Secondary 4 Pure Biology Preliminary Examination Paper 5

Answer Key - Pure Biology Preliminary Paper 2 (Version 5)

Section A: Cellular and Molecular Basis of Life

Question 1 (a) C (Mitochondrion) [1] (b) (i) Ribosomes [1] (ii) Site of protein synthesis [1] (c) 1. Plant cell has a cell wall; animal cell does not. [1] 2. Plant cell has chloroplasts; animal cell does not. [1] (Alternatively: Plant cell has a large central vacuole; animal cell has small temporary vacuoles).

Question 2 (a) Water moves out of the red blood cell [1] by osmosis [1] down a water potential gradient. The cell becomes shrunken/crenated [1]. (b) Plant cells have a rigid cell wall made of cellulose [1]. When water enters, the cell becomes turgid but the cell wall prevents the cell from bursting [1]. Red blood cells lack a cell wall [1], so the plasma membrane ruptures under osmotic pressure.

Question 3 (a) Reducing sugars (Brick-red) [1], Proteins (Purple) [1], Fats/Lipids (Cloudy emulsion) [1]. (b) Proteins are polymers [1] built from amino acids joined by peptide bonds [1].

Question 4 (a) The enzyme has a specific 3D shape with an active site [1]. The substrate has a complementary shape to the active site [1]. The substrate fits into the active site to form an enzyme-substrate complex [1], where the reaction is catalyzed to form products [1]. (b) At 60°C, the high thermal energy breaks the bonds holding the enzyme's tertiary structure [1]. The active site changes shape/is lost [1], meaning the substrate can no longer fit (enzyme is denatured) [1].

Question 5 (a) The movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration [1] using energy from respiration (ATP) [1]. (b) Mineral ion concentration in the soil is often lower than inside the root hair cell [1]. Ions must move against the concentration gradient [1], which requires active transport to ensure the plant gets sufficient nutrients [1].

Section B: Human Physiology and Plant Biology

Question 6 (a)

• One-cell thick walls / very thin walls $\rightarrow$ short diffusion distance [1]
• Large surface area $\rightarrow$ increases rate of gas exchange [1]
• Moist lining $\rightarrow$ allows gases to dissolve before diffusing [1]
• Rich network of capillaries $\rightarrow$ maintains steep concentration gradient [1] (Any 4 points) (b) Carbon monoxide binds more strongly/irreversibly to haemoglobin than oxygen [1]. This reduces the oxygen-carrying capacity of the blood [1], leading to oxygen deprivation in tissues/organs [1].

Question 7 (a) Blood flows past a dialysis fluid with a lower urea concentration [1]. A concentration gradient is established [1]. Urea moves from the blood into the dialysis fluid [1] across a semi-permeable membrane by diffusion [1]. (b) Total urea removed = $(50\text{ mg/dL} - 20\text{ mg/dL}) \times (2000\text{ mL} / 100\text{ mL}) = 30 \times 20 = 600\text{ mg}$ [1] Average hourly rate = $600\text{ mg} / 4\text{ hours} = 150\text{ mg/hour}$ [2] (1 mark for working, 1 mark for correct value, 1 mark for unit).

Question 8 (a) $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ (Light and Chlorophyll) [2] (b) As light intensity increases, the rate of photosynthesis increases [1] because more energy is available to drive the reaction [1]. Eventually, the rate plateaus/levels off [1]. At this point, light is no longer the limiting factor; other factors like $\text{CO}_2$ concentration or temperature are limiting [1].

Question 9 (a) The transport of organic solutes (sucrose and amino acids) through the phloem from source to sink [2]. (b) On a hot sunny day, stomata open to allow $\text{CO}_2$ in for photosynthesis [1]. This increases the rate of water vapor escaping the leaf [1], thereby increasing the rate of transpiration [1].

Question 10

• Photosynthesis converts light energy into chemical energy stored in glucose [1].
• Plants (producers) use this glucose for growth and respiration [1].
• Herbivores and carnivores (consumers) depend on eating plants or other animals for their energy source [1].
• Without photosynthesis, there would be no food/energy for the rest of the food chain [1].
• Photosynthesis releases oxygen as a byproduct [1].
• Oxygen is essential for the aerobic respiration of most living organisms to produce ATP [1]. (Max 6 marks)