O Level Biology Practice Paper 5

Answer Key - Biology O-Level Practice Paper (Version 5)

Section A: Foundational Concepts

Question 1 (a) Rough Endoplasmic Reticulum (RER) / Golgi Body [1] (b) Inner membrane is folded into cristae [1]; increases surface area for enzymes/electron transport chain [1]; allows for higher rate of ATP production [1]. (c) Bacterial cells have a cell wall (peptidoglycan) while liver cells do not [1]; Bacterial cells have plasmids/no nucleus (nucleoid) while liver cells have a nucleus [1].

Question 2 (a) Mass decreases [1]. Sucrose solution has lower water potential than cell sap [1]. Water moves out of the vacuole by osmosis [1]. (b) The net movement of water molecules [1] from a region of higher water potential to a region of lower water potential through a partially permeable membrane [1].

Question 3 (a) Carbon, Hydrogen, Oxygen, Nitrogen (Sulphur optional) [1] (b) Add Benedict's solution to the sample [1]. Heat the mixture in a water bath [1]. Observation: brick-red precipitate forms [1]. (c) Long-term energy storage [1]; thermal insulation/protection of organs [1].

Question 4 (a) The enzyme's active site has a specific 3D shape [1] that is complementary to the shape of the substrate molecule [1]. (b) High temperature breaks the bonds holding the enzyme's 3D shape [1]. The active site changes shape/is denatured [1]. The substrate can no longer fit into the active site to form an enzyme-substrate complex [1].

Question 5 (a) Active Transport [1] (b) Oxygen is required for aerobic respiration [1]. This process produces ATP/energy [1]. ATP is used to power the protein pumps that move ions against the concentration gradient [1].

Section B: Applied Systems

Question 6 (a) Amylase breaks down starch [1] into maltose [1]. (b) The acidic pH (HCl) provides the optimum pH for pepsin/protease to function [1] and helps denature proteins to make them easier to digest [1]. (c) Zygote divides by mitosis to form an embryo [1]. Embryo travels down the fallopian tube [1]. Blastocyst implants in the uterine wall [1]. Placenta develops for nutrient/oxygen exchange [1]. Umbilical cord connects fetus to placenta [1]. Amniotic fluid protects the fetus [1].

Question 7 (a) Right atrium receives deoxygenated blood from body [1]. Right ventricle pumps it to lungs via pulmonary artery [1]. Blood is oxygenated in lungs [1]. Oxygenated blood returns to left atrium via pulmonary vein [1]. Left ventricle pumps it to the body via the aorta [1]. This ensures high-pressure delivery of oxygenated blood to tissues [1]. (b) Arteries have thicker walls / Veins have valves [1].

Question 8 (a) Alpha cells of islets of Langerhans secrete glucagon [1]. Glucagon travels to the liver [1]. Liver converts stored glycogen back into glucose [1]. Blood glucose levels rise back to normal [1]. (b) It acts as the control center/thermostat [1]. It detects changes in blood temperature and triggers responses like sweating or shivering [1].

Question 9 (a) Glucose $\rightarrow$ Lactic acid + Energy [1] (b) Lactic acid accumulates in muscles during anaerobic respiration [1]. Oxygen is needed to break down/oxidize this lactic acid [1] back into pyruvate or transport it to the liver [1].

Section C: Continuity and Environment

Question 10 (a) Parents: Pp x pp. Gametes: (P, p) and (p, p). Offspring: Pp, Pp, pp, pp. Ratio: 1 Purple : 1 White [4]. (b) Mitosis produces 2 genetically identical cells [1]; Meiosis produces 4 genetically different cells [1]. (Alternative: Diploid vs Haploid).

Question 11 (a) Target human insulin gene is cut using a restriction enzyme [1]. Bacterial plasmid is cut using the same restriction enzyme [1]. Gene and plasmid are joined by DNA ligase [1] to form a recombinant plasmid [1]. Plasmid is inserted back into bacteria [1]. (b) Variation exists due to random mutation [1]. Some bacteria have a resistance gene [1]. Antibiotics kill non-resistant bacteria [1]. Resistant bacteria survive and reproduce [1]. The frequency of the resistance gene increases in the population [1].

Question 12 (a) Removal of metabolic waste products from the body [1]. (b) Increased protein intake leads to more amino acid deamination [1]. This increases urea production in the liver [1]. More urea in the blood increases the osmotic gradient, leading to more water reabsorption in the kidney and more concentrated urine [1].