Photosynthesis Laboratory
PSI Biology
Name____________________________
Background
Green plants use sunlight to make glucose. To do so, the plant must use carbon dioxide
and water in a process called photosynthesis. Plants and animals use the glucose made
by plants as a source of energy. To release the energy contained in the bonds of
glucose, the glucose must be converted to ATP. The process by which ATP is made
from glucose is called cellular respiration. Respiration also produces waste products
including carbon dioxide and water, which are the same substances that served as raw
materials for photosynthesis. In water, carbon dioxide dissolves into water, it forms a
weak acid. As a result, an acid-base indicator such as bromothymol blue can be used to
indicate the presence of carbon dioxide. In this laboratory investigation, you will use
bromothymol blue to explore the relationship between photosynthesis and respiration.
Problem
Some plants, such as Elodea, are aquatic. How does photosynthesis affect the pH of
the aquatic environment?
Safety
Aprons, gloves and goggles.
Materials
6 50mL test tubes
3 15cm-long sprigs of Elodea
250mL graduated cylinder
Straw
Funnel
Bromothymol blue
Dropper
Light source
Distilled water
3 beakers
Stopwatch
Bromothymol blue indicator color chart
Procedure:
1. Mark the six test tubes –
A – Experiment & A - Control
B – Experiment & B - Control
C – Experiment & C - Control
2. Using a graduated cylinder, pour 200 mL of distilled water into a beaker.
3. Add 20 drops of bromothymol blue to your solution.
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4. Using the straw, blow into the solution. The CO2 in your breath will decrease the
pH of the solution. Keep blowing until the solution turns yellow.
5. Pour 30 mL of your solution into each of the six test tubes using the funnel.
6. Obtain three 15-cm-long sprigs of freshly cut Elodea. Place one sprig of Elodea
in each experiment labeled test tube.
7. Place test tubes A in darkness, test tubes B in room light, and test tubes C in
front of a light bulb.
8. After 10, 20 and then 30 minutes, use the color chart to determine the
approximate pH of the solution in each test tube. Record your results in the
DATA TABLE as pH after experiment.
DATA
pH Start
Exp
Control
10 Min
Exp
Control
20 Min
Exp
Control
30 Min
Exp
Control
Sample A
Darkness
Sample B
Room light
Sample C
Bright Light
Analysis:
1. Graph the pH change.
2. What is the relationship between change in pH and rate of photosynthesis?
3. According to your experiment, how is photosynthesis affected by light intensity?
4. Why did we use bromothymol blue in this experiment?
5. Explain the purpose of the control tubes.
6. What is photosynthesis and how do our results demonstrate the requirements
necessary for this process to occur?
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