WLHS / Biology / Monson / UNIT 9 – Energetics
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LAB: Observing the Relationship between Photosynthesis & Respiration
Pre-Lab Discussion / Background info:
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 (like glucose!), 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 produce are used by
plants and animals alike as a source of energy to make ATP in their cells. 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 cellular respiration.
Respiration also produces the 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.
PRELAB QUESTIONS:
1) Write the balanced chemical equations for photosynthesis and cellular respiration below.
PHOTOSYNTHESIS:

CELLULAR RESP.:

2) Which gas is a product of photosynthesis, and a reactant for respiration?
3) Which gas is a reactant for photosynthesis, and a product of respiration?
4) Which sugar is a product of photosynthesis, and a reactant for respiration?
5) Your instructor will demonstrate exhaling through a straw into the bromthymol blue solution. Describe
what happens (and why!).
6) In which organelle does photosynthesis occur?
7) In which organelle does respiration occur?
8) Of the above organelles (in #6 and #7), which do plant cells have?
9) So, which process(es) do plant cells perform?
Problem / Research Question:
What is the relationship between the processes of photosynthesis and respiration?
Materials:
2 125mL flasks
2 #5 rubber stoppers
100 mL graduated cylinder
Bromthymol blue solution
2 Elodea
light source
drinking straw
Procedure:
1) Using a graduated cylinder, measure out 100 mL bromthymol blue solution for each of the two flasks.
CAUTION: Bromthymol blue is a dye and can stain your hands and clothing.
2) Insert one end of a drinking straw into the bromthymol blue in one of the flasks. 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 flask. Record your observation in the data table.
3) Place a sprig of Elodea into each flask. Stopper the flasks. Label each flask with your initials, class
period, and whether it will be left in the “DARK” or in the “LIGHT.”
4) Place one flask in the dark for 24 hours. Place the other flask on a sunny windowsill for the same
amount of time. Artificial light may be used to supplement the sunlight.
5) After 24 hours, examine each flask. Note any change in the appearance / color of the bromthymol blue
solution. Record your observations in the data table below. Complete the two “PREDICT” questions on
day 1!
DATA TABLE:
Bromthymol blue
solution in the:
DAY 1 – before
exhaling into it:
DAY 1 – after
exhaling into it:
DAY 2 – after sitting
24 hours:
FLASK left in the dark:
FLASK left in the light:
PREDICT:
1) What do you predict will be the color of the solution (after 24 hours) in the:
A) flask left in the dark?
B) flask left in the light?
2) EXPLAIN your predictions from above! WHY?? (HINT: which cellular process(es) will be happening
inside the plant cells in each flask?)
Analysis and Conclusions
1) What substance was released into the bromthymol blue solution when you exhaled into it? How is this
substance produced (in your body / cells)?
2) Explain why the color of the bromthymol blue solution changed after you exhaled into it.
3) Why was Elodea placed in both flasks?
4) Which flask is the control? Describe additional controls that you might use for this experiment.
5) Why are the results for the two flasks different?
6) How are photosynthesis and respiration related?
Critical Thinking and Application
1) Carbon dioxide was bubbled through 2 flasks of bromthymol blue until they became acidic. Then a
sprig of Elodea was placed in each flask. One flask was left in green light for 24 hours, the other flask
was left in red light. No change occurred in the flask left in green light. The bromthymol blue in the flask
that was left in the red light turned back to blue. Explain these results.
2) A sprig of Elodea was placed into each of the two flasks containing bromthymol blue. One flask was
left in the dark, the other was left in light for 24 hours. The bromthymol blue flask that was left in the
dark turned yellow. The other remained blue. Explain these results.
3) How did this lab activity demonstrate that Elodea carries out photosynthesis at a faster rate than it
carries out respiration? Explain using your results!
4) What would happen if plants carried out photosynthesis at the same rate as cellular respiration?
A) In this lab:
B) In general (i.e. in nature, what would be the implications of this):
5) Based on your understanding of the chemistry of photosynthesis, why do plants need animals in order
to survive? (HINT: what do animals produce that plants use? what process produces this?)
6) Why do animals need plants in order to survive? (there are two reasons!)