Secondary 4 Pure Biology Practice Paper 2

Answer Key - Pure Biology Secondary 4 Practice Paper (Version 2)

Section A

Question 1 (a) Ribosomes [1] (b) Golgi apparatus modifies and packages proteins/enzymes into vesicles for secretion [2]. (c) Cell membrane is partially permeable and present in both; cell wall is fully permeable, made of cellulose, and present only in plant cells [2].

Question 2 (a) The sucrose solution has a lower water potential than the potato cell sap [1]. Water moves out of the potato cells into the solution [1] by osmosis across a partially permeable membrane [1]. (b) The cell becomes turgid [1] as water enters the cell by osmosis, pushing the cytoplasm against the cell wall [1].

Question 3 (a) The enzyme has a specific 3D shape of its active site [1]. This shape is complementary to the shape of a specific substrate molecule [1], allowing them to fit together like a lock and key to form an enzyme-substrate complex [1]. (b) High temperature provides too much thermal energy, breaking bonds in the enzyme [1]. The active site changes shape (denatures), so the substrate can no longer fit [1].

Question 4 (a) Pepsin [1]; Acidic pH (approx pH 2) [1]. (b) One-cell thick epithelium/wall for short diffusion distance [1]; rich network of blood capillaries to maintain concentration gradient [1]; large surface area due to finger-like projections [1].

Question 5 (a) Arteries have thicker, more elastic walls / Veins have valves [1]. (b) The left ventricle must pump blood to the entire body (systemic circulation) [1], requiring higher pressure to overcome greater resistance compared to the right ventricle which only pumps to the lungs [1].

Question 6 (a) One-cell thick walls for short diffusion distance [1]; large surface area for maximum gas exchange [1]. (b) The process of breaking down glucose in the presence of oxygen to release energy [1]. $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ [2].

Question 7 (a) Ultrafiltration is the non-selective forcing of small molecules out of the glomerulus due to high pressure [1]. Selective reabsorption is the active/passive recovery of useful substances (e.g., glucose) back into the blood [1]. (b) All glucose filtered into the nephron is reabsorbed into the blood [1] via active transport in the proximal convoluted tubule [1].

Question 8 (a) Hypothalamus detects increase in blood temperature [1]. It triggers effectors: sweat glands to produce sweat for evaporative cooling [1] and skin arterioles to dilate (vasodilation) to increase heat loss via radiation [1]. (b) Stimulates the liver to convert excess glucose into glycogen for storage [1], thereby lowering blood glucose levels [1].

Question 9 (a) Antibiotics target bacterial structures/processes (e.g., cell wall synthesis) [1]. Viruses lack these structures and live inside host cells, making them unaffected by antibiotics [1]. (b) A weakened/inactive pathogen is injected [1]. This stimulates the immune system to produce antibodies and memory cells [1] without causing the disease.

Question 10 (a) Evaporation of water from stomata creates a transpiration pull [1]. This pulls water up the xylem from the roots [1]. Minerals dissolved in this water are transported upwards along with the water stream [1]. (b) The transport of organic solutes (sucrose/amino acids) through the phloem from source to sink [1].

Section B

Question 11 (a) For both temperatures, as light intensity increases, the rate of photosynthesis increases [1] until it reaches a plateau [1]. (b) Higher temperature increases the kinetic energy of molecules [1]. This increases the frequency of successful collisions between enzymes and substrates [1], increasing the rate of the light-independent reactions [1]. (c) Carbon dioxide concentration [1] and Temperature [1] (or chlorophyll availability) [1].

Question 12 (a) Energy is lost as heat during respiration [1], used for movement/growth [1], or lost as undigested waste/faeces [1]. (b) Toxins are non-biodegradable and accumulate in tissues [1]. Because energy is lost, the Osprey must eat many large fish, each containing toxins from many smaller fish [1]. This leads to biomagnification at the top of the food chain [1]. (c)

Generated diagram for this question.

. It is pyramid-shaped because biomass decreases at each level due to energy loss and inefficiency of transfer [2].

Question 13 (a) Isolation of human insulin gene using restriction enzymes [1]. Cutting a bacterial plasmid with the same restriction enzyme [1]. Joining the gene and plasmid using DNA ligase to form recombinant DNA [1]. Inserting the plasmid into E. coli bacteria [1]. Growing bacteria in a fermenter to express the gene and produce insulin [1]. (b) Potential for "playing God" or altering natural evolution [1]. Risk of creating "super-weeds" or unexpected ecological disruptions if transgenic organisms escape into the wild [2].

Question 14 (a) Combustion releases carbon stored in fossil fuels directly into the atmosphere as $\text{CO}_2$ [2]. Deforestation removes trees (carbon sinks) that would otherwise absorb $\text{CO}_2$ for photosynthesis [2]. (b) $\text{CO}_2$ and other greenhouse gases trap infrared radiation (heat) reflecting off the Earth's surface [2]. This prevents heat from escaping into space, increasing the average global temperature [2].

Question 15 (a) Blood is passed through a partially permeable membrane [1]. Urea, being a small molecule, moves from the blood (high concentration) to the dialysis fluid (low concentration) [1] by diffusion [1]. The fluid is constantly refreshed to maintain a steep concentration gradient [1]. (b) To prevent the net movement of essential solutes like glucose and salts [1]. If the fluid had lower concentrations, these nutrients would be lost from the blood by diffusion [2], leading to nutrient deficiency in the patient [1].