Background: Elodea is a genus of photosynthetic aquatic (water) plants often called the water weeds. Elodea is native to North America and is widely used as aquarium vegetation. It can be used, along with bromothymol blue, for science experiments in classrooms to demonstrate how plants use carbon dioxide, a waste product from aerobic cellular respiration, as a raw material
(reactant) for photosynthesis. Photosynthesis is a set of chemical reactions (aided by enzymes) that trap energy from the sun and using inorganic reactants (CO
2
and H
2
O) and store that energy in an organic molecule called sugar (C
6
H
12
O
6
). Only organisms with special pigments and organelles can perform these reactions. Green plants can perform photosynthesis because they have both the chlorophyll (a pigment) and chloroplasts (a specialized organelle). But once the sun’s energy is stored in the bonds of the sugar the plant must undergo cellular respiration, just like every other organism, to move that energy to ATP (Adenosine Triphosphate – the cell’s battery)
Photosynthetic plants use the products of photosynthesis (sugar and oxygen) as the reactants of aerobic cellular respiration. And, they use the waste products of aerobic cellular respiration
(carbon dioxide and water) as the reactants of photosynthesis. So as long as there is sunlight available (energy), photosynthetic plants can recycle the elements carbon, hydrogen, and oxygen that make up CO
2
, H cellular processes.
2
O, C
6
H
12
O
6
, and O
2
and provide itself with the energy it requires for its
Photosynthesis Cellular Respiration
Life Functions
1.
Transport
2.
Synthesis & Assimilation
3.
Growth
4.
Regulation
5.
Excretion
6.
Reproduction
7.
Respiration
8.
Nutrition
Write the chemical equation for photosynthesis
________________________________________________________________________
Write the chemical equation for aerobic cellular respiration.
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Through the following series of questions, you will determine how you can design an experiment to see under what conditions the waste products of cellular respiration are used as
reactants for photosynthesis in an Elodea plant and under what conditions they are not.
Remember: a.
You need 2 groups: i.
Control group = set-up with “normal” conditions ii.
Experimental group = set-up with ONE change (1 condition that is different from the control group). b.
Bromothymol blue is a CO
2
indicator. An Elodea plant is a green, water plant. CO
2
is released through the stomates in the leaves of the plant as a result of aerobic cellular respiration.
Materials: These are the materials you will have to work with for your experimental design. (Notice, you have 2 of each material, 1 for the control group and 1 for the experimental group)
 2 - beakers
 2 – 2 cm pieces of Elodea plant (you have to measure and cut)
 2 x 100 mL of Water
 2 x 1 mL Bromothymol blue (CO
2
indicator)
1.
What process (reaction) does a photosynthetic plant, including Elodea, perform only when there is sunlight? ___________________________________________________________
2.
Plant cells need energy for their cellular processes all the time, so what process (reaction) does Elodea (and every other living organism’s cells) perform ALL the time?
_________________________________________________________________________
3.
Rather than releasing them what 2 waste products from aerobic cellular respiration can an
Elodea reuse as reactants of photosynthesis? ______________________________________
4.
In addition to carbon dioxide (CO
2
) and water (H
2
O) what else does Elodea need to perform photosynthesis? ____________________________________________________________
5.
In order to stop photosynthesis from happening in an Elodea plant you must remove one of the
3 reactants. Since it lives in water, it would be difficult to remove that reactant from its environment. And since carbon dioxide (CO
2
) is dissolved in the water and constantly being produced in its cells during cellular respiration, it would be difficult to remove that reactant from its environment. Therefore, in order to stop photosynthesis in an elodea plant what reactant can you easily remove? __________________________________________________________
6.
How can you use bromothymol blue to see under what conditions the waste product of aerobic cellular respiration, CO
2
, is released from an Elodea plant rather than reused for photosynthesis?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________

7.
Hypothesis (in an “If… then…because” statement):
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
8.
IV (the condition that is given to the experimental group that is not given to the control group: ___________________________________________________________________
9.
DV (the results or the data you are collecting from both groups:
___________________________________________________________________
10.
Control Group conditions: ___________________________________________________
11.
Experimental Group conditions: ______________________________________________
12.
Constants (at least 5):
_________________________________________________________________________
_________________________________________________________________________
13.
Procedure (list your procedure in detailed steps numbered down the page):
1. __________________________________________________________________
2. __________________________________________________________________
3. __________________________________________________________________
4. __________________________________________________________________
5. __________________________________________________________________
6. __________________________________________________________________
7. __________________________________________________________________
8. __________________________________________________________________
9. __________________________________________________________________
10. _________________________________________________________________
11. _________________________________________________________________
12. _________________________________________________________________
13. _________________________________________________________________
14. _________________________________________________________________
15. _________________________________________________________________
16. _________________________________________________________________
17. _________________________________________________________________
18. _________________________________________________________________
19. _________________________________________________________________
20. _________________________________________________________________
21. _________________________________________________________________
22. _________________________________________________________________
23. _________________________________________________________________
24. _________________________________________________________________

14.
Data Table: Make a data table on a separate piece of paper to help organize the information you will be collecting. Your data table should have 2 columns. The headings on your data table should include your independent variable (condition you are changing), your dependent variable
(what you are measuring or looking at in your experiment), etc.
Analysis & Conclusion: (Answer in complete sentences where you restate your question in the
answer)
1.
(6 pts.) Cellular respiration was happening all the time in both groups of plants.
However, you could see the direct of effects of that process in only one group. a.
In which group of plants could you see that cellular respiration was taking place?
____________________________________________________________
____________________________________________________________ b.
What happened in one beaker that indicated cell respiration was taking place?
____________________________________________________________
____________________________________________________________ c.
Why did that change happen (what reacted with what)?
____________________________________________________________
___________________________________________________________ d.
In which group of plants could you NOT see that cellular respiration was taking place (even though it still was)?
____________________________________________________________
____________________________________________________________ e.
What did you NOT see happen?
____________________________________________________________
____________________________________________________________ f.
Why did that change NOT happen?
____________________________________________________________
____________________________________________________________
2.
(1 pt.) When does cellular respiration take place in the cells of plants?
_______________________________________________________________
3.
(1 pt.) When does cellular respiration take place in cells of animals?
_______________________________________________________________
4.
(1 pt.) When does cellular respiration take place in the cells of all living organisms?
_______________________________________________________________
5.
(2 pts.) Why is it necessary for all living things to undergo cellular respiration all the time?
_______________________________________________________________
_______________________________________________________________
6.
(1 pt.) The products of what process performed by photosynthetic plants supply the reactants for aerobic cellular respiration to living things?
_______________________________________________________________
_______________________________________________________________
7.
(1 pt.) The waste products of what process supplies the reactants for photosynthesis to photosynthetic plants?
_______________________________________________________________
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