Biology 5090/32 — May/June 2025

Walkthrough

When designing a data table for an experimental investigation, adhere to the following key criteria:

1. Ruled lines: The table must be enclosed with fully ruled grid lines for both rows and columns.
2. Independent variable: Time is recorded in the first column or top row, stating the unit once in the header (time / s or time / minutes). The intervals must match the instructions: every 30 seconds up to 2 minutes ($30$, $60$, $90$, $120$).
3. Dependent variable: The measured parameter is foam height, with its unit specified in the header (height / mm or height / cm).
4. Plant tissues: The three tissues tested (celery, apple, potato) must each have a designated, distinct column/row header and appear once only.

Key Takeaways

• Never repeat units in the data cells; place them only in the column or row headers using the slash notation (e.g., time / s).
• Keep the structure clean and logical, with the independent variable placed in the leftmost column.

Common Mistakes

• Writing units next to every numerical entry in the body of the table instead of in the header.
• Omitting one of the plant types or failing to write the full time intervals up to $120\,\text{s}$.

Things to Be Careful About

• Check that the lines are drawn neatly with a ruler.
• Ensure time is represented in seconds ($30, 60, 90, 120$) or minutes ($0.5, 1.0, 1.5, 2.0$) consistently.

Walkthrough

This is a hands-on practical task where candidates carry out the reaction and record their own measurements:

1. Data completeness: Every cell in the drawn results table must contain a numerical value.
2. Expected trends:
   • Potato and celery tissues contain active catalase, producing oxygen gas when mixed with hydrogen peroxide. The foam layer increases in height over time, meaning the reading at $120\,\text{s}$ must be greater than at $30\,\text{s}$.
   • Apple tissue has very low or negligible catalase activity under these conditions, so its foam height should be recorded as $0$ (or close to zero).

Key Takeaways

• Catalase activity varies considerably among different plant tissues.
• Measurements should be recorded to a consistent degree of precision.

Common Mistakes

• Leaving table cells blank or recording non-numerical descriptions instead of heights.
• Fabricating impossible trends (such as foam height drastically decreasing without explanation).

Things to Be Careful About

• Measure from the boundary between the liquid and foam up to the top surface of the foam column.

Walkthrough

Catalase is an intracellular enzyme found inside plant cells (specifically within peroxisomes/cytoplasm). Plant cells are enclosed by a rigid cellulose cell wall and cell membranes. Crushing the plant tissue ruptures the cell walls and membranes, allowing the intracellular catalase to be released into the aqueous suspension where it can come into direct contact with the hydrogen peroxide substrate.

Key Takeaways

• Intracellular enzymes require cell lysis or mechanical disruption of tissues to be extracted into solution.

Common Mistakes

• Giving vague answers such as "to make it smaller" or "to make it dissolve" without referring to breaking open cells or releasing the enzyme.

Things to Be Careful About

• Use the specific terms: break open cells or release catalase / enzyme.

Walkthrough

Suspensions are heterogeneous mixtures where insoluble cellular debris tends to settle at the bottom over time due to gravity.

1. Even distribution: Stirring resuspends the settled cell material, ensuring that any sample taken has an equal concentration of cells and catalase.
2. Contact and surface area: Dispersing the particles maximizes the surface area of cell fragments exposed to the hydrogen peroxide solution, ensuring consistent interaction between enzyme and substrate.

Key Takeaways

• Stirring a suspension prevents sedimentation and ensures representative sampling across repeated trials.

Common Mistakes

• Merely stating "to mix it" without explaining what is being mixed or why (e.g., failing to mention even distribution of cells/enzyme).

Things to Be Careful About

• Ensure both marking points are clearly stated: distribution of contents/enzyme, and maximizing/standardizing surface area for enzyme reaction.

Walkthrough

In a valid scientific investigation, only the independent variable (the type of plant tissue) is altered. All other potential variables must be kept constant (controlled variables):

• Depth / volume of plant suspension added ($2\,\text{cm}$ depth).
• Volume of hydrogen peroxide added ($5\,\text{cm}^3$).
• Time intervals between measurements ($30\,\text{s}$ intervals up to $2\,\text{minutes}$).
• Diameter/size of the test-tubes used.

Any one of these is acceptable.

Key Takeaways

• Controlled variables ensure that any difference in the dependent variable (foam height) is caused solely by the independent variable (tissue type).

Common Mistakes

• Stating the independent variable (type of plant) or dependent variable (foam height) instead of a controlled variable.
• Stating "amount" instead of the precise scientific term "volume".

Things to Be Careful About

• Use precise terms like "volume" rather than vague terms like "amount of liquid".

Walkthrough

Repeating an experiment multiple times under identical conditions allows the experimenter to:

1. Check for consistency between trials.
2. Spot and discard anomalous results (outliers) that deviate significantly from the general pattern.
3. Calculate a mean value, which reduces the impact of random errors and increases the reliability of the conclusions.

