Biology 5090/32 — May/June 2015

Walkthrough

Catalase is an enzyme that catalyses the breakdown of hydrogen peroxide ($\text{H}_2\text{O}_2$) into water and oxygen gas. The production of oxygen gas is observed as fizzing, bubbling, or frothing in the solution.

The mark scheme indicates that the testa contains little or no catalase, so there will be no visible reaction (no fizzing or bubbling). The cotyledons contain active catalase, so the hydrogen peroxide will be broken down rapidly, producing visible fizzing, bubbling, or a froth of oxygen bubbles.

Key Takeaways

• Enzyme activity can be observed qualitatively by looking for the products of the reaction. For catalase, the product is oxygen gas, seen as bubbles or froth.
• Observations must be descriptive and reference the visible change (bubbles, frothing), not just the chemical name (oxygen).

Common Mistakes

• Saying 'oxygen is produced' without referencing bubbles: The mark scheme ignores a reference to oxygen on its own. You must describe the visual observation, such as 'bubbles', 'fizzing', or 'frothing'.
• Saying 'no change' for both: The cotyledons definitely react. One test-tube must show a positive result.

Things to Be Careful About

• Use descriptive language: Write 'fizzing', 'bubbling', or 'frothing' rather than just 'gas produced'.
• Accuracy: Ensure the observation for the testa reflects the lack of enzyme activity (no reaction), while the cotyledons show a clear positive reaction.

Walkthrough

The observation of fizzing in the cotyledon test-tube indicates that hydrogen peroxide is being broken down, meaning catalase is present. The lack of fizzing in the testa test-tube indicates that hydrogen peroxide is not being broken down, meaning catalase is absent or present in negligible amounts.

The conclusion must be comparative and directly address the amount of catalase in each tissue based on the observations.

Key Takeaways

• A conclusion must follow logically from the observations. Observation: fizzing = enzyme present. Observation: no fizzing = enzyme absent.
• Use comparative language ('little or no' vs 'present') to score both marks.

Common Mistakes

• Not being comparative: Stating 'catalase is in cotyledons' but not mentioning the testa, or vice versa, may only score one mark.
• Confusing enzyme with substrate: Do not conclude about hydrogen peroxide; the question asks about catalase.

Things to Be Careful About

• Error carried forward (ecf): If you wrote a wrong observation in (b)(i) but your conclusion logically follows from that observation, you may still get credit. However, for the correct answer, base it on the expected observations (fizzing for cotyledons, no fizzing for testa).
• Wording: Use 'catalase' explicitly. Saying 'enzyme' is acceptable but 'catalase' is more precise.

Walkthrough

Why would one tissue have catalase and the other not? Catalase protects cells from oxidative damage during metabolism. The cotyledons in a germinating seed are metabolically active—they are respiring to provide energy for growth. Respiration produces reactive oxygen species, which are neutralised by catalase. The testa (seed coat) is a protective, often dead or dormant tissue that does not carry out active metabolism, so it does not require catalase.

Key Takeaways

• Enzyme presence often correlates with metabolic activity. Active tissues (respiring) have more enzymes like catalase to manage metabolic by-products.
• Distinguish between the protective, inactive role of the testa and the active, metabolic role of the cotyledons.

Common Mistakes

• Saying 'living vs non-living': The mark scheme explicitly rejects this. The testa may be living but inactive, or the cotyledons are simply more active. Use 'active' vs 'inactive' or 'metabolising'.
• Not explaining the difference: Simply stating 'cotyledons respire' is good, but linking it to the need for catalase or the metabolic state is required.

Things to Be Careful About

• Avoid 'living/dead': This is a common oversimplification that 5090 rejects. Focus on 'activity' and 'metabolism'.
• Single mark: Only one clear point is needed. 'Testa is inactive and cotyledons are respiring' is sufficient.

