Bubble Rush

Bubble Rush: the pondweed photosynthesis practical

This free game is part of our Pearson Edexcel iGCSE Biology (4BI1) course for exams in EXAM YEARS 2026-2029.

How do you show that a water plant gives off oxygen?

When a plant photosynthesises it makes glucose and gives off oxygen as a by-product:

carbon dioxide + water → glucose + oxygen (light energy, absorbed by chlorophyll)

In a land plant you can't see the oxygen, but in a water plant such as pondweed (Elodea) it comes out of the cut end of the stem as bubbles. The faster the bubbles come out, the faster photosynthesis is happening. To prove the gas is oxygen, collect it in an upturned test tube full of water and put a glowing splint into it. Oxygen relights the splint.

The method: light intensity and the rate of photosynthesis

1. Cut a fresh piece of pondweed about 10 cm long. Cut it underwater, so no air gets into the stem.
2. Put it in a boiling tube of water with a little sodium hydrogen carbonate added. This gives off carbon dioxide, so the plant doesn't run out.
3. Work in a dark room, so the lamp is the only light source. Place the lamp a measured distance away (for example 50 cm), using a ruler.
4. Wait a few minutes so the plant adjusts to the new light intensity.
5. Count the bubbles coming from the cut stem in one minute. Repeat the count three times and work out a mean.
6. Move the lamp closer (30, 20, 15 and 10 cm) and repeat.

The independent variable is light intensity (the lamp distance). The dependent variable is the number of bubbles per minute. Control variables include temperature, the piece of pondweed, and the concentration of sodium hydrogen carbonate (carbon dioxide).

How to play Bubble Rush

You run the practical yourself in a 3D lab.

• Move the lamp along the ruler to the distance shown, and watch the bubbles speed up as the light gets closer.
• Count every bubble for one minute (sped up three times). Press Space or tap the screen each time a bubble breaks free from the cut stem. A ring closes on each bubble to show the moment. Perfect timing builds a combo, but missed bubbles make your count too low and extra taps make it too high.
• Your count and your lab partners' repeats go into a results table and graph. In one run you have to spot the anomaly and leave it out of the mean.
• When the lamp gets close, the water starts to heat up, and you have to drag in a heat shield to keep the test fair.
• At the end your graph is re-plotted against light intensity, and a bonus round with extra carbon dioxide shows you what the limiting factor was.
• A quick question after each run checks the method, with a full explanation if you get it wrong.

What the results show

As the lamp moves closer, the light intensity goes up and so does the number of bubbles per minute, because light provides the energy for photosynthesis. Light intensity follows the inverse square law (light intensity ∝ 1 / distance²), so halving the distance gives four times the light.

At high light intensity the rate levels off. Light is no longer the limiting factor; something else is in short supply, usually carbon dioxide or a low temperature. Adding more sodium hydrogen carbonate makes the rate go up again, which shows carbon dioxide was limiting.

A lamp also gives off heat. Without a heat shield (a tank of water between the lamp and the plant) or a cool LED lamp, the water warms up and temperature changes the rate too, so the test is no longer fair.

Exam tips

• Always say how you control temperature (a heat shield or an LED lamp) and carbon dioxide (the same concentration of sodium hydrogen carbonate).
• Counting bubbles is not very accurate: bubbles are different sizes and fast ones are easy to miss. Collecting the gas in a gas syringe or measuring cylinder and measuring its volume is more accurate.
• Repeat each distance and calculate a mean, leaving out any anomalies, to make the results more reliable.
• Explain a levelling-off graph with limiting factors: "light intensity is no longer limiting; carbon dioxide concentration or temperature is."