Photosynthesis Virtual Lab: Light Intensity and Rate

This photosynthesis virtual lab matches the AQA GCSE required practical on light intensity and the rate of photosynthesis, and supports the AP Biology photosynthesis investigation. Students put a sprig of pondweed under a lamp, count the oxygen bubbles at seven light levels, and plot the rate against light intensity. Then they turn the light to full and raise the CO₂ to find the limiting factor. They come away with a rate curve and a clear picture of limiting factors. Every number below was read from the simulation.

Curriculum links

• AQA GCSE Biology: specification 4.4.1.2, rate of photosynthesis, and its required practical on the effect of light intensity on the rate of photosynthesis using pondweed. The CO₂ step covers limiting factors (higher tier).
• AP Biology: Unit 3 (Cellular Energetics), topic 3.5, photosynthesis (ENE-1), and the AP photosynthesis investigation. The AP lab times floating leaf disks; this simulation counts oxygen bubbles instead, with the same variables.
• NGSS: HS-LS1-5, how photosynthesis turns light energy into stored chemical energy.
• Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Ask: "CO₂ stays at 0.04 % and the water at 25 °C. As the light rises from 0 % to 100 %, how will the bubble rate change?" Students choose a straight line, a quick rise that levels off, a peak and a fall, or no change, and write one reason. On a class link this is question 1; the simulation unlocks after they answer.

Method in the simulation

Light is shown as a percentage of the lamp's full brightness, not as a distance in cm. Dragging the lamp closer raises the light, as in the real practical, and the Light slider follows. The simulation does not heat the water: temperature is a separate slider.

1. Set CO₂ = 0.04 % and Temperature = 25 °C with the sliders under the picture. Choose Graph vs. light.
2. Set Light to 10 %. Press Reset, wait 30 seconds, and read "Counted: … bubbles in … s". Double the count to get bubbles per minute.
3. Record the rate shown next to the buttons: "Photosynthesis rate ≈ … bubbles/min".
4. Repeat for every light level in the table. At 0 %, no bubbles form.
5. Limiting factor test. Set Light to 100 %. Change CO₂ to each value in the second table and record the rate.
6. Temperature check. Set Light to 60 % and CO₂ to 0.04 %. Record the rate at 20, 25 and 30 °C.
7. Plot bubbles per minute against light intensity.

Light (%)	0	10	20	40	60	80	100
Bubbles counted in 30 s
Bubbles per minute (count × 2)
Rate shown (bubbles/min)

CO₂ at 100 % light (%)	0.02	0.04	0.06	0.08	0.10
Rate shown (bubbles/min)

Expected results

With CO₂ at 0.04 % and 25 °C, the rate shown is 0, 21, 33, 45, 52, 56 and 59 bubbles/min at 0, 10, 20, 40, 60, 80 and 100 % light. The first 10 % of light adds 21 bubbles/min; the last 20 % adds only 3. The curve rises quickly, then levels off.

Counts vary because bubbles form at random. In a 30-second test at 60 % light, the counter read 27 bubbles (54 per minute) against the expected 52. Two counts and a mean bring students close to the curve.

At 100 % light, raising CO₂ gives 41, 59, 69, 75 and 80 bubbles/min at 0.02, 0.04, 0.06, 0.08 and 0.10 %. So at high light, CO₂ is now the limiting factor. With light at 60 %, the rate is 33 bubbles/min at 20 °C, 52 at 25 °C and 61 at 30 °C: a small temperature change has a big effect.

Questions for students

1. Prediction: as light rises from 0 % to 100 % (CO₂ 0.04 %, 25 °C), how will the bubble rate change?
2. In the light-intensity experiment, which are the control variables?
3. CO₂ 0.04 %, 25 °C, light 40 %. What is the rate in bubbles per minute?
4. At 100 % light and 25 °C, by how much does the rate rise when CO₂ goes from 0.04 % to 0.08 %?
5. A real lamp also warms the water. Use the simulation to explain why that matters, and suggest an improvement.

Answers for teachers:

1. It rises quickly at first, then levels off.
2. CO₂ concentration and temperature.
3. 45 bubbles/min (accepted ± 1).
4. 59 → 75, an increase of 16 bubbles/min (accepted ± 1).
5. From 25 °C to 30 °C the rate rises from 52 to 61 bubbles/min. A lamp that heats the water changes two variables at once. Use an LED or a water heat shield, and check the temperature with a thermometer.

Common misconceptions

• "More light always means proportionally more bubbles." Doubling light from 40 % to 80 % raises the rate from 45 to only 56.
• "The bubbles are carbon dioxide." They are oxygen, a product of photosynthesis.
• "Once the curve levels off, the plant is at its maximum." At 100 % light, more CO₂ raises the rate from 59 to 80.

Extension

• Temperature curve. Choose Graph vs. temperature and sketch the curve at full light. The rate peaks near 30 °C and stops above 45 °C. Ask why the shape differs from the light curve.
• Inverse square law (GCSE). In the wet lab, moving the lamp to twice the distance cuts the light intensity to a quarter. Students use their curve to predict the rate at a quarter of full light, then set Light to 25 % to check: the rate shown is 37 bubbles/min.

FAQ

Why does the simulation give light as a percentage instead of a distance?

Real lamps differ, so a distance in cm would not match a real light meter. The percentage is the light reaching the plant. Students can still drag the lamp closer or farther, as they would in the wet lab.

Should students count bubbles or use the rate shown?

Both. Counting trains the practical skill and shows why repeats matter. The questions use the rate shown, so every student gets the same value.

Can I set the starting values for my class?

Yes. On the Share page, create a link and set light, CO₂ and temperature in the starting values. Students can still move the sliders, so repeat the settings in your instructions.

Related

Photosynthesis rate – light, CO₂, temperature and the compensation point Enzyme activity vs. temperature, pH, substrate concentration and inhibitors

The 5E lesson plan guide uses this simulation for a full inquiry lesson. To build your own tables and number checks, see how to create a virtual lab activity.