Bright Light and Photosynthesis

water jug
lit splint
graduated cylinder
baking soda
pond weed
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6
0 1 2 3 4 5 6 7 8 9

Because plants contain the green pigment chlorophyll, they have the ability to trap sunlight. Plants can use the energy of sunlight to power the process of photosynthesis. Photosynthesis occurs inside of cellular structures called chloroplasts.

During photosynthesis, light energy captured in chlorophyll powers a complex, multi-stage chemical reaction in which carbon, hydrogen, and oxygen atoms are assembled into glucose and oxygen gas. The reaction occurs in two major steps: the light reaction and the dark reaction. The light reaction requires the presence of light, and it is during this phase of photosynthesis that oxygen is generated. The dark reaction, which can proceed without light, is the stage in which glucose molecules are assembled.

  • pond weed
  • small piece of cardboard
  • funnel
  • 400-ml beaker
  • gooseneck lamp
  • lightbulbs: 15 W, 45 W, 100 W
  • lit splint
  • baking soda
  1. Fill the 400-ml beaker with water until it is about 3/4 full. Add about 1/2 tsp of baking soda to the water and stir. Baking soda provides carbon dioxide for photosynthesis in the pond weed.
  2. Place the pond weed into the beaker and lower the funnel, mouth first, into the beaker of water. Immerse the entire funnel, including the stem.
  3. Place the beaker on the base of the ring stand.
  4. Place the mouth of the graduated cylinder underwater, remove the piece of cardboard and return it to the left side of the screen. Place the mouth of the graduated cylinder over the stem of the funnel.
  5. Allow the apparatus to sit undisturbed for 5 minutes. While it sits, count the number of bubbles produced during a period of 1 minute, and record that number in column A of the data table in the row labeled “First 5 minutes; no lightbulb.” As gas bubbles leave the stem, they will enter the graduated cylinder and displace some of the water in it.
  6. After 5 minutes, observe the volume of gas in the graduated cylinder. Record this volume in column B of the data table.
  7. Place a gooseneck lamp with a 15-watt lightbulb above the beaker containing the pond weed. Shine the light on the beaker of pond weed for 5 minutes. During this time, count the number of bubbles generated in a period of 1 minute, and record that number on the data table in column A, row “15 W.”
  8. At the end of 5 minutes, turn off the light and record the water level. Determine the volume of gas generated by subtracting the current volume from the previous volume.
  9. Repeat steps 7 and 8 with a 45-watt bulb. Record your data on the data table.
  10. Repeat steps 7 and 8 with a 100-watt bulb. Record your data on the data table.
  11. Determine the kind of gas generated in the graduated cylinder. Lift the graduated cylinder from the beaker of water and cover it with the square of cardboard.
  12. Remove the cardboard from the mouth of the graduated cylinder and insert the glowing splint. If the splint glows brightly, the gas in the graduated cylinder is oxygen. Carbon dioxide does not cause a splint to glow.
  1. What kind of gas was generated by the pond weed: oxygen or carbon dioxide? How do you know?
  2. How did the evolution of bubbles change as the wattage of light increased?
  3. On graph paper, create a line graph that shows how change in wattage affects the amount of oxygen gas evolved. Remember to label your axes and title your graph.
  4. Based on the findings in this experiment, explain how photosynthesis might be affected by a change in weather patterns or by seasonal changes.
  1. Oxygen. The wooden splint relit when lowered in the test tube of gas. If the gas had been carbon dioxide, this would not have occurred.
  2. The higher the wattage of the bulb, the faster bubbles were generated.
  3. The graph should show that as the wattage increases, the amount of oxygen gas will also increase, in a directly proportional relationship.
  4. Photosynthesis slows on cloudy days and during the winter season when days are short.
Data Table
Data table: Rate and volume of gas bubbles
Wattage of lightbulbs A
Rate at which bubbles were generated (no. of bubbles/minute)
Number of milliliters that water volume changed in graduated cylinder
First 5 minutes; no lightbulb
15 W
45 W