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Lesson 3-Dose-Response Relationships
Explain

When you are finished making a sample of the graph, it should look like the following graph:

Dose-Response Curve for Seed Germination Investigation

blank dose-response graphD

5. Distribute copies of Master 3.2, Graph Paper, one to each team. Direct teams to make a dose-response graph for their chemical on Graph A.

Remind students to label their graph with the name of their chemical.

6. Once teams have made a graph of their chemical, instruct them to get data from two other teams that tested different chemicals. Tell teams to graph the data on Graphs B and C, remembering to label each graph with the name of the chemical.

Tip from the field test: Included here are several dose-response graphs for chemicals tested by students who participated in the field test.

pine-scented cleaner dose-response graph D
coffee dose-response graphD
insect repellent dose-response graph D
fruit punch soda dose-response graph D
salt dose-response graph D
window cleaner dose-response graph D
water-soluble plant fertilizer dose-response graph D
assessment icon Listen to students' descriptions of the dose-response curves to see if they understand the concepts of threshold and potency.

7. Circulate around the room and, as the teams work, ask them to study the team graphs to decide which chemical had the highest potency or which chemicals have a clear threshold.

Of the chemicals graphed above, Lysol (pine-scented cleaner) is the most potent and coffee is the least potent. Field-test students found that it was difficult to determine threshold. They knew only that they exceeded the threshold below which there was no effect when they used Lysol, salt, window cleaner, bug repellent, fruit punch soft drink, and plant fertilizer. To pinpoint the exact threshold, students would need to repeat their investigation using additional concentrations between 0% and 6.25%. When they tested coffee, students found no threshold: Coffee had no effect on seed germination even at 100% concentration.

National Science Education Standards icon Content Standard A:
Students develop descriptions, explanations, predictions, and models using evidence.
Content Standard A:
Students think critically and logically to make the relationships between evidence and explanations.

8. Discuss with students what conclusions they can make, if any, about the safety or potential harm of the chemicals tested for humans. Ask students to evaluate the use of plants as a model system for toxicology testing.

Students should recognize that plants do not have the same structure as humans or other animals, so results from tests that determine toxicity to plants may not apply to humans. In fact, there are chemicals that are toxic to plants that do not harm humans and, conversely, there are chemicals that do not harm plants but are capable of harming animals like humans. For this reason, it is best to test chemicals on systems that most closely resemble the human system if knowledge about effects on humans is the goal. Even so, students may recognize that data from toxicity tests on seeds could help inform them of possible toxicants to humans.

Toxicologists use to their advantage their understanding that it is possible for some chemicals to cause injury to one kind of living matter without harming another kind of living matter. By understanding this biological phenomenon, scientists can, for example, help farmers develop pesticides that are lethal to fungi or insects but do not harm crops, or antibiotics that kill infectious bacteria (also living organisms) but have low toxicity in humans.

Extension Activity

Ask students to describe other questions they would like to answer about the effects of their chemical on plants. Allow space and time for students to investigate their questions.

For example, students might want to know if their chemical has an effect on the growth of plants. They can sprout seeds and then water the seedlings with varying concentrations of chemical solutions. By setting up an investigation that can last a longer time, students can experiment with the concept of exposure over time. They can administer a small dose of chemical repeatedly for many days and compare the responses to those in plants that receive only one large dose of chemical.


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