Biology 5090/32 — May/June 2024

Walkthrough

The question asks for a large biological drawing of a flower with two petals retained. In the Practical Test, biological drawings are scored heavily on conventions as much as on content. We must ensure the drawing meets the size requirement, uses the correct line quality, and includes the specific structures named in the mark scheme.

1. Line quality and size: The outline must be continuous, clean, and drawn with a sharp pencil. Shading is strictly rejected. The overall width must be at least 90 mm.
2. Petals: Two adjacent petals must be drawn, typically shown at the back to reveal the internal parts.
3. Stamens: These are the male parts. Each consists of a filament (drawn as a double line to represent the tube) and an anther at the tip (clearly delimited, often drawn as a small oval or capsule). They must be positioned below the stigma.
4. Pistil (female part): The style is drawn as a double line rising to the stigma at the top. The stigma is where pollen lands.

Key Takeaways

Biological drawings require strict adherence to conventions: continuous sharp pencil lines, no shading, minimum size requirements, and double lines for walls/stalks. Structural detail (delimited anthers, style double line) is often the difference between scoring and losing marks.

Common Mistakes

• Using shading, stippling, or cross-hatching.
• Drawing lines that are not continuous or using a blunt pencil.
• Forgetting to draw the anthers as clearly delimited structures separate from the filament.
• Drawing the anthers above the level of the stigma.
• Not meeting the minimum size requirement (90 mm).

Things to Be Careful About

• The mark scheme explicitly rejects shading. Keep the drawing clean.
• Filaments and the style must be drawn with a double line to represent their tubular nature.
• Anthers must be delimited (clearly separated from the filament), not just a blob at the end of a single line.
• Ensure the drawing is at least 90 mm wide; measure it before finishing.

Walkthrough

Pollination is the transfer of pollen to the stigma. The question asks to label where pollen must land. This is the stigma, which is the receptive tip at the top of the style.

1. Locate the stigma at the top of the central style in the drawing from (a)(i).
2. Draw a single straight label line from outside the drawing boundary to the top of the stigma.
3. Write the letter P at the end of the label line.

Key Takeaways

The stigma is the specific structure that receives pollen during pollination. Label lines must be straight, unbroken, and never cross each other or any drawn structures.

Common Mistakes

• Pointing the label line to the style or the ovary instead of the stigma.
• Using a curved or ruled line for the label.
• Forgetting to include the letter P.

Things to Be Careful About

• The label line must point to the top of the stigma, not the side or the style.
• Label lines must not touch the structures they are labelling except at the very tip where they indicate the structure.

Walkthrough

The candidate must design a table to record food test results. The mark scheme awards marks for table construction conventions, not just the content.

1. Ruled lines: The table must have clear, ruled rows and columns. No information should be written outside the table boundaries.
2. Headers: The substances being tested (A and B) must be column headers. The reagents used (Benedict's, biuret, iodine) must be row headers.
3. Overarching headers: The columns need a main header like "Colour / Observation / Results" and the rows need a main header like "Test / Reagent / Solution".

Key Takeaways

Results tables in the Practical Test are marked on structure. All data must be inside the table, headers must be clear, and units or observation types must be specified.

Common Mistakes

• Writing headers or data outside the table lines.
• Forgetting overarching headers (e.g., just writing "A" and "B" without "Colour" or "Observation").
• Repeating substance names in every row instead of using a single column header.

Things to Be Careful About

• Ensure the table has enough rows and columns for all three reagents and two substances.
• Use ruled lines for the table; freehand wobbly lines may not score.
• The mark scheme accepts variations in header wording (e.g., "Test" vs "Reagent"), but the concept must be clear.

Walkthrough

The question asks for the temperature of the water-bath used for the Benedict's test. This is a standard practical procedure.

