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Lesson 2-The Dose Makes the Poison
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PREPARATION

Activity 1

Pour 500 mL of water into each of the 3 1,000-mL beakers. Label the beakers #1, #2, and #3.

Put a handful of radish seeds in a resealable plastic sandwich bag.

Gather the materials you will need for the demonstration.

Make a transparency of Master 2.1, Opening Questions.

Activity 2

Gather the materials you will need for this activity.

Duplicate Master 2.2, Making Solutions for Toxicity Testing, 1 copy for each team.

Set up a computer center at which students can view the Web site.

Activity 3

Decide whether you will use the print or Web version of the laboratory investigation. It is tempting to avoid the preparation and materials that a laboratory investigation requires, but students benefit from conducting a scientific investigation, using tools to gather data, and developing a hands-on understanding of the use of models in scientific inquiry. The simulation presented on the Web site enables teachers and students without access to laboratory equipment to gather data to use in Lesson 3, but it should not replace actual laboratory experience. In addition, Lesson 2's laboratory investigation provides students with an opportunity to meet Content Standard A of the National Science Education Standards: All students should develop abilities necessary to do scientific inquiry and understandings about scientific inquiry.6

For the print version of the laboratory investigation:

Prepare the chemicals, 1 chemical for each team of 3 students:

Choose a wide variety of chemicals for testing:

Tip from the field test: During the field test, the following chemicals yielded data that made the most interesting dose-response curves for students to graph in Lesson 3: salt, Miracle Gro, fruit punch soft drinks, window cleaner, and Lysol. The results from other chemicals were also of interest to students, so be sure to include a wide variety of familiar chemicals, such as shampoo, soft drinks, coffee, and sweetener, even if the dose-response curves generated from the use of these chemicals in the investigation are less exciting. One of the reasons to use a variety of chemicals is to demonstrate the range of responses that are possible.

When available, follow directions on the container to make solutions of nonliquid chemicals, such as plant food or instant coffee. When no directions are available, make as saturated a solution as possible: Heat the water and slowly stir in a small amount of the chemical until it no longer dissolves easily in the water. In pilot testing this activity, we made a sugar solution with 40 g of sugar in 100 mL of water and a salt solution with 24 g of salt in 100 mL of water. Be sure to make enough solution for all your classes.

Purchase radish seeds. Put 60 radish seeds into a resealable plastic sandwich bag. Continue until you have a bag for each team of 3 students.

Radish seeds found in a local garden store work well for this investigation. They will germinate in 1 to 3 days. If you prefer faster germination (6-24 hours), you can purchase Wisconsin Fast PlantsTM, Brassica rapa seeds (which are close relatives of the radish) from Carolina Biological Supply (1-800-334-5551). Be aware that the Brassica rapa seeds are quite a bit smaller than radish seeds, so consider your students' dexterity when deciding which seeds to use.

Tip from the field test: Counting the 60 seeds can be time-consuming. Estimate the number of seeds by measuring approximately 1/4 teaspoon of regular radish seeds (less if you use Brassica rapa seeds) for each bag. There will be a little more than 60 seeds in each bag. Students tend to lose a few as they set up the investigation, so it doesn't hurt to have a few extra seeds in the bag or on hand at the materials table.

Activity 4

If your students conducted the Web version of Activity 3, arrange for students to have access to a computer at which students can view the Web site for this activity.

Extension Activity

Arrange for students to have access to the World Wide Web. Gather materials needed to design a bulletin board display or ask students to provide them.

Procedure

ACTIVITY 1: DOSE AND CONCENTRATION

1. Place the shoe box with the mystery chemical from Lesson 1 in it and the three beakers of water on a table in the front of the room.

2. Display a transparency of Master 2.1, Opening Questions.

3. Allow the students a few minutes to puzzle through the questions on the transparency. Assure them that it is perfectly acceptable not to know the answers to all the questions right now. Remind students that toxicologists study chemicals and perform toxicity tests because they don't know the answers to questions like these about every chemical.

