Different wavelengths of light are absorbed differently by photosynthetic pigments. By measuring the rate of photosynthesis in Elodea (Canadian pondweed) under light of different wavelengths, you can construct an action spectrum — a graph showing photosynthetic rate against wavelength — and compare it to the known absorption spectra of chlorophyll a, chlorophyll b, and carotenoids. This is an elegant and visually striking IB Biology IA.
This practical is suitable for IB Diploma Biology HL and SL.
Background Theory
Photosynthesis occurs in two stages: the light-dependent reactions (in the thylakoid membranes) and the light-independent reactions (Calvin cycle, in the stroma). In the light-dependent reactions, photons are absorbed by photosynthetic pigments. The main pigments and their peak absorption wavelengths are:
- Chlorophyll a: peaks at ~430 nm (blue-violet) and ~680 nm (red)
- Chlorophyll b: peaks at ~450 nm (blue) and ~640 nm (red-orange)
- Carotenoids: absorb broadly at ~400–500 nm (blue-violet)
Green light (~550 nm) is poorly absorbed and mostly reflected — which is why plants appear green. An action spectrum plots photosynthetic rate against wavelength and should show peaks corresponding to the absorption maxima of the pigments above.
Variables
- Independent variable (IV): Wavelength of light (nm) — e.g. 420, 450, 480, 510, 540, 570, 600, 630, 660, 690 nm using coloured filters or LED light sources
- Dependent variable (DV): Rate of photosynthesis measured as O₂ bubble count per minute, or O₂ concentration change per minute using an O₂ probe (continuous)
- Controlled variables (CV): Light intensity (same distance and bulb wattage), temperature (water bath), CO₂ concentration (sodium hydrogen carbonate solution), length of Elodea used, same individual plant throughout
Equipment
- Fresh Elodea (Canadian pondweed) — healthy, actively growing
- Sodium hydrogen carbonate solution, NaHCO₃ (0.5%) as CO₂ source
- Light source (LED lamp or fibre optic lamp of known wattage)
- Coloured filters at a range of wavelengths (420–690 nm) OR a set of coloured LED lights
- Ruler (to fix distance from light source to plant)
- Beaker or clear tank of water (to absorb heat from lamp)
- Stopwatch
- O₂ probe connected to data logger (preferred) OR method for counting O₂ bubbles
- Thermometer
Safety
Safety
⚠️ Take care with electrical equipment near water. Sodium hydrogen carbonate is low hazard but avoid ingestion. There are no significant chemical disposal requirements — pour solutions down the sink with water.
Method
- Cut a 10 cm length of healthy Elodea. Place it in a beaker containing 200 cm³ of 0.5% NaHCO₃ solution. Allow it to acclimatise under white light for 10 minutes.
- Position the light source at a fixed distance (e.g. 10 cm) from the beaker. Place your first coloured filter between the lamp and the beaker.
- Leave for 3 minutes to allow the plant to adjust to the new wavelength.
- Count the number of O₂ bubbles produced per minute for 3 minutes and calculate the mean. If using an O₂ probe, record the rate of O₂ increase (mg dm⁻³ min⁻¹).
- Change to the next filter and repeat. Work through all wavelengths in random order to reduce systematic bias.
- Repeat the full set of wavelengths three times and calculate mean rates.
- Check temperature at each wavelength using the thermometer and record.
Results Table
| Wavelength (nm) | Rate 1 (bubbles/min or mg dm⁻³ min⁻¹) | Rate 2 | Rate 3 | Mean Rate |
|---|---|---|---|---|
| 420 | ||||
| 450 | ||||
| 480 | ||||
| 510 | ||||
| 540 | ||||
| 570 | ||||
| 600 | ||||
| 630 | ||||
| 660 | ||||
| 690 |
Analysis
1. Plot mean rate of photosynthesis (y-axis) against wavelength (x-axis). This is your experimental action spectrum.
2. Identify the peaks and troughs. Do they correspond to the known absorption maxima of chlorophyll a (~430 and ~680 nm) and chlorophyll b (~450 and ~640 nm)?
3. Overlay your action spectrum with published absorption spectra for chlorophyll a, chlorophyll b, and carotenoids. Discuss the degree of correspondence.
4. Identify the trough at ~540–560 nm (green light). Calculate the percentage decrease in rate between your highest and lowest wavelengths.
Discussion Points
- Why is photosynthetic rate low under green light? Why do plants not absorb green light efficiently?
- Why are there two peaks in the action spectrum rather than one?
- What is the role of carotenoids in photosynthesis and how does this show up in the action spectrum?
- Why is CO₂ concentration kept constant using NaHCO₃? What would happen without it?
- Why is counting O₂ bubbles a less reliable measure than using an O₂ probe?
Guidance
IA Guidance
The action spectrum comparison to published absorption spectra makes this IA particularly strong for the conclusion criterion. To score highly:
- Research Design: Justify your wavelength range and intervals. Explain why you use NaHCO₃ rather than plain water and what CO₂ concentration you are providing. Explain why the experiment is conducted in random wavelength order.
- Data Analysis: Include error bars. Overlay your action spectrum with published chlorophyll absorption spectra (cite the source). Calculate percentage differences between predicted and observed peak positions.
- Conclusion: Discuss the extent to which your action spectrum matches the absorption spectrum, and what discrepancies suggest about accessory pigment contributions.
- Evaluation: Discuss the limitations of bubble counting (bubbles vary in size, some O₂ dissolves in water). Recommend using a calibrated O₂ electrode for continuous, quantitative measurement. Discuss how filter bandwidth affects the precision of your wavelength control.
Discover more from Practical Science
Subscribe to get the latest posts sent to your email.