A leaf is a small power station.
Drag the sliders. Watch the rate of carbon fixation climb until something else becomes the bottleneck. Three regimes take turns: the enzyme Rubisco, the regeneration of the substrate it works on, and the rare case where the sugar exits the leaf too slowly. The model under the hood is Farquhar, von Caemmerer and Berry from 1980, simplified to the essentials.
Model · FvCB 1980 · Bernacchi 2001 temperature kinetics · C3 leaf · Ci/Ca fixed at 0.7
The budget
A C3 leaf at noon in temperate climate fixes about 20 micromoles of CO2 per square metre per second. Each fixed CO2 also releases one molecule of O2, and six fixed CO2 make one molecule of glucose. The sliders below drive the model. The numbers above the curve are what comes out the other end.
How the rate climbs with light
Solid line is net rate (after respiration). Dashed line is gross rate. The dot marks the current setting.
Which limit takes over where
What happens inside the leaf
Two photons, two photosystems, a fall down the energy ladder twice. Photosystem II splits water and lifts the electron high. The chain that follows builds the gradient that drives ATP. Photosystem I lifts the electron again and hands it to NADP+. The diagram below shows the real Z-shape on the redox axis, with electrons flowing live as the photon rate scales with the slider.
Slide left and right to change the photon influx. At low light the chain runs slowly; at full sun a constant rain of photons keeps both photosystems firing.
What the model leaves out
Every working model trades realism for clarity. The two notes below mark where this one cuts corners that matter if you are pushing it past textbook conditions.
What is in the model
Three regimes (Rubisco, electron transport, triose phosphate) with Bernacchi 2001 temperature kinetics. The ratio of internal to external CO2 is held fixed at 0.7. Heat stress above 35 °C is a single linear factor; the real shape is a smoother bell. Quantum yield α is 0.24 mol e⁻ per mol photon, the curvature θ is 0.7. C3 only.
What is not in the model
C4 plants like maize and sugarcane concentrate CO2 around Rubisco and barely care about ambient ppm. CAM plants like cactus open their stomata at night and shift the chemistry across the diurnal cycle. Both are coming as their own page. Stomatal conductance is fixed here, but in reality water stress closes the stomata and starves the enzyme of CO2.
Sources
- Farquhar G.D., von Caemmerer S., Berry J.A. (1980). A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 149, 78–90.
- Bernacchi C.J. et al. (2001). Improved temperature response functions for models of Rubisco-limited photosynthesis. Plant, Cell & Environment 24, 253–259.
- Sharkey T.D. et al. (2007). Fitting photosynthetic carbon dioxide response curves for C3 leaves. Plant, Cell & Environment 30, 1035–1040.
- Govindjee (1999). On the requirement of minimum number of four versus eight quanta for evolution of one molecule of oxygen in photosynthesis. Photosynthesis Research 59, 249–254.
- Wullschleger S.D. (1993). Biochemical limitations to carbon assimilation in C3 plants — A retrospective analysis of the A/Ci curves from 109 species. Journal of Experimental Botany 44, 907–920.