Walkthrough

The original experiment is qualitative (observing fizzing). To make it quantitative and fair, several improvements can be made:

1. Fair test (controlled variables): The amount of tissue matters. If you add a large piece of cotyledon and a small piece of testa, the difference in fizzing might be due to mass, not enzyme concentration. Use the same mass or surface area.
2. Reaction rate: Grinding the tissue increases the surface area, allowing more enzyme to contact the substrate, making the reaction faster and easier to compare.
3. Temperature: Enzyme activity is temperature-dependent. Keeping the temperature constant ensures it doesn't affect the results.
4. Quantitative measurement: Instead of just 'observing', measure the rate of oxygen production (e.g., using a gas syringe to measure volume, or counting bubbles over time, or measuring froth depth).

Key Takeaways

• Qualitative observations can be improved by making them quantitative (measuring volume of gas, counting bubbles).
• Fair test principles require controlling variables like mass, surface area, and temperature.
• Increasing surface area (grinding) can speed up the reaction for easier comparison.

Common Mistakes

• Saying 'use more samples': This is about repeats, not improving the method itself for this specific comparison. The mark scheme looks for controlling variables or quantitative measurement.
• Saying 'amount of tissue': The mark scheme rejects 'amount' as vague. Use 'mass', 'weight', or 'surface area'.
• Saying 'to avoid mistakes': This is not a valid scientific improvement. Name the specific variable or measurement.

Things to Be Careful About

• Match the improvement to the experiment: The experiment compares two tissues. The most obvious flaw is that 'pieces' were cut, which could have different masses. Controlling mass is the most direct improvement.
• Acceptable alternatives: The mark scheme lists several valid improvements. Any one of them scores. Choose the one you are most confident explaining.

Walkthrough

To test for starch, you use iodine solution (iodine dissolved in potassium iodide). The test requires the starch to be accessible, so the tissues (testa and cotyledons) must be separated. You can add the iodine solution directly to small pieces of each tissue, or make a suspension/extraction and add iodine.

The mark scheme specifically looks for 'separating tissues' and 'iodine solution added'.

Key Takeaways

• Starch test uses iodine solution, turning from brown/orange to blue-black in the presence of starch.
• Tissues must be separated to test each one individually.

Common Mistakes

• Forgetting to separate tissues: If you test the whole seed, you might get a mixed result or the testa might interfere. The mark scheme requires 'reference to separating tissues'.
• Wrong reagent: Do not use Benedict's (for reducing sugars) or biuret (for protein). Starch requires iodine.

Things to Be Careful About

• Procedure description: Keep it concise. 'Add iodine solution to the separated tissues' is sufficient for 2 marks.
• Observation: The question only asks to 'describe how you carried out this test', not the expected result (that's in part ii).

Walkthrough

Bean seeds (dicots) store their food (starch) in the cotyledons. The testa is a protective outer layer and does not store starch. Therefore, the iodine test will turn blue-black for the cotyledons and remain brown/orange for the testa.

The conclusion must be comparative or state the result for both tissues to score the mark.

Key Takeaways

• Dicots (like beans) store food in cotyledons. Monocots (like maize) store food in the endosperm.
• The testa is non-nutritive and does not contain stored starch.

Common Mistakes

• Saying starch is in both: The testa does not contain starch.
• Not being comparative: Saying 'starch is in cotyledons' is good, but 'no starch in testa' or 'more starch in cotyledons' ensures both marks are covered if the scheme requires a comparative statement.

Things to Be Careful About

• Specific observation: If you were to describe the colour change, it would be 'blue-black for cotyledons, brown/orange for testa'. But the question asks for 'conclusions', so state the presence/absence of starch.

Walkthrough

The data shows amylase activity over 10 days of germination. You must plot this as a line graph.

1. Axes: X-axis is 'germination / days' (0 to 10). Y-axis is 'amylase activity / arbitrary units' (0 to at least 8.0, since the max value is 8.0).
2. Scale: The scale must be suitable, using at least half the grid in both directions. For example, if the grid is 10 large squares wide, each large square could represent 1 day. If the grid is 8 large squares high, each could represent 1 arbitrary unit.
3. Plotting: Plot the points: (0, 0.2), (2, 0.8), (4, 2.0), (6, 3.0), (8, 8.0), (10, 6.5). Allow $\pm \frac{1}{2}$ small square for plotting accuracy.
4. Line: Join the points with neat, ruled, straight lines. Do NOT draw a curve of best fit or extrapolate beyond 10 days.
5. Origin: At least one point at the origin (0,0) or close to it (0, 0.2 is acceptable as a starting point, but the axis must start at 0).

