O Level Biology Practice Paper 3

Answer Key - Biology O-Level Practice Paper (Version 3)

Question 1 (a) Cell wall; Chloroplasts; Large central vacuole (Any two) [2] (b) Function: Muscle cell / Sperm cell / Secretory cell [1]. Reasoning: Mitochondria are the site of aerobic respiration to produce ATP/energy; a high number indicates a high energy demand for the cell's specific function [1].

Question 2 (a) The net movement of water molecules from a region of higher water potential to a region of lower water potential [1] through a partially permeable membrane [1]. (b) (i) Decrease in mass [1]. (ii) The salt solution has a lower water potential than the cell sap of the potato [1]. Water moves out of the vacuole/cell by osmosis [1] across the partially permeable membrane [1].

Question 3 (a) Protein [1]. (b) Add Benedict's solution to the sample [1], heat the mixture in a water bath [1], observe a brick-red precipitate [1]. (c) The enzyme has a specific 3D shape of its active site [1] which is complementary only to a specific substrate molecule [1].

Question 4 (a) Long extension/projection of the cell [1] increases the surface area [1] for faster absorption of water and mineral ions [1]. (b) Active transport [1]. Yes, it requires energy (ATP) [1].

Question 5 (a) Amylase [1]; Starch [1]. (b) Bile emulsifies fats [1], breaking large fat droplets into smaller droplets [1], which increases the surface area for lipase to act upon [1].

Question 6 (a) Deoxygenated blood is pumped from the right ventricle [1] via the pulmonary artery [1] to the lungs [1]. Oxygenated blood returns to the left atrium via the pulmonary vein [1]. (b) Arteries have thick, muscular, and elastic walls [1] to withstand high pressure from the heart [1]. Veins have thinner walls and contain valves [1] to prevent the backflow of blood under low pressure [1].

Question 7 (a) Glucose + Oxygen $\rightarrow$ Carbon dioxide + Water + Energy [2]. (b) Occurs when oxygen supply is insufficient for the energy demand [1]. Lactic acid is formed [1] (or Glucose $\rightarrow$ Lactic acid + Energy) [1].

Question 8 (a) High pressure in the glomerulus [1] forces small molecules (water, glucose, urea, salts) [1] out of the blood into the Bowman's capsule [1]. (b) Islets of Langerhans secrete glucagon [1]. Glucagon travels to the liver [1] and stimulates the conversion of glycogen back into glucose [1], which is released into the blood to raise glucose levels [1].

Question 9 (a) Receptor in skin detects heat [1] $\rightarrow$ impulse travels along sensory neurone [1] $\rightarrow$ impulse reaches the spinal cord/relay neurone [1] $\rightarrow$ impulse crosses synapse via neurotransmitter [1] $\rightarrow$ impulse travels along motor neurone [1] $\rightarrow$ effector (muscle) contracts to pull hand away [1]. (b) Infectious diseases can be spread from one person to another via pathogens [1]. Non-infectious diseases cannot be transmitted between individuals [1].

Question 10 (a) Light intensity; Carbon dioxide concentration; Temperature (Any two) [2]. (b) Presence of stomata for gas entry/exit [1], spongy mesophyll with air spaces [1] to allow CO2 to diffuse to cells [1], and thin leaf to reduce diffusion distance [1].

Question 11 (a) Evaporation of water from the leaves [1]. It creates a transpiration pull [1] that draws water and minerals up from the roots through the xylem [1], and helps in cooling the plant [1]. (b) Energy is lost as heat during respiration [1], lost in undigested materials/faeces [1], or lost when organisms die and are not consumed [1].

Question 12 (a) Mitosis: 2 daughter cells [1], genetically identical [1]. Meiosis: 4 daughter cells [1], genetically different [1]. (b) An individual having two different alleles for a particular gene [2].

Question 13 (a) Target gene for insulin is cut from human DNA using a restriction enzyme [1]. A plasmid is cut using the same restriction enzyme [1]. The gene and plasmid are joined by DNA ligase [1] to form recombinant DNA [1]. This is inserted into a bacterial cell [1]. The bacteria are grown in a fermenter to produce insulin [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], passing the gene to offspring, increasing the frequency of resistance in the population [1].

Question 14 (a) Secreted by the corpus luteum [1]. It maintains the thickness of the uterine lining [1] to prepare for the implantation of a fertilised egg [1]. (b) Protects the fetus from mechanical shock/injury [2].

Question 15 (a) Parents: Pp x Pp. Gametes: P, p and P, p. Offspring: PP, Pp, Pp, pp [2]. Phenotypic ratio: 3 Purple : 1 White [3]. (b) A gene is a section of DNA that codes for a specific protein/trait [1]. An allele is an alternative form of a gene [1].