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Photosynthesis Rate Simulation

IGCSE Biology — Investigating the rate of photosynthesis using pondweed

Investigation Mode
0
Oxygen bubbles per minute
Light Intensity50%
Temperature25°C
CO₂ Concentration0.04%
6CO₂ + 6H₂O  →  C₆H₁₂O₆ + 6O₂
Light
50%
Temperature
25°C
CO₂
0.04%
Limiting Factor: All factors adequate

How to use this simulation

This is the pondweed experiment. Oxygen bubbles stand in for the rate of photosynthesis while you change light intensity, temperature or carbon dioxide. The lamp, the bubble count and the graph all use the same rate. Choose one investigation mode so you can see that factor on the graph while the other two still affect the result.

  1. Choose Light intensity, set temperature to 25 °C and carbon dioxide to about 0.04%, then increase the light. The bubble rate rises and then levels off.
  2. Switch the mode to Temperature. From a cool start the rate rises toward about 35 °C and then falls. Above about 45 °C the model treats the enzymes as inactive and the rate drops to zero.
  3. Switch to CO₂ and increase it from a very low value. The rate rises, then stops rising once carbon dioxide is no longer the scarcest factor.
  4. Read the limiting-factor line under the controls. It names the factor that is holding the rate down. Change that one, and only that one, if you want a fair test.

Key ideas

The reaction

Photosynthesis stores light energy in glucose. The overall equation used on the page is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Oxygen is released in the light-dependent stage, which is why counting oxygen bubbles is a usable measure of the rate in this experiment.

Limiting factors

Light, temperature and carbon dioxide all have to be available. The rate is held down by whichever factor is in shortest supply. Once that factor is plentiful, another one takes over and the graph levels off. Raising a factor that is not limiting does not increase the rate.

Light and carbon dioxide

In the model the light response saturates by about 75% of the slider, and the carbon dioxide response saturates by about 0.07%. Extra light or CO₂ beyond those regions does not raise the bubble count further. Atmospheric carbon dioxide is about 0.04%, so the default is a realistic starting point.

Temperature and enzymes

The reactions are enzyme-controlled. Below 10 °C the model gives no measurable rate. It rises to an optimum near 35 °C and then falls as the enzymes denature. By 45 °C the rate is back to zero. That is the same shape as an enzyme curve, not a steady rise with temperature.

Where this sits in the course

GCSE and IGCSE ask for the word and symbol equations, the pondweed method, and light, carbon dioxide and temperature as limiting factors. A level and IB add the idea that oxygen production belongs to the light-dependent reactions, while the overall equation also includes glucose from the light-independent reactions. This page shows the overall rate, not the two stages separately.

What the model leaves out

Bubble size is assumed constant, and the water is not given a changing mineral supply or chlorophyll concentration. The numbers are a teaching model of the shapes of the graphs, not a data table from one species of pondweed.

More on the Biology simulations page, or the full SciSim library.