SimuVeda

Biology · Simulator 038

Photosynthesis

A leaf makes sugar from light, carbon dioxide and water. Turn up the light, the CO₂ or the temperature and see the photosynthesis rate respond, until another factor becomes the bottleneck. Count the oxygen bubbles from a pondweed to “measure” the rate.

How the model works

Overall: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. The rate is set by whichever input is in shortest supply: Blackman's principle of limiting factors.

This model combines three effects:

  • Light: a saturating curve, J = Jmax · I / (I + I½). Increasing light helps until the light reactions saturate.
  • CO₂: carbon fixation by the enzyme Rubisco follows Michaelis–Menten kinetics, Vc · C / (C + Kc).
  • Temperature: enzyme activity rises to an optimum (about 25–30 °C for many temperate plants) and falls steeply beyond it as enzymes denature.

Gross rate = min(light-limited rate, CO₂-limited rate) × temperature factor. Net rate subtracts respiration, which keeps going in the dark and grows with temperature. The light level where net rate is zero is the compensation point.

Illustrative leaf-level model with rounded parameters. It shows the shape of real responses, not values for a particular species.

Guided experiments

  1. Light curve. Raise light from 0 to 1500 µmol/m²/s at 400 ppm CO₂. Prediction: steep rise at low light, then a plateau where CO₂ is limiting. In darkness the net rate is negative (respiration).
  2. Lift the CO₂ ceiling. At high light, raise CO₂ from 400 to 1000 ppm. Prediction: the plateau rises. This is why commercial greenhouses add CO₂.
  3. Too hot. Push the temperature to 40 °C. Prediction: the rate falls despite plenty of light and CO₂, as enzymes denature and respiration rises.

Challenge

On a cool, sunny morning (10 °C, full sun, 400 ppm), would adding CO₂ or warming the greenhouse raise photosynthesis more?

Reveal the explanation

At 10 °C the temperature factor is low, so enzyme speed limits both pathways and warming gives the bigger boost. Once near the optimum, CO₂ becomes the main limit. Compare the readout's “limiting factor” at 10 °C with and without extra CO₂.

References

  • OpenStax, Biology 2e, ch. 8 “Photosynthesis”.
  • Farquhar, von Caemmerer & Berry (1980), Planta 149: 78–90 (biochemical model of C3 photosynthesis).