O Level Combined Science Life Sciences Quiz

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

Section A: Cell Biology and Movement

1. Mitochondria function: Site of aerobic respiration / production of energy (ATP). [1]
2. (a) Appearance: The cell membrane shrinks away from the cell wall (plasmolysis). [1] (b) Explanation: The salt solution has a lower water potential than the cell sap. Water moves out of the cell by osmosis from a region of higher water potential to lower water potential across a partially permeable membrane. [2]
3. Active Transport: Movement of substances against a concentration gradient (from low to high concentration) using energy (ATP). [1] Example: Absorption of glucose/amino acids in the small intestine or minerals in root hair cells. [1]
4. Differences:
   • Plant cells have a cell wall; animal cells do not. [1]
   • Plant cells have large permanent vacuoles/chloroplasts; animal cells do not. [1]
5. Surface Area to Volume Ratio: A larger ratio means there is more membrane surface available for transport relative to the volume of the cell, allowing nutrients to reach the center of the cell more quickly. [2]

Section B: Human Physiology and Enzymes

6. Enzyme: Salivary amylase. [1] Substrate: Starch. [1]
7. (a) Optimum Temp: Approximately $37^\circ\text{C}$. [1] (b) Explanation: High temperature breaks the bonds holding the enzyme's tertiary structure. [1] The active site changes shape and is no longer complementary to the substrate. [1] The enzyme is denatured. [1]
8. Bile role: Emulsifies fats (breaks large fat droplets into smaller droplets) to increase the surface area for lipase action. [2]
9. Villi Adaptations:
   • Finger-like projections increase surface area for absorption. [1]
   • One-cell thick wall (epithelium) provides a short diffusion distance. [1]
   • Rich network of blood capillaries to maintain a steep concentration gradient. [1]
10. Valves: To prevent the backflow of blood / ensure blood flows in one direction. [1]
11. Blood Composition: Arterial blood is generally oxygenated (high $\text{O}_2$); venous blood is generally deoxygenated (low $\text{O}_2$). [2]
12. Gas Exchange: Oxygen diffuses from the alveoli (high concentration) into the blood capillaries (low concentration). [1] Carbon dioxide diffuses from the blood capillaries into the alveoli. [1] This occurs across a thin, moist membrane. [1]

Section C: Molecular Genetics and Inheritance

13. DNA Structure: Double helix structure. [1] Composed of two strands of polynucleotides made of sugar, phosphate, and nitrogenous bases (A, T, C, G). [1]
14. Mitosis vs Meiosis: Mitosis produces two genetically identical daughter cells. [1] Meiosis produces four genetically different daughter cells (gametes). [1]
15. (a) Genotypes: Parent 1: Pp; Parent 2: pp. [1] (b) Working:
    • Gametes: P, p and p, p.
    • Offspring: Pp, Pp, pp, pp.
    • Probability of white (pp) = 2/4 = 50%. [3]
16. Genetic Engineering:
    • Use restriction enzymes to cut the human insulin gene. [1]
    • Use the same restriction enzyme to cut a bacterial plasmid. [1]
    • Use DNA ligase to join the gene into the plasmid. [1]
    • Insert the recombinant plasmid back into the bacterial host cell. [1]
17. Mutation: A spontaneous change in the DNA sequence/base sequence of a gene. [1] Example: Sickle cell anemia / Cystic fibrosis. [1]

Section D: Ecology and Conservation

18. Deforestation:
    • Fewer trees mean less $\text{CO}_2$ is removed via photosynthesis. [1]
    • Burning of trees releases stored carbon as $\text{CO}_2$. [1]
    • This increases the concentration of greenhouse gases in the atmosphere. [1]
19. Chain vs Web: A food chain is a single linear sequence of organisms through which nutrients and energy pass. [1] A food web consists of multiple interconnected food chains. [1]
20. Conservation:
    • Establishing protected areas/national parks. [1]
    • Implementing laws against illegal logging/poaching. [1]