Biology 5090/62 — October/November 2020

Walkthrough

The student is investigating the effect of light and dark on seedling growth. The independent variable is the presence or absence of light, and the dependent variable is the growth (e.g., stem length). To ensure a fair test, all other factors that could affect growth must be kept constant (controlled variables).

Common factors that affect plant growth include:

• Water / moisture: Essential for turgor pressure and photosynthesis.
• Temperature: Affects enzyme activity and metabolic rates.
• Soil / growing medium: Provides minerals and anchorage.
• Species / type of seed: Different plants grow at different rates.
• Carbon dioxide / oxygen: Required for photosynthesis and respiration.
• Number of seeds / seedlings: Competition for resources if too many are in one pot.

Any two of these will score.

Key Takeaways

In any investigation, only the independent variable should change. All other potentially affecting factors must be controlled to ensure the results are valid.

Common Mistakes

• Giving the independent variable (light/dark) or dependent variable (growth/length) as a controlled variable.
• Giving vague answers like "same care" without specifying what that means (e.g., same amount of water).

Things to Be Careful About

• The question asks for factors apart from light and dark.
• Give exactly two points as there are two answer lines.

Walkthrough

The candidate is given a diagram (Fig. 1.2) of a seedling grown in the light. Line A is at the top of the stem (below the leaves) and line B is at the base of the stem (above the roots/cotyledon). The candidate must use a ruler to measure the distance between these two lines in millimetres.

In the official exam paper, this distance is designed to be approximately $23\,\text{mm}$. The mark scheme accepts a range of $23 \pm 1\,\text{mm}$ (i.e., 22, 23, or 24 mm) to account for slight differences in printing or reading the ruler.

Key Takeaways

When measuring from a diagram or photograph in an exam, always use a ruler and read the scale in the correct units (mm here). Record the value to the nearest millimetre unless the scale demands more precision.

Common Mistakes

• Measuring the wrong distance (e.g., including the roots or the leaves in the measurement). The question specifies "between A and B".
• Forgetting the unit ($\text{mm}$).
• Reading the ruler incorrectly (e.g., starting from the edge of the paper instead of the zero mark, or reading from the wrong end).

Things to Be Careful About

• The answer must be a number within the acceptable range. If measuring on a screen, the value will differ; the candidate must measure the physical paper or the provided image at the correct scale. In this generated context, the mark scheme value is $23 \pm 1$.
• Ensure the ruler is aligned exactly with the dashed lines A and B.

Walkthrough

The table requires 10 data points: 5 for dark and 5 for light. The dark data is given directly: 27, 25, 33, 35, 30. The light data has 4 values given (13, 12, 19, 13) and the 5th value must be taken from the measurement in part (a)(ii). Let's assume the measurement from (a)(ii) is $23\,\text{mm}$ (the mark scheme value).

1. Enter the data: Fill in the rows with the given values and the measured value.
2. Calculate the dark mean: Add the five dark values and divide by 5.
   $\text{Dark mean} = \frac{27 + 25 + 33 + 35 + 30}{5} = \frac{150}{5} = 30\,\text{mm}$
3. Calculate the light mean: Add the four given light values and the measured value, then divide by 5.
   $\text{Light mean} = \frac{13 + 12 + 19 + 13 + 23}{5} = \frac{80}{5} = 16\,\text{mm}$

The mark scheme awards marks for entering all 10 measurements correctly and for calculating both means correctly.

Key Takeaways

Always use the data from previous parts of the question. The mean is the sum of all values divided by the number of values. Ensure you include the unit in the table header if not already present, but the mean value itself is just the number (the unit is in the header).

Common Mistakes

• Forgetting to include the measurement from (a)(ii) in the light column.
• Calculating the mean incorrectly (e.g., dividing by 4 instead of 5 for the light mean if they forget the 5th value).
• Not carrying forward the value from (a)(ii) even if it was wrong (error carried forward is often accepted, but here the value is needed for the mean).

Things to Be Careful About

• The table has 5 rows for seedlings. Ensure all 5 rows are filled before calculating the mean.
• The mean should be rounded to a sensible number of decimal places (here, whole numbers are exact).
• Check that the sum and division are correct.

Walkthrough

The original investigation used a small number of seedlings (4 in light, 5 in dark). Increasing the sample size to 20 in each group is a standard way to improve an experiment.

1. Reliability: Larger sample sizes reduce the effect of random variations (e.g., one seedling might be genetically weaker or have a defect). The results become more representative of the population.
2. Anomalies: With a larger dataset, it is easier to spot an anomalous result (a value that doesn't fit the pattern). If one seedling in a group of 20 has a very long stem due to a defect, it won't skew the mean as much as one anomalous seedling in a group of 5 would.

Key Takeaways

Always link sample size to reliability and the ability to spot anomalies. "More data is better" is the principle, but use the biological/experimental terms: reliability, anomalies, mean.

