Name _____________________________________
Date _________________________
Observing the Relationship between Photosynthesis and Respiration
Green plants are autotrophic. They use the solar energy to make food. The production of food also requires
raw material from the environment. When plants synthesize food, in the form of plant carbohydrates (glucose),
they use carbon dioxide and water. The process of making carbohydrates with the aid of solar energy 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 reaction of
photosynthesis must be reversed. The process in which energy is released from the chemical bonds of glucose is
called respiration. Since all living things need a constant flow of energy, all living things undergo Respiration
all of the time. Respiration also produces waste products, carbon dioxide and water, which are the same
substances that served as the reactants for photosynthesis.
In this experiment, you will examine the reactions of photosynthesis and respiration and also see how
photosynthesis and respiration are the reverse processes.
Materials: per group
2 125 mL flask
2 pieces of aluminum foil
Bromothymol blue
2 Elodea springs
Water
drinking straw
GOGGLES
Light source
Procedure:
1. Measure100mL of water into two flasks and put 10 drops of bromothymol blue into each of two flasks.
2. Insert one end of a drinking straw into the bromothymol blue water of one of the flasks. Covering up
the top of the flask, GENTLY blow through the straw.
Keep blowing until there is a change in appearance from blue to yellow. Record this information on
your data sheet. Repeat this procedure for the other flask.
3. Place a sprig of Elodea into each flask. Secure a piece of aluminum foil over the opening
4. Place one flask in the dark for 24 hours and the other an area with light.
5. After 24 hours, remove the flasks from their locations and make observations on the data table
Observations: Change in color of flask in light and in dark.
Treatment
Water and
Bromothymol Blue
After you breathed
into the flask
Elodea flask
(In the light)
Elodea flask
(In the dark)
Bromothymol Blue
(color)
Analysis:
1. What was the color of the bromothymol blue solution before you exhaled into it and then after you
exhaled into it?
Before ___________________________
After ______________________________
2. What gas was added to the bromothymol blue solution when you blew into it? __________
3. What process produced the above substance? ____________________________________________
4. What was the color of the bromothymol blue solution in the flask that was placed in the dark and the one
in the light after 24 hours?
Dark ____________________________
Light ______________________________
5. What gas was removed by the Elodea plant in the bromothymol blue solution in the light flask as it
underwent photosynthesis? _________________________
6. What gas was added to the bromothymol blue solution by the Elodea plant in the light?
__________________________________
7. What gas was removed from the flask by the Elodea plant in the dark from the bromothymol blue
solution? ____________________________
8. What gas was added to the flask by the Elodea plant in the dark to the bromothymol blue solution?
_______________________________
9. For the plant in the light, what carbohydrate molecule did it produce as a result of photosynthesis?
________________________________________________
10. For the plant in the dark, what food substance did it break down to release energy for it to stay alive?
___________________________________________________
11. In the space below, write the general formula for photosynthesis
___________ + ____________ + ________Energy
___________ + ____________
12. In the space below, write the general formula for respiration
____________ + ___________
___________ + ___________ + _______Energy
13. What do you notice about the two formulas above?
14. When does a plant undergo photosynthesis to make plant carbohydrates?
15. When does a plant undergo respiration to release the energy found in the bonds of plant carbohydrates?