Question 1(a)(i): Describing Appearance After Sedimentation

Test tube R initially contains a uniform suspension of yeast cells. After being left undisturbed for some time, sedimentation occurs, resulting in the appearance observed in test tube S. Which statement below incorrectly describes the appearance of test tube S?

• The mixture in the tube separates into visibly distinct upper and lower regions.
• A clear or clearer liquid layer, called the supernatant, is visible.
• A packed layer of yeast cells, called the sediment, forms.
• The clear supernatant layer is located at the bottom of the test tube.
• The sediment layer, composed of settled yeast cells, is found at the bottom of the tube.

Question 1(a)(ii): Preparing Ethanol Dilutions

You need to prepare 10 cm³ of a 40% ethanol solution using a 100% ethanol stock solution (E) and distilled water (W) via proportional dilution. Which step below describes an incorrect calculation or procedure for this specific task?

• Identify the total final volume required for the dilution, which is 10 cm³.
• Calculate the volume of 100% ethanol (E) needed: 40% of 10 cm³ = (40/100) * 10 cm³ = 4.0 cm³.
• Calculate the volume of distilled water (W) needed by subtracting the required ethanol volume from the total volume: 10 cm³ – 4.0 cm³ = 6.0 cm³.
• Accurately measure 4.0 cm³ of distilled water (W) using a suitable measuring cylinder or pipette.
• Accurately measure 6.0 cm³ of distilled water (W) separately, then combine the measured 4.0 cm³ of ethanol (E) and 6.0 cm³ of water (W) and mix thoroughly.

Question 1(a)(iii): Setting Up the Results Table

You are investigating the effect of various ethanol concentrations (including a 0% control) on the height of yeast sediment (measured in mm) over 20 minutes, with readings taken every 4 minutes. Which step describes an incorrect convention for setting up the results table or recording the data?

• Use ‘Time / min’ as a heading for the independent variable (or columns), listing the measurement times 0, 4, 8, 12, 16, and 20 minutes.
• Use ‘Percentage concentration of ethanol (%)’ as labels to distinguish the different experimental conditions (e.g., as row headings or in the main title).
• Record the measured ‘Height of sediment’ (the dependent variable) in the body of the table for each time point and concentration.
• Ensure all measurements for the height of the sediment are recorded to a consistent and appropriate precision, such as the nearest whole millimetre (e.g., 11 mm, 13 mm).
• Omit the units (mm) from the table heading for sediment height (e.g., just ‘Height of sediment’) to avoid clutter, as the units are stated in the method description.

Question 1(a)(iv): Forming a Possible Conclusion

Considering that ethanol can affect yeast cell properties like membrane permeability, which statement below is not a plausible or correctly formulated conclusion about the effect of ethanol concentration on yeast sedimentation height after 20 minutes?

• The final height of sediment after 20 minutes generally increases as ethanol concentration increases from 0% up to about 60%.
• Yeast cells settle out of suspension over the 20-minute period due to gravity.
• There appears to be an optimal ethanol concentration (e.g., around 40-60%) that results in the maximum sediment height after 20 minutes.
• Very high ethanol concentrations (e.g., 100%) might result in less final sediment height after 20 minutes compared to intermediate concentrations.
• The percentage concentration of ethanol clearly influences the degree of yeast sedimentation observed within the 20-minute timeframe.

Question 1(a)(v): Evaluating the Hypothesis ‘Ethanol is needed for sedimentation’

A student hypothesizes, ‘Ethanol is needed for sedimentation of yeast cells to occur.’ The experiment includes a control (C) with 0% ethanol. Which step below describes an incorrect piece of reasoning or evaluation regarding this hypothesis?

• Observe the control tube (C) containing yeast suspension with 0% ethanol at the end of the 20-minute experimental period.
• Carefully note whether a visible layer of sediment has formed at the bottom of this control tube (C).
• Reason logically that if sediment formation is observed in the complete absence of ethanol (in the control tube C), then ethanol cannot be an absolute requirement for sedimentation to occur.
• Assume that because the results clearly show ethanol affects the amount of sediment formed in the other tubes, it must therefore be essential for the process of sedimentation itself.
• Conclude that the student’s hypothesis (‘Ethanol is needed…’) should be rejected if sedimentation occurs in the 0% ethanol control, as this demonstrates settling happens via gravity even without ethanol.

