Photosynthesis Lab: Effect of Light Distance on the Rate of Photosynthesis
Purpose
To investigate how the distance from a light source affects the rate of photosynthesis in an
Elodea plant by counting the number of oxygen bubbles produced.
Background Information
Photosynthesis is the process by which green plants use light energy to convert carbon
dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂).
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
In aquatic plants like Elodea, oxygen is released as bubbles from the cut end of the stem.
The rate of bubble production can be used as an indicator of photosynthetic rate.
Hypothesis
If the light source is moved farther from the Elodea, then the number of oxygen bubbles
released per minute will decrease, because light intensity decreases with distance.
Materials
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Elodea sprig (about 10 cm long)
Beaker (250 mL)
Water (pond or dechlorinated tap water)
Sodium bicarbonate (baking soda) – source of CO₂
Ruler or meter stick
Lamp with a 40–100 W bulb (constant wattage for all trials)
Stopwatch or timer
Scissors
Thermometer
Procedure
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Fill the beaker with water and add a small pinch of sodium bicarbonate.
Cut the end of the Elodea stem at an angle under water to prevent air bubbles.
Place the Elodea in the beaker with the cut end facing upward.
Position the lamp 5 cm away from the beaker.
Allow the plant to adjust for 1–2 minutes.
Count the number of oxygen bubbles released in 1 minute.
Repeat the test with the lamp placed at 10 cm, 15 cm, 20 cm, and 25 cm from the plant.
Record your results in the data table.
Keep all other conditions (temperature, amount of bicarbonate, size of Elodea) constant.
Data Table
Distance
from Light
(cm)
Trial 1
Trial 2
Trial 3
Average
Notes
5 cm
10 cm
15 cm
20 cm
25 cm
Plot a line graph of the average number of bubbles per minute (y-axis) vs. distance from the
light source (x-axis).
Analysis Questions
1. What was the relationship between the distance from the light source and the number of
bubbles produced?
2. Why was sodium bicarbonate added to the water?
3. What variable was changed (independent variable)?
4. What variable was measured (dependent variable)?
5. List two factors that were kept constant (controlled variables).
6. Why is it important to keep other factors constant during the experiment?
7. What might happen if the light is placed too close to the Elodea?
8. Explain why counting bubbles is only an estimate of the rate of photosynthesis.
9. How could you improve the accuracy of this experiment?
10. What would happen to the rate of photosynthesis if the experiment were done in a dark
room?
Answer Key
1. As the distance from the light increased, the number of bubbles produced decreased.
2. Sodium bicarbonate provides carbon dioxide, which is needed for photosynthesis.
3. The distance between the light source and the Elodea plant.
4. The rate of photosynthesis, measured by the number of oxygen bubbles released per
minute.
5. The amount of sodium bicarbonate and the size/type of Elodea plant.
6. To ensure changes in bubble count are due to light distance only.
7. The water might heat up and affect the plant or reaction rate.
8. Bubble size varies and some oxygen dissolves in water, making it only an estimate.
9. Use a gas syringe or graduated tube to measure actual oxygen volume.
10. The rate of photosynthesis would drop to nearly zero without light.