Question 1(a)(i): Preparing Salicylic Acid Dilutions

You need to prepare 10 cm³ each of 0.8%, 0.5%, and 0.2% salicylic acid solutions using a 1.0% stock solution (A) and distilled water (W). Which line below describes an incorrect volume combination or calculation for preparing one of these specific concentrations?

• For 0.8% solution: Calculate volumes as (0.8 / 1.0) * 10 cm³ = 8.0 cm³ of A, and 10 cm³ – 8.0 cm³ = 2.0 cm³ of W. Mix 8.0 cm³ A + 2.0 cm³ W.
• For 0.5% solution: Calculate volumes as (0.5 / 1.0) * 10 cm³ = 5.0 cm³ of A, and 10 cm³ – 5.0 cm³ = 5.0 cm³ of W. Mix 5.0 cm³ A + 5.0 cm³ W.
• For 0.2% solution: Calculate volumes as (0.2 / 1.0) * 10 cm³ = 2.0 cm³ of A, and 10 cm³ – 2.0 cm³ = 8.0 cm³ of W. Mix 2.0 cm³ A + 8.0 cm³ W.
• For 0.8% solution: Measure 8.0 cm³ of distilled water (W) and add 2.0 cm³ of 1.0% stock solution (A).
• Ensure accurate measurement using appropriate apparatus (e.g., pipettes or measuring cylinders) and thorough mixing for each prepared dilution.

Question 1(a)(ii): Tabulating Colour Intensity Results

When setting up a results table for the experiment testing colour intensity (+ to ++++++) produced by adding iron(III) chloride to different salicylic acid concentrations (e.g., 1.0%, 0.8%, 0.5%, 0.2%, 0.0%), which step below describes an incorrect way to set up the table or predict the trend?

• Use ‘Percentage concentration of salicylic acid (%)’ as the heading for the independent variable column.
• Use ‘Intensity of colour (Symbol)’ or similar as the heading for the dependent variable column where results are recorded.
• Include a row for 0.0% salicylic acid (control) and expect it to show the most intense colour reaction (++++++).
• Predict that the number of ‘+’ symbols recorded (representing colour intensity) will generally increase as the concentration of salicylic acid increases.
• Record the observed colour intensity for each tested concentration using the provided qualitative symbol scale (+ to ++++++).

Question 1(a)(iii): Recording Observations for Unknown Samples

Two unknown samples, S1 and S2, were tested with iron(III) chloride. The observation was that S1 produced an intense purple colour evaluated as ‘+++++’ on the scale, while S2 produced a less intense purple colour evaluated as ‘+++’. Which step below represents an incorrect way to record these specific observations?

• Prepare a results table or section in your notes clearly labelled to distinguish results for the unknown samples S1 and S2.
• Record the result for sample S1 under a suitable heading, such as ‘Intensity of colour (Symbol)’.
• Enter the observed intensity symbol for sample S1 accurately as: +++++.
• Record the result for sample S2 under the same heading (‘Intensity of colour (Symbol)’).
• Enter the observed intensity symbol for sample S2 accurately as: +++++.

Question 1(a)(iv): Estimating Concentration from Colour Intensity

Using the expected trend (higher concentration gives more ‘+’) and your standard results (e.g., 1.0% gives ++++++, 0.8% gives ++++, 0.5% gives +++, 0.2% gives ++, 0.0% gives +), you observe that unknown sample S1 gives ‘+++++’ and S2 gives ‘+++’. Which statement below represents an incorrect estimation or interpretation based on these results?

• Compare the observed colour intensity of S1 (+++++) to the intensities produced by the known standard concentrations.
• Estimate the concentration of salicylic acid in S1 to be high, likely close to the standard that gave ‘+++++’ (or potentially between the standards giving ‘++++’ and ‘++++++’ if ‘+++++’ wasn’t a standard, e.g., around 0.9% if 1.0% was ++++++ and 0.8% was ++++). Assuming ‘+++++’ was observed for 1.0% standard, estimate ~1.0%.
• Compare the observed colour intensity of S2 (+++) to the intensities produced by the known standard concentrations.
• Estimate the concentration of S2 by matching its intensity (+++) to the standard concentration that gave the lowest observed intensity (++).
• Estimate the concentration of salicylic acid in S2 to be moderate, corresponding to the standard concentration that produced the matching intensity ‘+++’ (which is 0.5% in the example standards provided).

