Absorption and action spectra of photosynthesis – pigments, Engelmann's experiment and chromatography

What it shows

This simulation links the light that pigments absorb to the light that drives photosynthesis, for ages 15–18. In a virtual practical, students measure O₂ production by pondweed or a green alga under light of chosen wavelengths at equal photon flux and plot the action spectrum. They then compare it with the absorption spectra of chlorophyll a, chlorophyll b and carotenoids, watch aerobic bacteria cluster in red and blue-violet light as in Engelmann's 1882 experiment, and separate leaf pigments by paper chromatography. Model: spectra are smooth approximations built from standard absorption bands, and respiration is ignored.

How to use

On Action spectrum lab, choose the Organism, set Wavelength and Photon flux, then press Measure (or Sweep 400–700 nm) and read the results table and graph. On Absorption spectra, drag the cursor and tick the curves to compare. On Engelmann's experiment, change Light on the filament and watch the bar chart. On Pigment chromatography, press Run solvent, then Stop, click each spot and choose the pigment.

Parameters you can change

• Starting screen Action spectrum lab, Absorption spectra, Engelmann's experiment, Pigment chromatography
• Organism Pondweed (count bubbles), Green alga Chlorella (O₂ sensor)
• Wavelength of light 400–700 nm
• Photon flux 20–200 µmol m⁻² s⁻¹
• Overlay absorption spectra on the action spectrum graph
• Light on the algal filament (Engelmann's experiment) Spectrum from a prism, White light, Dark
• Number of aerobic bacteria 100–800 cells
• Leaf used for the pigment extract Green leaf, Yellow autumn leaf
• Random seed (same number, same results) 1–9999

Questions to explore

1. Why is the rate of O₂ production highest in red and blue-violet light and lowest in green light?
2. Why must the photon flux and temperature be kept the same at every wavelength?
3. Why does an autumn leaf show only the carotene and xanthophyll spots on the chromatogram?