Name: ___________________
Background:
All organisms are dependent on a healthy carbon dioxide-oxygen balance. Photosynthesis and cellular respiration are key processes in maintaining this balance. Plants, through the process of photosynthesis, use energy absorbed from sunlight, water, and carbon dioxide to produce carbohydrates and oxygen. Animals and plants, through the process of cellular respiration, use oxygen and sugars to produce carbon dioxide, water, and energy needed to maintain life.
Carbon dioxide dissolves in (reacts with) water, forming carbonic acid H
2
CO
3
. Carbonic acid then immediately dissociates into a hydrogen ion and a bicarbonate ion. The reaction occurring in solution is:
CO
2
(g) + H
2
O (l) H
2
CO
3
H + (aq) +HCO
3
- (aq)
The free hydrogen ions (H + ) lower the pH of the solution, making it more acidic. The degree to which the pH changes is proportional to the amount of CO
2
. That dissolves in the water. In other
is words, as more Co
2
dissolves in water, the pH of the solution will continue to decrease. If CO
2 removed from the solution, the pH will increase. A pH indicator such as BTB can therefore indicate the relative amount of CO
2
dissolved in water based on the color of the solution (When placed into a liquid substance, it will turn yellow in acid solutions and blue with base solutions)
Problem 1: Do plants respire, photosynthesize or both?
Materials: Complete on your own based on the procedure below.
Hypothesis: The Elodea plant will ______________ (photosynthesis/respiration) when left in the ____________ (light/dark ) and will _______________ (photosynthesis/respiration) when left in the ____________ (light/dark ).
Set up your experiment in the following manner:
You will use
4 test tubes in this set-up. Set-up the test tubes in the following way:
Rack 1 – place in the dark
!
Test tube 1: only Bromothymol Blue (BTB) solution
!
Test tube 2: 1 Elodea plant
Rack 2 – place in the light
!
Test tube 1: only Bromothymol Blue (BTB) solution
!
Test tube 2: 1 Elodea plant

Name: ___________________
1.
Dark: only
Bromothymol
Blue (BTB) solution
2.
Dark
1 Elodea plant
1.
Light
only
Bromothymol
Blue (BTB) solution
2.
Light
1 Elodea plant
Add stoppers
Place the test tubes in the appropriate environments indicated in your procedure.
Record the final results in the table above
Analysis:
1.
What happened when the elodea was kept in the dark? Did the color change? Why or why not?
2.
What happened when the elodea was left in the light? Did the color change? Why or why not?

Name: ___________________
Problem 2:
You are setting up a snail aquarium at home. Your aquarium kit contains a bag of snail eggs on an Elodea plant. The instructions say to pour the content, including the Elodea, into the aquarium, but you are not sure why you need the Elodea. Why might Elodea plants be helpful in maintaining a health ecosystem?
Purpose: To determine how carbon dioxide cycles through a biological system.
Procedure: Follow the steps to the lab given on the screen.
Hypothesis: If there is ______________ (Elodea/snail) then it will release ____________ (O
2
/
CO
2
) and the _______________ (Elodea/snail) will take it in.
Set up your experiment in the following manner:
Dependent variable: Amount of carbon dioxide present
Operation Definition : Color of the solution
Independent variable : number of snails
Control: only Bromothymol Blue (BTB) solution
You will use
4 test tubes in this set-up. Set-up the test tubes in the following way:
!
Test tube 1: only Bromothymol Blue (BTB) solution
!
Test tube 2: 0 snails, 1 Elodea plant
!
Test tube 3: 1 snail, 1 Elodea plant
!
Test tube 4: 2 snails, 1 Elodea plant
1.
only
Bromothymol
Blue (BTB) solution
2.
0 snails, 1
Elodea plant
3.
1 snail, 1
Elodea plant
4.
2 snails, 1
Elodea plant
Add stoppers
Place the test tube under the grow light

Name: ___________________
Analysis and Conclusion:
1.
What is the relationship between snails and Elodea
2.
Why did the color of the Bromothymol Blue (BTB) solution change?
3.
What was the importance of a control in your experiment?
What would you conclude if the color of the solution in the control changed?
4.
When you began the experiment, was there CO2 in the water?
In the test tubes that contained Elodea, where did the CO2 go?
5.
Which gas did the snails release?
What observations support this?
6.
Based on the results of your experiment, explain why you need to add the Elodea to your snail aquarium.