeper breathing increases the rate of gas exchange, supplying more oxygen to muscles and removing excess carbon dioxide [1]

15. Glucose + Oxygen → Carbon dioxide + Water (+ energy) [1]

Section D: Photosynthesis & Plant Transport (Questions 16–20)

16.

• Water is absorbed by root hair cells from the soil by osmosis [1]
• Water moves through the cortex and endodermis of the root [1]
• Water enters the xylem vessels in the root [1]
• Water is transported up the stem through the xylem vessels [1]
• Water reaches the leaves and exits through the stomata as water vapour via transpiration [1]

17. (a)

• 1 mark for correct plotting of all points [1]
• 1 mark for smooth curve or appropriate line of best fit connecting the points [1]
• (Graph should show rate increasing and then plateauing at light intensity 5-6)

(b) Change the distance of the light source from the plant / Change the number of light bulbs / Change the power/wattage of the light bulb [1] (Accept any one valid method)

(c) To prevent heat from the light source from affecting the temperature of the experiment / To ensure temperature remains a controlled variable [1]

18. Transpiration pull [1]

19. (a)

• Only the green parts of the leaf would test positive for starch [1]
• Chlorophyll is present only in the green parts, and chlorophyll is necessary for photosynthesis to produce starch [1]

(b) To remove/decolourise the chlorophyll so that the colour change with iodine solution can be clearly observed [1]

20. (Any two valid conditions, 1 mark each)

• Presence of light/sunlight
• Presence of chlorophyll
• Presence of carbon dioxide
• Suitable temperature
• Presence of water

END OF ANSWER KEY