UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/
Developed by: K. Thomas, C. Davis & J. Thomas
California State Standards

Biology, 6d. Students know how water, carbon, and nitrogen cycle between abiotic resources and biotic matter in the ecosystem and how oxygen cycles through photosynthesis and respiration.

Investigation and Experimentation, 1a. Select and use appropriate tools and technology to perform tests, collect data, analyze relationships, and display data.

Investigation and Experimentation, 1c. Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions.

Investigation and Experimentation, 1d. Formulate explanations by using logic and evidence.

Investigation and Experimentation, 1l. Analyze situations and solve problems that require combining and applying concepts from more than one area of science.
Synopsis The purpose of this inquiry-based lab/lesson is to have students discover the interdependence of the processes of cellular respiration and photosynthesis to living organisms.
Background
Oxygen and carbon dioxide are gases used and released by living organisms. Animals and plants use oxygen for cellular respiration and give off carbon dioxide as a waste product. Plants also use carbon dioxide for photosynthesis.
Photosynthesis: CO
2
+ H
2
0 + light energy  C
6
H
12
O
6
+ O
2
+ H
2
0
Respiration: C
6
H
12
O
6
+ O
2
 CO
2
+ H
2
0 + energy
Oxygen, which is needed by most living things for respiration, makes up about 21 percent of the atmosphere. It is also dissolved in water and is part of water molecules themselves. The movement of oxygen through an ecosystem is called the oxygen cycle. One phase of the oxygen cycle occurs during the process of photosynthesis.
During photosynthesis, molecules of water are split, releasing oxygen into the atmosphere. Oxygen in the atmosphere is used by most organisms for respiration.
One of the waste products of respiration is carbon dioxide. Carbon dioxide is used in photosynthesis. Another waste product of respiration is water. Water is released into the atmosphere and may later fall to the earth as precipitation. This water can be absorbed by plants, which break down the water and release oxygen during photosynthesis, thus continuing the oxygen cycle. In this lesson, students should learn that plants produce oxygen during photosynthesis, oxygen is used during respiration, carbon dioxide is given off during respiration, and carbon dioxide is used during photosynthesis.
In this activity, photosynthesis by the elodea removes CO
2
from the solution, raising the pH. This higher pH is indicated by the blue color of the indicator. The snail, on the
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/ other hand, respires, producing CO
2
, thereby lowering the pH. This lower pH is indicated by the yellow color of the indicator. In containers that have both elodea and snails, the pH should be relatively neutral since the snail is respiring and the plant is photosynthesizing. While we will be exposing all of our tubes to light, note that if we put a tube with only elodea in the dark, the solution will become acidic and turn yellow.
Why? The CO
2
was produced as a result of cellular respiration by the elodea. Both cellular respiration and photosynthesis occur in plants when light is available and photosynthesis is the dominant process. In the absence of light, the plant cannot photosynthesize, only respire and there is a net increase in CO
2
concentration.
To ensure/encourage inquiry, be sure to provide the purpose but not the answers to the investigation. Do not provide the hypotheses to the students. If you want to increase the inquiry, don’t provide the experimental setup- allow them the opportunity to do it themselves.
Objectives
1. To understand and be able to relate the interdependence of the processes of cellular respiration and photosynthesis to living organisms.
2. Become familiar with how oxygen cycles through photosynthesis and respiration.
3. Students will become more familiar with the scientific method; students will make observations and hypotheses, follow a procedure, collect and record data, and make conclusions.
4. Students will develop writing skills by completing the required sections of the worksheet.
Suggested Timeline
This activity needs to take place on two consecutive days.
Materials
Tubes that can be sealed (4 per group)
Masking tape (for labeling tubes)
Bromothymol blue solution in dropper bottles (1 per group)
Sprigs of elodea (4 per group)
Aquatic snails (2 per group)
Drinking straws (1 per group)
De-chlorinated water (spring or aged tap)
Teaching Tips
**It is important that you try this activity BEFORE implementing it in a classroom. You may need to alter the BTB concentration, light exposure, etc. to fit your particular needs and materials.**
DAY ONE
Pre-lab and Lab

You can start the lesson by showing students a picture of a biosphere. Ask if they know what it is, how it works, etc. If we were to be self-sustained, what are some
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/ factors we would have to consider? If the students don’t understand, ask them how we would get our food and what we would do with our waste.