Key Takeaways

• Repeating investigations increases reliability and allows identification of anomalies; it does not change the fundamental accuracy of the instruments used.

Common Mistakes

• Writing "to make it accurate" or "to avoid human error" (Cambridge mark schemes routinely reject or ignore vague references to "accuracy" and "human error").

Things to Be Careful About

• Use the specific terms: identify anomalies / outliers or increase reliability.

Walkthrough

When measuring gas bubbles forming foam in a test-tube, several practical difficulties arise:

• The upper surface of the foam layer is irregular, bumpy, and uneven rather than flat, making it difficult to decide where to position the ruler.
• Foam bubbles continuously burst or form rapidly during the reaction, changing the height while reading.
• Plant debris may cling to the sides of the tube, obscuring the boundary between the liquid suspension and the foam layer.

Key Takeaways

• Evaluating practical limitations requires pointing out specific physical observations rather than general mistakes.

Common Mistakes

• Vague answers like "it was hard to measure" without specifying what caused the difficulty.
• Stating personal error ("I forgot to look at the clock") rather than an inherent methodological difficulty.

Things to Be Careful About

• Focus on the physical nature of the foam: uneven top, fading/bursting bubbles, or unclear interface.

Walkthrough

To construct a full-marks graph for Cambridge O Level Biology:

1. Orientation and labels: The independent variable (time / minutes) goes on the $x$-axis. The dependent variable (total volume of contents / cm^3) goes on the $y$-axis. Both labels must include their units using slash notation.
2. Scale selection: The grid has $7 \times 7$ large squares (each large square contains $10 \times 10$ small squares, so $70 \times 70$ small squares total).
   • For the $x$-axis (0 to 5 minutes): Scale of $1\text{ large square} = 1\,\text{minute}$ uses 5 large squares ($> 50\%$ of the grid width).
   • For the $y$-axis (0 to 20.5 $\text{cm}^3$): Scale of $1\text{ large square} = 4\,\text{cm}^3$ or $1\text{ large square} = 5\,\text{cm}^3$ (giving 25 $\text{cm}^3$ at 5 large squares) uses $> 50\%$ of the grid height.
3. Plotting points:
   • $(0, 4.0)$
   • $(1, 11.0)$
   • $(2, 16.0)$
   • $(3, 19.5)$
   • $(5, 20.5)$
     Mark each point with a crisp $\times$ or a dot inside a small circle.
4. Joining lines: The question specifically instructs: "Join your plotted points with ruled, straight lines." Use a clear ruler to connect consecutive points. Do not draw a line of best fit or smooth curve here, and do not extrapolate beyond $(5,\, 20.5)$.

Key Takeaways

• Always follow the explicit instruction regarding the line style (ruled straight lines vs smooth curve).
• Never extrapolate a graph line past the first or last plotted point unless explicitly requested.

Common Mistakes

• Inverting the axes (putting volume on $x$ and time on $y$).
• Plotting a smooth freehand curve when the question explicitly demands ruled, straight lines.
• Extrapolating the line beyond $5\,\text{minutes}$ or before $(0, 4.0)$.

Things to Be Careful About

• Scales must be strictly linear (regular numerical increments per major division).
• Ensure $(0, 4.0)$ is plotted on the $y$-axis at $4.0$, not at the origin $(0,0)$.

Walkthrough

1. Linear interpolation: The straight line between $(3,\, 19.5)$ and $(5,\, 20.5)$ represents the rate across that 2-minute interval. At the midpoint ($t = 4\,\text{minutes}$), the volume is exactly the midpoint between $19.5$ and $20.5$, which is $20.0\,\text{cm}^3$.
2. Showing working: Clearly draw dashed or solid construction lines on the graph (a vertical line from $x = 4$ to the line, and a horizontal line from the intersection to the $y$-axis).
3. Unit: State the correct unit, $\text{cm}^3$.

Key Takeaways

• When asked to "Show your working on your graph", construction lines on the grid are mandatory to gain full marks.

Common Mistakes

• Forgetting to draw construction lines on the graph grid.
• Omitting the unit $\text{cm}^3$ in the final answer.

Things to Be Careful About

• Ensure the read-off precisely matches the intersection on your drawn graph.

Walkthrough

This question requires two distinct parts: a description of the graph shape and a biological explanation of why that shape occurs.

1. Describe the shape (2 marking points):

   • Overall trend: The total volume increases with time.
   • Rate change: The reaction is fastest initially (steepest gradient from 0 to 1 minute), then the rate decreases / slows down (gradient becomes less steep between 1 and 3 minutes), eventually levelling off / plateauing towards 5 minutes.

