Question 1(a)(i): Tabulating Plasmolysis Observations

You observed 15 onion epidermal cells in each of three unknown solutions (U1, U2, U3) and recorded the number showing plasmolysis: U1=13, U2=1, U3=8. You need to present these results in a table. Which step below describes an incorrect element or procedure for creating an appropriate table?

• Give the table a clear, descriptive title (e.g., “Table showing the effect of different solutions on plasmolysis in onion cells”).
• Use ‘Solution’ (or ‘Solution ID’) as the heading for the column containing U1, U2, and U3.
• Use ‘Extent of Plasmolysis’ as the heading for the column containing the numerical data (13, 1, 8).
• Include the total number of cells observed per sample (i.e., ‘out of 15’) within the column heading for the results or in the table title.
• List the solutions (U1, U2, U3) in the first column and their corresponding counts (13, 1, 8) in the second column.

Question 1(a)(ii): Identifying Solutions by Plasmolysis Level

Given that the unknown solutions U1, U2, and U3 were actually 0.10 mol dm⁻³, 0.50 mol dm⁻³, and 1.00 mol dm⁻³ NaCl, and knowing that higher external solute concentration causes more plasmolysis, you need to match your observations (U1=13 plasmolyzed, U2=1, U3=8) to the correct concentrations. Which reasoning step below is incorrect?

• Identify that solution U1 caused the highest number of cells (13/15) to plasmolyze.
• Identify that solution U2 caused the lowest number of cells (1/15) to plasmolyze.
• Recall the principle: Higher NaCl concentration → Lower external water potential → More water leaves cell → More plasmolysis.
• Therefore, the solution causing the most plasmolysis (U1) must correspond to the lowest NaCl concentration (0.10 mol dm⁻³).
• Match the results: U1 (most plasmolysis) = 1.00 M; U2 (least plasmolysis) = 0.10 M; U3 (intermediate) = 0.50 M.

Question 1(a)(iii): Describing Plasmolyzed Onion Cells

Imagine observing four adjacent onion epidermal cells under a microscope after they have undergone significant plasmolysis in a 1.00 mol dm⁻³ NaCl solution. Which statement below provides an incorrect description of their likely appearance?

• The cells generally retain their original rectangular shape due to the presence of the rigid cell wall.
• The cell wall of each cell remains intact and stays in contact with the walls of adjacent cells.
• The protoplast (cell surface membrane and cell contents) appears swollen and pressed tightly against the cell wall.
• The cell surface membrane is visibly pulled away from the inside of the cell wall, creating a gap.
• The volume occupied by the protoplast is noticeably smaller than the volume enclosed by the cell wall.

Question 1(a)(iv): Explaining Plasmolysis using Water Potential

Explain, using the concept of water potential (ψ), why onion epidermal cells show significant plasmolysis when placed in a 1.00 mol dm⁻³ NaCl solution. Which statement below contains an incorrect element in this explanation?

• The 1.00 mol dm⁻³ NaCl solution has a very low (more negative) water potential due to its high solute concentration.
• The water potential inside the onion cell cytoplasm and vacuole is higher (less negative) than that of the external NaCl solution.
• Water molecules move passively by osmosis down the water potential gradient (from higher ψ to lower ψ).
• Therefore, there is a net movement of water from the external solution into the onion cell.
• This net loss of water from the cell causes the protoplast volume to decrease, leading to the pulling away from the cell wall observed as plasmolysis.

Question 1(b)(i): Calculating Percentage Change in Mass

A piece of potato tissue had an initial mass of 2.3 g. After soaking in sucrose solution, its final mass was 2.4 g. Which step below shows an incorrect method for calculating the percentage change in mass?

• Calculate the absolute change in mass: Final mass – Initial mass = 2.4 g – 2.3 g = +0.1 g.
• Divide the absolute change in mass (+0.1 g) by the initial mass (2.3 g).
• Divide the absolute change in mass (+0.1 g) by the final mass (2.4 g).
• Multiply the result of the division (Change / Initial mass) by 100 to express it as a percentage.
• The standard formula for percentage change is: ((Final mass – Initial mass) / Initial mass) × 100.

Question 1(b)(ii): Calculating Mean Percentage Change

Three trials of soaking potato in 0.2 mol dm⁻³ sucrose yielded percentage mass changes of +4.2%, +4.2%, and +4.3%. Which step describes an incorrect way to calculate the mean percentage change?

• Sum the three percentage change values obtained from the trials: 4.2 + 4.2 + 4.3 = 12.7.
• Identify the total number of trials performed, which is 3.
• Calculate the mean by dividing the sum of the values (12.7) by the number of trials (3).
• Calculate the mean as (4.2 + 4.3) / 2, ignoring the first +4.2% value as it is a repeat.
• The correct calculation procedure is (Sum of all values) / (Number of values) = (4.2 + 4.2 + 4.3) / 3.

