Secondary 4 Combined Science Biology Preliminary Examination Paper 3

TuitionGoWhere Practice Paper - Combined Science Biology Secondary 4

Answer Key & Marking Scheme Paper: Preliminary Examination (Version 3 of 5)

Section A: Structured Questions

1. (a) Cell wall / Chloroplast / Vacuole (Accept any one correct plant-specific structure if labelled correctly in standard diagrams, but typically X refers to Cell Wall or Chloroplast in this context. Assuming standard diagram: Chloroplast or Cell Wall). Note: Without the visual, standard answer is usually Cell Wall or Chloroplast. Let's assume X is Chloroplast for specificity. -> Chloroplast [1] (b) Nucleus (if Y is nucleus): Controls cell activities / Contains genetic material. OR Cell Membrane: Controls entry/exit of substances. [1] (c) Muscle cells require more energy (ATP) for contraction [1]; Mitochondria are the site of aerobic respiration/ATP production [1].

2. (a) The time taken decreases / Rate of reaction increases [1]. (b) At 60°C, the enzyme is denatured [1]; The shape of the active site changes [1]; Substrate (starch) can no longer fit/bind to the active site [1]. (c) pH is a variable that affects enzyme activity / To ensure it is a fair test / To keep pH as a controlled variable [1].

3. (a) Palisade mesophyll [1]; Contains many chloroplasts / Tightly packed / Columnar shape to maximize light absorption [1]. (b) Air spaces allow for rapid diffusion of gases (CO₂ and O₂) [1]; They connect to stomata for gas exchange with the atmosphere [1]. (c) Transpiration [1].

4. (a) Active transport [1]. (b) Glucose is moved against the concentration gradient (from low to high concentration) [1]; This requires energy to pump molecules [1]. (c) Respiration (producing ATP) [1].

5. (a) Oestrogen [1]. (b) Maintains the thickness of the uterine lining [1]; Prepares the uterus for implantation of the embryo [1]. (c) Progesterone levels drop [1]; The uterine lining breaks down/sheds (menstruation) [1].

6. (a) Reduced surface area for gas exchange / Cells block capillaries / Shorter lifespan [1]. (b) Less hemoglobin can be carried / Blood flow is restricted/blocked in capillaries [1]; Less oxygen is delivered to tissues [1].

7. (a) Carbon dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll) [1]. (b) Plants use carbon dioxide as a raw material for photosynthesis [1]; Carbon is stored in the plant biomass (stems/leaves/roots) [1]. (c) Burning releases the stored carbon dioxide back into the atmosphere [1].

8. (a) The net movement of water molecules [1] from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane [1]. (b) The potato cylinder will lose mass [1]; Water moves out of the potato cells by osmosis [1] because the sugar solution has a lower water potential than the cell sap [1].

9. (a) ATP (Adenosine Triphosphate) [1]. (b) Sperm cells need energy for movement/swimming [1]; Mitochondria provide the ATP required for the tail to move [1].

10. (a) Starch and Protein [1] (Must identify both if implied by "nutrient present" generally, but usually asks for specific. A has Starch (Iodine +) and Protein (Biuret +). Question asks for "the nutrient". If singular, accept Starch or Protein. Better: Starch and Protein). Correction: Question asks for "the nutrient" singular. Usually, these questions target the positive result. Let's assume it asks for the main carbohydrate or just identification. Accept Starch or Protein. Given A is Blue-black (Starch) and Purple (Protein), listing both is safest. If forced to one, context usually implies the carbohydrate test first. Let's accept Starch or Protein. [1] (b) Reducing Sugar (e.g., Glucose) [1]. (c) Add Biuret reagent (Sodium hydroxide and Copper(II) sulfate) [1]; Solution turns purple/lilac if protein is present [1].

Section B: Free Response Questions

11.

• Oxygen diffuses from alveoli into blood capillaries [1].
• Binds to hemoglobin in red blood cells [1].
• Transported via pulmonary vein to left atrium [1].
• Left atrium to left ventricle [1].
• Left ventricle pumps blood into aorta [1].
• Aorta branches into arteries leading to the leg [1].
• Diffuses from capillaries in muscle tissue into muscle cells [1]. (Max 6 marks. Must mention diffusion, hemoglobin, heart chambers, aorta, and final diffusion into cell.)