Question 1(a)(vi): Refining the Estimate of Optimal Concentration

Initial results suggest that the maximum height of yeast sediment after 20 minutes occurs at an ethanol concentration of approximately 40%. Which procedural modification described below is incorrect if the aim is to determine this optimal concentration more accurately?

• Prepare a new series of ethanol solutions with concentrations closely centred around the initial estimate of 40%.
• Use narrower concentration intervals for these new tests, for instance, testing 30%, 35%, 40%, 45%, and 50% ethanol.
• Repeat the entire original experiment using only the initial concentrations (0%, 20%, 40%, 60%, 80%, 100%) but increase the number of replicate tubes for each concentration to improve reliability.
• Ensure all other experimental variables (like temperature, yeast source and concentration, timing, measurement technique) are kept strictly consistent between the initial and refinement experiments.
• Carry out the sedimentation measurements using these new, closely spaced concentrations and analyze the results to identify the peak sediment height more precisely.

Question 1(a)(vii): Identifying a Source of Measurement Error

When measuring the height of the yeast sediment in millimetres using a ruler held against the outside of the round test tube, which of the following is least likely to be a significant source of random or systematic error in the height reading itself?

• The top surface of the sediment layer not being perfectly flat or forming a sharp, easily identifiable line.
• Viewing the ruler scale mark and the sediment level from slightly different angles or eye levels between readings (parallax error).
• The inherent limitation in the precision of the ruler itself (e.g., markings only every 1 mm).
• Variations in the ambient room temperature affecting the metabolic rate of the yeast cells during the 20 minutes.
• The possibility that the yeast sediment packs down to a slightly different density or tightness in different tubes or at different times.

Question 1(a)(viii): Classifying an Experimental Error

A syringe is used to measure 7.0 cm³ of ethanol for preparing a specific dilution, but due to a calibration flaw, it consistently delivers 7.1 cm³ every time it is used for this measurement. Which statement below incorrectly describes this type of error or its consequence?

• This consistent deviation (+0.1 cm³) from the intended or true volume is an example of a systematic error.
• This error will mean that the actual concentration of the prepared ethanol solution is consistently slightly higher than the intended calculated concentration.
• This type of error, because it affects the ethanol measurement consistently, is classified as a random error.
• The effect of this systematic error on the sediment height result for this specific (intended) concentration will be consistent across different repeats using the same faulty syringe.
• While this error affects the absolute accuracy of the data point for that concentration, it is unlikely to fundamentally change the overall shape or trend observed in the graph plotting sediment height against the series of intended ethanol concentrations.

Question 1(b)(i): Constructing a Bar Chart

You are given data for the percentage of ethanol produced from 100g of different carbohydrate sources (Molasses, Oranges, Grapes, Beetroot, Rice). You need to create a bar chart to display this information visually. Which step describes an incorrect method or convention for constructing this bar chart?

• Use the ‘Source of carbohydrate’ as the label for the x-axis, listing the names of the five distinct sources (these are categories).
• Label the y-axis ‘Percentage ethanol produced / %’ (or similar) and use a continuous, linear scale starting from zero to represent the ethanol yield values.
• Draw separate, rectangular bars for each carbohydrate source, ensuring the height of each bar accurately corresponds to the percentage ethanol value for that source on the y-axis scale.
• Make the width of each rectangular bar proportional to the percentage ethanol produced by that specific source (e.g., a source producing 20% has a bar twice as wide as one producing 10%).
• Ensure there are equal-sized gaps between the adjacent bars representing the different, distinct carbohydrate sources on the x-axis.

Question 1(b)(ii): Explaining Lower Ethanol Yield in a Different Yeast

The fermentation experiment is repeated using Schizosaccharomyces pombe instead of Saccharomyces cerevisiae at the same temperature (30°C) and pH (5). The ethanol yield is consistently lower for S. pombe. Which of the following statements provides an unlikely or incorrect biological explanation for this lower yield?

