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Lesson 2-The Dose Makes the Poison
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4. Once teams have completed their solutions and have six small beakers of chemical solutions on their work tables, ask students to discuss how they might test the toxicity of these solutions on seeds.

National Science Education Standards icon Content Standard A:
Students design and conduct a scientific investigation.

This discussion can be brief, but it helps students design their own investigation rather than simply following directions without giving a thought to their purpose. Students will recognize that they want to put the seeds in contact with the chemical solutions in some way. Many will remember seed germination activities from previous years and will suggest moist paper towels in petri dishes or plastic bags. Show students the assembly they will use in this investigation.

Ask students how they will know if a chemical was toxic to seeds.

Ask students what they predict should happen to healthy seeds in a nontoxic environment (the seeds will germinate). Help students recognize that they will want to look at nongermination as an indicator of toxicity in their investigation.

Ask students how they will know it was the chemical or a particular dose of the chemical that caused the observed effect and not some other factor.

Take this opportunity to discuss experimental design:

5. Distribute copies of Master 2.3, Toxicity Testing on Seeds, one to each student. Direct teams to work together to set up the investigation.

Tip from the field test: During the field test, some students and teachers tried to short-cut the solution preparation and set up steps by adding water and then chemical directly to the bag in which they already had placed the paper napkins and seeds. This resulted in an unequal distribution of chemical; some areas on the napkins had a high concentration of chemical and others had a low concentration. In addition, the seeds flowed to the bottom of the bag and got lost under the layers of napkin. The seeds should be added to the bag only after the napkins are saturated with completely mixed solutions of chemicals. Encourage your students to work carefully and methodically through the preparation of the chemical solutions (see Step 3 above) and then the preparation of the seed bags as outlined on Master 2.3, Toxicity Testing on Seeds.

pouring the solution into a bag placing seeds on the napkin the end result - seeds on the saturated napkin in a labeled plastic bag
assessment iconWhen students return to class with their worksheets complete, review their answers to assess their understanding of the investigation and underlying toxicological principles. Do they know the primary route of exposure of a human to their chemical? Can they describe how concentration is determined in a beaker? Are they able to make a reasonable prediction about the outcome of the investigation based on an understanding of dose, concentration, and response?

6. Provide space for the trays of seeds from each team. Tell the students that they will check on their seeds during the next class.

stacked trays of seed bagsTip from the field test:

Storing multiple sets of seeds for multiple science classes can be a challenge. Remember that germinating seeds do not need light, so the seed bags for each team can be stacked. One field-test teacher gave each team a dissecting tray and told the teams to stack their seed bags in the tray. She then stacked the trays for each class. In this way, she was able to fit the ongoing investigations for four science classes on one shelf in her classroom.

7. Students might need to complete the rest of the work on Master 2.3 at home. Remind students to construct a data table in their science notebooks (or to put the data table from Master 2.3 in their notebooks). Ask them to fill out Day 1 on the data table with observations of their seeds at the start. Instruct them to answer the questions at the end of the worksheet.

National Science Education Standards icon Content Standard A:
Students use appropriate tools and techniques to gather, analyze, and interpret data.

ACTIVITY 4: GATHERING DATA

1. As students come into the room, ask them to get their team's seed investigation from Activity 3 and bring it to their team's table.

2. Ask students to observe their seeds and share with the class what is happening with their seeds.

Ask students to make observations that are qualitative (descriptive) and quantitative (measurable). An example of a qualitative observation might be that the seeds seem to be "puffed up" and are turning the paper napkin yellow underneath them. An example of a quantitative observation might be that 6 out of the 10 seeds in Bag #2 have germinated.

bag number two

For those classes not equipped to conduct a laboratory investigation, the Web site includes a simulation of the results students can expect from the Dose-Response Seed Germination Experiment using three different chemicals.

Web activity iconOpen 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 Lesson 2—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. Click the Next button to access the simulation of the experiment.

3. Direct the students to the question that guided the seed investigation:

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?

Note to teachers: The term dose is used rather loosely here because it is difficult to know how much chemical penetrates each seed. It is reasonable to assume, however, that seeds exposed to a higher concentration of chemical receive a higher dose. Getting students accustomed to thinking about the term dose will prepare them for activities later in the unit.

National Science Education Standards icon Content Standard A:
Students develop descriptions, explanations, predictions, and models using evidence.

By asking them the following questions, help them see that they can begin to answer this question based on their observations:

4. Instruct students to fill in Day 2 of their data table on Master 2.3, which they placed in their science notebooks (see Activity 3, Step 7). Remind students that they will observe the seeds again on the next day to see if there are further changes.

5. Direct students to place their trays of seeds in the same spot as the day before.

Extension Activity

The hazard of chemicals in the environment is one that often is exaggerated. Because we are exposed to many chemicals, both naturally occurring and synthetically produced, it makes sense to understand when to worry and when not to worry. The problem arises when people do not know the dose of a chemical required to cause an adverse effect. In many cases, people assume that any exposure to a chemical that can cause harm is harmful.

To illustrate the flaw in this reasoning, the American Council on Science and Health publishes a Holiday Dinner Menu each year. In this menu, to determine our exposure to natural chemicals known to cause adverse effects in rats, scientists from the council analyze the foods we eat at Thanksgiving. For example, people consume natural chemicals such as ethyl benzene in their coffee, hydrogen peroxide in their tomatoes, and furan in their sweet potatoes. How are humans not being poisoned by their own food supply?

1. Invite students to log on to the American Council on Science and Health Web site to learn more about the importance of dose with respect to the dangers of chemicals in our food source:

www.acsh.org

2. Ask those students who choose to explore this extension to share their findings with the class.

3. Encourage students to design a bulletin board that illustrates how much of a food containing a harmful chemical a human would have to consume to get a toxic dose of the chemical. Such calculations are presented on the council's Web site. Past years' examples have included the amount of turkey a person would have to eat to get a toxic dose of malonaldehyde and the amount of bread a person would have to eat to get a toxic dose of furfural. The calculations for the bread example are shown in the box below.

The bread in Thanksgiving stuffing contains furfural, a chemical that can cause cancer in rats when they are fed high doses of it. But, before you start to worry about your Thanksgiving dinner, take into account the difference in body weight between a human and a rodent. How much bread would you have to eat to consume an amount of furfural equal to the amount that increased the risk of cancer in rodents?

Here are the facts:

  • One slice of white bread contains 167 micrograms of furfural.
  • The carcinogenic dose of furfural for a rodent is 197 milligrams per kilogram of body weight per day, fed every day of its life. (This is the same as 197,000 micrograms/kilogram/day because there are 1,000 micrograms in a milligram.)
  • The weight in kilograms of a middle school student who weighs 110 pounds is 50 kilograms because there are 2.2 pounds in every kilogram.

Here is the solution:

equation: 197,000 micrograms divided by 1 kilogram multiplied by 50 kilograms divided by person equals 9,850,000 micrograms per day, a carcinogenic dose for an average middle school student

equation: 9,850,000 micrograms divided by 167 micrograms per slice equals 58,982 slices of bread

a person being overwhelmed by a huge pile of breadWhat does this mean?

The average middle school student would have to eat 58,982 slices of bread a day to get a carcinogenic dose of furfural, assuming he or she responded the same way experimental animals do. When looking at this example, remember the conditions of animal studies: Doses are fed every day of the rodent's life (usually two years). To get an equivalent carcinogenic dose, a human would have to consume those 58,982 slices every day for years.


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