Question 1(b)(iii): Describing How to Plot an Osmosis Graph

You have data for mean percentage change in potato mass (y-axis) versus sucrose concentration (x-axis): (0.0 M, +8.2%), (0.2 M, +4.2%), (0.4 M, +1.3%), (0.6 M, -6.7%), (0.8 M, -9.9%). Which instruction for plotting this data is incorrect?

• Label the x-axis appropriately (e.g., “Sucrose concentration / mol dm⁻³”) and choose a sensible linear scale that covers the range 0.0 to 0.8 M.
• Label the y-axis appropriately (e.g., “Mean percentage change in mass / %”) and choose a linear scale that accommodates the range of values from approximately +9% to -10%.
• Plot each data point accurately on the graph using small, clear symbols like crosses (x) or encircled dots (⊙).
• Draw a line of best fit (which may be slightly curved or straight depending on the trend) that best represents the overall pattern shown by the plotted points.
• Ensure the line of best fit is drawn with a ruler connecting exactly the first plotted point (0.0, +8.2) to the last plotted point (0.8, -9.9).

Question 1(b)(iv): Estimating the Isotonic Point from Data

Using the osmosis data [(0.0 M, +8.2%), (0.2 M, +4.2%), (0.4 M, +1.3%), (0.6 M, -6.7%), (0.8 M, -9.9%)], you want to estimate the sucrose concentration where the potato tissue would experience no change in mass (isotonic point). Which method below is incorrect or least appropriate?

• Accurately plot the mean percentage change in mass (y-axis) against sucrose concentration (x-axis) on graph paper.
• Draw a suitable line of best fit through the plotted points that represents the overall trend.
• Identify the point where the line of best fit intersects the x-axis (where percentage change in mass = 0%) and read the corresponding sucrose concentration value from the x-axis scale.
• Observe from the data table that the point of zero change must lie between 0.4 M (positive change) and 0.6 M (negative change).
• Calculate the isotonic concentration by finding the exact midpoint concentration between the point showing the smallest positive change (0.4 M) and the point showing the largest magnitude negative change (0.8 M): (0.4 + 0.8) / 2 = 0.6 M.

Question 2(a): Describing Tissue Arrangement in a Dicot Root Plan Diagram

When drawing a plan diagram of a typical dicotyledonous root in transverse section, showing the main tissue regions, which statement below provides an incorrect description?

• The outermost tissue layer is the epidermis (sometimes called the piliferous layer if root hairs are present).
• Internal to the epidermis is a wide region called the cortex, typically composed mainly of parenchyma cells for storage.
• The innermost layer of the cortex is a distinct ring of cells called the endodermis, often showing Casparian strips.
• The central vascular cylinder (stele) contains xylem arranged in a distinct ring surrounding a central pith made of parenchyma.
• The vascular cylinder typically features a central, star-shaped (stellate) core of xylem, with phloem tissue located in the regions between the arms of the xylem star.

Question 2(b)(i): Performing a 1-in-10 Serial Dilution

You need to perform a 1-in-10 serial dilution starting with 1.0% starch solution (S) to obtain four further concentrations (0.1% down to 0.0001%), ensuring 9 cm³ of each is available for testing. Which step described below is incorrect for this procedure?

• Label four clean test tubes appropriately, for example, ‘0.1%’, ‘0.01%’, ‘0.001%’, and ‘0.0001%’.
• Using a measuring cylinder or pipette, add exactly 9 cm³ of distilled water (the diluent) into each of the four labelled tubes.
• Using a clean pipette, transfer 1 cm³ of the initial 1.0% starch solution into the tube labelled ‘0.1%’ (containing 9 cm³ water) and mix thoroughly. This creates a 1/10 dilution (1 cm³ in a total volume of 10 cm³).
• Using a clean pipette (or the same one after rinsing), transfer 1 cm³ from the freshly prepared 0.1% solution into the tube labelled ‘0.01%’ (containing 9 cm³ water) and mix thoroughly.
• To prepare the 0.001% solution, transfer 1 cm³ of distilled water using a pipette into the tube labelled ‘0.001%’ (which already contains 9 cm³ of the 0.01% solution from the previous step).

Question 2(b)(ii): Recording Iodine Test Colour Changes

You perform an iodine test on the starch serial dilution (1.0% down to 0.0001%). You expect the colour intensity to decrease with concentration, potentially ranging from blue-black (intense, represented by ++++++) to yellow-orange (no reaction, represented by +). Which representation of the expected qualitative results in a table format is incorrect?

• Include a column with the heading ‘Percentage concentration of starch / %’ listing the standard concentrations used (1.0, 0.1, 0.01, etc.).
• Include a column with a heading like ‘Observed colour intensity symbol’ or ‘Iodine test result (+ scale)’ to record the qualitative colour symbol.
• Show the highest concentration (1.0%) corresponding to the most intense colour symbol (e.g., ++++++ for blue-black).
• Show the lowest concentration (0.0001%) corresponding to the least intense or negative reaction symbol (e.g., + for yellow-orange).
• Show the 0.01% starch solution corresponding to the symbol +++++ (representing a dark blue colour), identical to the result expected for 0.1%.