4. Once students have answered the opening questions, draw their attention to the shoe box that holds the mystery chemical from Lesson 1. Announce to the students that a toxicologist partially analyzed the mystery chemical and informed you that the chemical would not enter your body through inhalation or contact with the skin, so it is safe to handle the chemical as long as you don't ingest it.

5. Demonstrate appropriate laboratory safety measures by putting on safety glasses and a pair of latex gloves. Open the shoe box and remove the jar of mystery chemical.

teacher demonstrating Activity 16. Direct students' attention to the three large beakers of water. Let students know that each beaker holds exactly the same amount of water as the other two. As the students watch, use the eyedropper to add

Gently swirl or stir each beaker until the drops of mystery chemical are totally distributed in the water. Use the white poster board as a backdrop and ask students to tell you what they can observe.

Students will observe that the water in the three beakers has turned a shade of blue. In Beaker #1, the water is a very pale blue. In Beaker #2, the water is a darker blue. In Beaker #3, the water is the deepest blue.

7. Suggest that the beakers of water represent human bodies. Ask students to tell you what they know about the relative sizes of the three "bodies." Once they recognize that all the bodies are the same size, discuss the added chemicals using questions such as these:

8. Point out to students that the questions they have about how much of a chemical is harmful and how much is beneficial are the same kinds of questions that toxicologists study. Help students understand some basic vocabulary of toxicology by asking them these questions and writing the terms on the board.

Beaker #3 has the highest concentration of all the beakers because it has the largest dose of chemical compared with the constant volume of water. For the same-size person, ingesting more of a chemical results in a higher concentration of the chemical in that person's body. It follows that if several people of various sizes take the same amount of a chemical, the final concentration to which each person is exposed can vary, depending on the size of the person.

To help clarify the difference between dose and concentration, you can use a fourth beaker. This time, fill it with only 250 mL of water, half the amount of the other three beakers. Ask students to describe to you the relative size of this new "body" compared with the other beakers. Tell students that you are going to give this new body the same dose as you gave Beaker #2: 4 drops. Add that dose to the beaker. Ask students to observe the color and compare the concentration of chemical in the fourth beaker with the concentrations in the other three beakers. Is this concentration more or less than the others? Students should see that the water in the fourth beaker does not look like the water in Beaker #2, even though the dose was the same. The color is deeper, showing that the concentration of chemical in the smaller body is greater for the same dose than it is in the larger body.

9. Ask students to come up with ways to find out if the mystery chemical at different doses (1, 4, and 16 drops) is harmful or beneficial to living things.

Students might suggest that someone could drink the three solutions of chemical and see what happens. If you ask for volunteers, however, you probably will not get any; you can discuss with students why this method may provide information but would not be a responsible way to test the toxicity of a particular dose of chemical. Ask students to think of other living things that they could use for testing. Use the Background Information in this lesson to discuss how animals are used in research. Be sure to discuss the Society of Toxicology's policies and guidelines for animals in research.

animals in research
Photo: Courtesy American Association for Laboratory Animal Science

10. Help students recognize that they are not equipped to safely and ethically handle toxicology tests on animals. Tell them that there are many animal welfare laws, regulations, guidelines, and policies that govern the use of animals in research. Let students know that there are other living organisms that they can use for testing the mystery chemical in the classroom. Hold up a bag of seeds and ask students to tell you if they think the seeds are living organisms.

Seeds are living organisms that grow into plants. As living organisms, plants respond to chemicals in their environment, particularly in water. Plants are not the same as humans or other animals in structure, however, so results from tests that determine toxicity to plants might not be applicable to humans. Students will discuss this problem with the model system of plants when they analyze their data in Lesson 3.

11. Tell the students that they are going to perform toxicology tests on seeds, but first they need to formulate a question and design an investigation.

ACTIVITY 2: PLANNING THE SEED INVESTIGATION

1. Put on your desk some of the chemicals from Lesson 1. Show the students the mystery chemical. Show the students the seeds in the bag. Ask the students what they want to know about the chemicals and the seeds.

Conduct this discussion as a brainstorming session, perhaps writing on the board all of the questions the students formulate. Help the students arrive at a general question, such as this one:

What effect does the chemical have on the seeds?

