Biology 5090/31 — May/June 2025
Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme
Topics Experimental Contexts · Planning Experiments and Investigations · Observations and Measurements · Analysis, Conclusions and Evaluation · Microscopy and Biological Drawing
Catalase is an enzyme found in living cells. This enzyme catalyses the breakdown of hydrogen peroxide into oxygen and water. Some plant cells are a source of catalase.
If some material from a plant is crushed and added to water, a suspension of the contents of the plant's cells can be obtained.
When hydrogen peroxide solution is added to this suspension, any oxygen produced is released as bubbles of gas. These bubbles collect to form a foam on top of the suspension, as shown in Fig. 1.1. Greater height indicates greater catalase activity.
You are going to investigate the activity of catalase in three different suspensions of plant cells, celery, apple and potato.
Read these instructions and then answer (a)(i):
- use a clean stirring rod to stir the celery cell suspension in the beaker
- pour the celery cell suspension into a clean, large test-tube to a depth of
- use a clean syringe to add of hydrogen peroxide to this large test-tube
- immediately start timing
- measure the total height of any foam layer produced every 30 seconds for 2 minutes, recording your measurements
- repeat these instructions using the apple cell suspension and then the potato cell suspension.
Answer
| time / s | height of foam / mm | ||
|---|---|---|---|
| celery | potato | apple | |
| 30 | |||
| 60 | |||
| 90 | |||
| 120 |
All rows and columns must be drawn with ruled lines. The time column must use 30 s intervals up to 120 s. The height column must have units (mm or cm). Each plant type is listed as a separate column header once.
See working
Walkthrough
The question asks for a table to record the height of the foam layer over time for three different plant suspensions. The time intervals are every 30 seconds for 2 minutes, which gives four data points per plant (30, 60, 90, 120 seconds). The three plant types (celery, potato, apple) need to be compared, so they are best placed as column headers. The dependent variable is the foam height, which must include a unit (mm or cm).
Key Takeaways
A results table must have clear headers with units, the independent variable (time) and dependent variable (height) clearly labelled, and space for all conditions being tested. Ruled lines must be used to separate rows and columns.
Common Mistakes
Forgetting to include units in the column headers. Using non-ruled lines (e.g. pencil shading or freehand). Not including all required time intervals or plant types. Placing the plant types in rows instead of columns (though this is acceptable, it must be consistent).
Things to Be Careful About
The mark scheme specifically requires "single table with columns and rows drawn with ruled lines". Ensure the time column goes from 30 to 120 seconds in 30-second increments. The height unit must be stated.
Answer
All boxes in the table must be completed with measured values. Based on typical catalase activity:
- Celery and potato: The foam height at 120 s must be greater than at 30 s (e.g., celery: 30 s = 5 mm, 120 s = 45 mm; potato: 30 s = 4 mm, 120 s = 35 mm).
- Apple: The foam height should be close to zero throughout the 2 minutes, as apple tissue contains very little catalase.
| time / s | height of foam / mm | ||
|---|---|---|---|
| celery | potato | apple | |
| 30 | 5 | 4 | 0 |
| 60 | 15 | 12 | 0 |
| 90 | 30 | 25 | 0 |
| 120 | 45 | 35 | 0 |
Candidate-dependent measurement. See working for expected result patterns.
Walkthrough
This part requires the candidate to actually perform the experiment and fill in the table. The mark scheme checks for two things: that all boxes are filled, and that the results reflect biological reality. Celery and potato are rich in catalase, so the foam height will increase over the 2 minutes. Apple contains very little catalase, so the foam height will remain near zero.
Key Takeaways
In practical papers, you must record all data, even if it is zero. The biological context (catalase distribution in plants) should guide your expectations.
Common Mistakes
Leaving cells blank. Recording decreasing foam heights for celery/potato. Recording high foam heights for apple.
Things to Be Careful About
Ensure all cells in the table are filled. The mark scheme awards marks for the celery and potato results at 120 s being greater than at 30 s, and for apple results being close to zero.
Suggest why the three plant tissues were crushed before adding the hydrogen peroxide solution.
______
Answer
To break open the plant cells and release the catalase enzyme (which is located inside the cells) into the suspension.
break open cells to release catalase
Walkthrough
Catalase is an intracellular enzyme. In intact plant cells, the cell wall and cell membrane prevent the enzyme from contacting the hydrogen peroxide in the surrounding water. Crushing the tissue breaks these barriers, releasing the enzyme so it can catalyse the breakdown of hydrogen peroxide.
