Photosynthesis – counting oxygen bubbles in pondweed
Integrated ScienceLife ProcessesAges 12–13Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Life Processes
A classic experiment: a sprig of pondweed (Elodea) sits in a beaker of water under an inverted funnel with a collecting test tube. In light, the plant photosynthesizes and releases oxygen as countable bubbles. Students drag the lamp closer or farther, or drag sliders for light, CO₂, and temperature, to see how the bubble rate changes; a graph of photosynthesis rate against the selected factor shows the current point and either a saturating curve (light, CO₂) or a curve with an optimum (temperature).
Lesson: Photosynthesis in plants; Factors affecting photosynthesis
What it shows
Pondweed releases oxygen as a by-product of photosynthesis (carbon dioxide + water → glucose + oxygen, using light absorbed by chlorophyll), so the number of bubbles per minute is a practical measure of its rate. The model combines three limiting factors. Raising light intensity or CO₂ concentration increases the rate until it levels off, because another factor then becomes limiting. Temperature acts through enzymes: the rate peaks at about 30 °C, falls on either side and stops above 45 °C. With no light or no CO₂ there are no bubbles.
How to use
Treat it as a fair test: change one factor and keep the other two fixed. Set Graph's horizontal axis to Light intensity, then drag the lamp or the Light intensity slider in steps and read the bubbles/min from the bubble counter at each value. Repeat with CO₂ concentration in water and then Temperature as the variable, choosing the matching axis each time, and compare the shapes of the three curves.
Parameters you can change
- Light intensity 0–100 %
- CO₂ concentration in water 0–0.1 %
- Temperature 0–50 °C
- Graph's horizontal axis Light intensity, CO₂ concentration, Temperature
- Show bubble counter
Questions to explore
- Which gas forms the bubbles, and why does counting them per minute measure the rate of photosynthesis?
- Why does the bubble rate stop rising when the light intensity is already high?
- What happens to the bubble rate above about 35 °C, and why?
Common questions
How does the pondweed photosynthesis experiment work?
A sprig of pondweed is placed in water under a lamp, and the oxygen bubbles it releases are counted for a fixed time. The bubble rate shows how fast the plant photosynthesizes. One factor, such as light intensity, is changed while CO₂ concentration and temperature are kept constant, then the rate is plotted against that factor.
What does a graph of photosynthesis rate against distance from the light source look like?
The graph is a falling curve: the rate is highest near the lamp, drops as the distance grows and approaches zero far from the lamp. Light intensity falls with the square of the distance, so doubling the distance leaves a quarter of the light. A graph of rate against light intensity looks different: it rises, then levels off.
What can I use instead of counting oxygen bubbles?
Measuring the volume of gas collected is more accurate, because bubbles vary in size and some can be missed. The gas can be collected in a test tube, a gas syringe or a capillary tube with a scale. A dissolved oxygen sensor also works and records the concentration over time without relying on bubbles.
Teaching ideas
All guides- 5E Lesson Plan with Interactive SimulationsA 5E lesson plan with interactive simulations: what each E looks like, a timed photosynthesis lesson with real data, a reusable template and adaptations.
- Photosynthesis Virtual Lab: Light Intensity and RatePhotosynthesis virtual lab: count oxygen bubbles from pondweed at seven light levels, plot the rate, then raise CO₂ to find the limiting factor.
- Transpiration Virtual Lab: Light, Humidity, Wind and WaterA transpiration virtual lab for the AP Biology investigation: change wind, light, humidity and soil water, measure the rate and explain it with stomata.
- Seed Germination Virtual Lab: Conditions and Flower PartsA seed germination virtual lab: set up five tubes of cress seeds to test water, oxygen, warmth and light, then dissect a flower and a bean seed.