2. Explain the shape (1 marking point):

   • At the start, there is a high concentration of substrate ($ext{H}_2 ext{O}_2$), leading to frequent successful collisions between substrate and active sites of catalase.
   • As the reaction proceeds, hydrogen peroxide is progressively broken down into water and oxygen gas. Substrate concentration decreases (becomes limiting), resulting in fewer collisions per unit time and a slower rate of oxygen production.
   • Eventually, almost all substrate is exhausted, so no more oxygen gas is produced and the volume reaches a plateau.

Key Takeaways

• In enzyme reaction progress curves, the decreasing slope over time is caused by substrate depletion, not enzyme denaturation (unless high heat was applied).

Common Mistakes

• Claiming the enzyme is "denatured" or "dies" over time (at room temperature, enzymes are not denatured; the reaction simply slows because substrate runs out).
• Giving only a description without explaining why the rate decreases.

Things to Be Careful About

• Address both parts of the command: describe the gradient/volume change and explain using substrate consumption.

Walkthrough

The candidate is asked to measure the width of the celery section on the printed photomicrograph between the labelled points A and B. This is a direct reading from the exam paper.

1. Place a ruler along the line joining A and B.
2. Read the measurement to the nearest millimetre.
3. The mark scheme accepts any value between 59 mm and 61 mm. This range accounts for minor parallax errors or slight variations in how the paper was printed or scaled.

Key Takeaways

• Measurements from printed diagrams are checked against an accepted range rather than a single exact value.
• Always measure to the precision required by the question (here, to the nearest mm).

Common Mistakes

• Reading the wrong axis or line: Ensure the measurement is strictly between the vertical marks at A and B, not the outer edges of the entire image.
• Wrong precision: Recording 59.5 mm when the question implies mm precision, or failing to record the unit.

Things to Be Careful About

• Accepted range: The mark scheme gives 59–61 mm. Any value in this range scores 1 mark. If you measure 58 mm or 62 mm, you will lose the mark.
• Unit: The question already provides the unit ($\text{mm}$) in the answer line, so you only need to write the number.

Walkthrough

This part tests whether the candidate has maintained the correct scale in their drawing from part (a).

1. Replicate the A–B line from the photomicrograph onto your own drawing, ensuring it is in the same relative position (across the widest point of the crescent).
2. Measure this new line with a ruler.
3. Record the value in mm. This value will be used in part (b)(iii) to calculate magnification.

Key Takeaways

• The line on the drawing must correspond exactly to the line on the specimen.
• This measurement is the 'image size' in the magnification formula.

Common Mistakes

• Drawing the line in the wrong position: If the line is not across the widest part or not horizontal, the measurement is invalid.
• Not measuring: Forgetting to actually measure the line and leaving the answer blank.

Things to Be Careful About

• Consistency: The value you record here must be the exact value you use in part (b)(iii). If you measure 120 mm here but use 115 mm in the calculation, you will lose marks for error carried forward or incorrect working.
• Precision: Record to the nearest mm, consistent with part (b)(i).

Walkthrough

The candidate must calculate the magnification of their drawing relative to the photomicrograph. This is a standard application of the magnification equation.

1. Recall the formula: Magnification = Image size / Actual size. In this context, Image size is the length of the line on your drawing, and Actual size is the length of the line on the photomicrograph.
2. Substitute values: Use the value you recorded in (b)(ii) as the numerator and the value from (b)(i) as the denominator. Note that the units ($\text{mm}$) cancel out, so magnification is a dimensionless number.
3. Calculate and round: Perform the division and round the final answer to exactly 2 decimal places as required by the question.

The mark scheme awards 1 mark for correctly carrying forward the answer from (b)(ii), 1 mark for the correct magnification calculation (which can include error carried forward from an incorrect measurement in (b)(ii) or (b)(i)), and 1 mark for the answer being to 2 decimal places.

Key Takeaways

• Magnification is a ratio and has no units.
• Error carried forward (ecf) is awarded if the formula is applied correctly to an incorrect measurement from a previous part.
• Rounding must be exact to the number of decimal places specified.

Common Mistakes

• Inverting the formula: Calculating Actual size / Image size instead of Image size / Actual size. This gives a number less than 1, which is incorrect for a drawing that is larger than the specimen.
• Wrong rounding: Rounding to 1 decimal place or rounding the intermediate calculation instead of the final answer.
• Forgetting ecf: If a candidate measures 120 mm in (b)(ii) but writes 110 mm in the calculation, they lose the calculation mark but may still get the 'correct magnification' mark if they use 120 mm.

Things to Be Careful About

• 2 decimal places: The question explicitly asks for 2 d.p. An answer like 2.0 or 2 is incorrect; it must be written as 2.00.
• Units in calculation: Do not include $\text{mm}$ in the final magnification value. Magnification is just a number (e.g., $\times 2.00$).
• Scissors/Calculator errors: Ensure your calculator is set to the correct mode and you are dividing the correct numbers. The mark scheme accepts the ecf value, so show your working clearly.