1. Range: The water-bath must be hot enough to drive the reaction but not boiling. The mark scheme accepts any temperature between 30 and 90 $^\circ\text{C}$ inclusive.
2. Unit: The unit must be $^\circ\text{C}$. Writing just a number without units will lose the mark.

Key Takeaways

Always include units when recording measurements. For food tests involving heating, a water-bath at 30–90 $^\circ\text{C}$ is standard.

Common Mistakes

• Forgetting the unit $^\circ\text{C}$.
• Recording a temperature outside the 30–90 $^\circ\text{C}$ range (e.g., room temperature or boiling water at 100 $^\circ\text{C}$).
• Writing "degrees" instead of the symbol $^\circ\text{C}$.

Things to Be Careful About

• The mark scheme explicitly requires the unit $^\circ\text{C}$.
• Any value between 30 and 90 inclusive scores. Do not overthink this; just record a plausible water-bath temperature.

Walkthrough

The candidate must record the expected observations for food tests on substances A and B. Based on the mark scheme, Substance A tests positive for reducing sugar, and Substance B tests positive for protein.

1. Benedict's test: Tests for reducing sugars. Positive result: blue $\rightarrow$ green $\rightarrow$ yellow $\rightarrow$ orange $\rightarrow$ brick-red precipitate (on heating). Negative result: remains blue.
2. Biuret test: Tests for proteins. Positive result: blue $\rightarrow$ purple / lilac / violet. Negative result: remains blue.
3. Iodine test: Tests for starch. Positive result: brown/yellow $\rightarrow$ blue-black. Negative result: remains brown/yellow.

Filling the table:

• Substance A: Benedict's is positive (green/yellow/orange/red), biuret is negative (blue), iodine is negative (brown/yellow).
• Substance B: Benedict's is negative (blue), biuret is positive (purple/lilac), iodine is negative (brown/yellow).

Key Takeaways

Memorise the starting and ending colours for the three main food tests. A positive result always involves a distinct colour change from the reagent's original colour.

Common Mistakes

• Confusing the positive colour for biuret (purple/lilac) with iodine (blue-black).
• Forgetting that Benedict's test requires heating, so the initial colour is blue, not the final colour.
• Writing "positive" or "negative" instead of the actual colour observation.

Things to Be Careful About

• The mark scheme accepts multiple correct colours for a positive Benedict's test (green, yellow, orange, or brick-red) depending on the concentration of reducing sugar.
• For biuret, accept purple, lilac, violet, or mauve.
• Always record the colour of the final mixture, not the reagent's original colour.

Walkthrough

This part asks to state the nutrients present based on the food test results. We link the positive test results to the specific nutrients they detect.

1. Substance A: Positive Benedict's test $\rightarrow$ reducing sugar (e.g., glucose, maltose). Negative biuret and iodine tests rule out protein and starch.
2. Substance B: Positive biuret test $\rightarrow$ protein. Negative Benedict's and iodine tests rule out reducing sugar and starch.

Key Takeaways

Each food test is specific to a nutrient:

• Benedict's $\rightarrow$ reducing sugar
• Biuret $\rightarrow$ protein
• Iodine $\rightarrow$ starch

Common Mistakes

• Stating "sugar" instead of "reducing sugar" (not all sugars give a positive Benedict's test; sucrose is negative).
• Stating "amino acid" instead of "protein" (biuret tests for peptide bonds in polypeptides/proteins, not free amino acids).
• Forgetting to answer for both substances.

Things to Be Careful About

• Use the precise term reducing sugar, not just "sugar" or "glucose" (though glucose is accepted as an example).
• Use protein, not "peptide" or "amino acid".

Walkthrough

The candidate must measure the diameter of the pollen grain from the provided photomicrograph (Fig. 1.1). This is a direct measurement task.

1. Place a ruler against the printed image (or use the on-screen measurement tool if digital).
2. Measure the widest point of the pollen grain. In Fig. 1.1, the grain is roughly oval with a division; measure across the maximum width.
3. The mark scheme provides an accepted range of 24 – 29 mm to account for slight variations in print size or screen resolution.

