Instructor’s Copy
Lab Worksheet – Let The Sun In
Background: Green plants bask in sunlight. They use the energy in the sun’s rays to make food. The
production of food also requires raw materials. When plants synthesize food, more precisely
carbohydrates, they use carbon dioxide and water. The process of synthesizing carbohydrates with
the aid of the energy in light is known as photosynthesis. The carbohydrates plants make are used
by plants and animals alike as a source of energy. To release the energy contained in the bonds of
carbohydrate molecules, the chemical reactions of photosynthesis must be reversed. The process in
which energy is released from food is called respiration. Respiration also produces waste products,
carbon dioxide and water, which are the same substances that served as raw materials for
photosynthesis.
In water, carbon dioxide dissolves to form a weak acid. As a result, an acid-base indicator such as
bromthymol blue can be used to indicate the presence of carbon dioxide. In this laboratory
investigation, you will use bromthymol blue to explore the relationship between photosynthesis and
respiration.
Materials (per group) -
2 test tubes
2 rubber stoppers
10 ml graduated cylinder
bromthymol blue solution
2 Elodea
light source
drinking straw
Procedure:
A. Using a graduated cylinder, measure out 10 mL of bromthymol blue solution for each of the two
tubes. CAUTION: Bromthymol blue is a dye and can stain your hands and clothing.
B. Insert one end of a drinking straw into the bromthymol blue in one of the tubes. Gently blow
through the straw. Keep blowing until there is a change in the appearance of the bromthymol
blue solution. Repeat this procedure with the other tube. Record your observations in the data
table.
C. Place a sprig of Elodea into each tube. Stopper the tubes.
D. Place one tube in the dark for 24 hours. Place the other tube on a sunny windowsill for the same
amount of time. Artificial light may be used to supplement the sunlight.
E. After 24-48 hours, examine each tube. Note any change in the appearance of the bromthymol
blue solution. Record your observations in the data table.
Condition
Breath
Elodea (light)
Elodea (dark)
Data Table 1: Photosynthesis and Respiration
Color of Bromthymol Blue Solution
Initial
Final
Blue
Green to yellow
Blue
Blue
Blue
Green to yellow
Observation Questions:
1. What was the color of the bromthymol blue solution:
(a) before you exhaled into it? blue
(b) after you exhaled into it? Green to yellow
2. What was the color of the bromthymol blue solution in the tube that was placed in the dark for 24
hours? Green to yellow In the tube that was placed in the light for 24 hours? blue
Analysis and Conclusion:
1. What substance was released into the bromthymol blue solution when you exhaled into it?
Carbon dioxide In what process is this produced? Respiration
2. Explain why the color of the bromthymol blue solution changed after you exhaled into it.
The carbon dioxide formed a weak acid when mixed with the water and made the water
acidic so the color of the bromthymol changed.
3. What process was occurring in the tube containing the Elodea which was placed in the light?
Photosynthesis
What organelle is responsible for this process? Chloroplast
4. What raw materials were being used in this process? Carbon dioxide and water
5. What chemical compounds were being generated in this process? Glucose and oxygen
6. Why was the color of the solution in the tube in the dark not the same as that in the light?
The Elodea was not undergoing photosynthesis so it used respiration to release energy to
stay alive. The respiration generated carbon dioxide which turned the bromthymol blue to
green-yellow.
7. Why were the tubes stoppered? To show that nothing entered or left the tube.
8. What served as a control in this experiment? The plant that received normal light. Describe
at least one additional control for this experiment. May include same type of plant, same type
of tubes, same amount of time, same indicator.
(in the presence of light)
9. Write the chemical equation for photosynthesis. CO2 + H2O  C6H12O6 + O2
Write the chemical equation for respiration. C6H12O6 + O2  CO2 + H2O + energy
10. Explain, based on your observations and the chemical equations, how photosynthesis and
respiration are related. Photosynthesis is the reaction in one direction and respiration was
the reaction in the other direction.