Question 1(a)(v): Relating Concentration to Sample Origin

Given that salicylic acid is absorbed into the blood after ingestion and later metabolised and excreted in urine, and that sample S1 showed a higher colour intensity (estimated concentration around 0.8-1.0%) than sample S2 (estimated concentration around 0.5%), which conclusion or reasoning step is incorrect?

• Identify sample S1 as having the higher estimated salicylic acid concentration based on its more intense colour reaction.
• Identify sample S2 as having the lower estimated salicylic acid concentration based on its less intense colour reaction.
• Reason that following ingestion and absorption, the concentration of a substance in the blood is likely to peak relatively early.
• Reason that concentration in urine often peaks later than in blood, as the substance is filtered and excreted over time by the kidneys.
• Conclude that S2, which has the lower estimated concentration, is therefore more likely to be the blood sample taken shortly after ingestion compared to S1.

Question 1(a)(vi): Identifying the Independent Variable

In the investigation examining the effect of different salicylic acid solutions on the intensity of colour produced with iron(III) chloride, what is the independent variable? Which option below incorrectly identifies or defines the independent variable?

• The variable that is intentionally changed or selected by the experimenter to test its effect is known as the independent variable.
• In this specific investigation, the experimenter is deliberately using different known concentrations of salicylic acid.
• Therefore, based on the definition, the independent variable in this experiment is the Percentage concentration of salicylic acid (%).
• The variable that is measured or observed as the outcome, which is expected to depend on the independent variable, is known as the dependent variable.
• Therefore, the independent variable in this experiment is the Intensity of colour (Symbol) observed.

Question 1(a)(vii): Error Analysis in Visual Colour Comparison

When visually comparing the intensity of purple colour in test tubes to estimate concentration, several issues can arise. Which statement below describes something that is NOT a significant source of error specifically related to the visual comparison process itself, OR misidentifies the standard improvement?

• Accurately judging subtle differences in colour intensity or shade by eye is inherently subjective and prone to error.
• Variations in colour perception between different observers can lead to inconsistent results when comparing samples to standards.
• Ensuring that exactly the same volume of iron(III) chloride reagent was added to each tube is important for consistency, but variations here represent a procedural error rather than an error inherent in the visual comparison itself.
• The ambient lighting conditions (e.g., fluorescent vs. natural light, brightness) under which the colour comparisons are made can influence the perceived intensity.
• A significant improvement to reduce subjectivity is to use a colorimeter, which provides highly subjective readings based entirely on the user’s visual perception of colour intensity.

Question 1(b)(i): Plotting Concentration vs. Time Data

You are given data for salicylic acid concentration in urine (µg mL⁻¹) at various times (minutes) after aspirin ingestion: (30, 36.0), (60, 65.5), (120, 42.5), (180, 33.0), (240, 31.5). How should you plot this data on a graph? Which instruction below is incorrect?

• Plot the dependent variable, ‘Concentration of salicylic acid in urine / µg mL⁻¹’, on the vertical y-axis.
• Plot the independent variable, ‘Time / minutes’, on the horizontal x-axis.
• Use logarithmic scales for both the x-axis and the y-axis to better visualise the initial rapid rate of change.
• Choose appropriate linear scales for both axes that allow the plotted points to occupy a substantial portion (e.g., >50%) of the graph paper area for clarity.
• Plot each data point accurately using small crosses (x) or circled dots (⊙) and consider joining them with a ruled line or a smooth curve to show the trend.

Question 1(b)(ii): Interpolating Data from a Table or Graph

Using the urine concentration data (Time/min, Conc/µg mL⁻¹): (60, 65.5) and (120, 42.5), you need to estimate the concentration at 105 minutes. This involves interpolation. Which step below describes an incorrect procedure or assumption for interpolation?

