O Level Biology Practice Paper 2

Answer Key - Biology O-Level Practice Paper (Version 2)

Section A: Cells and The Chemistry of Life

Question 1 (a) Plant cell [1] (b) Provides structural support / prevents the cell from bursting [1] (c) Stores cell sap [1]; maintains turgidity to support the plant/leaf [1]

Question 2 (a) Long extension/large surface area [1]; increases the rate of water absorption [1] (b) Active transport [1]; uses energy from ATP produced by aerobic respiration [1]; ions move against the concentration gradient from soil into the cell [1]

Question 3 (a) Decrease in mass [1] (b) Sucrose solution has lower water potential than the cell sap [1]; water moves out of the vacuole/cell [1] by osmosis across the partially permeable membrane [1]

Question 4 (a) Enzyme has a specific 3D shape with an active site [1]; substrate has a complementary shape [1]; they fit together to form an enzyme-substrate complex [1] (b) High temperature causes the enzyme to denature [1]; the active site changes shape [1]; substrate can no longer fit/no enzyme-substrate complex formed [1]

Question 5 (a) Benedict's solution [1]; brick-red precipitate [1] (b) Starch: Add iodine solution $\rightarrow$ blue-black color [2]. Protein: Add Biuret solution $\rightarrow$ purple color [2].

Section B: The Human Body - Maintaining Life

Question 6 (a) Amylase [1]; Starch [1]; Maltose [1] (b) Emulsifies fats [1]; breaks large fat droplets into smaller droplets to increase surface area for lipase action [1]

Question 7 (a) Right atrium $\rightarrow$ Right ventricle [1] $\rightarrow$ Pulmonary artery [1] $\rightarrow$ Lungs [1] $\rightarrow$ Pulmonary vein $\rightarrow$ Left atrium $\rightarrow$ Left ventricle $\rightarrow$ Aorta [1] (b) Left ventricle pumps blood to the entire body (systemic circulation) [1]; requires higher pressure to overcome resistance [1]

Question 8 (a) One-cell thick wall (short diffusion distance) [1]; large surface area / rich blood supply (maintains concentration gradient) [1] (b) Carbon monoxide binds strongly/irreversibly to haemoglobin [1]; forms carboxyhaemoglobin [1]; reduces the capacity of blood to carry oxygen [1]

Question 9 (a) Removal of metabolic waste products from the body [1] (b) High pressure in glomerulus [1]; forces small molecules (water, glucose, urea, salts) [1] through the basement membrane into Bowman's capsule [1]

Question 10 (a) Islets of Langerhans detect high glucose [1]; insulin is secreted [1]; liver converts glucose to glycogen [1]; blood glucose levels decrease [1] (b) Hypothalamus detects drop in temperature [1]; triggers shivering of muscles [1] and vasoconstriction of skin arterioles to reduce heat loss [1]

Section C: Living Together and Continuity of Life

Question 11 (a) $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ (with light and chlorophyll) [2] (b) $\text{CO}_2$ is present in very low concentrations in the atmosphere [1]; it limits the rate of the Calvin cycle/carbon fixation [1]

Question 12 (a) Increases rate of transpiration [1]; wind removes the water vapour layer from the leaf surface, increasing the water potential gradient [1] (b) Lignified walls prevent collapse [1]; hollow tubes allow continuous column of water movement [1]

Question 13 (a) Energy lost as heat during respiration [1]; energy lost in undigested waste/faeces [1]; energy spent on movement/metabolism [1] (b) Pyramid of numbers shows the total number of individuals [1]; pyramid of biomass shows the total dry mass of organisms [1]

Question 14 (a) Double helix [1]; two antiparallel strands [1]; complementary base pairing (A-T, C-G) [1] (b) Target gene for insulin cut using restriction enzyme [1]; bacterial plasmid cut with same enzyme [1]; gene and plasmid joined by DNA ligase [1]; recombinant plasmid inserted into bacteria [1]; bacteria divide and produce insulin protein [1]

Question 15 (a) Mitosis: 2 identical daughter cells / Meiosis: 4 genetically different cells [1]. Mitosis: diploid / Meiosis: haploid [1] (b) Zygote divides by mitosis to form embryo [1]; embryo moves down fallopian tube [1]; blastocyst implants in uterine wall [1]; placenta develops [1]; provides nutrients/oxygen via umbilical cord [1]; amniotic fluid protects fetus [1]

Question 16 (a) Homozygous: two identical alleles for a trait [1]; Heterozygous: two different alleles for a trait [1] (b) Parents: Tt x tt. Gametes: (T, t) and (t). Offspring: Tt, Tt, tt, tt. Ratio: 1 Tall : 1 Short [4]

Question 17 (a) Continuous: range of phenotypes (e.g., height) [1]; Discontinuous: distinct categories (e.g., blood group) [2] (b) Random mutation creates resistance gene [1]; antibiotic acts as selective pressure [1]; non-resistant bacteria die, resistant ones survive [1]; survivors reproduce and pass gene to offspring [1]; frequency of resistant phenotype increases in population [1]

Question 18 (a) Low light detected by retina [1]; circular muscles contract / radial muscles relax [1]; pupil dilates to allow more light in [1] (b) Contains photoreceptors (rods and cones) to detect light and form images [1]

Question 19 (a) Bacteria: have cell wall and nucleus-free DNA [1]; Virus: protein coat and genetic material only [1]; Bacteria are larger/living cells, viruses are non-living until in host [1] (b) Antibiotics target bacterial cell walls or metabolic pathways [1]; viruses do not have these structures/pathways [1]

Question 20 (a) Photosynthesis [1]; Dissolution in oceans [1] (b) Fewer trees to absorb $\text{CO}_2$ via photosynthesis [1]; burning of trees releases stored carbon as $\text{CO}_2$ [1]; $\text{CO}_2$ is a greenhouse gas that traps heat in the atmosphere [1]