12. (a)

• Enzyme has a specific active site shape [1].
• Substrate has a complementary shape [1].
• Substrate fits into active site like a key in a lock to form an enzyme-substrate complex [1].

(b)

• Changes in pH affect the charges on the amino acids in the enzyme [1].
• This alters the shape of the active site [1].
• If the shape changes significantly (denaturation), the substrate no longer fits [1].

13.

• Diffusion: Passive process (no energy) [1]; Moves down concentration gradient (high to low) [1]; Does not require carrier proteins (simple diffusion) or uses channel proteins (facilitated) [1].
• Active Transport: Requires energy (ATP) [1]; Moves against concentration gradient (low to high) [1]; Requires specific carrier proteins [1]. (1 mark for each correct contrasting point. Max 6.)

14.

• Important to ensure cells have a constant supply of glucose for respiration [1] and to prevent damage to cells/organs (e.g., kidneys, eyes) from high glucose [1].
• High Glucose: Pancreas (beta cells) detects high glucose [1]; Secretes insulin [1]; Insulin causes liver/muscle cells to take up glucose and convert it to glycogen [1]; Blood glucose decreases.
• Low Glucose: Pancreas (alpha cells) detects low glucose [1]; Secretes glucagon [1]; Glucagon causes liver to convert glycogen to glucose [1]; Blood glucose increases. (Max 7 marks. Need clear negative feedback loop explanation.)

15. (a)

• Palisade mesophyll cells are located in the upper part of the leaf [1].
• They contain more chloroplasts than the spongy mesophyll cells in the lower part [1].
• Chlorophyll in chloroplasts is green [1].

(b)

• Upper surface receives more direct sunlight [1].
• More chloroplasts maximize light absorption for photosynthesis [1].

Section C: Data-Based Question

16. (a) The rate of photosynthesis increases linearly/proportionally with light intensity [1]; Light is the limiting factor [1]. (b) Another factor becomes limiting (e.g., CO₂ concentration or temperature) [1]; Increasing light further cannot increase the rate because the enzymes are working at maximum capacity [1]. (c) Higher temperature increases the kinetic energy of enzyme and substrate molecules [1]; More frequent successful collisions occur [1]; (Up to the optimum temperature). (d) The rate would decrease significantly/drop to zero [1]; Enzymes would be denatured [1]; Active site shape changes, preventing substrate binding [1].

17. (a) Carbohydrates in the meal are digested into glucose [1]; Glucose is absorbed into the bloodstream from the small intestine [1]. (b) Pancreas detects high blood glucose [1]; Secretes insulin [1]; Insulin promotes uptake of glucose by cells and conversion to glycogen in the liver [1]. (c) Pancreas [1].

18. (a) An allele that is only expressed in the phenotype when two copies are present (homozygous) / Masked by a dominant allele [1]. (b) Nn (Heterozygous) [1]. (c) Individual 5 is unaffected (shows dominant phenotype) [1] but has an affected parent/child (must inherit recessive allele) OR Individual 5 has an affected child (nn), so 5 must contribute an 'n' allele [1]. Note: Depending on pedigree structure. Standard logic: If 5 is unaffected but has affected offspring or parents, 5 is Nn. (d) If 3 and 4 are both Nn (carriers):

• Genetic cross: Nn x Nn [1].
• Offspring probabilities: NN, Nn, Nn, nn.
• Probability of disease (nn) is 1/4 or 25% [1].

19. (a) Duodenum / Small intestine [1]. (b) Bile salts emulsify fats [1]; Breaks large fat droplets into smaller droplets [1]; Increases surface area for lipase action [1]. (c) It does not break down fats chemically / It does not change the chemical structure of fats / It is not a protein catalyst [1].

20. (a) Mitosis [1]. (b) Growth / Repair of tissues / Asexual reproduction [1]. (c) Chromosomes are visible as distinct threads [1]; They are duplicated (consist of two chromatids) [1].