Harvesting Sunlight
Every oxygen molecule you breathe was produced by the light reactions of photosynthesis. In thylakoid membranes of chloroplasts, an extraordinary molecular machine captures photons, splits water, and channels electrons through a series of protein complexes to produce the ATP and NADPH that power carbon fixation. This electron transport chain is the engine of virtually all life on Earth.
The Z-Scheme of Electron Transport
Electrons follow a Z-shaped energy path through two photosystems. Photosystem II (P680) absorbs light and uses that energy to extract electrons from water, releasing O₂ as a byproduct. These electrons pass through the cytochrome b6f complex (generating a proton gradient for ATP synthesis) to Photosystem I (P700), which absorbs a second photon and reduces ferredoxin, ultimately producing NADPH. The two-photon requirement explains why the quantum yield of photosynthesis is roughly 0.125 (one O₂ per 8 photons).
Light Response and Saturation
At low light, photosynthetic rate increases linearly with irradiance — each additional photon contributes to electron transport. Above the light saturation point, downstream carbon fixation becomes limiting, and excess absorbed energy must be safely dissipated. Plants use non-photochemical quenching (NPQ) — converting chlorophyll excited states to heat via the xanthophyll cycle — to protect PSII from photodamage. This simulation models the light response curve and how stress factors shift it.
Environmental Regulation
Temperature, CO₂ concentration, and water availability all modulate light reaction efficiency. Heat destabilizes the oxygen-evolving complex of PSII. Drought causes stomatal closure, reducing CO₂ supply and backing up the electron transport chain. Low CO₂ increases photorespiration, wasting energy. Understanding these interactions is crucial for predicting crop productivity under climate change — and this simulator lets you explore each factor independently.