Aim: to investigate the uptake and production of carbon dioxide and oxygen in an aquatic plant.
One or more aspects of the following questions will be investigated.
1. Does a green plant exposed to sunlight use carbon dioxide?
2. Is light necessary for a green plant to carry out photosynthesis?
3. Is carbon dioxide taken in, given off or involved in any way when a green plant is not photosynthesising?
Materials& Apparatus (per group):
Ø 4 X 10 cm strands of elodea
Ø 8 large test tubes with tight fitting lids
Ø 2 X test tube racks
Ø Bromothymol Blue solution
Ø Carbon dioxide generator ( marble chips + HCl)
Ø pH meter/ indicator strips
Ø Oxygen meter (shared)
Bromothymol Blue (BB) is an indicator which turns yellow in the presence of carbon dioxide.
Elodea is an aquatic plant.
Bromothymol Blue
• Bromothymol blue is an indicator that turns yellow-green in the presence of acidic substances.
• When carbon dioxide is breathed into water it reacts with the water to form carbonic acid.
CO2 + H2O H2CO3
• Carbonic acid itself is in equilibrium with water and carbon dioxide.
• As more and more carbonic acid is produced, the pH of solution decreases causing the indicator to change color to yellow-green.
• The color change of the indicator thus serves to demonstrate the presence of carbon dioxide by showing the change in pH.
Bromthymol blue (BTB) as an indicator of Carbon Dioxide (CO2)
The amount of CO2 in a solution changes the pH.
An increase in CO2 makes a solution more acidic (the pH gets lower).
A decrease in CO2 makes a solution more basic (the pH gets higher).
This BTB can indirectly measure the amount of CO2 in a solution.
More CO2 Less CO2
pH 5.5 6.0 6.5 7.0 7.5
More acidic More basic
Experimental Design :
Control 1 / Control 2 / Experimental 1 / Experimental 2Light
/ H2O+
BTB
/ H2O+
BTB
+
CO2 / H2O
+
BTB
+
Elodea / H2O
+
BTB
+
CO2
+
Elodea
Control 3 / Control 4 / Experimental 3 / Experimental 4
Dark
/ H2O+
BTB
/ H2O+
BTB
+
CO2 / H2O
+
BTB
+
Elodea / H2O
+
BTB
+
CO2
+
Elodea
Hypotheses and predictions:
H1 :
When elodea is placed in light it will remove carbon dioxide and add oxygen to the water. Experimental tube 1 will: remain blue as any carbon dioxide present is being removed.
Experimental tube 2 will: change to blue as the carbon dioxide present is removed.
Control tube 1 will: remain blue
Control tube 2 will: remain yellow
H2:
When elodea is placed in the dark it will remove oxygen and add carbon dioxide to the water.
Experimental tube 3 will: turn yellow as the elodea releases carbon dioxide from respiration
Experimental tube 4 will: remain yellow as elodea is not removing carbon dioxide for photosysnthesis
Control tube 3 will: remain blue
Control tube 4 will: remain yellow
Procedure : (rewrite in past tense)
• Set up the tubes according to the experimental design.
• About 10 cm of elodea should be placed in the appropriate tubes.
• Each tube should be filled to within a few mm of the rim and sealed with an airtight cap.
• Measure the pH of the solution in each tube using test strips or pH meters
• Record your observations in the results table immediately and again after 24 hrs.
o Note: colour changes
o Emanation of bubbles
o pH changes
Results:
Light
/ H2O+
BTB
/ H2O+
BTB
+
CO2 / H2O
+
BTB
+
Elodea / H2O
+
BTB
+
CO2
+
Elodea
Initial colour
/ Blue / Yellow / Blue / YellowFinal colour
Inferences
Initial pH
Final pH
Inferences
Control 3 / Control 4 / Experimental 3 / Experimental 4
Dark
/ H2O+
BTB
/ H2O+
BTB
+
CO2 / H2O
+
BTB
+
Elodea / H2O
+
BTB
+
CO2
+
Elodea
Initial colour / Blue / Yellow / Blue / Yellow
Final colour
Inference
Initial pH
Final pH
Inferences
Photosythesis / Respiration
Research photosynthesis and respiration. Rewrite and incorporate the questions into your answers.
1. Write a one paragraph description of photosynthesis.
2. Write a one paragraph description of respiration.
Discussion:
3. Describe why the control tubes were included.
4. Explain how experimental variables were controlled.
5. Could the indicator have changed colour due to light alone? Describe any evidence for this.
6. Was atmospheric carbon dioxide or oxygen involved in your experiment in any way? Discuss.
7. What conclusions can you make from your results.
Ponds and lakes often experience large daily fluctuations in dissolved oxygen, carbon dioxide, pH, and water temperature.
At night:
• dissolved oxygen is consumed by both plant and animal respiration,
• carbon dioxide is produced by plant and animal respiration
Carbon dioxide reacts with water and causes the water to become more acidic ie lowers pH:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
carbon dioxide water carbonic acid hydrogen ions hydrogen carbonate ions
During the day:
• animal respiration consumes oxygen and produces carbon dioxide,
• plant respiration consumes oxygen and produces carbon dioxide
• photosynthesis by plants consumes carbon dioxide and produces oxygen.
• The net effect may be a drop in CO2, an increase in pH, and an increase in O2.
Complicating the relationship described above are the effects of temperature.
• During the day, water temperatures tend to climb, especially in shallow lakes and ponds.
• Both oxygen and carbon dioxide are less soluble in warm water than in cool water.
• A 10°C increase in temperature, for example, reduces the solubility of oxygen by about 20%.
8. Write a paragraph describing why :
a. Oxygen builds up during the day
b. Carbon dioxide builds up at night: