ZHSS 2024 P2 MS ANS

ZHSS 2024 Prelim Paper 2 Answers Section A [70 marks] Q/no. Answer Mark 1ai • Cell wall [1] 1aii • Deoxyribonucleic acid / DNA [1] 1aiii • Attaching/sticking to surfaces of host cells/objects (for infection/colonisation). • Joining other bacteria to exchange genetic material (i.e., gene transfer). Any 1 [1] 1b • Kissing. • Sexual activity/intercourse. • Sharing of eating utensils. • Touching surfaces contaminated by infected person then puts hand in mouth. Any 2 [2] 1c • Bacteria will evolve to become resistant to antibiotics / May increase the number of antibiotic-resistant strains of the bacteria. • May cause antibiotics to become less effective/ineffective when it is used to treat severe cases. • Antibiotics do not shorten the illness / reduce the duration of symptoms (diarrhoea, fever, stomach cramps). [2]

• Antibiotics can prolong the time a person carries the bacteria, which increases the risk of infecting others. Any 3 1d • Nucleus controls cell activities/cell growth/repair of worn-out parts / essential for cell division / contains DNA/genetic material/chromosomes/chromatin. • Chloroplast contains chlorophyll for photosynthesis. • Vacuole contains cell sap which stores dissolved substances/sugars/minerals salts/amino acids. • Mitochondrion is the site of aerobic respiration to provide energy for the cell. • Golgi body/apparatus chemically modifies substances made by the ER / stores and packages substances in vesicles for secretion out of the cell. • Rough endoplasmic reticulum transports proteins made by ribosomes to Golgi body/apparatus for secretion out of the cell. • Smooth endoplasmic reticulum synthesises fats/steroids/hormones / detoxification/converts harmful substances into harmless substances. Any 3 [4] 2a Diffusion Active transport • Does not require energy. Requires energy. • Molecules/particles/substances move from a region of higher concentration to a region of lower Molecules/particles/substances move from a region of lower concentration to a region of higher concentration / against a concentration gradient. [3]

concentration / along a concentration gradient. • Occurs regardless of the presence of a partially permeable membrane. Occurs in the presence of a partially membrane. 2bi • All the glucose and amino acids, and some salts are selectively reabsorbed from the tubules/nephron into the blood/capillaries. • Glomerular filtrate (in tubules) has a higher water potential than the blood (in capillaries). • Water molecules leaves the glomerular filtrate through the partially permeable membrane of cells and into the blood by osmosis. • Most water is reabsorbed at the loop of Henle by osmosis and forms part of the urine [4] 2bii • ADH increases the permeability of the cells in the walls of the collecting duct (V). • More water is reabsorbed from the collecting duct into the blood capillaries. [2] 3a • When a stimulus disrupts a system/body, a sequence of events/corrective mechanism will occur. • This results in the reverse effect of the stimulus and restore the system/body to its original state. [2] 3bi • Sweat gland. [1] 3bii • When body temperature rises above normal, arterioles in the skin dilate. [3]

• This allows more blood to flow through blood capillaries near the skin surface. • More heat will be lost through the skin by conduction, convection and radiation and body temperature will decrease back to normal. 3biii • Fat tissues do not contain thermoreceptors/temperature receptors to detect changes in body temperature. • Fat tissues do not actively respond to temperature changes in the body. • Any 1 [1] 4a • During photosynthesis, light energy is absorbed by chlorophyll in the chloroplasts. • The energy is used to convert carbon dioxide into glucose (chemical energy). • Glucose is converted to cellulose to synthesise cell wall / Glucose is converted to sucrose and transported by the phloem to storage organs and excess stored as starch [4] 4b Similarity • Blue cobalt chloride paper on the upper surfaces of leaves from both plants took more time to turn pink than the lower surfaces. • Fewer stomata on upper surfaces than lower surfaces, so water vapour took a longer time to exit. Difference • Blue cobalt chloride paper on leaves of plant Q took less time to turn pink than on leaves of plant R. • Leaves of plant Q have more stomata than leaves of plant R. / Plant Q has larger leaves than plant R, so more stomata are present on the leaves of plant Q than on the leaves of plant R. [4]

5ai • Drawn to scale accurately. • Occupies more than half the grid, all trophic levels have the same height and labelled. [2] microscopic plants microscopic animals fish

5aii • Metabolic heat / heat released during respiration • Uneaten body parts • Faeces • Excretory products Any 2 [2] 5aiii • Eutrophication enhances the growth of algae and water plants, causing an increase in their numbers. • Overgrowth of algae and floating water plants prevent light energy from reaching the submerged plants. • Submerged plants are unable to carry out photosynthesis and die • Dead plants are decomposed by aerobic bacteria and fungi which grow rapidly and uses up the dissolved oxygen in the lake. Any 3, in correct order [3] 5bi • Decomposers [1] 5bii • C6H12O6, H2O [1] 6a • Complementary base pairing [1] 6b • Genes carry the genetic code that is used to synthesise specific polypeptides, • by transcription and translation. • Polypeptides are folded into proteins in cells, and the cells are used to form organs. [3]

6c Social • (-) Access may be limited to those who can afford, leading to potential social inequalities. • (-) Cultural and religious beliefs. (A: examples) • (-) Economic impact on healthcare costs, pharmaceutical industries, and agricultural sectors. • (+) Alleviation of critical shortage of human organs for transplantation, potential to save many lives. • (+) Improved quality of life for patients with organ failure as they need not be limited by treatments that are temporary (e.g. dialysis). • (+) Could lead to development of new medical technologies and treatments, stimulating economic growth (medical tourism) and job creation in healthcare sector. Ethical • (-) Concerns about animal rights / moral status of animals (exploitation of animals for human gains, killing of animals to extract organ, testing on animals). • (-) Potential for disease transmission and immunological rejection. • (-) Allocation of organs fairly and equitably is challenging. • (-) Patients may not have the capacity to fully understand the risks and benefits / make autonomous decisions. • (+) Alleviation of critical shortage of human organs for transplantation, potential to save many lives. Any 1 from each category (no repetition) [2]

7a • When muscles of the left atrium contract, oxygenated blood is forced/pumped through the bicuspid valve into the left ventricle. • When muscles of the left ventricle contract, blood is forced/pumped through the semilunar valve into the aorta and around the body. [2] 7bi [4]

7bii • Based on graph in 7ai, showing extrapolated dotted line on graph. [1] 7biii • As speed of running machine/running speed increases, there is a greater demand for energy for muscular contractions. • Rate of aerobic respiration increases to supply more energy. • More oxygen is needed for aerobic respiration. • Heart rate increases to transport oxygen faster to the muscles. Any 3 [3] 8ai • Mitosis is a type of cell division that produces genetically identical cells. • Chromosome number of cells is maintained. [2] 8aii • Mitosis produces new cells, which are needed for plant growth • Mitosis produces new cells, which are needed for the plant to produce flowers/fruits. • Mitosis produces new cells to replace cells that are dead/damaged / to repair tissues. • Mitosis allows for asexual reproduction to occ