12/10/10
Elodea Photosynthesis Lab (BSCS Investigation 4)
Partners: Trevor Hopper
Problem: How does the color of light affect the rate of photosynthesis?
Background Research: The problem or question being answered in this experiment is: how does the color of light change the rate of photosynthesis in an elodea plant. The color of light may affect the rate of photosynthesis because different light waves have different amounts of energy.
Elodea is a photoautotroph that lives in a watery environment. Since Elodea performs photosynthesis in water it is placed in a test tube that gets filled with water. Carbon Dioxide is needed for photosynthesis so carbonated water is added to the water. Bromothymol blue is added to the solution because in this experiment, photosynthesis is detected by changes in ph and bromothymol blue is able to show changes in ph. The CO
2
and H
2
O combine to make carbonic acid (H
2
CO
3
) which makes the environment more acidic. This is because the carbonic acid releases H
+
ions into the environment. If photosynthesis is performed rapidly then there will be less carbon dioxide in the water so there will be less carbonic acid. This will make the solution more basic since there are less H
+
ions. This will turn the solution blue. If little or no photosynthesis is performed then there will be more carbonic acid because less carbon dioxide is being used for photosynthesis. This will make there more H
+
ions in the solution so it will be more acidic. This makes the bromothymol blue turn yellow.
Independent Variable: Type of light plant receives
Dependent Variable: Amount of photosynthesis performed
Hypothesis: If a plant is only subjected to black light then it will perform very little photosynthesis, however if a plant is subjected to only green light then it will perform a small amount of photosynthesis. I think this because although plants cannot absorb very much energy to perform photosynthesis from green light, they get more energy from green light than from black light, which has no energy. Accessory pigments can absorb some green light so that will also help with photosynthesis. In the environment of the Elodea subjected to black light the solution will be a greenish yellow and be a little acidic because little photosynthesis is being performed since there is not enough light to perform photosynthesis. This will make it so that there are more H + ions, which makes it acidic. The bromothymol blue of the plant with the green cellophane around it will probably be slightly more green than the dark light plant because some of the light energy will help with photosynthesis. This will make less H
+
ions so the solution will be slightly more basic, but it will still be a little acidic. If a plant is subjected to blue light then it will perform photosynthesis, however if a plant is subjected to white light then it will

perform the most photosynthesis. I think this because even though the blue light is a very high energy light, after a certain point the plant will not be able to receive any more of the same color of light so the white light will help to perform photosynthesis because it is composed of all light colors. The color of the bromothymol blue of the solution of the elodea exposed only to blue light will be blue because a fair amount of photosynthesis is being produced because blue light is a high energy light and lots of energy is being absorbed by the plant. More carbon dioxide will be used so there will be less carbonic acid. This means there will be a very small amount of H
+ ions in the water making it basic. For the Elodea exposed to white light the color of the solution will be a deeper blue because with the high energy white light it will be performing photosynthesis at optimal levels. There will be a very small amount of CO
2
in the solution and a very small amount of H
+
ions.
Control Groups:
Positive Control Group: Test tube A (Elodea exposed to white light)
Negative Control Group: Test tube B (Elodea exposed to black light)
Experimental groups:
Experimental Group 1: Test tube C (Elodea exposed to blue light)
Experimental Group 2: Test tube D (Elodea exposed to green light)
Constant Variables:
Constant Variable 1: Amount of water in the test tubes (10 mL per test tube)
Constant Variable 2: Amount of Polar Seltzer Carbonated water in each test tube ( 1 mL of soda per test tube)
Constant Variable 3: Time available to perform photosynthesis (3 days)
Constant Variable 4: Length of Elodea Plant (3 cm)
Materials:

2 Desk Lamps

4 test tubes

40 mL of water (10 mL per tube)


1 mL pipettes
4 3cm pieces of elodea




2 test tube racks
1 bottle of Polar Seltzer carbonated water
1 piece of blue cellophane
(enough to measure around test tube C)

1 piece of green cellophane
(enough to measure around test tube D)

1 grease pencil

12 mL of Bromothymol blue solution

2 30mm (12 in) rulers






4 graduated cylinders
Clear tape
Source of water
1 pair of scissors
1 window
Area with very little light
Lab safety precautions:
Safety goggles
Lab apron (optional)
Using graduated cylinder and water source
Procedure:
1.
Put on safety equipment
2.
Gather materials
3.
Using grease pencil label the four test tubes A, B, C, and D
4.
Using graduated cylinder and water source fill up graduated cylinder with 10 ml of water
5.
Pour 10 mL of water into test tube A
6.
Repeat steps 3 and 4 for the other 3 test tubes
7.
Fill graduated cylinder with 1 mL of Polar seltzer carbonated water
8.
Pour into test tube A
9.
Repeat steps 6 and 7 for the other 3 test tubes
10.
Fill plastic pipette with 1 mL of bromothymol blue
11.
Empty bromothymol blue into test tube A
12.
Fill plastic pipette with 1 mL of bromothymol blue twice more and empty each time into test tube A
13.
Repeat steps 9, 10, and 11 for the other 3 test tubes (each test tube should have a total of
3 ml of bromothymol blue in them)
14.
Using a ruler measure 4 pieces of elodea each at 3 cm each
15.
Use scissors to cut each piece of elodea at 3 cm length
16.
Place one piece of elodea in each test tube
17.
Wrap test tube C with 1 layer of blue cellophane
18.
Tape it down with clear tape
19.
Wrap test tube D with 1 layer of green cellophane
20.
Tape it down with clear tape
21.
Place test tube A, C, and D in a test tube rack
22.
Place test tube rack with test tube A, C, and D 60 cm away from a window (use rulers)
23.
Place a desk lamp 30.5 cm away from the test tube rack to its left (use rulers)
24.
Place a desk lamp 27.5 cm away from the test tube rack to its right (use ruler)
25.
Turn off all other lights in the room

26.
Put the blinds on the window down
27.
Place test B in the second test tube rack
28.
Place the test tube rack containing test tube B in a dark area with very little light
29.
Leave all 4 test tubes for 3 days
30.
Record the colors of each of the bromothymol blue solutions in the test tubes on an organized data table
31.
Analyze the data
32.
Draw conclusions
Raw Data: Figure 2: Effect of Color of light on color of bromothymol blue with elodea plant inside test tube performing photosynthesis
Test tube
A
B
C
D
Color of light
White
Black
Blue
Green
Color of bromothymol blue
Light blue
Light yellow green
Lighter blue
Light green
Final Data: Figure 3: Class average of the effect of color of light on the color of bromothymol blue with elodea plant inside test tube performing photosynthesis
Test tube
A
B
Color of light
White
Black
Color of bromothymol blue
Deep blue
Pale greenish yellow

C
D
Blue
Green
Light blue
Light green
Conclusion:
The hypothesis was well supported because it was hypothesized that white light would perform the most photosynthesis followed by blue light, then green light, then black light. The white light solution was a deep blue color because it was the most basic. The blue light solution was a light blue color because it was the next basic. The green light solution was slightly acidic so it was a light green color. The black light solution was a greenish yellow, and it was the most acidic solution. The data suggests that in Elodea plants it has some accessory pigments, like carotenoids. This is suggested because the elodea exposed to only green light performed a decent amount of photosynthesis, even though normally plants can’t absorb that much green light. This means there must be carotenoids helping the plant to absorb green light. The data also suggests that after a certain point the pigments can’t keep absorbing the same color light (in this case blue), so it is more efficient to give plants white light which has all lights, and in this experiment the plant exposed to white light performed the most photosynthesis. The data is not valid. It has not been repeated 3 times, each time coming up with the same results. Also, this experiment was not performed by another scientist. The variables were kept constant throughout the experiment, and also the results do answer the problem being asked. To make the experiment more valid I would have to repeat the experiment twice more and get the same results, and give someone the exact same lab procedure and have them get the same results.

Works Cited:
BSCS Biology a Molecular Approach. Columbus: Glencoe / McGraw - Hill Companies,
2006. Print.
Miss Ziegler, Biology Teacher at Ridgefield High School