Common Mistakes

• Saying "to get a better result" or "to be more accurate" without explaining why (accuracy is about the instrument, reliability is about consistency/repeats).
• Saying "to avoid errors" (errors can still happen; repeats help deal with them).

Things to Be Careful About

• The mark scheme specifically mentions "reliability" and "anomalies / outliers". Use these terms.
• Do not say "to make it more accurate"; sample size affects reliability, not accuracy (unless systematic error is reduced, which isn't the case here).

Walkthrough

In the dark, seedlings exhibit a growth response called etiolation: they grow very tall and thin, with pale leaves (lack of chlorophyll). This is a survival mechanism.

In a dense, shady forest, light is blocked by the canopy above. A seedling germinating on the forest floor needs to reach the light quickly to survive. By growing rapidly upwards (positive gravitropism / negative phototropism in the dark, or simply rapid stem elongation), the seedling increases its chances of emerging above the leaf litter or competing plants to reach sunlight.

Once it reaches light, it will turn green and start photosynthesising.

Note: The mark scheme explicitly rejects "Sun" as an answer and ignores references to photosynthesis. The advantage is about finding the light, not the process of photosynthesis itself (which happens once light is found).

Key Takeaways

Etiolation (growth in dark) leads to tall, pale stems. This is an adaptation to reach light in competitive environments like forests.

Common Mistakes

• Saying "to get more sunlight" (acceptable) but not "to find/reach light" (the mechanism is reaching it).
• Saying "for photosynthesis" (rejected by mark scheme; the growth allows photosynthesis later, but the advantage of the growth itself is reaching the light).
• Confusing this with phototropism (bending towards light); here, the whole stem is growing tall in the absence of directional light.

Things to Be Careful About

• The mark scheme says "R Sun" and "Ig photosynthesis". Focus on the reaching or finding of light.
• Use the term "shady, forest environment" to contextualise the answer.

Walkthrough

The question asks for a visible difference from the photograph (Fig. 1.1), apart from the height (which was measured in part a).

Looking at the photograph:

• Light pot: Seedlings are shorter, have dark green leaves (chlorophyll produced due to light), and look bushy.
• Dark pot: Seedlings are very tall, thin, pale/white/yellow (no chlorophyll produced in dark), and have small, undeveloped leaves.

Acceptable observations include:

• Colour: Greener/darker (light) vs pale/white (dark).
• Leaf development: Larger/more leaves (light) vs small/undeveloped (dark).

Key Takeaways

Always look at the provided image carefully. Qualitative observations (colour, shape, size) are as important as quantitative ones (length, mass).

Common Mistakes

• Repeating the height difference (taller/shorter), which was already established in part (a).
• Saying "the dark ones have no chlorophyll" (this is an explanation, not a visible observation; the observation is "paler" or "yellow/white").

Things to Be Careful About

• The question asks for "one other visible difference". Do not mention height.
• Describe what you see, not the biological reason (unless asked to explain). "Greener" is an observation; "chlorophyll is present" is the reason, but often accepted together or as ORA (Opposite Reverse Argument).

Walkthrough

The candidate must plot the data from the table on the grid (Fig. 1.3).

1. Label axes: The x-axis is the independent variable (time / days). The y-axis is the dependent variable (mean height of plants / cm). Both axes must be fully labelled with the variable name and unit.
2. Scales: Use linear scales. The x-axis goes from 0 to 110. The y-axis goes from 8 to 73 (so 0 to 80 or 0 to 100 is appropriate). The scales must occupy at least half the grid in both directions to ensure accuracy.
3. Plot points: Carefully plot each coordinate pair (time, height):
   • Day 0: 8 cm
   • Day 10: 10 cm
   • Day 30: 29 cm
   • Day 60: 58 cm
   • Day 90: 73 cm
   • Day 110: 73 cm
4. Draw line: The mark scheme specifies "join your points with ruled, straight lines". This is a line graph, but not a smooth curve; use a ruler to connect adjacent points. Do not extrapolate the line beyond the last point (day 110).

Key Takeaways

Graph plotting is heavily marked in 5090. Key marks are awarded for: correct axis labels with units, linear scales starting at 0 (usually) and using >50% of the grid, correct plotting (within half a small square), and correct line type (ruled lines vs smooth curve).

Common Mistakes

• Forgetting units on the axes (e.g., just "time" and "height").
• Using a non-linear scale (e.g., 0, 10, 20, 40, 80).
• Plotting points inaccurately (more than half a small square off).
• Drawing a smooth curve through the points instead of ruled straight lines (the question specifies "ruled, straight lines").
• Extrapolating the line beyond the data (e.g., drawing a line continuing upwards after day 90).

Things to Be Careful About

• The grid in Fig 1.3 is blank. The candidate must choose the scale. A good choice for x-axis: 1 large square = 10 days. For y-axis: 1 large square = 10 cm.
• Ensure the origin (0,0) is used if appropriate, or at least the scales start at 0.
• The line between day 90 and 110 should be horizontal (flat) as the height is constant (73 cm).