Key Takeaways

When measuring from printed images, always measure the widest or longest dimension as specified. Acceptance ranges are provided to accommodate minor measurement differences.

Common Mistakes

• Measuring the wrong axis (e.g., the narrower length instead of the widest diameter).
• Recording a value outside the 24–29 mm range.
• Forgetting to include the unit mm.

Things to Be Careful About

• The question asks for the diameter at its widest point. Do not measure the length if the grain is oval.
• Ensure the unit mm is included in the final answer.

Walkthrough

This part tests the candidate's ability to calculate actual size from an image size and magnification. The magnification is given as $\times 400$ in Fig. 1.1.

1. Formula: $\text{actual size} = \frac{\text{image size}}{\text{magnification}}$
2. Substitution: Use the measured value from (c)(i). Let's assume $26 \text{ mm}$.
   $\text{actual size} = \frac{26}{400} = 0.065 \text{ mm}$
3. Unit conversion: Actual sizes of pollen grains are typically expressed in micrometres ($\mu\text{m}$). Multiply by 1000: $0.065 \times 1000 = 65 \mu\text{m}$.
4. Precision: The question asks for the answer to 2 decimal places. So, $65.00 \, \mu\text{m}$ or $0.07 \text{ mm}$ (if keeping in mm, though $\mu\text{m}$ is standard).

The mark scheme awards marks for: the correct division by 400, the correct numerical answer to 2 decimal places, and the inclusion of a unit.

Key Takeaways

Always use the correct formula: $\text{actual size} = \frac{\text{image size}}{\text{magnification}}$. Convert units to make the answer biologically meaningful (mm to $\mu\text{m}$ for cells/pollen).

Common Mistakes

• Multiplying by magnification instead of dividing.
• Forgetting to convert mm to $\mu\text{m}$ (or failing to include a unit).
• Not rounding to 2 decimal places as requested.
• Using the wrong magnification (e.g., 40 instead of 400).

Things to Be Careful About

• The question asks for the answer to 2 decimal places. Ensure your final number reflects this (e.g., $65.00$, not just $65$).
• The unit is mandatory. $65$ without $\mu\text{m}$ will lose a mark.
• Error carried forward (ecf): if the candidate used a wrong value in (c)(i) but applied the formula correctly in (c)(ii), they still get credit for the calculation.

Walkthrough

The candidate must describe two visible structural differences between the pollen grains in Fig. 1.1 and Fig. 1.2. This tests observation skills and the ability to articulate differences.

1. Shape: Fig. 1.1 is oval or elongated. Fig. 1.2 is round, circular, or spherical.
2. Surface texture: Fig. 1.1 has a smooth or granular surface with no spikes. Fig. 1.2 has a rough surface covered in spikes, prickles, or projections.
3. Internal features (optional): Fig. 1.1 shows an internal division or line. Fig. 1.2 appears solid with no internal division.

The mark scheme accepts any two of these differences, as long as they are clearly stated for both figures.

Key Takeaways

When comparing specimens, look for differences in shape, size, surface texture, and internal structures. Use precise descriptive language (e.g., "spikes" instead of "bumps").

Common Mistakes

• Describing similarities instead of differences.
• Using vague language like "different" or "not the same".
• Failing to state the difference for both figures (e.g., only saying "Fig. 1.2 has spikes" without mentioning Fig. 1.1 is smooth).
• Confusing the two figures (attributing Fig. 1.2 features to Fig. 1.1).

Things to Be Careful About

• The table has columns for Fig. 1.1 and Fig. 1.2. Ensure you fill in the correct description for each column.
• The mark scheme explicitly accepts "spikes present / rough surface" for Fig. 1.2 and "spikes absent / smooth surface" for Fig. 1.1.
• Only two differences are required; providing more will not earn extra marks but ensure the two you provide are correct.