• Identify the two known data points that bracket the time point you want to estimate (105 min), which are the values at 60 min and 120 min.
• Determine the fraction of the time interval between 60 min and 120 min that corresponds to 105 min. (105 – 60) / (120 – 60) = 45 / 60 = 0.75 or 3/4.
• Calculate the total change (decrease) in concentration over the 60 to 120 minute interval: 65.5 µg mL⁻¹ – 42.5 µg mL⁻¹ = 23.0 µg mL⁻¹.
• Assume, for the purpose of linear interpolation, that the concentration changes at a constant rate between the two known points (60 min and 120 min).
• Estimate the concentration at 105 min by taking the concentration at the later time (120 min, 42.5) and adding 3/4 (0.75) of the total concentration change (23.0). Calculation: 42.5 + (0.75 * 23.0).

Question 1(b)(iii): Describing Change in Concentration Over Time

Using the urine concentration data (Time/min, Conc/µg mL⁻¹): (60, 65.5), (120, 42.5), (180, 33.0), (240, 31.5), describe the change in concentration between 60 minutes and 240 minutes. Which statement below is incorrect?

• The concentration of salicylic acid in urine at 60 minutes was 65.5 µg mL⁻¹.
• The concentration of salicylic acid in urine at 240 minutes was 31.5 µg mL⁻¹.
• Overall, the concentration at 240 minutes is lower than the concentration at 60 minutes, indicating a net decrease over this period.
• The data shows that the concentration of salicylic acid in urine increases steadily and consistently between 60 minutes and 240 minutes.
• The rate of decrease in concentration appears to slow down over time; the drop between 60 and 120 minutes (23.0) is greater than the drop between 180 and 240 minutes (1.5).

Question 1(b)(iv): Suggesting Aspirin’s Mechanism of Action

Aspirin (active component related to salicylic acid) is known to reduce inflammation and blood clotting, processes which involve enzyme-controlled reactions. How might aspirin achieve this effect? Which suggestion below is biochemically implausible or incorrect as a primary mechanism?

• Aspirin likely functions as an inhibitor of specific enzymes that are key components of the biochemical pathways responsible for inflammation and blood clotting (e.g., cyclooxygenase (COX) enzymes).
• One possible mechanism of inhibition is competitive inhibition, where aspirin binds to the enzyme’s active site, blocking access for the normal substrate.
• Another possible mechanism is non-competitive inhibition, where aspirin binds to an allosteric site on the enzyme, causing a conformational change that inactivates the active site. (Note: Aspirin’s actual primary mechanism is irreversible acetylation, a form of covalent modification/inhibition).
• By inhibiting these key enzymes, aspirin reduces the rate at which their respective reactions proceed, thus diminishing the processes of inflammation and clotting.
• Aspirin might achieve its effects by directly causing the chemical breakdown or degradation of the essential substrate molecules (e.g., arachidonic acid or thromboxane precursors) required for these pathways.

Question 2(a)(i): Plan Diagram of Leaf Structure (M1)

Slide M1 shows a transverse section through a plant structure composed of leaves wrapped around a central area (like an onion bulb or leek). You need to draw a plan diagram showing the arrangement of tissues in a region displaying parts of the outer leaf layers. Which step below describes an incorrect procedure or feature for this plan diagram?

• Draw the outlines representing the overall shape and arrangement of sections through several concentric leaf layers visible in the field of view.
• For each leaf section shown, use single, clear lines to represent the position and boundaries of the outer (abaxial) and inner (adaxial) epidermal tissues.
• Indicate the region between the epidermal layers as the ground tissue (mesophyll) without drawing the individual parenchyma cells that compose it.
• Draw numerous, accurately shaped, and detailed parenchyma cells filling the ground tissue region within each depicted leaf layer.
• Show the approximate size and position of the vascular bundles (represented as small circles or ovals) embedded within the ground tissue of the leaf layers and label at least one clearly.

Question 2(a)(ii): High Power Drawing of Epidermal Cells (M1)

You are to draw a line of four adjacent epidermal cells from the lower surface of one of the leaves in section M1, as viewed under high power. Which instruction below represents an incorrect convention or technique for this type of biological drawing?