Walkthrough

The candidate must use the graph drawn in part (c)(i) to find the height at 50 days. This is interpolation (reading a value within the range of the data).

1. Find 50 on the x-axis. Since 30 and 60 are marked, 50 is two-thirds of the way from 30 to 60.
2. Go up to the line graph.
3. Go across to the y-axis to read the height.

Looking at the data: at day 30, height is 29 cm. At day 60, height is 58 cm. The growth is roughly linear between these points. 50 days is $\frac{20}{30} = \frac{2}{3}$ of the way from 30 to 60. The height increase is $58 - 29 = 29\,\text{cm}$. $\frac{2}{3}$ of 29 is $\approx 19.3$. $29 + 19.3 = 48.3\,\text{cm}$.

The mark scheme accepts $48 \pm 1\,\text{cm}$ (i.e., 47, 48, 49 cm). The candidate must show construction lines on the graph to demonstrate how the value was read.

Key Takeaways

Interpolation requires clear construction lines (vertical from x-axis to graph, horizontal from graph to y-axis). The value read must be consistent with the plotted line.

Common Mistakes

• Not showing construction lines on the graph (marks lost for "working shown").
• Reading the wrong axis.
• Extrapolating incorrectly (though 50 is within the range 30-60, so it's interpolation).

Things to Be Careful About

• The mark scheme requires "working shown". This means drawing the lines on the graph paper.
• The unit is cm.

Walkthrough

The question asks to describe how the rate of growth changed. Rate of growth is the gradient (slope) of the line graph.

1. 0 to 10 days: Height went from 8 to 10 cm (2 cm in 10 days). Gradient is shallow. Rate is slow.
2. 10 to 60 days: Height went from 10 to 58 cm (48 cm in 50 days). Gradient is steep. Rate increased / is fast.
3. 60 to 90 days: Height went from 58 to 73 cm (15 cm in 30 days). Gradient is less steep than before. Rate slowed down / decreased.
4. 90 to 110 days: Height stayed at 73 cm (0 cm change). Gradient is zero (flat line). Growth stopped.

The candidate must describe these changes in the slope of the line.

Key Takeaways

Rate of growth = gradient of the height-time graph. A steeper line means faster growth. A flat line means no growth (growth has stopped).

Common Mistakes

• Just stating the heights at each time point without describing the rate (e.g., "height was 29 at day 30").
• Saying "growth was fast" without specifying the time period.
• Not mentioning that growth stopped at the end (flat line).

Things to Be Careful About

• Use terms like "rate", "gradient", "slow", "fast", "increased", "decreased", "stopped".
• Cover the whole range 0 to 110 days.
• The mark scheme gives marks for: slow to day 10; increased/fast 10-60; slowed 60-90; stopped/rate=0 90-110.

Walkthrough

The current experiment only has one group of plants (with nitrogen fertiliser). To prove that the fertiliser caused the growth (and not just that wheat grows normally), you need a comparison group.

1. Control group: Grow wheat seeds in soil with no nitrogen fertiliser (or soil without added fertiliser).
2. Controlled variables: Everything else must be identical to the fertilised group. This includes:
   • Same type/variety of wheat seeds.
   • Same amount of water.
   • Same temperature and light conditions.
   • Same soil type (except for the fertiliser).
   • Same measurement method and intervals.
3. Comparison: After the same time period, compare the mean heights. If the fertilised group is significantly taller, the fertiliser had an effect.

Key Takeaways

A control group lacks the independent variable (fertiliser) but has all other conditions the same. This allows a valid comparison.

Common Mistakes

• Saying "grow plants in the dark" (wrong variable).
• Not specifying that the control should have no fertiliser.
• Forgetting to mention that other conditions must be the same (controlled variables).

Things to Be Careful About

• The mark scheme awards marks for "no fertiliser" and "same method / same experiment / same type of wheat seeds / same soil / same water supply" or any reference to a controlled condition.
• Do not change the dependent variable; measure the same thing (height).

Walkthrough

Height is a common but not the only way to measure plant growth. Other measurable features include:

1. Leaves: Number of leaves, length of leaves, width of leaves, or total surface area of leaves (can be estimated by tracing on graph paper).
2. Biomass / Mass: Weighing the plant. Fresh mass (including water) is easy but variable. Dry mass (after drying in an oven) is more accurate as it reflects actual biological material grown.
3. Reproduction: Number of flowers or seeds produced (yield).

Key Takeaways

Growth can be measured by increase in size (length, area) or increase in mass (biomass).

Common Mistakes

• Saying "colour" (this is a quality, not a measure of growth, though lack of colour indicates health).
• Saying "number of roots" (hard to measure without disturbing the plant).
• Just saying "mass" without specifying dry mass is okay, but dry mass is better.

Things to Be Careful About

• The question asks for "features that could have been used to measure growth". These must be quantifiable (measurable).
• Give two distinct features.