O Level Combined Science Life Sciences Quiz

O-Level Combined Science Quiz - Life Sciences (Answer Key)

Total Marks: 40
Topic: Life Sciences (Biology)

Section A

1. [1] Mitochondrion (or mitochondria).
Teaching note: Aerobic respiration occurs in the mitochondria, where glucose and oxygen are used to release energy as ATP. Common mistake: confusing with chloroplast (plant photosynthesis site).

2. [1] Diffusion is passive (no energy needed) and moves down concentration gradient; active transport requires energy and moves against gradient.
Accept any one clear difference.
Teaching note: Diffusion needs no ATP; active transport uses ATP from respiration.

3. [1] Lowers blood glucose level (by promoting glucose uptake into cells / storage as glycogen).
Teaching note: Insulin is secreted when glucose is high; it reduces blood sugar.

4. [1] Transpiration.
Teaching note: Water vapour leaves leaf stomata; this is transpiration.

5. [1] Protein synthesis.
Teaching note: Ribosomes read mRNA to build proteins.

Section B

6. [4]
(a) [2] Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to lower water potential (concentrated solution) through a partially permeable membrane.
(b) [2] It swells and may burst (lysis). Because water enters by osmosis (outside is hypotonic), increasing internal pressure.
Teaching note: Animal cells have no wall; pure water is low solute, high water potential.

7. [3]
(a) [1] Grass.
(b) [1] Secondary consumer (or 3rd trophic level).
(c) [1] Snake population decreases due to less food.
Teaching note: Grass producer (1st), grasshopper primary consumer, frog secondary.

8. [3]
(a) [1] Optimum temperature.
(b) [2] High temp denatures enzyme (breaks bonds, changes shape of active site), so substrate no longer fits.
Teaching note: Beyond optimum, protein structure unfolds.

9. [3] Best pH = 7 (shortest time 4 min) [1]. Average for pH5 & 9 = (12+9)/2 = 10.5 min [2].
Working: (12 + 9) ÷ 2 = 21 ÷ 2 = 10.5 min.
Teaching note: Lower time = faster digestion = better pH.

10. [3] Oxygenated blood returns from lungs to left atrium → left ventricle → aorta → body. [3 marks: 1 each for chamber, vessel, path]
Teaching note: Double circulation; left side pumps to body.

11. [3]
(a) [1] Carbon dioxide and water (accept light).
(b) [2] Carbon dioxide + water → glucose + oxygen (with light/chlorophyll).
Teaching note: Endothermic reaction in chloroplasts.

12. [3] Thin walls (short diffusion distance) [1]; many alveoli (large surface area) [1]; moist lining (gases dissolve) [1].
Teaching note: Adaptations follow FICK principles.

13. [2] Genotype Aa.
Teaching note: Carrier has one recessive allele but unaffected (dominant masks).

14. [2] Removes urea (excretion) [1]; regulates water and ion balance (osmoregulation) [1].
Teaching note: Nephron filters blood.

15. [2] Cell wall and chloroplast (or large vacuole) [1+1].
Teaching note: Animal cells lack rigid wall and plastids.

Section C

16. [3]
(a) [1] 14:00.
(b) [2] Daylight allows photosynthesis in pond plants producing O₂; at night respiration uses O₂ so level falls.
Image note: Graph peaks midday, matches photosynthesis rate.

17. [3]
(a) [1] Eutrophication.
(b) [2] Algae block light, die, bacteria decompose using O₂ → fish suffocate.
Teaching note: Nutrient runoff causes bloom.

18. [3]
(a) [2] Drop = (5.0 − 4.2)/5.0 × 100 = 0.8/5.0 × 100 = 16%.
(b) [1] Glucagon (or adrenaline).
Working shown line by line.

19. [3] Mitosis: 2 daughter cells, same chromosome number (diploid) [1.5]; Meiosis: 4 daughter cells, half chromosome number (haploid) [1.5].

20. [2] Villi and microvilli (or long thin shape, folded wall) [1+1].
Image note: Diagram shows villi projecting into lumen increasing area.