Question 2(b)(iii): Identifying the Dependent Variable in Iodine Test

When performing an experiment where different known concentrations of starch solution are tested with a standard amount of iodine solution under controlled conditions, what is the dependent variable?

• The percentage concentration of starch solution being tested.
• The volume of iodine solution added to each starch sample.
• The temperature at which the reaction is carried out.
• The time allowed for the colour to develop before observation.
• The observed colour intensity or the symbol assigned based on the colour developed in the test.

Question 2(b)(iv): Recording Unknown Iodine Test Results Using a Key

Root extract R1 gave a dark blue colour (++++) symbol) and R2 gave a brown colour (++ symbol) when tested with iodine. Using the key (+ +++++ blue-black, +++++ dark blue, ++++ purple, +++ dark brown, ++ brown, + yellow-orange), which recording below is incorrect?

• Recording for R1: +++++
• Recording for R2: ++
• Recording for R1: Dark blue
• Recording for R2: Brown
• Recording for R1: ++++ (Purple)

Question 2(b)(v): Estimating Starch Concentration from Iodine Colour

Your standards gave: 1.0%(++++++), 0.1%(+++++), 0.01%(++++), 0.001%(++/+++), 0.0001%(+). Your unknowns gave: R1(+++++) and R2(++). Which statement makes an incorrect estimation or comparison based on these results?

• The colour intensity observed for R1 (+++++) matches the intensity observed for the 0.1% standard starch solution.
• Based on the colour match, the estimated starch concentration for R1 is approximately 0.1%.
• The colour intensity observed for R2 (++) falls within or matches the intensity range observed for the 0.001% standard starch solution (++/+++).
• Based on the colour match, the estimated starch concentration for R2 is approximately 0.001%.
• Comparing the results, the colour for R2 (++) indicates a significantly higher starch concentration than the colour for R1 (+++++).

Question 2(b)(vi): Improving Accuracy of Starch Concentration Estimation

To get a more accurate estimate of starch concentration in extracts R1 and R2 using the iodine test, several improvements could be made to the semi-quantitative visual comparison method. Which suggestion below is the least likely to lead to a significantly more accurate quantitative result compared to the others?

• Prepare standard starch solutions with much narrower concentration intervals (e.g., 0.05%, 0.07%, 0.09%, 0.11%, 0.13%, etc.) around the initial estimates to allow for finer visual comparison.
• Use a colorimeter or spectrophotometer to measure the light absorbance of the iodine-starch complex at a suitable wavelength (e.g., ~600 nm) for both standards and samples, allowing for objective quantification.
• Ensure the volume of iodine solution added, the total reaction volume, and the time allowed for colour development are precisely controlled and identical for all standards and unknown samples.
• Carry out multiple independent tests (replicates) for each standard and unknown sample and calculate the mean colour reading or absorbance value to reduce random error.
• Ensure the iodine solution used is freshly prepared and stored correctly before use.

Question 2(b)(vii): Relating Starch Concentration to Season

Given that starch storage in roots is generally lower in summer (when sugars are used for growth) than in winter (when starch is stored), and your estimates were R1 ≈ 0.1% starch and R2 ≈ 0.001% starch, which conclusion is incorrectly reasoned?

• Comparing the estimates, root extract R2 has a significantly lower starch concentration than root extract R1.
• Based on typical plant physiology, lower starch storage levels are expected in root samples collected during the active growing season (summer).
• Therefore, it is reasonable to conclude that sample R2, exhibiting the lower starch level, was likely collected in the summer.
• Conversely, higher starch storage levels are expected during the dormant season (winter), suggesting sample R1 (higher starch) was likely collected in winter.
• Since R2 (summer sample) has less stored starch (solute), its cells will necessarily have a significantly higher (less negative) overall water potential compared to the cells of the winter sample R1.

Question 2(c): Comparing Two Root Micrograph Descriptions

Micrograph 1 description: Dicot root TS, small stele, wide cortex, no root hairs visible. Micrograph 2 description: Root TS, prominent root hairs visible, larger stele (relative to root diameter), narrower cortex, distinct endodermis identified.

• Micrograph 2 clearly shows root hairs extending from the epidermis, while these are absent or not visible in Micrograph 1.
• The cortex region in Micrograph 1 is described as being wider (occupying a larger proportion of the radius) compared to the cortex in Micrograph 2, which is described as narrower.
• The central vascular cylinder (stele) is described as being relatively larger in Micrograph 1 compared to Micrograph 2.
• The endodermis is specifically mentioned as distinct in Micrograph 2, suggesting it might be less clearly distinguishable or wasn’t highlighted in the description of Micrograph 1.
• The presence of prominent root hairs in Micrograph 2 suggests it likely represents a region closer to the root tip (zone of differentiation) compared to Micrograph 1, or potentially a younger root.