Explain that today we are going to do a lab that discovers how plants and animals affect the level of CO2 in a tube of water. We are going to set up an experiment to find out if a plant or snail will change the CO2 levels. Pick a student to describe what we will be doing in today’s lab.

Start the teacher demo, explaining what you are doing as you go:
1. Fill two 50mL beakers about 1/3 full with water.
2. Add about 10-15 drops of bromothymol blue (BTB) to each (the water should turn blue).
3. Gently blow through a straw into the solution of one beaker (it should turn yellow). Continue to blow until there is no more color change.

Have the class think about what is going on and propose their ideas. Then, explain that when we breathe, we exhale CO2. BTB is an indicator that is blue when there is no CO2 and yellow when there is CO2. Indicators are substances that show the presence of certain chemicals by changing color. BTB is an indicator that turns green or yellow in the presence of a weak acid. CO2 reacts with water, forming a weak acid, so BTB can indicate the presence of CO2 in water. Keep the “before” and “after” beakers visible throughout the lab.

Have the students answer the following questions by filling in the blanks on their worksheet.
1. When carbon dioxide is not present in the water (the level of CO2 is low) what color will the BTB indicator be? (blue)
2. When carbon dioxide is present in the water (the level of CO2 is high) what color will the BTB indicator be? (yellow)

Explain to the students that in this activity, they will be examining the question: What is the biological relationship between an aquatic snail and an aquatic plant? (Define aquatic.) To do this, they will discover how each organism affects the level of CO2 in a tube of water.

Explain that they will be using living organisms and ask what things they need to consider when using living organisms (respect life, treat the snails gently).

Model the following procedure (or have the class guide you through it using their handouts) discussing where the materials are located, how to transport them to their desks, and other important points (see italics below):
1. Using the masking tape, label four tubes as follows:
Tube 1
2 pieces Elodea
1 snail
Tube 2
1 snail
Tube 3
2 pieces Elodea
Tube 4
No organisms
Period: Period: Period: Period:
Group: Group: Group: Group:
2. Add de-chlorinated water (sprin g or aged tap water) to each tube until about ¾ full.
3. Add 8-10 drops of BTB solution to each tube and using the straw, very carefully blow into the tube until the solution is yellow-green. Make sure to discuss that they should not suck the liquid in or blow too hard and make the liquid splatter! If
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/ you are concerned for their safety, have them wear safety goggles when working with the open tubes.
4. IMMEDIATELY close all of the tubes; the BTB should remain yellow-green.
Discuss how you need to make sure all four of the tubes are the same color and ask why this is important.
5. Quickly add 2 sprigs of Elodea and 1 snail to tube #1 and tightly close the lid.
Remind them to treat living things gently and with respect.
6. Quickly add 1 snail to tube #2 and tightly close the lid.
7. Quickly add 2 sprigs of Elodea to tube #3 and tightly close the lid.
8. Set the tubes near a bright light and leave them there for 24 hours. The lights must remain on overnight and the sides and tops of the tubes should not be covered.
9. Record the contents of each tube and your hypotheses in the chart below. Draw their attention to the hypothesis and observation chart. You can fill in the contents column now as a class or have them complete it later.
10. After 24 hours, observe the tubes. Place a piece of white paper behind each tube to determine the color of the water more easily. Record the color of the solutions in the chart below.

Then, describe the clean up procedure.

During the lab, circulate the room and monitor activity. Check that all the proper safety measures are being taken and that the snails are being handled respectively.

Make sure the students complete the appropriate columns of their chart.
End of day 1 discussion:

Ask the students what the purpose of the lab was.

Ask them to describe the lab set up.

Ask them what color the BTB was in their vials and what this means.

Ask what are some possible changes that can occur overnight and what it would look like.

Describe some scenarios that might be seen tomorrow and ask them what they mean. Use these responses to judge how deeply the students understand the set up of the lab and the role of BTB as an indicator.
DAY TWO
Review of yesterday
 Ideally, have the vials somewhere where the students can’t see them when they come into the classroom.

Ask students what the purpose of the lab is. Ask for a volunteer to refresh our memories of how we set up the lab. Draw the vials on the board and ask what they think they will see today. Ask what kind of data they are going to be gathering today.
Data collection and clean up
 Have one person pick up their group’s vials and have them start collecting their data.

Walk around the room asking students about their data, checking for understanding.
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/

Get the classes attention and explain the clean up procedure. You may want to have a discussion about not releasing the snails into the wild (non-native species etc).
Post-lab
 Have one person from each group report the group’s data. You can write each group’s data on one large table on the board or an overhead.