Key Takeaways
Enzymes are often trapped inside cells. To study their activity in vitro, the cells must be disrupted to release the enzyme.
Common Mistakes
Saying "to mix the enzyme" or "to make the enzyme work faster". The primary reason is release, not activation or mixing.
Things to Be Careful About
Use the precise term "break open cells" or "release enzyme". Do not say "destroy the enzyme".
Suggest why the plant suspensions were stirred before adding the hydrogen peroxide solution.
______
Answer
- To ensure that the cells (or cell contents / catalase) are evenly distributed throughout the suspension.
- To provide a maximum and consistent surface area for enzyme activity across each species.
ensure even distribution of catalase and maximize surface area for enzyme activity
Walkthrough
Stirring achieves two things. First, if the crushed plant material settles at the bottom, the suspension will be uneven; stirring keeps the catalase evenly distributed so that any sample taken or any reaction occurring has a consistent enzyme concentration. Second, stirring ensures that the surface area exposed to the hydrogen peroxide is maximised and is the same for each plant species, making it a fair test.
Key Takeaways
In suspension experiments, stirring prevents settling and ensures uniform conditions (concentration and surface area) for all reactants.
Common Mistakes
Saying "to mix the hydrogen peroxide". The hydrogen peroxide is added after stirring. Saying "to dissolve the enzyme" — enzymes are not dissolved, they are suspended.
Things to Be Careful About
The mark scheme requires two points: even distribution AND maximum/same surface area. Both are needed for full marks.
Answer
Any one of:
- The volume of hydrogen peroxide added.
- The volume of plant cell suspension used.
- The time intervals between measurements.
- The diameter (or size) of the test-tubes used.
volume of hydrogen peroxide
Walkthrough
A controlled variable is one that is kept the same across all experimental groups (celery, apple, potato) to ensure that any difference in foam height is due only to the plant type (the independent variable). The instructions mention using a syringe to add 5 cm³ of hydrogen peroxide and pouring the suspension to a depth of 2 cm. These are controlled variables. The time intervals (every 30 s) and the apparatus (test-tubes) are also controlled.
Key Takeaways
Identify what is kept constant in the method. These are your controlled variables.
Common Mistakes
Naming the plant type (this is the independent variable). Naming the foam height (this is the dependent variable). Naming temperature (not mentioned in the instructions, though it should be controlled if not stated).
Things to Be Careful About
Only name variables that are actually controlled or mentioned in the instructions. Do not invent variables like "temperature" unless the question implies it.
Suggest one reason why repeating the investigation would give you more confidence in your results.
______
Answer
To identify anomalies or outliers in the data, and to make the results more reliable (by calculating a mean).
to identify anomalies and make the results more reliable
Walkthrough
Repeating an experiment allows you to check for consistency. If one result is very different from the others, it is an anomaly (outlier) and can be ignored or investigated. Repeats also allow you to calculate a mean, which gives a more reliable average result and reduces the effect of random errors.
Key Takeaways
Repeats improve reliability and help identify anomalies.
Common Mistakes
Saying "to avoid mistakes" or "to make it accurate". 5090 specifically rejects vague answers like "human error" or "to avoid making errors". Use "reliable" and "anomalies".
Things to Be Careful About
Use the precise terms "anomalies" (or "outliers") and "reliable". Do not use "accurate" or "avoid errors".
Answer
Any one of:
- The top of the foam was not level, making it difficult to measure the height.
- The suspension stuck to the sides of the test-tube.
- It was difficult to identify the bottom of the foam layer.
- The level of the foam was changing rapidly, making it hard to read at exactly 30-second intervals.
top of foam not level
Walkthrough
This part asks for a genuine difficulty encountered. Foam is unstable. The top surface is rarely flat, making a single height measurement subjective. The foam can also collapse or change level quickly. The plant suspension can stick to the glass, obscuring the boundary between the liquid and the foam.
Key Takeaways
Practical difficulties often relate to the physical properties of the materials being measured (e.g., foam, bubbles, colour changes).
Common Mistakes
Saying "the apparatus broke" or "I didn't have enough time". These are not scientific difficulties. Focus on measurement or observation issues.
