Biology IA · C · Interaction and interdependence · topic 17 of 25

Plants that bend toward blue · IB Biology IA idea: guide, sample graph and step-by-step plan

cheap, visual, lots of data!

Shine a light from one side and a seedling leans toward it. Now change the colour and see who notices.

Difficulty●●○ Medium
KitCress or mustard seeds, coloured LEDs, a dark box, phone, image software
TimeAbout 10 hours
FitsSL and HL (phototropism and auxin are HL content)
  • ✓ you like plants and time-lapse
  • ✓ you can build a light-proof box
  • ✓ you like measuring from photos

The big idea

← this is your RQ hook!

Seedlings bend toward light because cells on the shaded side elongate faster. This depends on photoreceptors that respond to certain colours, with blue light the classic trigger. Your job: expose seedlings to one colour from one side, photograph them, measure the angle of curvature carefully and compare colours. It is cheap, visual and gives you a numerical angle for each seedling, so you have real statistics to think about.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Time course

Photograph the seedlings every 30 minutes for the whole exposure.

→ Angle against time, with lag and speed of response. A bigger data set, so keep the analysis focused.
Vary the intensity

Use one colour at several light levels.

→ A dose-response curve; the graph shows threshold and saturation.
Two colours at once

Blue from one side and red from the other, or blue and green together.

→ Tests how seedlings combine signals; the analysis compares a mixed result with each alone.
Different seedling parts

Measure the bend in the stem separately from the cotyledons if you can.

→ A more detailed measurement, so you must define it clearly.
Age of seedlings

Compare seedlings of different ages.

→ A different independent variable: response against age, with everything else held fixed.
Change the context

Link to windowsill plants, greenhouses or crops in rows.

→ Gives your colours a purpose; the experiment stays the same.

Your Biology IA research question: run the check

write it FIRST, then check it

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable (light colour, with wavelength) and the colours and intensities you will use.
  • Names the dependent variable (angle of curvature, defined from a stated line) and the time of exposure.
  • Says how many seedlings per colour, and how seedlings were chosen for equal age and height.
  • Says what stays fixed (intensity, distance, temperature, humidity, direction) and how each is checked.
  • Is narrow enough for about 10 hours, with enough seedlings to give a meaningful average.

Get your RQ checked · S$29

The set-up

One way to set it up. Draw your own version, with your real distances, for the Method section.

θLED on one side only,one colour per traydark box: no stray lightfrom any other sideangle θ from vertical:use image softwareseedlings: same age and height at the startphone side-on,ruler in the shot

How your Biology IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question, why plants orient toward light (windowsill plants, crops in rows), and what you expect.
Background biology~450
Phototropism, photoreceptors and auxin; why blue light might matter most; how you expect angle to change with colour (cite your source).
Variablestable
Independent, dependent and controlled variables, with how each is set, measured or kept constant, and why.
Method~450
How you grew the seedlings, how you built the dark box, how you set up the lights and camera, how you measured the angle, repeats; enough detail to copy.
Safety, ethics, environment~60
Do not look into bright LEDs; electrical kit; hygiene with seedlings; any allergy risk. Short and specific.
Raw datatable
One clear table: colour, seedling number, angle, units and uncertainties in the headers.
Processing~250
How you measured angles, how you averaged, the spread and sample size, and one worked example of each calculation.
Graphs and analysis~400
The graph you choose and why, error bars (say SD or SE); ANOVA across colours (Kruskal-Wallis if the angles are skewed), with its conditions checked. Seedlings that share one box run are not fully independent, so repeat each colour in separate runs.
Conclusion~400
Answer the question with numbers and uncertainties, link to photoreceptors, compare with published work and state limits.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on your result (seedling spread, stray light, measuring angle), and improvements linked to each.
Referencesnot counted
Every source for the biology and the method.

What your Biology IA graph could look like

error bars on EVERY point ✓

Made-up data, to show you the form of a strong graph, not the answer. It uses five LED colours; with only blue, green and red you would have three points or a bar chart. example only

4004505005506006507007508000102030405060Peak wavelength of LED λ / nm (± 10)Mean angle of curvature after 6 h / ° (± 2 per seedling)one possible pattern (a prediction, not a result)no effect of colour (baseline)mean of n seedlings, with error bars (SE)do the bars on green and redoverlap the baseline?ILLUSTRATIVE · MADE-UP DATA
  • Each point is a mean of many seedlings, since seedlings differ. Say n and what the bars are (SD or SE).
  • The error bar on each mean is the spread of seedlings, not the angle-reading error. Mention both.
  • The baseline shows what 'no response' looks like for your seedlings: some bend a little even in uniform light or in the dark.
  • Because intensity matters, say how you matched it across colours, and check that your match is good enough that differences are about colour.
  • Another graph to consider: angle against time for each colour from your time-lapse. It shows how quickly bending starts, and whether 6 hours is the best time to compare.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result (the mean angle for blue, assumed 40°), not on raw readings. With seedlings, spread between individuals is usually the biggest. The fix is more seedlings, and that lowers the standard error.

Seedling-to-seedling spread (SD 12°, n = 12)9%Measuring the angle from a photo (±2°)5%Camera angle and perspective (±1°)2.5%biggest effect: discuss this first!% uncertainty in mean angleILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

don't skip the trial run! it saves you weeks

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictFind out which colours trigger phototropism and what receptors are involved. Write down your prediction before you start.
  2. Grow seedlingsDecide where and how the seedlings are grown (light or dark) and how you will pick seedlings of equal height. Grow more than you need.
  3. Build a light-tight boxMake a dark box where light comes from one side only. Check for leaks by looking from inside with the lamp off.
  4. Trial run (1 lesson)Run one colour and take photos. Can you see and measure the bend? How long does it take to get a clear angle? Fix the problems now.
  5. Decide range, repeats and timingChoose colours, the number of seedlings per colour, and the exposure time. Write down why for each choice.
  6. Match intensityDecide how you will keep light intensity the same across colours, and how you will check it.
  7. Collect the dataPhotograph every seedling from the same position at the start and the end, with a ruler or reference in each image.
  8. Measure and processMeasure the angle from the same reference line each time. Calculate the mean and spread per colour, and the standard error.
  9. Graph and analysePlot with error bars and decide which comparison and statistic are justified. Check the conditions of any test.
  10. Evaluate, then writeRank the weaknesses using your own spread and checks. Write in the order of the layout and run the checklist below before your teacher's draft.

Biology you'll need

Traps to avoid in this Biology IA

  • Light leaking in from other sides, so seedlings bend toward the wrong lamp.
  • Colours with different intensities, so the effect you see is brightness, not colour.
  • Seedlings of different heights, so a taller one bends more just because of its length.
  • Measuring the angle from different reference lines, or from photos taken at different angles.
  • Running a test that assumes normal data and equal spread with few seedlings, without checking.
  • Calling a colour 'ineffective' because the mean angle was smaller, with overlapping error bars.
  • Not defining 'angle of curvature' clearly enough for someone to repeat it.

More Biology IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

Get your RQ checked · S$29