Photosynthetic state space
The Z-scheme
Oxygenic photosynthesis uses two photosystems in series to move electrons from water to NADP+. The energy diagram traces a Z-shaped path: electrons are excited by PSII (P680, ~1.82 eV), lose energy through the electron transport chain while pumping protons for ATP synthesis, then are re-excited by PSI (P700, ~1.77 eV) before reducing NADP+ to NADPH.
Why two photosystems?
The redox span from water () to NADP+ () is about 1.14 V. A single photon at 680 nm provides ~1.82 eV — enough in principle, but not enough in practice because each step in the chain dissipates energy. Two photosystems in series solve this by providing two energy boosts, each smaller but collectively sufficient.
Species strategies
The five species presets represent distinct evolutionary solutions to the same energetic problem. C3 plants are the most common strategy but are vulnerable to photorespiration. C4 plants concentrate CO2 via bundle-sheath cells, tolerating higher light and temperature. CAM plants separate CO2 fixation in time (night) from light reactions (day), enabling extreme drought tolerance. Cyanobacteria use carboxysomes as a prokaryotic CCM with fast D1 repair. Red algae use phycobilisomes for large antenna cross-sections in low-light aquatic environments.
Photoprotection and homeostasis
When light exceeds the capacity of carbon fixation, excess excitation generates reactive oxygen species (ROS) that damage proteins and membranes. Plants defend against this through non-photochemical quenching (NPQ), which dissipates excess energy as heat; D1 protein repair turnover, which replaces damaged PSII reaction centers; and cyclic electron flow, which adjusts the ATP:NADPH ratio and contributes to photoprotection via the proton gradient.
Path dependence and bifurcation
The same steady-state parameters can produce different outcomes depending on the history of light exposure. Damage memory accumulates when ROS production exceeds repair capacity, creating path dependence: a system that experienced a high-light pulse may behave differently from one that reached the same nominal conditions gradually. When ROS production persistently outruns repair, the system can bifurcate into a photoinhibited regime — an attractor from which recovery requires reducing light or increasing repair.
Speculative control nodes
Three speculative controls explore mechanisms that are not established textbook levers but represent active areas of research. Adaptive excitonic routing imagines fast redistribution of excitation away from overloaded subdomains. Dynamic thylakoid topology imagines membrane restructuring to rebalance transport and repair access. Protonic micro-homeostasis imagines ultrafast buffering of local proton motive fluctuations. These are prompts for systems thinking, not established mechanisms.
Model changelog
- Five species presets: C3, C4, CAM, Cyanobacteria, Red Algae with distinct metabolic strategies
- Z-scheme energy ladder visualization with tunable PSII/PSI wavelengths
- Light-response landscape showing fixation, ROS, and homeostasis across intensities
- Path dependence via five perturbation profiles with damage memory accumulation
- Regime classification: efficient, homeostatic, protected, transition, photoinhibition
- Invariant window analysis: ATP:NADPH ratio, excitation balance, repair-vs-ROS, homeostatic slack
- Three speculative control nodes: adaptive routing, dynamic topology, protonic homeostasis
- Parameter sweep and sensitivity analysis for six key control parameters