Things to Be Careful About
The difficulty must be specific to this experiment (foam height measurement). Vague answers like "it was hard" do not score.
Another student used the same procedure using a celery cell suspension and hydrogen peroxide. Instead of using a test-tube, they used a measuring cylinder. They measured the total volume of the contents of the measuring cylinder every minute for 5 minutes.
These measurements are shown in Table 1.1. The student did not record the measurement at 4 minutes.
Table 1.1
| time / minutes | total volume of contents / |
|---|---|
| 0 | 4.0 |
| 1 | 11.0 |
| 2 | 16.0 |
| 3 | 19.5 |
| 5 | 20.5 |
Construct a graph of the data in Table 1.1 on the grid below.
Join your plotted points with ruled, straight lines.
Answer
Axes:
- x-axis: time / minutes (linear scale from 0 to 5, using at least half the grid width).
- y-axis: total volume of contents / cm³ (linear scale from 0 to 20 or 25, using at least half the grid height). Both axes must be fully labelled with units.
Plots (marked as crosses or encircled dots):
- (0, 4.0)
- (1, 11.0)
- (2, 16.0)
- (3, 19.5)
- (5, 20.5)
Line:
- Join the plotted points with ruled, straight lines. Do NOT extrapolate beyond the plotted points (i.e., do not draw the line past 5 minutes or before 0 minutes).
See working
Walkthrough
The graph must have time on the x-axis (independent variable) and total volume on the y-axis (dependent variable). Both axes need full labels including units. The scales must be linear and start at 0 (or close to it, but the mark scheme says "value(s) at origin" — for volume, 0 is appropriate; for time, 0 is the origin). Use at least half the grid in both directions. Plot the 5 points accurately. Join them with ruled straight lines. The mark scheme explicitly rejects extrapolation, so stop the line at the last plotted point (5 min).
Key Takeaways
Graph construction rules: x-axis is independent variable, y-axis is dependent variable. Both fully labelled with units. Linear scales. Correct plotting. Ruled lines, no extrapolation.
Common Mistakes
Putting time on the y-axis. Using a non-linear scale. Not labelling axes with units. Extrapolating the line beyond 5 minutes. Connecting points with a smooth curve instead of ruled straight lines.
Things to Be Careful About
The mark scheme requires "plots correct marked as crosses or encircled dots". Ensure your points are clearly marked. The line must be ruled (straight segments between points), not a smooth curve through all points.
Use your graph to estimate the total volume of the contents of the measuring cylinder at 4 minutes.
Show your working on your graph.
total volume at 4 minutes = ______
Answer
Total volume at 4 minutes = 20.0 cm³ (accept values between 19.8 and 20.2 cm³).
Working on graph:
Draw a vertical construction line from x = 4 minutes up to the graph line. From that intersection, draw a horizontal construction line across to the y-axis. Read the value, which is approximately 20.0 cm³. Include the unit cm³ in the final answer.
20.0 cm³
Walkthrough
The student did not record the value at 4 minutes. We must interpolate between the data points at 3 minutes (19.5 cm³) and 5 minutes (20.5 cm³). On the graph, locate 4 on the x-axis, draw a vertical line up to the graphed line (which is a straight ruled line between the 3 and 5 minute points), then draw a horizontal line to the y-axis. The value will be exactly halfway between 19.5 and 20.5, which is 20.0 cm³. The mark scheme requires the correct value, the graph working (construction lines), and the correct unit (cm³).
Key Takeaways
Interpolation is estimating a value between two known data points. Always show construction lines on the graph when doing this.
Common Mistakes
Extrapolating the line (e.g., extending the initial steep slope). Forgetting the unit cm³. Not showing construction lines on the graph.
Things to Be Careful About
The mark scheme awards marks for the correct value, the graph working, and the correct units. All three are needed for full marks. The value should be read from your own graph, so allow a small range (e.g., 19.8 – 20.2 cm³).
Answer
Description:
- The total volume increases with time.
- The rate of increase reduces (the graph becomes less steep / flattens out).
Explanation:
- The hydrogen peroxide (substrate) is getting used up.
- Most or all of the hydrogen peroxide has been broken down by the catalase, so less oxygen is being produced.
Volume increases with time but the rate of increase reduces because the hydrogen peroxide substrate gets used up.
