Biology 1:
An investigation into photosynthesis and cellular respiration.
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):
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4 X 10 cm strands of elodea
8 large test tubes with tight fitting lids
2 X test tube racks
Bromothymol Blue solution
Carbon dioxide generator ( marble chips +
HCl)
 pH meter/ indicator strips
 Oxygen meter (shared)
Bromothymol Blue (BB) is an indicator which turns yellow in the presence of carbon dioxide.
Elodea is an aquatic plant.
Carbon dioxide
Hydrochloric acid
water
Calcium carbonate
2HCl
hydrochloric acid
+
Ca CO3
calcium carbonate
Calcium chloride
CaCl2
calcium chloride
+
H2O
water
+
CO2
carbon dioxide
Bromothymol Blue
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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
pH
5.5
More acidic
Less CO2
6.0
6.5
7.0
7.5
More basic
Experimental Design :
Light
Control 1
Control 2
Experimental 1
Experimental 2
H2 O
+
BTB
H2 O
+
BTB
+
CO2
H2 O
+
BTB
+
Elodea
H2 O
+
BTB
+
CO2
+
Elodea
Control 3
Control 4
Experimental 3
Experimental 4
H2 O
H2 O
+
BTB
+
CO2
H2 O
+
BTB
+
Elodea
H2 O
+
BTB
+
CO2
+
Elodea
Dark
+
BTB
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)
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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
Initial
colour
Final colour
Inferences
Initial pH
Final pH
Inferences
Control 1
Control 2
H2O
+
BTB
H2O
+
BTB
+
CO2
Blue
Yellow
Experimental Experimental
1
2
H2O
H2O
+
+
BTB
BTB
+
+
Elodea
CO2
+
Elodea
Blue
Yellow
Dark
Initial
colour
Final colour
Inference
Initial pH
Final pH
Inferences
Control 3
Control 4
H2O
+
BTB
H2O
+
BTB
+
CO2
Blue
Yellow
Experimental Experimental
3
4
H2O
H2O
+
+
BTB
BTB
+
+
Elodea
CO2
+
Elodea
Blue
Yellow
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:
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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
+
carbon dioxide
H2O
water
↔
H2CO3
carbonic acid
↔
H+
+
hydrogen ions
HCO3hydrogen carbonate ions
During the day:
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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.
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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: