Name: ______________________________________________________
Period: ____
Photosynthesis Lab - Elodea and Bromthymol Blue
Photosynthesis occurs in the chloroplast, an organelle in plant cells that contains the
molecule chlorophyll. Chlorophyll absorbs the energy of sunlight. That light energy is
converted to chemical energy through the steps of photosynthesis.
The reactions of photosynthesis can be divided into two major types: light-dependent
reactions and light-independent reactions. The light-dependent reactions convert energy
from the sun into a form that the chloroplast can then use to make sugar from carbon
dioxide; in the process producing oxygen as a waste product. The light-independent
reactions use that energy to make glucose from carbon dioxide.
In this lab, you will use bromothymol blue as an indicator to show how much CO 2 is left
in test tubes containing plants and exposed to lights. A small piece of elodea will be
introduced to a solution containing bromothymol blue and CO2. The point is to determine
whether the elodea will have an effect on CO2 levels.
Write out the equation for photosynthesis:
Write out the equation for cellular respiration:
Hypothesis: Write a hypothesis about what will happen to CO2 levels in a solution if an
aquatic plant is photosynthesizing.
If _____________________________________________________, then _____________________________
__________________________________________________________________________________________
Material Preparation:
If concentrated bromthymol blue (BTB liquid) is available, dilute with water (distilled works
best) and test the concentration by adding 10 ml of your BTB solution to 15 ml water and
bubbling one lung full of air through a straw into the water. It should turn greenish. (If it stays
blue, the BTB is too concentrated; if it turns yellow, the BTB is too diluted.) Adjust your solution
as necessary and place in dropper bottles for lab teams to use.
Photosynthesis Lab - Elodea and Bromthymol Blue
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Procedures:
1. Combine 15 ml of water (distilled works best) with 10 ml of BTB solution and GENTLY bubble
one lungful of air through the straw. The liquid should turn a greenish color.
2. Label four test tubes: "Control," "1.25 cm Elodea" and "4 cm Elodea." And "4 cm dark"
The test tube marked "control" will be sealed with masking tape and no plant will be added.
3. The test tube marked "Control" is sealed with masking tape. To the other test tubes, add
the designated length of elodea, gently pushing each down into the solution with the straw.
4. Wrap the "4 cm dark" test tube in aluminum foil, so that no light gets in.
5. Place test tubes in rack and take the rack outside and set it in sunlight.
6. View the colors of the test tubes after a half hour (or even overnight). You should notice
differences.
Results: Record your results in the table below.
Sample
Control
1.25 cm
Elodea
4 cm Elodea
4 cm dark
Color
Conclusions: Write 2 to 3 sentences based on your results. __________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
Questions: Answer on a separate page in complete sentences and staple paper to this lab.
1. Did the plants alter the level or CO2 in the test tubes? What is the relationship between the
size of the plant and the difference in C02 levels?
2. Why did you initially blow into the solution to turn in green?
3. What is the purpose of the control?
4. Compare the 4cm dark to the 4cm in the light test tube. How do you account for any
differences in color?
5. In the tubes in which photosynthesis can occur, what gas would eventually replace all of
the carbon dioxide in the solution?
6. Can the plants placed in the “dark” test tube perform light independent reactions if it has
never been exposed to light? What if it was exposed to light for an hour before being
wrapped in aluminum foil?
Photosynthesis Lab - Elodea and Bromthymol Blue
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