Walkthrough
The graph shows volume increasing over time, which makes sense as oxygen gas is produced and adds to the total volume. However, the line is steepest at the start and becomes less steep as time goes on. This means the rate of reaction is highest initially and slows down. The biological reason is that the substrate (hydrogen peroxide) is being consumed. As its concentration decreases, the rate of enzyme-substrate complex formation decreases, so the reaction slows. Eventually, when all the hydrogen peroxide is broken down, the volume will stop increasing (the graph would flatten completely).
Key Takeaways
A reaction rate graph that starts steep and flattens indicates substrate limitation. The description must state both that volume increases and that the rate reduces. The explanation must mention substrate being used up.
Common Mistakes
Saying "the enzyme gets used up" — enzymes are not used up, they are catalysts. Saying "the temperature changes" — not relevant here. Failing to describe the rate reduction (just saying "it goes up").
Things to Be Careful About
The mark scheme requires both a description (volume increases, rate reduces) and an explanation (substrate gets used up). Ensure you use the word "substrate" or "hydrogen peroxide" in the explanation. Do not say the enzyme is used up.
Fig. 2.1 is a photomicrograph of a section of a celery plant.
In the space below, make a large drawing of the plant section as it appears in the photomicrograph.
Answer
- Draw the section with clear, clean, continuous outlines using a sharp pencil, with no shading, stippling or cross-hatching.
- Make the drawing large: at least 90 mm wide between the positions corresponding to A and B.
- Show the overall shape accurately: a crescent-shaped section with a concave outer (lower) edge and a scalloped upper edge.
- Draw the ridges on the upper edge in detail: the scalloped upper edge drawn with a double line, with each 'bump' (collenchyma ridge) clearly delimited.
- Draw the vascular bundles in their correct positions within the section.
Large, clean, unshaded pencil drawing of the celery petiole section, at least 90 mm wide at A–B, showing the concave lower edge, double-lined scalloped upper edge with delimited collenchyma ridges, and the vascular bundles.
Walkthrough
This is a classic 'make a large drawing' task, and most of the 5 marks are awarded for HOW you draw, not just WHAT you draw.
-
Line quality (1 mark). Use a sharp HB pencil and draw one clear, clean, continuous outline. Never sketch in faint broken lines and go over them; never use shading, stippling (dots) or cross-hatching to show tone. Biological drawings are line drawings only.
-
Size (1 mark). 'Large' always means a stated minimum — here the drawing must be at least 90 mm wide measured between the points corresponding to A and B. That is most of the width of an A4 answer space, so draw boldly.
-
Shape and proportions (1 mark). Look carefully at Fig. 2.1: the section is crescent-shaped, with a smooth concave outer (lower) edge and a scalloped (bumpy) upper edge. Reproduce that shape and its proportions faithfully — do not round the bumps off or flatten the curve.
-
Detail of the ridges (1 mark). The scalloped upper edge carries ridges of collenchyma tissue (the 'strings' you chew in celery). Draw the scalloped edge as a double line (two parallel lines, like the cell walls they represent) and make each bump clearly delimited — a separate, defined ridge, not a wavy line.
-
Vascular bundles (1 mark). The darker oval patches scattered through the section are the vascular bundles (xylem and phloem). Draw them as outlined shapes in their correct positions — roughly in a ring following the curve of the section — not as shaded blobs.
Key Takeaways
- A biological drawing is judged on conventions: continuous sharp-pencil lines, no shading, correct proportions, minimum size, and labels/detail where asked.
- Celery petiole structure: a crescent cross-section with collenchyma ridges on the upper edge (support) and vascular bundles distributed through the ground tissue.
- 'Large' on 5090 always means a specific minimum size — here 90 mm — so measure your drawing.
Common Mistakes
- Shading, stippling or cross-hatching to show the grey tones of the photomicrograph — this loses the line-quality mark outright.
- Drawing too small: anything under 90 mm wide at A–B loses the size mark.
- Drawing the scalloped edge as a single wavy line instead of a double line with delimited bumps.
- Omitting the vascular bundles, or shading them in as dark patches instead of drawing outlined shapes.
- Sketching with several overlapping 'sketchy' lines instead of one clean continuous outline.
Things to Be Careful About
- The mark scheme joins features with '+': the line-quality mark needs BOTH clean continuous lines AND no shading; the detail mark needs BOTH the double line on the scalloped edge AND delimited bumps.
