- What is the response that you want to graph?
This is sometimes counterintuitive for students. The response that
they want to graph is the one caused by the chemical (the response that
was different from the response of the control group). In most cases, the
response is the lack of germination, so students would graph the number
of seeds in the population that did not germinate.
- What measurement should be on the x-axis?
The dose is recorded along the x-axis, in increasing concentrations
from 0% to 100%. Help students recognize that they need to space their measurements
accurately along the x-axis, but not at even intervals since their
concentrations are 0%, 6.25%, 12.5%, 25%, 50%, and 100%.
- What measurement should be on the y-axis?
The response is recorded along the y-axis, as the number of
seeds in the population that did not germinate, from 0 to 10.
When you are finished making a sample of the graph, it should look like
the following graph:
Dose-Response Curve for Seed Germination Investigation
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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.
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Listen to students' descriptions of the dose-response curves to
see if they understand the concepts of threshold and potency. |
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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.
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Content Standard A:
Students develop descriptions, explanations, predictions, and models using evidence. |
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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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