2. Ask the students to refine their question by considering what they learned in the beaker demonstration about dose.

The students' question should address both effect and dose. If students do not provide such a question on their own, suggest one like this:

What effect does the chemical have on seeds, and how is the effect different if the seeds receive different doses of the chemical?

3. Refresh students' memories about the function of seeds in the life cycle of a plant. In doing so, discuss with students what they would expect a seed to do if it is exposed to moisture.

progression from a seed to a plant

Seeds are the most important part of a plant. In fact, the roots, leaves, and flowers of a plant all exist to produce seeds. Seeds have three parts: the protective outer seed coat, the embryo that becomes the new plant, and the endosperm that provides the food to nourish the embryo. Most students have looked at the germination of a bean seed in elementary school. Remind students that before a seed germinates, it absorbs moisture so that it swells and bursts its seed coat. At this time, the embryo starts to grow and the root tip, or radicle, pushes through the eye of the seed. This "sprouting" is the first evidence of growth that students can observe with the naked eye. Several days later, seed leaves emerge and the embryonic stem begins to extend upward. At this time, chlorophyll pigment is visible. If planted in favorable conditions with the right moisture, temperature, and light, the plant will develop leaves, lengthen its stem, flower, produce fruit, and develop mature seeds. Once the seeds dry, the cycle is complete and the seeds are ready to produce new plants.

4. Ask students to further refine their question from Step 2 based on the discussion of what seeds do under normal conditions.

Guide students to a question like this one:

How does the chemical affect the germination of the seeds, and how is the effect different if the seeds receive different doses of the chemical?

5. Distribute copies of Master 2.2, Making Solutions for Toxicity Testing. Ask students to interpret the information presented in the handout by asking questions similar to these:

6. Instruct teams to put their copies of Master 2.2 in their science notebooks so that they can use them during the next class period.

7. Make the Web site available at a computer center for students who want more practice determining concentrations of chemicals.

Web activity icon Open the Web site in your browser (see instructions for using the Web site). From the main page, click on Web Portion of Student Activities, then The Dose Makes the Poison. This brings up the Dose-Response Relationships page. Click the Start button to run the Dose-Response Seed-Germination Experiment. On this page, students will be able to manipulate chemical concentrations. Instruct the students not to click Next at this time.

ACTIVITY 3: SETTING UP THE SEED INVESTIGATION

The following procedures describe how to conduct a hands-on laboratory investigation, which is the preferred method of instruction for this activity.

Important note to teachers: Activity 3 requires at least three consecutive school days (without interruption by a weekend or holiday). Usually, that means you need to start no later than the Wednesday of a full week of school.

1. Direct students' attention to the beakers of chemicals you prepared. Distribute chemicals on the trays, one chemical to each team.

Prepare at least one chemical per team and perhaps more if you want to give teams a variety of choices for testing. Alternatively, you can assign chemicals to teams. Ask one team to test the mystery chemical or you can test it for the class.

2. Ask teams to get out their copy of Master 2.2, Making Solutions for Toxicity Testing, from the previous class period. Ask students, How are you going to label your beakers of solutions so that you know what is in each one?

Answers will vary, but impress upon students that it is important to keep accurate records in toxicology testing. At the minimum, students should number the beakers 1 to 6 to correspond with the beaker numbers on the chart on Master 2.2.

beakers labeled 1 through 6

measuring fluids in a graduated cylinder3. Direct students to gather their materials, put on their gloves and safety glasses, and follow the chart on Master 2.2 to make their solutions.

If necessary, review proper measuring techniques for fluids in a graduated cylinder before the students attempt to make their solutions. Make sure they know how to measure at eye level so that they can see when the meniscus is right on the line that demarcates the target volume. Demonstrate to students how they can use the eyedropper to add or subtract small amounts of volume.

Tip from the field test: If students have not measured liquid volume before, they might need to practice before completing this step of the activity. Refer to measurements from Master 2.2, Making Solutions for Toxicity Testing, and, using clear and colored water, set up a practice lab for inexperienced students. For further tips from the field test, see Step 5.



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