Uploaded by cristabusche

Observing Photosynthesis & Respiration Lab

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
In this experiment, you will examine the reactions of photosynthesis and respiration and also see how
how photosynthesis and respiration are the reverse processes.
Materials: per group
2 125 mL flask
2 pieces of aluminum foil
100-mL graduated cylinder
bromothymol blue
2 Elodea springs
Light source
drinking straw
1. Using a graduated cylinder, measure out 100mL of water 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. GENTLY blow
through the straw. What substance were you adding to the bromothymol blue solutions as you blew into
it? ________________________. 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
Bromothymol Blue
After you
breathed into
the flask
(In the light)
(In the dark)
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 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 ______________________________
3. What substance was added to the bromothymol blue solution when you blew into it? __________
4. What process produced the above substance? ____________________________________________
5. What substance did the Elodea plant remove from the bromothymol blue solution in the light flask as it
underwent photosynthesis? __________________________________________________
6. What substance did the Elodea leaf in the flask in the light add to the bromothymol blue solution?
7. What substance did the Elodea plant remove from the flask in the dark from the bromothymol blue
solution? ____________________________________________________________
8. What did the Elodea plant add to the flask 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 formula for photosynthesis
___________ + ____________ + _____________
____________ + ____________
12. In the space below, write the formula for respiration
____________ +
+ ___________
___________ + ___________ + _______
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?
16. Which process, photosynthesis or respiration, must occur at a faster rate and why?