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Exploration Guide : Interdependence of Plants and Animals
Every time you take a breath, thank a plant! All of the oxygen in Earth's atmosphere was created by plants in the process of photosynthesis. Plants are also responsible for producing, directly or indirectly, all the food that we eat. Want to give something back to show your appreciation? Just breathe out!
Along with sunlight and water, plants depend on the carbon dioxide that animals exhale to power photosynthesis. In cellular respiration (the reverse of photosynthesis), animals and plants combine oxygen and sugars to produce water, carbon dioxide, and energy for life.
In this Gizmo tm
, you will see how plants and animals maintain a balance of gases in Earth's atmosphere. To indicate the presence of carbon dioxide, you will use brom thymol blue. This liquid turns yellow when the concentration of carbon dioxide increases, and blue when the concentration of carbon dioxide decreases. In this model, the animal kingdom is represented by the common pond snail.
The plant kingdom is represented by the aquatic weed Elodea.
Measuring Oxygen and CO
2
Concentrations
In this activity, you will create and monitor closed ecosystems with snails and plants. The bromothymol blue indicator in the test tubes will show carbon dioxide concentrations by changing color (yellow for high concentrations, blue for low concentrations). Because plants convert carbon dioxide to oxygen, you can infer that low carbon dioxide levels correspond to high oxygen levels.
Answer all Questions on a separate sheet of paper.
1.
In the Gizmo, click on a snail and drag it into a test tube, either in the Light room or the Dark room. Drag the remaining snails and sprigs of Elodea into test tubes of your choice. (You can add more than one organism to a test tube.) Do not start the simulation yet! a. What gas do you think will be produced by the snail? What color change will this trigger in the bromothymol blue? b.
What gas will be produced by the Elodea plant in the light? What color change will this trigger in the bromothymol blue? c.
What do you think will happen for a plant in the dark? d.
In your notes, create a table like the one below. Describe the contents of each test tube, and fill in the starting colors. Under Predicted Color, write your hypothesized color change for each test tube in the experiment.

2. Click Play ( ) and observe the color changes over a 24-hour period (simulated time). Fill in the
Final Color column in your table. a. Based on the results, what gas was produced by a plant in the light? A snail in the light? b.
What gas was produced by a plant in the dark? A snail in the dark? c.
From your answer to the previous question, what condition is necessary for plants to produce oxygen?
3. Click Clear test tubes
4. Make a table. Using various combinations of Elodea and snails, chart what happens to the contents of the test tubes in both the light and the dark. Some combinations are suggested below.
Be sure to pause the experiment at 6, 12, 18, and 24 hours and record the appearance of the test tube.
Do each of the following in both the light and the dark . Record your observations in a table
Run # 1
1.
1 sail, no Elodea
2.
1 snail, 2 sprigs of elodea,
3.
2 sprigs of elodea, no snail
4.
empty
Run # 2
1. 1 snail no eloda
2. 1 snail 3 sprigs of elodea
3. 1 sprig of elodea
4. empty
Run # 3
1.
1 snail, no elodea
2.
2 snails 3sprigs of elodea
3.
1 sprig of elodea
4.
empty
Run #4
1. 4 snails, 3 sprigs of elodea
Run # 5
1. 4 snails, 4 sprigs of elodea

Answer the questions :
1. Based on your experiments, what combinations yield the most rapid increases in oxygen concentration? Carbon dioxide concentration?
2. Suppose a closed test tube containing an Elodea sprig and a snail were left in a lighted room for several days. Would the snail and the plant survive? Why or why not?
3. Did the snail produce different results in the light and in the dark? How about the plant? Explain your answer and discuss it with your classmates. Was this what you expected?
4. What would happen to an aquatic ecosystem that lost all of its producers. Would this be a rapid change. Justify your answer.
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