Scivra
Pro 🔒~18 min

Photosynthesis: Light & Calvin Cycle

Capturing sunlight to build sugars

How it works

Photosynthesis occurs in two stages within chloroplasts. Light Reactions (thylakoid membranes): Photosystem II absorbs light at 680 nm and splits water (photolysis), releasing O₂ and electrons. Electrons flow through the ETC (plastoquinone, cytochrome b6f, plastocyanin) to Photosystem I (700 nm), reducing NADP⁺ to NADPH. The proton gradient drives ATP synthase (photophosphorylation). The Calvin Cycle (stroma): RuBisCO fixes CO₂ onto RuBP (5C) to form 3-PGA (3C). ATP and NADPH from light reactions reduce 3-PGA to G3P. Every 3 turns fix 3 CO₂, producing 1 net G3P. Two G3P form glucose. Limiting factors: light intensity, CO₂ concentration, and temperature each limit the rate.

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Step-by-step

  1. Adjust light intensity and watch photon absorption rates change in both photosystems.
  2. See O₂ bubbles from water splitting.
  3. Switch to the Calvin Cycle view to track CO₂ fixation by RuBisCO.
  4. Use the wavelength slider (Pro) to see the absorption spectrum — why red (680 nm) and blue (450 nm) are most effective, and why green is reflected.

Key formulas

  • 6CO2+6H2O+light→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2Overall equation for photosynthesis
  • Light reactions: H2O→hνATP+NADPH+O2\text{Light reactions: } \text{H}_2\text{O} \xrightarrow{\text{hν}} \text{ATP} + \text{NADPH} + \text{O}_2Light reactions split water, produce ATP and NADPH
  • 3CO2+9ATP+6NADPH→G3P→glucose3\text{CO}_2 + 9\text{ATP} + 6\text{NADPH} \to \text{G3P} \xrightarrow{} \text{glucose}Calvin cycle: 3 CO₂ → 1 G3P (requires 9 ATP + 6 NADPH)

Frequently asked questions

Which gas is produced in the light reactions and where does it come from?
O₂ is produced from the splitting (photolysis) of water in Photosystem II.
How many turns of the Calvin cycle are needed to produce one glucose molecule?
Each turn fixes 1 CO₂. Glucose (C6) needs 6 CO₂, so 6 turns. But producing 1 net G3P takes 3 turns; glucose needs 2 G3P = 6 turns total.
A plant is exposed to increasing light intensity. At what point does the rate of photosynthesis stop increasing?
The light saturation point — after this, CO₂ fixation (Calvin cycle) or RuBisCO becomes the limiting factor, not light.
Herbicide DCMU blocks plastoquinone in the ETC. Predict its effects on: (a) O₂ production, (b) ATP synthesis, (c) NADPH production, (d) Calvin cycle activity.
DCMU stops electron flow after PSII. (a) O₂ still produced (PSII works), (b) ATP drops (no proton gradient from ETC), (c) NADPH drops (PSI starved), (d) Calvin cycle halts without ATP and NADPH.