Science · Middle school

The leaf factory

Set the light and the carbon dioxide, and see that whichever is scarcest sets the pace of photosynthesis.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1The leaf factory

Turn the lamp’s knob or use the sliders; watch the shortest stave

21.0 µmol CO₂ / m²·s

Light100% sun

CO₂420 ppm

Light

CO₂

Photosynthesis rate

Limited by: CO₂

21.0 µmol CO₂ / m²·s

The barrel’s staves: The rate stops level with the shortest stave.

Plenty of light but little CO₂: more light does nothing, more CO₂ helps.

O₂ out: 21.0 µmol / m²·s; 126 bubbles/min
Glucose: 3.5 µmol / m²·s

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

In forest shade (10% sun), what happens to the rate of photosynthesis if you give the leaf more CO₂?

The numbers come from Blackman’s (1905) limiting-factor model: rate = min(0.05 × light, 0.05 × CO₂, 30). Light is in µmol photons / m²·s (full sun ≈ 2000), CO₂ in ppm (today’s air ≈ 420); 30 µmol / m²·s is the ceiling of the leaf’s enzymes at 25 °C. Each CO₂ fixed uses one water and releases one O₂; six CO₂ make one glucose. It is a simplified single-leaf model: respiration, temperature and water shortage are left out. The bubble count is a display proportional to the rate.

What is photosynthesis? How plants make food out of thin airRead the article