- Keep the orientation the same as the photomicrograph — concave edge at the bottom.
- Use a sharp pencil and a ruler only for label lines, never for the outline itself.
- Measure the A–B width of your drawing before moving on, because (b)(ii) depends on it.
Draw a straight line to join A and B on Fig. 2.1. This is the length of the plant section in the photomicrograph. Measure and record this length.
length A–B = ______
Answer
Draw a straight line joining A and B on Fig. 2.1 and measure it with a ruler.
length A–B = 60 mm (any value in the range 59–61 mm scores)
59–61 mm
Walkthrough
Place a ruler so it joins point A to point B exactly, draw the straight line with a sharp pencil, and read the length to the nearest millimetre. The printed distance is about 60 mm, and the mark scheme accepts 59–61 mm because small differences in printing and ruler placement are expected. Always record the unit (mm) with the reading.
Key Takeaways
- Measure between the exact marked points, not to the edge of the image.
- Record the reading with its unit; the accepted range here is 59–61 mm.
Common Mistakes
- Measuring along the curve of the section instead of the straight line A–B.
- Omitting the unit mm.
- Reading to the nearest half-centimetre instead of the nearest millimetre.
Things to Be Careful About
- The line must be straight and must start and end exactly at the ticks at A and B, because the same line is used again in (b)(ii) and (b)(iii).
- Any value from 59 to 61 mm inclusive scores; outside that range the mark is lost.
On your drawing, draw a straight line in the same position as the line A–B you have drawn on the photomicrograph. Measure and record the length of this line.
length of line on drawing = ______
Answer
On the drawing, draw a straight line in the same position as A–B on the photomicrograph (between the two lower tips of the section) and measure it.
length of line on drawing = e.g. 150 mm (the correct measurement of your own line scores)
Correct measurement of the line drawn on the candidate's own drawing, with unit mm
Walkthrough
Copy the A–B line onto your drawing: it runs between the same two points — the two lower tips of the crescent-shaped section. Draw it straight with a ruler, then measure it to the nearest millimetre. Because the drawing must be at least 90 mm wide at this position (part (a)), your measurement should be at least 90 mm — typically 100–160 mm depending on how large you drew. The mark is for a correct measurement of the line you actually drew, checked by the examiner against your drawing.
Key Takeaways
- The line goes in the SAME position as on the photomicrograph — between the two lower tips.
- The measurement must be consistent with your drawing; since the drawing is at least 90 mm wide, the line must measure at least 90 mm.
Common Mistakes
- Drawing the line in a different position (e.g. across the widest part of the bumps instead of tip to tip).
- Recording a measurement that does not match the actual size of the drawing — examiners check this.
- Forgetting the unit mm.
Things to Be Careful About
- This value is the numerator of the magnification calculation in (b)(iii), so measure carefully.
- The mark is 'correct measurement and line drawn' — both the line on the drawing AND the recorded measurement are needed.
Use your measurements in (b)(i) and (b)(ii) to calculate the magnification of your drawing compared to the photomicrograph. Record your answer to 2 decimal places.
Show your working.
magnification ______
Working
Using the example measurements:
Answer
magnification 2.50 (using your own measurements; answer to 2 decimal places)
magnification × (drawing length ÷ 59–61 mm), to 2 decimal places, e.g. ×2.50
Walkthrough
Magnification compares the size of your drawing with the size of the original image:
Here the 'specimen' is the photomicrograph itself, so you divide your measured drawing length (b)(ii) by the measured photomicrograph length (b)(i). With the example values, 150 mm ÷ 60 mm = 2.5, written as ×2.50. The question demands 2 decimal places, so 2.5 must be written as 2.50 — that final decimal-place mark is separate in the mark scheme. Because the two lengths are in the same unit (mm), no unit conversion is needed, and the answer itself has no unit — magnification is a ratio, which is why it is written with a × sign.
Key Takeaways
- Magnification = image (drawing) size ÷ actual (photomicrograph) size; both measurements in the same unit.
- Magnification is a ratio with no unit, written as '× value'.
- 'To 2 decimal places' means exactly two digits after the point: 2.50, not 2.5.
Common Mistakes
- Dividing the wrong way round (photomicrograph ÷ drawing), which gives a value less than 1.
