Name:____________________________________________ Date:_________________ Block:______
Do Plants Consume or Release CO2?
1. Background
In this lab, you will use phenol red as an indicator to show whether CO2 is
being consumed or produced in a reaction within plants.
It is well known that in the presence of light plants perform photosynthesis.
It is less well understood that at the same time plants are also performing
cell respiration. To demonstrate this we will determine whether CO2 is
consumed or produced as elodea (anacharis) is placed in either a light or
dark environment. The change in CO2 will be detected by the pH indicator
phenol red.
Think about the chemical equations for respiration and photosynthesis.
۞ Which one releases CO2 causing an increase and which one uses up CO2which will cause a decrease?
Phenol red is yellow under acidic conditions (high H ion concentration), pink under basic or alkaline
conditions (low H ion concentration) and orange under neutral conditions. A change in CO2 will cause a
directly proportional change in H ion.
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If the CO2 concentration decreases, the H ion concentration will also decrease and the solution
will change to pink, becoming basic.
If the CO2 concentration increases, the H ion concentration will also increase and the solution
will change to yellow, becoming acidic.
Neutral solutions of phenol red will be orange.
2. Preparation
A. Create a solution of phenol red by adding two dropper-fulls of
phenol red to about 80 ml of water into a 150 mL beaker.
B. The phenol red may change color as a result of adding water
(depending on how acidic or basic your tap water is). You must get it
to a neutral state (orange) so that you can observe a change in pH in
either direction (more acidic or more basic). To accomplish this, use
a straw and create bubbles in the beaker until it changes from pinkish
red to orange. CAUTION: don’t blow too hard or you will end up
with phenol red on your face! Also, be careful not to blow too much.
You want orange, NOT yellow!
C. Once you have the solution prepared and mixed, transfer it to 4
test tubes. Put the same amount of fluid in all four tubes.
D. Place a cut piece of elodea (2 inches long, cut end up) into two of the four tubes. The other two test
tubes will not have elodea and will serve as controls. One set will be placed in the light, and one set in
the dark (see data table).
E. Once all materials have been added to the tubes, all test tubes should be stoppered or covered with
parafilm to minimize reactions with the air.
F. Lastly, aluminum foil can be used to cover the tubes that will be in the “dark”.
3. Predictions
Write your predictions about what will happen to the color of the water in the tubes.
Write the initial color on the line below. On the other side of the vertical line, write your prediction of
what color the solution will be at the end of the experiment. Then, in the lines provided below, explain
your predictions! Why do you think the color will be what you predicted? Back your prediction up with
logic!
Tube
Initial color Prediction (color)
1. phenol red solution, light, NO ELODEA
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2. phenol red solution, no light, NO ELODEA
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3. phenol red solution, light, has elodea
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4. phenol red solution, no light, has elodea
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Explain your predictions:
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Tube 2:_____________________________________________________________________________
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Tube 3:_____________________________________________________________________________
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Tube 4:_____________________________________________________________________________
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4. Data
Record the colors of the solutions in the test tubes after 1-2 days.
Elodea + PR/ dark
Elodea + PR / light
PR / dark (control)
PR / light (control)
5. Analysis
Explain what happened in this lab, use complete sentences and be sure to provide supporting data or
statements. Use the results of this lab to answer the experimental question (Do Plants Consume or
Release CO2?).