• The optimal temperature for the key fermentation enzymes (e.g., pyruvate decarboxylase, alcohol dehydrogenase) in S. pombe may be significantly different from 30°C, leading to reduced activity.
• S. pombe might be less tolerant to the inhibitory effects of accumulating ethanol compared to S. cerevisiae, causing fermentation to slow down or stop earlier.
• The experimental pH of 5 might be further from the optimal pH range required for efficient fermentation by S. pombe enzymes compared to those of S. cerevisiae.
• S. pombe inherently possesses a significantly more efficient set of enzymes for converting the specific carbohydrates used in the experiment directly into ethanol compared to S. cerevisiae.
• S. pombe, under these conditions, might divert a larger proportion of the carbohydrate substrate towards producing cellular biomass (growth) or other metabolic byproducts (e.g., glycerol, acetate) instead of ethanol.

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

When preparing a plan diagram (low power, showing tissue layout) of a transverse section through a typical herbaceous dicot stem, which feature or arrangement described below is incorrect?

• An outermost single protective layer of cells representing the epidermis.
• A region of ground tissue called the cortex located immediately beneath the epidermis, often containing parenchyma and sometimes collenchyma or sclerenchyma for support.
• The vascular bundles (containing xylem and phloem) are typically arranged in a distinct ring formation separating the cortex from the central pith.
• A large central region composed primarily of parenchyma tissue, known as the pith, is located internal to the ring of vascular bundles.
• Within each vascular bundle, the xylem tissue (responsible for water transport) is typically located towards the outer side of the stem, while the phloem tissue (sugar transport) is located towards the inner (pith) side.

Question 2(a)(ii): Drawing High-Power View of Cortex Parenchyma Cells

You are drawing four adjacent parenchyma cells from the cortex region of a plant stem slide as seen under high power. Which instruction below describes an incorrect technique or representation for these cells?

• Draw the outlines of four cells showing shared, contiguous walls where they touch, representing their typical polygonal or roughly isodiametric shapes in TS.
• Use clear, sharp, continuous lines for all cell walls, avoiding sketchy lines, artistic shading, or colouring.
• Represent the relatively thin primary cell walls typical of parenchyma using heavy, dark double lines to emphasize their structure.
• Include numerous, detailed drawings of chloroplasts within the cytoplasm of each cortical parenchyma cell, as they are the primary site of photosynthesis in the stem.
• Add a clear label line pointing accurately to one of the depicted cell walls and label it appropriately as ‘Cell wall’.

Question 2(b): Comparing Dicot and Monocot Stem Structures

Micrograph 1 represents a typical dicot stem (like L1, with vascular bundles in a distinct ring, separating a clear cortex and central pith). Micrograph 2 shows a stem with vascular bundles scattered throughout the ground tissue (typical monocot structure). Which statement makes an incorrect structural comparison between these two types of stem?

• A key difference in tissue organization is the arrangement of vascular bundles: arranged in a ring in the dicot stem (Micrograph 1) versus scattered throughout the ground tissue in the monocot stem (Micrograph 2).
• A distinct central pith region, composed of parenchyma, is clearly visible inside the vascular ring in the dicot stem (Micrograph 1), whereas such a distinct central pith is typically absent in the monocot stem (Micrograph 2).
• Vascular cambium is usually present between the xylem and phloem within the vascular bundles of the dicot stem (Micrograph 1), enabling secondary growth (increase in diameter), but is absent in monocot bundles (Micrograph 2).
• The scattered vascular bundles observed in the monocot stem (Micrograph 2) are typically uniform in size and structure regardless of their position within the stem.
• A distinct cortex region, located between the epidermis and the vascular ring, is identifiable in the dicot stem (Micrograph 1), while in the monocot stem (Micrograph 2), the tissue surrounding the scattered bundles is generally referred to simply as ground tissue.

Question 2(c): Calculating Mean Actual Stem Diameter

A photomicrograph of a plant stem section was taken at x40 magnification. The diameter of the stem image was measured at three different points as 80 mm, 82 mm, and 78 mm. Which step below shows an incorrect procedure or calculation when determining the mean actual diameter of the stem?

• Sum the three image diameter measurements: 80 mm + 82 mm + 78 mm = 240 mm.
• Calculate the mean image diameter by dividing the sum by the number of measurements: 240 mm / 3 = 80 mm.
• Identify the magnification used for the photomicrograph, which is given as x40 (M = 40).
• Recall or apply the formula relating actual size, image size, and magnification: Actual Diameter = Mean Image Diameter / Magnification.
• Perform the calculation using the mean image diameter and magnification: Actual Diameter = 80 mm * 40 = 3200 mm or 3.2 meters.