- Writing the answer as 2.5 instead of 2.50 and losing the decimal-place mark.
- Giving the answer a unit (e.g. '×2.50 mm') — magnification has no unit.
- Using a measured value that does not match the line actually drawn; the examiner checks consistency.
Things to Be Careful About
- Show the working: the mark scheme awards a mark for the division itself, a mark for the correct magnification and a mark for 2 decimal places — three separate marks.
- ecf applies: if your (b)(i) or (b)(ii) measurement is slightly off, you can still score the calculation marks for dividing your own values correctly.
- Keep both measurements in mm so the units cancel.
Plan an investigation to find out the effect of varying light intensity on the increase in height of mustard plant seedlings provided in Petri dishes, as shown in Fig. 3.1.
Answer
- Place at least three Petri dishes in a dark room so that the only light source is a lamp.
- Vary the light intensity by placing the lamp at three or more different distances from the dishes (e.g. 10 cm, 20 cm, 30 cm), ensuring the light source is positioned directly above them.
- Place a heat shield (e.g. a beaker of water) between the lamp and the dishes to prevent the lamp from heating the seedlings and altering the temperature.
- Measure and record the starting height of each seedling in millimetres.
- Leave the seedlings for a fixed time (e.g. 7 days) and then measure and record their final height.
- Keep control variables constant, such as providing the same volume of water / nutrients and keeping the temperature the same for all dishes.
- Calculate the mean increase in height (final height minus starting height) for each light intensity.
See working
Walkthrough
The question asks for a plan to investigate the effect of light intensity on the increase in height of mustard plant seedlings. This is a 6-mark planning question on Paper 3. The mark scheme rewards a structured method that covers the independent variable, how it is varied, the dependent variable, how it is measured, the control variables, the time frame, and how the data is processed. We also need to include AVP (alternative valid points) like using a dark room or a heat shield to ensure only light intensity is changing.
- Independent variable and its levels: We need at least three different light intensities. Using a dark room with only a lamp as the light source provides a controlled environment. Varying the distance of the lamp (e.g., 10 cm, 20 cm, 30 cm) is the standard method to change light intensity.
- Controlling confounding variables: A lamp emits heat. If the lamp is closer, the temperature will be higher, which could affect seedling growth. We must place a heat shield (like a beaker of water) between the lamp and the dishes, or state that the room temperature is controlled. We also must ensure the light comes from above only, as side-lighting would cause phototropism (bending), which would confound the height measurement.
- Dependent variable and measurement: The dependent variable is the increase in height. This requires measuring the starting height and the final height in millimetres. Leaving them for a fixed time (e.g., 7 days) ensures all groups are measured at the same point in time.
- Control variables: We must name specific variables to keep constant, such as the volume of water / nutrients, temperature, and the type / initial size of the seedlings.
- Data processing: To make the results reliable, we calculate the mean increase in height (final minus starting) across the seedlings or dishes for each light intensity.
Key Takeaways
- A planning question must explicitly state at least three values or levels of the independent variable.
- The method for varying the independent variable must be specific (e.g., changing distance from the lamp, not just "varying light").
- Control variables must be named and explained (e.g., same volume of water and temperature).
- The dependent variable must be measured at two points (start and end) to calculate the change.
- Data processing (calculating a mean) is often required and scores a mark.
Common Mistakes
- Stating "vary the light intensity" without explaining HOW (e.g., changing distance from the lamp).
- Forgetting to measure the STARTING height, only measuring the final height.
- Listing control variables without naming them (e.g., saying "keep everything the same" instead of "same volume of water and temperature").
- Not calculating a mean or change in height; just recording final heights.
- Forgetting that a lamp emits heat, so a heat shield or dark room is needed to isolate light intensity as the only changing factor.
Things to Be Careful About
- The mark scheme awards up to 6 marks from 8 possible points, so you must provide at least 6 distinct, correct points.
- Use the exact terminology from the syllabus: "control variables", "independent variable", "dependent variable".
- When measuring height, use a consistent unit (millimetres) and measure from the same point (e.g., base of the stem to the top of the cotyledons).
- Ensure the light source is from above only, as seedlings exhibit phototropism and will bend towards the light if it is from the side, which would confound the height measurement.
- Remember to calculate the MEAN increase in height across multiple seedlings or dishes for each light intensity to make the results reliable.