1. Planting Science How does light intensity affect photosynthesis?
Annalise Wulf, Thomas Farrell, and Vann Adrian-Hage

2. Hypothesis
If the bulb has a lower wattage, then photosynthesis will occur more rapidly in the spinach leaves because the fewer the wattage, the less heat is produced. Spinach grows best in a cooler climate.

3. Experimental Control
The control was having each end of the bulb 6 inches above the lab table, as well as using the same plant (spinach) and solution for all experiments. This allowed the data to only show how the independent variable affected photosynthesis, without any other variables that could affect the data.The independent variable was the different light bulbs used with 60, 100 and 150 Watts.

4. Procedure Making the bicarbonate and soap solution:
Retrieve two plastic cups and add a pinch of baking soda to each, then add water to about 6 cm deep. Stir to dissolve.
Add a drop of liquid soap to each cup and stir, but do not make suds.
Take two more plastic cups and add half of the solution to each, making four plastic cups with an equal amount of the solution in each. One cup of the solution will be left over, but can be used for further experimentation.
Making the spinach leaf disks:
Select enough spinach leaves to make about 36 disks.
Using a paper hole punch, punch out 36 leaf disks and split them into 3 piles of 12 leaf disks.
Removing the gases inside the leaf disks:
Take a syringe and remove the plunger. Pour one pile (12 leaf disks) into the barrel of the syringe. Make sure all the leaf disks are at the narrow end of the syringe. Push the plunger back into place until about 1/10 th of the way from the tip.
Place the syringe into the bicarbonate and soap solution and pull up some solution until the syringe is ⅓ rd of the way full. Make sure all the leaf disks are in the solution.

5. Procedure (cont.)
c Hold the syringe with the tip upwards and cover the opening with your finger. Pull back on the plunger, which will create a vacuum. Hold the vacuum for seconds. Repeat this three times, or until all leaf disks sink in the solution.
d Pour the infiltrated leaf disks into the cup that you took the solution out of for this section of the procedure. The cup should have about 3 cm of solution in it.
e Repeat this entire process for all three sets of twelve leaves, using a different cup of solution for each one.
4. Place each cup under a different light bulb intensity (60 Watts, 100 Watts, and 150 Watts). Make sure the distance between the table and the bottom of the light bulb is the same for all three (6 inches). Record the number of leaf disks in each cup.
Start the stopwatch as soon as you turn on the light and record the number of leaf disks that float to the top at one minute intervals.
b Stop recording when all leaf disks float to the top, or until the cap of 25 minutes is reached.

6. Experiment
To test our hypothesis of the effect of light intensity on photosynthesis, three different light bulbs were used with 60, 100 and 150 Watts. The distance was measured from the bottom of the lightbulb to the surface of the table, and remained the same for each trial (6 inches).

7. 60 Watt Light Bulb Table Trial 1 Trial 2 Time # of Floating Leaf Disks1 2 3 4 8 5 12
Time (minutes)
# of Floating Leaf Disks 1 2 3 4 9 5 10 6 12

8. Graph

9. 100 Watt Light Bulb Table Time (min) 1 2 3 4 5 6 7 Time (min) 8 4 9 6
# of Floating Leaf Disks 1 2 3 4 5 6 7
Time (min)
# of Floating Leaf Disks 8 4 9 6 10 11 12 13 14 15
Time (min)
# of Floating Leaf Disks 16 10 17 18 19 20 21 22 23
Time (min)
# of Floating Leaf Disks 24 10 25

10. Graph

11. 150 Watt Light Bulb Table Time (min) 1 2 3 4 5 6 7 Time (min) 8 9 10
# of Floating Leaf Disks 1 2 3 4 5 6 7
Time (min)
# of Floating Leaf Disks 8 9 10 11 12 13 14
Time (min)
# of Floating Leaf Disks 15 16 1 17 18 19 3 20 21 4
Time (min)
# of Floating Leaf Disks 22 4 23 24 25 6

12. Graph

13. Analysis and Interpretation of Data
As seen by the data collected, the less intense the light bulb, the faster the process of photosynthesis occurred. This is subject to the type of plant used. Spinach plants grow best in cooler temperatures, but still with full to partial sun. The lower intensity light bulb created the perfect environment for photosynthesis to occur in spinach. When the 60 Watt light bulb was used as the light source, all the spinach leaf disks produced oxygen gas, which caused the leaves to float, in 6 minutes and less. The first spinach leaf disks with the 60 Watt began floating around 2 to 3 minutes into the experiment. In the experiment that used a light bulb with 100 Watts, not all of the leaf disks floated by the cap of 25 minutes, and they did not begin floating until 6 minutes. When a light bulb with 150 Watts was used, the leaves did not begin floating until 15 minutes and only 6 leaves were floating in total at the end of 25 minutes. The leaf disks float because they consume carbon dioxide from the bicarbonate solution and produce oxygen gas. This occurs when the spinach leaves are exposed to conditions in which photosynthesis occurs, therefore the effect on photosynthesis can be measured through this experiment. The bulbs with a higher wattage created a condition in which the spinach leaves were unable to perform photosynthesis to their potential. This can be seen by the larger gap before photosynthesis occurred in the leaves. Unusual patterns, such as the point of no activity beginning around 14 minutes in the experiment for the 100 Watt light bulb, could be caused by possible errors like damaged leaves. The environment created by the 60 Watt light bulb could be interpreted as the best condition for spinach leaves because of the rapid photosynthesis that occurred. The lower intensity gave off a lower temperature, but still created full sun, which is the best environment for spinach plants. Higher intensity light bulbs had a negative effect on the spinach leaves and slowed down their process of photosynthesis.

14. Possible Errors
In the experiment, there were many possible errors that could have occurred. Such errors may have affected the data collected. For example, as part of the procedure, one must add a pinch of baking powder and a drop of liquid soap to create the solution necessary for the experiment to take place. Because these are eye measurements and can be influenced by other factors, for instance the size of the pinch, different variations of the substance can occur causing different results. Distorting the ratios of the bicarbonate solution affects how fast the leaf disks consume carbon dioxide, produce oxygen, and begin to float. Furthermore, because there were two different sets of solutions made, if the ratios were off, the solutions would affect the different experiments and the timing of photosynthesis. If the same solution was used for all three (four including the second trial for the 60 Watt light bulb) trials, this would not need to be categorized under possible errors because all experiments would be effected in the same way. The time photosynthesis would take place would still be seen as faster or slower in some trials. Another possible error is that the heat of the light bulbs could have skewed the data slightly.

15. Conclusion
The original hypothesis for this experiment was that if the light bulb has a lower wattage, then photosynthesis will occur more rapidly in the spinach leaves because the fewer the wattage, the less heat is produced and spinach grows best in a cooler climate with partial to full sun. After finishing the experiment, one can see that the hypothesis was correct. The data collected shows how the experiment with the low-light bulb was the most successful and was completed in 6 minutes. With both the 100 Watt and 150 Watt light bulbs, the trial reached 25 minutes without having all 12 leaf disks float to the top. Because spinach is better adapted to cooler climates with still partial to full sun, the 60 Watt light bulb was best suited to speed up photosynthesis. In conclusion, light intensity does affect photosynthesis by either speeding up or slowing down the process, depending on the plant.