2021 O Level 6093 Ans Key - Paper 3

2021 O’ Levels 6093/3 – Recommended Answers 1a. 179 [1] (in whole numbers) 1b. dimensions of agar cube / mm surface area / mm2 [1] volume / mm3 surface area : volume ratio [1] distance from surface to the centre of agar cube / mm [1] 2x2x2 24 8 3 1 4x4x4 96 64 1.5 2 6x6x6 216 216 1 3 8x8x8 384 512 0.75 4 10x10x10 600 1000 0.6 5 1c. [1m – values for time taken fall within 20% of range 1m – time taken increases as SA:V decreases] surface area : volume ratio Time taken for whole cube to change colour / s 3 27 1.5 96 1 189 0.75 393 0.6 557 1d. [1] each. Total: [4] - Vertical axis labelled “time taken for agar cube to change colour (s)” & horizontal axis labelled “surface area to volume ratio” [1] - Scale: Appropriate and graph occupies at least ½ of grid [1] - All points plotted in accurately [1] - Best fit line: Smooth curve that does not extrapolate [1] Page 1 of 3

[1] each. Total: [4] - Proportion: the thickness of the stem is proportional to the length of the plant + size of the leaves is proportional - Quality of lines: clean & continuous drawing lines Resemblance to specimen: - Shape of leaves - Distribution of root hairs: fine root hairs found on the tap root of the plant Note: - Draw seedling upright (in the orientation that it is in natural form) - No mark awarded if the size of the drawing is too small (For long structure, generally ≥¾ vertical space used) 1e. As surface area to volume ratio increases, time taken for the agar cube to change colour decreases [1]. 1f. [1] for each key variable and explanation key variable 1: concentration of hydrochloric acid explanation: To ensure that the concentration gradient for diffusion is the same for all cubes. key variable 2: type of agar used explanation: To ensure that the density of all cubes are the same. 1g. source of error: It is difficult to determine the exact time when the entire cube changes colour completely [1]. improvement: Repeat the experiment to obtain an average time taken for the time taken for the cube to change colour to improve the reliability of result [1]. source of error: Before the timing of the stopwatch started, diffusion of the acid has already started for the first four cubes, so their measured time taken will be shorter than expected [1]. improvement: Use separate stopwatches for each cube and start timing each one immediately after the cube is immersed in acid. / Conduct the experiment for each cube one after another [1]. 1h. [1] each. Total: [3] - The agar blocks are cubes but not all cells are cuboidal in shape, so the rate of diffusion will be different. - The agar block has the same consistency throughout but cells contain organelles which will affect the rate of diffusion differently. - The material that the agar block is made up of is different from that of cells, so the rate of diffusion will be different. 2a. i. Drawing of white mustard plant ii. [1m for working. 1m for answer in 2 sf] Magnification = length of drawing / length of actual 2b. NIS 2c. NIS 2d. i. The leaves and stems are stained red [1]. Page 2 of 3

ii. Aerobic respiration occurs in mitochondria, which releases energy [1]. This energy is used in cell division and making of new protoplasm [1]. 2e. [1] for each area of discussion. Total: [5] [P: For not describing the steps explicitly] Constant variables (≥3) - age of seeds - species of white mustard plants - light intensity provided to plants* - volume of water provided to plants* * : Must be included Independent variables Time gap of observing the roots Dependent variable Extent of staining of roots by TTC Significant apparatus - At least 5 specimens* - Water* - TTC* - Light source* - Receptable to grow seedlings (e.g. Petri dish) - Medium (e.g. filter paper/ wool) - Measuring cylinder * : Must be included Interpretation The darker the stain of the root, the higher the rate of respiration which indicates a higher activity of mitochondria 1. Grow seeds of the same age and species of white mustard plants in separate petri dishes with same mass of wool 2. Expose the petri dish to the same light intensity. [Note: Light, not a requirement in seed germination, is essential during photosynthesis by seedling] 3. Water the seeds daily with equal volume of water. 4. After the seeds germinate, remove 5 plants and soak them in a fixed volume of TTC for 30 minutes at the same time each day for the next 10 days. 5. Observe the roots of the plants daily for the extent of staining and record the observations. [Note: The no. of plants, frequency & period of observation may deviate from the recommended] 6. The darker the stain of the root, the higher the rate of respiration in the mitochondria of root cells indicating a higher level of activity of mitochondria. Page 3 of 3