What it does
A green water plant (pondweed) under a lamp makes food by photosynthesis: carbon dioxide + water → glucose + oxygen, using light trapped by chlorophyll. The oxygen comes off as bubbles you can count. This lab lets you change three things and see the bubbles change:
- Light: slide the lamp nearer or further. Light gets weaker with the square of the distance, so half the distance gives four times the light.
- Carbon dioxide: add baking soda (sodium hydrogencarbonate), which gives off carbon dioxide in water.
- Temperature: warm or cool the water. The enzymes that build sugar speed up as it warms, work best near 30 °C in this model, then break down.
The page tells you which of the three is the limiting factor: the one in shortest supply, which sets the rate (Blackman’s law). It fits Class 10 Science: life processes (nutrition, photosynthesis and the factors it needs) and Class 11 Biology: photosynthesis in higher plants (factors affecting photosynthesis and the law of limiting factors). It is a teaching model with made-up round numbers. Before this, see how water gets into plant cells in the osmosis lab; after it, fit real light-response data with photosynthesis curve fitter, or see how enzyme speed depends on substrate in enzyme kinetics fitter.
How to use it
- Pick the level. Class 10 shows the bubble rate, the limiting factor and the light sum. Class 11 adds how fast each factor would allow on its own, gross rate minus respiration, and the light compensation point.
- Slide the lamp, carbon dioxide and temperature. The picture shows bubbles rising (4 times faster than real life, so you do not have to wait).
- Count for 1 minute to add a row to your results table, as in the real experiment. Bubbles come at random, so repeat counts differ a little. Download the table as a CSV file for a graph or a report.
- Rate graph: choose Light, CO₂ or Temperature along the bottom. The light graph has three lines for three carbon dioxide levels; the carbon dioxide graph has three lamp distances.
- Check yourself gives fresh questions every time and shows the working.
Worked example
The page opens with the lamp at 20 cm, 0.25 % baking soda and water at 25 °C. Light = (10 ÷ 20)² × 100 = 25 units. In this model light allows 0.6 × 25 = 15 bubbles a minute, carbon dioxide allows 80 × (0.05 + 0.25) = 24, and the enzymes at 25 °C allow 50 × 0.71 = 35.4. The smallest is 15, so light is the limiting factor. Respiration uses 2 bubbles’ worth of oxygen, so you see 13.0 bubbles a minute. Move the lamp to 10 cm: light allows 60, so now carbon dioxide (24) limits, and you see 22.0. Adding baking soda now helps; a closer lamp would not.
Tips
- Change one thing at a time and keep the others the same, so you know what caused the change.
- In a real lab, a lamp also warms the water. Put a beaker of water between lamp and plant as a heat screen.
- If no bubbles come off in dim light, the plant is still photosynthesising: it is using all its oxygen in respiration. That light level is the compensation point.
FAQ
Why does the light graph go flat? Once there is plenty of light, something else (carbon dioxide or temperature) is in short supply and holds the rate down.
Why does the rate fall above 30 °C? Enzymes are proteins; heat changes their shape, so they work more slowly and finally stop. Real plants differ in their best temperature.
Are the bubbles pure oxygen? Mostly, but they also carry some dissolved gases, so counting bubbles is a rough measure. Measuring the volume of gas collected is better.
Where the numbers come from
- Inverse square law for light from a small lamp, and Blackman's (1905) law of limiting factors: written by fibub from these standard ideas.
- The plant's numbers (bubbles per light unit, carbon dioxide cap, top rate, optimum 30 °C, respiration) are made-up round values for teaching. They are not measurements of any real plant.
- The rates were checked in the tests against a separate Python version of the same model (40 random settings) and hand-worked cases.
- Drawings are fibub's own simple shapes. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
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Last updated 2026-11-09.