Photosynthesis Rate Simulation

About this experiment

This is the classic pondweed experiment. Move a lamp closer, add sodium bicarbonate or warm the water and count the oxygen bubbles the plant gives off. The graph shows how the rate rises and then levels off, so students can find the limiting factor at any moment and see why adding more of the other factors makes no difference.

Learning goals

Measure the rate of photosynthesis by counting oxygen bubbles
Describe how light intensity, carbon dioxide and temperature affect the rate
Identify the limiting factor from the shape of a rate graph
Explain why the rate falls at high temperature

How to use it

1Start with the lamp at 20 cm and press Count for 1 minute. Record the number of bubbles.
2Move the lamp to 10, 30, 40 and 60 cm and repeat the count each time.
3Raise the CO₂ level and repeat the light series. Watch the plateau on the graph move up.
4Switch the graph to temperature and find the optimum, then see what happens above 40 °C.

The model

6 C O_{2} + 6 H_{2} O light C_{6} H_{12} O_{6} + 6 O_{2}

Overall equation of photosynthesis

I \propto \frac{1}{d ^{2}}

Light intensity falls with the square of the lamp distance

rate = P_{m a x} \cdot min (f_{light}, f_{C O_{2}}, f_{T}) - R

The scarcest factor sets the rate. R is oxygen used by respiration.

Assumptions

This is a qualitative teaching model. The numbers show trends, not values from a particular plant.
Each bubble is assumed to hold the same volume of oxygen.
The lamp does not heat the water. In a real experiment a heat shield is used for this.

Try this in class

Q1Why does moving the lamp from 10 cm to 20 cm reduce the light to a quarter?
Q2At very high light intensity the rate stops rising. Which factor could be limiting now?
Q3Why are no bubbles produced when the lamp is far away, even though the plant is still alive?
Q4Why does the rate drop sharply above about 40 °C?

Frequently asked questions

What is a limiting factor?

It is the factor in shortest supply. Increasing it raises the rate, while increasing any other factor has no effect until the limit changes.

Why add sodium bicarbonate to the water?

It dissolves to release carbon dioxide, so it lets you control how much CO₂ the plant can use.

Are the bubbles pure oxygen?

Mostly, but they also contain some nitrogen and carbon dioxide dissolved in the water. Counting them is a quick way to compare rates, not an exact measurement.