To investigate in an introductory way how toxic chemicals affect biological systems.
This lesson is part of a three-part series on toxicology, the scientific study of poisons and their affect on biological systems. These lessons are based on Toxicology Enrichment Materials developed by Suzanne Conklin and found on the Society of Toxicology’s website.
In the first lesson, Toxicology 1: Toxicology and Living Systems, students are introduced to the basic concepts and terminology of the science. Toxicology 2: Finding the Toxic Dose allows students the opportunity to conduct a toxicology experiment on a plant. Specifically, students determine the toxic dose of a chemical that will inhibit seed germination in Brassica rapa, a relative to cabbages and mustards. In the third lesson, Toxicology 3: Toxicology and Human Health, students investigate the effect of environmental tobacco smoke on human lung development. These lessons can be done in a series or they can stand alone.
In the United States, thousands of chemicals are consumed and utilized in everyday items, such as food, personal care products, prescription drugs, household cleaners, and garden/lawn care products. The effects of many of these chemicals on humans are unknown. Moreover, our use and disposal of these products ultimately contaminates our planet’s soil, water, and air. Safeguarding public health and the quality of the environment depends on identifying the effects of these chemicals and the levels of exposure at which they may become hazardous.
As high-school students study the flow of matter and energy in natural systems, they develop an understanding that important molecules and elements are continually recycled. By studying toxicology, students add to this understanding by examining that hazardous chemicals in natural systems also are passed along the food chain. Thus, an herbicide used to kill weeds or a pesticide used to kill insects are not isolated to their target organisms. We are, in fact, all connected on this planet to each other and the risk to our own health must always be weighed against the benefit attained by using hazardous chemicals.
Two of the student sheets and readings are available online at the Society of Toxicology website. However, students should not do these readings or worksheets online since links are provided at the bottom of the pages to the Teacher’s Notes. Student sheets and readings for these two activities are also available in PDF format. It is recommended that you download these activities and provide copies to the students rather than having students do the readings online.
Write the following quote on the board:
“All substances are poisons: there is none which is not a poison. The right dose differentiates a poison and a remedy.”
Read the quote aloud to the class, telling students that Paraclesus was a Swiss physician.
Ask students these questions. They are designed to elicit students’ ideas and misconceptions about toxic substances:
- Do you agree or disagree with this statement?
- Why or why not?
- Why does the dose determine if a substance is a poison or a remedy?
- What is a toxic substance?
- What does it mean for something to be toxic?
- What is the effect of a toxic substance?
- Do all substances have the potential of being toxic?
- Are toxic substances natural or man-made?
- Who or what is affected by toxic substances?
- How would you define toxicology?
- What sciences does toxicology incorporate?
Through a discussion guided by these questions, students should develop an understanding that all chemicals and substances can be toxic. The dose delivered determines whether or not the chemical is toxic to its target. Also, a substance that is toxic to one organism may not be toxic to another. For example, the herbicide 2,4-D is toxic to dandelions but not grass. The antibiotic penicillin kills bacteria by disrupting the cell wall-building process. Thus, bacteria that lack cell walls are unaffected by penicillin. Not all toxic substances are lethal. Toxic chemicals can cause disease, behavioral abnormalities, cancer, genetic mutations, physiological or reproductive malfunctions, or physical deformities in any organism or its offspring. Toxic chemicals can also become poisonous after concentration in the food chain, in the environment, or in combination with other substances.
Toxicology overlaps with many aspects of the sciences, including biology, chemistry, physiology, microbiology, public health, and pharmacology. Overall, it is a science that studies toxins and their effect on biological systems.
Distribute the Toxicology at Every Level of Biological Organization student sheet to students. If desired, have one worksheet ready as a transparency to fill out with the class using an overhead projector. Tell students that each category represents the different biological levels that can be affected by toxic substances.
Review each biological level with students and ask them:
- How do you think toxic chemicals affect molecules in the body?
- Can you give an example of a molecule in the body that is affected by a toxic chemical?
Use the Teacher's Notes to supplement student answers and to provide students with more examples. Ask the above two questions for each biological level. Have students write down how toxic chemicals affect each biological level on their worksheets.
Distribute the five-page Some Basic Principles of Toxicology student sheet. Have students answer the questions posed in the reading. This activity can be done in a school computer lab or as homework.
Students probably will not be able to answer the second question: "Describe, in general terms, the target and/or mechanism of action for one of the poisons you named above." While students are encouraged to do research, this question will be investigated deeper in the Assessment section of the lesson. Before students do the reading, the term xenobiotic must be defined.
- What do you think a xenobiotic is?
- Read the term in the context of the question: Name at least five xenobiotics to which you have been exposed in the last 24 hours. What do you think xenobiotic means in this context?
As students try to determine what the term xenobiotic means, tell students that the Greek prefix xeno- means foreign or different. The term biotic refers to something living, like the body. Thus a xenobiotic is a chemical substance that is foreign to the body or not produced by the body itself.
In class, review students’ answers to the questions using the Teacher’s Notes.
Have students choose one of the poisons they named in their Some Basic Principles of Toxicology student sheet. Tell students that they will research this poison to answer these questions:
- How is this chemical traditionally used?
- Where is this chemical found?
- At what dose is this chemical toxic?
- What organism(s) does this toxic chemical affect?
- Describe, in general terms, the mechanism of action for this toxic chemical.
- Using the diagram Toxicology at Every Level of Biological Organization, state how this toxic chemical affects each level of the biological system.
Refer students to the Toxic Chemicals and Living Systems student esheet. Have students answer the questions posed above on this sheet. Alternatively, you can have students write a short essay and/or draw the mechanism of action of the poison.
Students also can do this activity in small groups of two to three. Have each group choose one toxic chemical and answer the questions posed in the student esheet. Each group can then make a poster presentation of their toxic chemical to the rest of the class.
The Society of Toxicology suggests the following topics for more extensive student work on the Topics in Toxicology page:
- Gulf War Syndrome
- Domoic Acid
Each topic is associated with a number of questions that require students to perform independent research. The questions range from factual to interpretive to evaluative. The goal is for students to gain enough information on a particular topic to be able to form an opinion about it. The site also provides Teacher’s Notes with a brief description of each of the topics and suggestions for use in a classroom toxicology discussion.