Biology IA · C · Interaction and interdependence · topic 15 of 25

Colours of light and photosynthesis · IB Biology IA idea: guide, sample graph and step-by-step plan

a stretch topic, but a great one :)

Plants look green, so do they really ignore green light? You're going to test it fairly, with the same number of photons in every colour.

Difficulty●●● Stretch
KitLEDs (red, green, blue, white), leaf punch, syringe, bicarbonate, thermometer, light meter
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like light and colour
  • ✓ you enjoy careful controls
  • ✓ you can calculate photon numbers from power and wavelength

The big idea

← this is your RQ hook!

Chlorophyll absorbs red and blue strongly and green less, which is why leaves look green. But absorbing less is not the same as doing nothing, and green light can penetrate deeper into a leaf. A fair test needs the same number of photons per second, not the same brightness, because a blue photon carries more energy than a red one. Your job: equalise photon flux and say how, time how fast leaf discs float as oxygen builds up, and keep heat out of it. This is a stretch topic because the controls are hard.

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 →

More colours

Add amber, cyan or far-red LEDs to get more points on the spectrum.

→ The graph gets a numerical wavelength axis and a shape you can describe.
Change the photon flux

At one colour, vary the number of photons per second.

→ A light-response curve; it shows where you are on the curve when you compare colours.
Two species

A shade plant and a sun plant under the same colours.

→ A comparison across species; each species needs its own trial.
Dim light and bright light

Repeat at two photon fluxes.

→ Tests whether the colour effect depends on how much light there is.
Colour mixes

Red alone, blue alone, then the two together.

→ Tests whether the effect of colours adds up; the analysis compares a sum with a measured value.
Change the context

Link to LED grow lights, vertical farms or plants under tree canopies.

→ 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 (LED colour, with the peak wavelength) and the colours you will use.
  • Names the dependent variable (time for half the discs to float, and why it stands for rate) and defines it exactly.
  • Says how photon flux is held equal across colours, and how that was measured or calculated.
  • Says what stays fixed (distance, disc size and number, bicarbonate concentration, temperature) and how each is checked.
  • Is narrow enough for about 10 hours, with several repeats of each colour.

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The set-up

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

done LED colour at a time,matched for photonsper secondwater-tank heat shield:check the temperaturediscs sink at first;floating means O₂sensor: read it where the discs sit

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 plant colour and LED lights matter (grow lights, canopies), and what you expect.
Background biology~450
Photosynthesis and the light reactions; absorption of pigments; why photon number, not brightness, is the fair measure; how you convert power and wavelength into photons (cite your source).
Variablestable
Independent, dependent and controlled variables, with how each is set, measured or kept constant, and why.
Method~450
How you cut and sank the discs; how you equalised photon flux; how you shielded heat; how you timed; repeats; enough detail to copy.
Safety, ethics, environment~60
Do not look into bright LEDs; hot lamps near water; electrical kit; disposal of leaf material.
Raw datatable
One clear table: colour, wavelength, photon flux, each time, units and uncertainties in the headers, and the temperature.
Processing~250
How you calculated the photon flux, the rate, and the uncertainty of each, with one worked example of each calculation.
Graphs and analysis~350
The graph you choose and why, error bars (say SD or SE), and whether the data support an effect of colour: ANOVA across colours, or t-tests on chosen pairs, with each run of discs (not each disc) as one replicate, and its conditions checked.
Conclusion~400
Answer the question with numbers and uncertainties, link to pigment absorption, compare with published work and state limits.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on your result (heat, flux match, disc variation), and improvements linked to each.
Referencesnot counted
Every source for the biology and the photon calculation.

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 red, green, blue and white, you would plot four points or a bar chart. example only

4004505005506006507000.000.010.020.030.040.050.06Peak wavelength of LED λ / nm (± 10)Rate: 1 / time to half-float / min⁻¹ (mean ± SD)one possible pattern (a prediction, not a result)no effect of colouryour data (mean of repeats), with error barsdoes green really do nothing?your error bars decideILLUSTRATIVE · MADE-UP DATA
  • The x-axis is a number (wavelength), so you can see where along the spectrum the rate changes. That is more informative than just naming colours.
  • An LED is not one wavelength. State its peak and the width from the data sheet, and show that as the horizontal uncertainty.
  • All points are at equal photon flux. Say how you made them equal and how well, because a 10% mismatch could be as large as the effect you are testing.
  • Both a trend and a flat line are drawn, so the reader can judge whether colour matters at all.
  • Another graph to consider: plot rate against photon flux for one colour, in a separate pilot. That shows whether you are in the light-limited region, where photon number controls the rate.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result (rate = 1/time, assuming 25 min), not on raw readings. If lamp heat warms the discs by a few degrees, the rate can change by more than your colour effect. That is why the water-tank heat shield and a thermometer matter.

Lamp heat: 3 °C rise (Q₁₀ assumed 2)23%Photon flux match (sensor ±10%)10%Reading half-floated (±0.5 min of 25 min)2%biggest effect: discuss this first!% uncertainty in the rateILLUSTRATIVE · 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 how pigments absorb light and why photons, not brightness, are the fair comparison. Write down your prediction before you start.
  2. Learn the leaf-disc methodPractise cutting discs and sinking them in a bicarbonate solution. How long do they take to float in plain white light? Do all discs sink?
  3. Plan photon matchingDecide how you will make photon flux equal: a photon sensor if you have access, or a calculation from LED power, wavelength and distance. Say what you will check.
  4. Trial run (1 lesson)Test two colours. Does the temperature rise? Do discs float in a sensible time? Can you judge half the discs floating the same way twice? Fix the problems now.
  5. Decide range, repeats and timingChoose the colours, how many discs per run, and how many repeats. Write down why for each choice.
  6. Control temperature and stray lightPut a heat shield between lamp and discs, and measure the temperature. Run the experiment in a dark room or box.
  7. Collect the dataRecord raw times straight into your table with units. Randomise the order of colours.
  8. Process and graphConvert time to rate with uncertainty. Plot with error bars, with the wavelength on the x-axis if you can.
  9. EvaluateWhich weakness mattered most? Use your own temperature and flux data to rank them. Make every improvement specific.
  10. Write, then checkWrite in the order of the layout. Run the checklist below before your teacher's draft.

Biology you'll need

Traps to avoid in this Biology IA

  • Matching brightness in lux and calling it fair. Lux is built for the human eye, not for plants.
  • Lamp heat warming the discs, so the colour with the hottest LED looks best.
  • Different disc batches or leaves for different colours, so the leaf, not the light, is changing.
  • Discs that were not fully infiltrated, or that stick to the beaker, and float for the wrong reason.
  • Stray room light that adds white light to every colour.
  • Calling a colour 'useless' because the time was longer, with no error bars to say how much longer.
  • Quoting a calculated photon flux with no uncertainty or assumptions.

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.

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