Light Intensity and Rate of Photosynthesis (Carol Cao)

Research Question and Hypothesis

Does light intensity affect the rate of photosynthesis in Elodea plants?

Hypothesis: Yes, light intensity does affect the rate of photosynthesis.  Too much light may cause plants to "burn out" while too little light will not provide enough light for optimal photosynthesis activity.


           Biology Standards
    • Students know usable energy is captured from sunlight by chloroplasts and is stored through the synthesis of sugar from carbon dioxide.       
Experimental Design

           This experiment will look at the affect of light intensity on the rate of photosynthesis on Elodea plants.  
            The rate of photosynthesis is determined by the number of bubbles formed by the Elodea plants. The 
            oxygen bubbles are a product of photosynthesis. Plants have shown to produce more bubbles when 
            placed closer to a light source than further away. The efficiency of photosynthesis can be measured
            by the amount of oxygen bubbles formed during a given amount of time. The bubbles counted were 
            only the ones that floated to the top of the test tube.  Baking soda is added to the test tubes to provide 
            adequate amounts of carbon dioxide for photosynthesis to occur.

Independent variable

Time is the independent variable and will be measured with a stop watch.

Dependent variables

The dependent variable will be the number of bubbles formed and floated to the top of the test tube.  This will be measured by counting


Study the number of bubbles formed (dependent variable) as a function of time (independent variable) at various light distances from the test tube. (series). 

Constants and Controls


  • Length of Elodea Plant-8cm
  • Amount of water in each test tube-40mL
  • Amount of baking soda in each test tube-1g
  • Lab room light will be used for the control light.


1) Aquatic plant (Elodea) 
2) Test tubes 
3) Watch 
4) Water (at room temperature) 
5) Lamp (light source) 
6) Knife/Scissors 
7) Rulers
8) baking soda


1) Cut a segment of the Elodea plant approximately 8cm with scissors.

2) Crush the end of the stem at the site of incision gently.

3) Submerge the plant into a test tube filled with 40mL room temperature water and 1g baking soda.

4) Set the test tube on the test tube rack.

5) Place a light source 50cm away facing the test tube.

6) Turn on the light source and begin taking observations.

7) Count the number of bubbles generated the following intervals:10 second, 30 second, and 60 seconds for each trial.

8) Repeat steps one through seven for the following light distances:

  • 40cm
  • 30cm
  • 20cm
  • 10cm

9) Lastly, lab room conditions will be used as the control group. Observe the plant without the use of the light source. Count the bubbles for 10 seconds, 30 seconds, and 60 seconds. 

Sample data and graphs

rate of photosynthesis lab results

Light Intensity and the Rate of Photosynthesis

Analysis & Conclusions

Based on the data collected, the distance the lamp was placed from the test tube with the Elodea plant in it did affect the number of bubbles that formed and floated to the top of the test tube.  The closer (from 50cm to 10cm) the lamp was placed to the test tube, the more bubbles formed over 60 seconds (from 110 to 201 bubbles). Since the oxygen bubbles are a product of photosynthesis, one can conclude that the more bubbles formed, the more photosynthesis occurred, thus increasing the rate of photosynthesis. 

Photos & Movies

Rate of Photosynthesis Lab

Oxygen Production in Elodea Plants