• Accurately draw the observed shape and relative size of four epidermal cells positioned adjacent to each other as seen in the microscope field of view.
• Use sharp, clear, continuous lines for all cell outlines and walls, avoiding any form of shading, stippling, or sketchy lines.
• Represent the cell walls separating adjacent cells and the outer cell wall using thin, single lines.
• Unless specifically requested or clearly visible and identifiable, omit internal cell contents such as the nucleus, cytoplasm granularity, or chloroplasts, focusing only on the cell walls and shape.
• Add a clear label line pointing precisely to a cell wall shared between two adjacent cells and write the label ‘Cell wall’.

Question 2(b): Comparing Plant Sections (Micrographs 1 & 2)

You compare Micrograph 1 (representing slide M1, showing concentric leaf layers with vascular bundles within each layer) with Micrograph 2 (Fig 2.2, described as a stem section with scattered vascular bundles and trichomes on the epidermis). Which statement makes an incorrect structural comparison between these two sections?

• Section M1 exhibits a layered or concentric organization characteristic of multiple wrapped leaves, while Micrograph 2 shows a more solid internal structure typical of a stem axis.
• In M1, vascular bundles are located within the distinct tissue layers (leaves), whereas in Micrograph 2, the vascular bundles appear scattered throughout the ground tissue, typical of a monocot stem or some dicot stems.
• Trichomes (epidermal hairs) are visibly present on the outer surface (epidermis) in Micrograph 1 (leaf layers) but are clearly absent in Micrograph 2 (stem section).
• The overall structure depicted in M1 represents multiple sections through leaf structures, while Micrograph 2 represents a section through a single, continuous stem structure.
• The tissue organisation in M1 clearly shows repeating units corresponding to individual leaf structures, while Micrograph 2 displays general ground tissue without such clear demarcation into distinct leaf units.

Question 2(c)(i): Recording Measurements from a Photomicrograph

On a photomicrograph (magnification x40) of a plant stem, a line X-Y is drawn across the diameter. The measured width of the entire stem section image along this line is 90 mm, and the width of the central region (e.g., pith or stele) image along the same line is 50 mm. Which option below shows an incorrect way to state these measurements or related calculations?

• Image width of the entire stem section = 90 mm.
• Image width of the central region = 50 mm.
• Width of the stem section image can also be stated as 9.0 cm.
• Width of the central region image can also be stated as 5.0 cm.
• Actual width of the entire stem section = 90 mm.

Question 2(c)(ii): Determining the Ratio of Widths

Using the image measurements from part (c)(i) – stem section width 90 mm, central region width 50 mm – what is the ratio of the width of the stem section to the width of the central region? Which option below represents an incorrect expression or simplification of this specific ratio?

• The required ratio is expressed as: Width of stem section : Width of central region.
• Substituting the measured image values, the ratio is: 90 mm : 50 mm.
• Ignoring the units (as they cancel out), the ratio is: 90 : 50.
• Simplifying the ratio 90:50 by dividing both sides by their greatest common divisor (10) gives the simplest whole number ratio: 9 : 5.
• Therefore, the ratio of stem width to central region width is 5 : 9.

Question 2(c)(iii): Improving Accuracy of Mean Width Measurement

To determine the mean width of the central region of the stem shown in the photomicrograph more accurately than just using the single 50 mm measurement along line X-Y, which procedure below is least effective or represents an incorrect approach to improving accuracy?

• Take several (e.g., 3-5) independent measurements of the width of the central region across different representative diameters or lines on the photomicrograph.
• Use a measuring instrument with finer resolution (e.g., a ruler with 0.5 mm markings or digital calipers on the image if possible) for each individual measurement taken.
• Carefully measure the width along the original line X-Y just once more, but concentrate intensely to be extremely precise in reading the scale to the nearest 0.1 mm.
• Calculate the mean (average) width by summing all the individual width measurements obtained and then dividing by the total number of measurements made.
• Ensure consistency in technique, for example, by always measuring the diameter perpendicularly between identifiable boundaries of the central region.