Discuss the data- what are the trends? Are there any groups that didn’t fit in with the trend?

Allow students to form their own ideas about the results of the lab and what they mean…ask them why there were/weren’t differences between CO2 levels in the different tubes. Guide them through the worksheet questions if necessary.
 Draw a plant and a snail on the board. Ask the students what the plant “breathes” and what the snail breathes. Ask the students what the snail exhales and what the plant “exhales”. Make a cyclical diagram showing the interdependence of the two based on the gases each relies on. Recommend that the students write this down.

Put up the picture of the biosphere and remind the students of the things they suggested that would be important for having a self-sustaining environment. Have them think about what they learned from the lab and ask them to describe the interdependence between plants and animals. Discuss what effects there would be on a balanced ecosystem if the number of plants or animals is dramatically or suddenly altered.
***This lesson should be followed by a power point lecture/discussion on photosynthesis, respiration, and the oxygen cycle with many references to what occurred in the lab.***
Note
Images in student handout are from the following web sites: http://www.prickleypear.com/_store_images/60_store_image_single-snail-lg.gif
http://www.dnr.state.wi.us/org/land/er/invasive/targetplants/images/ELodea_diagam_small.jpg
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/
(STUDENT HANDOUT BEGINS)
Snail
What is the biological relationship between an aquatic snail and an aquatic plant?
Elodea
Purpose
In this lab, you will set up closed systems containing plants and animals to observe interrelationships. You will discover how each organism affects the level of
CO
2
in a tube of water.
Materials
4 tubes with lids
2 pond snails
4 pieces of Elodea (an aquatic plant)
1 straw
CO
2
LOW
Bromothymol blue (BTB) solution
Water
Masking tape
Procedure
1. Using the masking tape, label the four tubes as follows:
CO
2
HIGH
Tube 1
2 pieces Elodea
1 snail
Period:
Group:
Tube 2
1 snail
Period:
Group:
Tube 3
2 pieces Elodea
Period:
Group:
Tube 4
No organisms
Period:
Group:
2. Add water to each tube until about ¾ full.
3. Add 8-10 drops of BTB solution to each container.
4. Using the straw, have one person very carefully blow into each vial until the color changes.
5. IMMEDIATELY close all of the tubes.
 What color is the water in each tube? __________________________________
 What does this tell you about the level of CO
2
? __________________________
___________________________________________________________________
6. Quickly add 2 sprigs of Elodea and 1 snail to tube #1 and tightly close the lid.
7. Quickly add 1 snail to tube #2 and tightly close the lid.
8. Quickly add 2 sprigs of Elodea to tube #3 and tightly close the lid.
9. Set the tubes near a bright light and leave them there for 24 hours.
10. Record the contents of each tube and your hypotheses in the chart below.
11. After 24 hours, observe the tubes. Place a piece of white paper behind each tube to determine the color of the water more easily. Record the color of the solutions in the chart below.
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/
Hypotheses and Observations
Tube # Contents
Hypothesis: What color do you think the BTB will be after
24 hours?
Observations:
Actual color of BTB after 24 hours.
1
2
3
4
Analysis and Conclusions (after 24 hours)
1. Using your knowledge of bromothymol blue and how it changes color in the presence of CO
2
, explain what happened in each tube: a. Tube 1- _______________________________________________________
______________________________________________________________
______________________________________________________________ b. Tube 2- _______________________________________________________
______________________________________________________________
______________________________________________________________ c. Tube 3- _______________________________________________________
______________________________________________________________
______________________________________________________________ d. Tube 4- _______________________________________________________
______________________________________________________________
______________________________________________________________
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UCLA, GK-12 Science & Mathematics in Los Angeles Urban Schools http://www.nslc.ucla.edu/STEP/GK12/
2. How do snails affect the level of CO
2
in the water? __________________________
___________________________________________________________________
3. How does Elodea affect the level of CO
2 in the water? ________________________
___________________________________________________________________
4. What is the purpose of tube #4? _________________________________________
___________________________________________________________________
5. If the water had changed color in tube #4, how would you explain it? ____________
___________________________________________________________________
___________________________________________________________________
6. Which of the tubes is an example of a balanced system? Why? ________________
___________________________________________________________________
___________________________________________________________________
7. How are plants and animals dependent on each other? _______________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
8. What do you think would happen if we repeated the experiment but kept the tubes in the dark? ___________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(STUDENT HANDOUT ENDS)
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