2024 TYS Paper 3 Answers

2024 Paper 3 Answers S/N Possible solutions Marks Cambridge Comments 1 ai percentage concentration of sodium chloride solution Volume of stock solution, S / cm 3 Volume of deionised water / cm 3 Total volume of solution / cm 3 0.0 0.0 30.0 30.0 2.5 7.5 22.5 30.0 5.0 15.0 15.0 30.0 7.5 22.5 7.5 30.0 10.0 30.0 0.0 30.0 1 1 1 The majority of candidates completed the table with the correct volumes aii 1. Used the 1 cm 3 syringe to measure volumes that are in 1 decimal place ; 2. Ensured that the syringe is fully immersed in deionised water and stock solution when extracting respective volumes to prevent presence of air bubbles ; 3. Turn the syringe upside down and flick it or tap it to remove any air bubbles present ; 4. Volume readings were read at eye level to prevent parallex error ; AVP Any two valid points to get the 2 marks 2 Most candidates correctly stated the precautions for ensuring accurate dilutions. Some candidates mistakenly referred to reading the meniscus when using syringes. Step 14: 5 to 15 ° 1

b Concentration of sodium chloride solution / % Angle of bending of potato strips / ° Strip 1 Strip 2 Mean 0.0 2.5 5.0 7.5 10.0 Unknown solution U Correct headers with units ; Total of two readings taken ; Correct number of decimal places (1 dp in independent variable. Whole number for strips 1 and 2 and 1 dp for mean) ; Correct trend (as concentration increases, angle of bending increases) Value unknown solution U ; 1 1 1 1 1 All candidates drew a table to record their results. Stronger responses included detailed headings with appropriate units. The majority of candidates adhered to the instructions, recording two results for each concentration of sodium chloride solution and the results for solution U of unknown concentration. Most candidates accurately calculated the mean angle of bending for the two potato strips in each sodium chloride solution. c Scale - Ensure that the graph takes up more than 50 % of space provided ; Line - best fit curve ; Axes - labelled fully with the correct units and 1 dp for both axes ; Plots - plots plotted accurately ; 1 1 1 1 The majority of candidates demonstrated a strong ability to transpose data from a table into graphical form. Most responses had correctly labelled axes with appropriate interval

markings. Stronger responses exhibited appropriate scales, precise plotting and a clear line. Most candidates were able to draw a line or curve of best fit, accurately following the expected trend without extrapolation. Some candidates showed a lack of understanding of the aim of the investigation, leading to errors such as plotting the unknown value U on the graph or drawing a bar chart instead of a line graph. di Dependent on student values for U and step 14 respectively ; 2 Most candidates were able to use their graphs to obtain the required concentrations. dii Dotted lines on graph for both values ; 1 Most candidates showed clearly how they derived the required concentrations on the graph. e 1. There are greater impact of anomalies on the average value ; 2 Some candidates

2. Reduce reliability of data obtained as two data points is not enough to identify a trend ; 3. With only two data, it is difficult to determine the precision of the attempts per concentration ; 4. Small sample size that is not representative of the concentration of the entire potato ; Any two valid points confused this question with sources of error. Stronger responses typically referenced anomalies and associated reliability to the mean angle of bending. f Source of error How you would minimise the source of error Using of knife to cut the potato strips Use a precision cutter or use a template to cut the potatoes accurately Solutions may evaporate while the strips were immersed in the solution Cover the petri dishes Using of ruler to measure the dimensions of the potato strips Use digital calipers instead Any two valid points - 2m Correct method to minimise both sources of error - 2m 4 A broad range of errors were identified by candidates. The majority were able to recognise at least one source of error and suggest a reasonable approach to minimise it. These candidates demonstrated a sound understanding of the equipment that could have been used or the procedures that could help reduce errors. Some responses included sources of error or suggested improvements that were either vague or not directly relevant to the investigation, for example, ‘potato strips of different sizes can affect the investigation’. Weaker responses on how to minimise the source of error lacked accurate

references to the procedure or did not propose meaningful changes to the data collection process, for example, ‘ensure that potato strips are totally immersed in the solutions’. g Dependent on data obtained for both 0.0 % and 10.0 %. If angle of bend is more than value seen in step 14, Water potential in sodium chloride is lower than that of the cells in the potato strips ; Therefore, more water molecules exited the cells through the partially permeable membrane via osmosis. This makes the potato strips cells be more flaccid therefore bending more. ; If angle of bend is less than value seen in step 14, Water potential in sodium chloride is higher than that of cells in potato strips ; Therefore, more water molecules entered the cells through the partially permeable membrane via osmosis This increased turgidity of the potato strip cells, making it bend less ; If angle of bend is the same value as seen in step 14, Water potential of sodium chloride solution is the same as that of the cells in the potato strips ; There is no net movement of water molecules into or out of the cells through the partially permeable membrane via osmosis ; 1 m for water potential 1m for movement of water from where to where via osmosis 1m for either flaccid / turgid Stronger responses exhibited a strong understanding of the concepts of water potential and osmosis, with appropriate application of theoretical knowledge to the experimental results. Most candidates used appropriate biological terminology, such as ‘increase in turgidity of cells in 0.0% solution’. Weaker responses did not make reference to the 0.0% and 10.0% sodium chloride solutions. 2

a 1. Xylem has a thicker cell wall 2. Xylem has a larger lumen than the other cells AVP 2 Most candidates were able to state the visible features of xylem vessels shown in Fig. 2.1. Weaker responses gave features that were not visible. These responses simply listed what was taught in theory, for example, ‘presence of lignin’. Some responses included inaccurate features, such as referring to the xylem vessel as having a ‘thick cell wall’. bi Xylem vessel