Photosynthesis /Respiration Lab
Background concepts:
Photosynthesis and Respiration are opposite reactions. Photosynthesis makes glucose using
carbon dioxide, water, and sunlight. Oxygen is also given off as a waste product.
Respiration is the “burning” of glucose using oxygen to produce the main product – energy. The
waste products of respiration are carbon dioxide and water.
Plants can carry out both photosynthesis and respiration simultaneously. However, light and the
amount of CO2 available can alter these processes.
In this lab, you will try to demonstrate the net change in carbon dioxide when a plant is placed
under different conditions. You will be using bromthymol blue (BTB) which is a pH indicator, as
a means of determining the presence or absence of carbon dioxide. A solution of bromthymol
blue changes color when carbon dioxide is introduced due to a change in pH. When carbon
dioxide is dissolved in water, it forms carbonic acid, which lowers the pH of the solution and
causes the bromthymol blue to change its color. Adding carbon dioxide to bromthymol blue
changes the solution to yellow. When the carbon dioxide is removed, the solution turns back to
blue.
Pre-Lab
1.
2.
3.
4.
5.
What is the balanced equation for photosynthesis?
What is the balanced equation for respiration?
During what process above is carbon dioxide a reactant?
During what process above is carbon dioxide a product?
Under what conditions does bromthymol blue turn yellow – adding carbon dioxide to the
solution or taking it away?
6. When do plants perform photosynthesis?
7. When do plants perform respiration?
LAB
Purpose: Demonstrate that environmental carbon dioxide is used during photosynthesis.
Demonstrate that there is a net production of carbon dioxide when a plant respires in the absence
of photosynthesis.
Question: Write a question to be answered relating to the purpose of the lab.
Hypothesis: If……Then…….Because……..(write one!)
Procedure:
Day 1
1. Gather 4 test tubes and mark with tape and a marking pen with your initials and period.
2. Pour 50 mL of the BTB solution into an Erlenmeyer flask.
3. Using a straw, place the tip of the straw above the liquid and blow air into the flask until
the BTB turns yellow. This will assure every sample that needs carbon dioxide will get
the same amount.
4. Pour 15 mL yellow BTB solution (using the graduated cylinders) into tubes labeled A
and B.
5. Pour 15 mL blue BTB solution (using a rinsed graduated cylinder) into tubes labeled C
and D.
6. Put equal sized sprigs of plant (as your teacher shows you) in tubes B and D.
7. Cover all tubes with Para film
8. Tubes A and B will go in a rack in the Greenhouse window.
9. Tubes C and D will go in a rack and be covered with a towel.
10. Record your observations on color for day 1 in table below.
Day 2
11. Remove your 4 tubes and bring back to your station.
12. Record your observations in a data chart like the one below:
Tube
A
Day 1 Color
Day 2 Color
B
C
D
Results/Summation of data:
1.
2.
3.
4.
5.
6.
What was the purpose of tube A?
What was the purpose of tube C?
What was consumed in tube B?
What process happened overnight in tube B?
What was produced in tube D?
What process happened overnight into tube D?
Conclusion: Write a conclusion paragraph. Did your data support your hypothesis? What
happened in each of your tubes, AND WHY!?