Key Takeaways

• Line graphs show changes over time. The independent variable (days) goes on the x-axis.
• Axes must be fully labelled with the variable name AND the unit (e.g., 'germination / days').
• Points must be plotted accurately and joined with straight ruled lines, not a smooth curve.

Common Mistakes

• Wrong axis orientation: Putting 'amylase activity' on the x-axis and 'days' on the y-axis.
• Missing units: Writing 'germination' instead of 'germination / days'.
• Extrapolation: Drawing the line beyond x=10. The mark scheme rejects extrapolation.
• Curved line: Joining points with a smooth curve instead of ruled straight lines.

Things to Be Careful About

• Scale: Ensure the y-axis goes up to at least 8.0. If you start at 0 and go to 5, you will run out of space for the point at 8.0.
• Plotting accuracy: The point at (10, 6.5) is between 6 and 7. Ensure you plot it correctly.
• Ruled lines: Use a ruler to join the points. Freehand lines lose marks.

Walkthrough

The question asks for the amylase activity after 5 days. On the graph, 5 days is halfway between 4 days (activity = 2.0) and 6 days (activity = 3.0).

Since the line joining (4, 2.0) and (6, 3.0) is straight, the value at x=5 is exactly halfway between 2.0 and 3.0, which is 2.5.

You can find this by drawing a vertical line up from x=5 to the graph line, then a horizontal line across to the y-axis, and reading the value.

Key Takeaways

• Interpolation is reading a value between two known data points.
• For a straight line between (4, 2.0) and (6, 3.0), the midpoint is (5, 2.5).

Common Mistakes

• Reading the wrong axis: Ensure you read the y-axis value, not the x-axis.
• Extrapolation error: Do not extend the line beyond the data to find the value at 5 days; 5 days is within the data range (interpolation).
• Ignoring the graph: If you just average 2.0 and 3.0, you get 2.5, which is correct, but the mark scheme expects you to 'use your graph'. Always show construction lines if possible in an exam.

Things to Be Careful About

• Precision: The answer is 2.5. Do not write '2.50' unless the graph precision demands it, but 2.5 is the exact interpolated value here.
• Acceptable figures: The mark scheme says 'accept figure consistent with graph'. If your graph is slightly off, a value like 2.4 or 2.6 might be accepted, but 2.5 is the correct mathematical interpolation.

Walkthrough

Barley is a monocot grain. It stores carbohydrates as starch in the endosperm. During germination, the embryo needs energy for growth. Starch is insoluble and cannot be used directly in respiration. Amylase is an enzyme that hydrolyses (breaks down) the stored starch into maltose, which can be further broken down into glucose. Glucose is then used in respiration to produce ATP for germination.

The graph shows amylase activity increasing during germination (peaking at day 8), which matches the need for sugar release as the seedling grows.

Key Takeaways

• Amylase is a hydrolytic enzyme that breaks down starch.
• The products are maltose (a disaccharide) or glucose (a monosaccharide) / reducing sugars.
• This provides soluble sugars for respiration in the germinating seed.

Common Mistakes

• Saying 'amylase makes glucose': Amylase primarily produces maltose. Maltase then breaks maltose into glucose. However, 'glucose' or 'reducing sugars' is often accepted as an alternative in the mark scheme.
• Not mentioning starch: The substrate must be named. 'Amylase breaks down food' is too vague.
• Missing the purpose: While 'breaks down starch to sugar' is the core mark, adding 'for respiration' or 'during germination' shows complete understanding, though the mark scheme mainly credits the breakdown reaction.

Things to Be Careful About

• Word equation: You could write 'starch $\rightarrow$ maltose' or 'starch $\rightarrow$ glucose'. The mark scheme accepts 'mono/disaccharides/reducing sugars'.
• Context: The question is about 'germinating barley grains'. Linking the enzyme activity to the needs of germination (respiration) is good practice.