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Light-dependent reactions and the Calvin cycle – electron transport, ATP synthase, RuBP/GP/TP and the ¹⁴C tracer experiment

BiologyPlant PhysiologyAges 16–17

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A thylakoid membrane with photosystems II and I, photolysis of water, an electron transport chain that pumps H⁺ into the thylakoid lumen, ATP synthase making ATP and NADP being reduced to NADPH; in the stroma the Calvin cycle fixes CO₂ with RuBisCO, reduces GP to TP and regenerates RuBP. Switch the light or CO₂ off to watch RuBP, GP and TP levels change over time, and vary light intensity, CO₂ and temperature to find the limiting factor. A pulse-chase mode repeats Calvin's ¹⁴C experiment: add H¹⁴CO₃⁻, sample at timed intervals, read the chromatogram and record the percentage of ¹⁴C in each compound.

Lesson: Photosynthesis: light-dependent reactions (photolysis, electron transport, photophosphorylation, reduced NADP) and the light-independent reactions of the Calvin cycle (carbon fixation, reduction, regeneration of RuBP)

What it shows

Photosynthesis happens in two linked stages inside the chloroplast. In the light-dependent reactions on the thylakoid membrane, light excites photosystems II and I, water is split to give O₂, H⁺ and electrons, and the electron transport chain pumps H⁺ into the lumen; H⁺ flowing back through ATP synthase makes ATP, and NADP is reduced to NADPH. The Calvin cycle in the stroma uses that ATP and NADPH: RuBisCO fixes CO₂ onto RuBP to make GP, GP is reduced to TP, and most TP regenerates RuBP. Calvin's ¹⁴C pulse-chase experiment revealed this order.

How to use

Set Light, CO₂ and Temperature with the sliders and watch the electrons, H⁺ and the Calvin cycle pools. Press Light off or CO₂ off and read how RuBP, GP and TP change on the graph; press again to switch back. In the ¹⁴C tracer mode, choose Sample after and a chase, then press Run one sample or Run series 2–120 s, and read the chromatogram and the results table.

Parameters you can change

  • Mode Photosynthesis pathway (light-dependent reactions + Calvin cycle), Calvin's ¹⁴CO₂ tracer experiment
  • Light intensity 0–2000 µmol photons m⁻² s⁻¹
  • CO₂ concentration 0–1500 ppm
  • Temperature 5–45 °C

Questions to explore

  1. Why does GP build up and RuBP fall when the light is switched off?
  2. What happens to RuBP and GP when CO₂ is removed, and why?
  3. Which compound is labelled first in the ¹⁴C experiment, and what does that show?

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