Toxicology and Living Systems

OVERVIEW

Mercury is toxic – what does that mean? How does mercury get into organisms, how does it move through their systems, and how does it affect them?  This activity begins to introduce students to answers to these questions. This activity is especially useful for chemistry classes.

The activity is an adaptation of a lesson titled Toxicology 1: Toxicology and Living Systems, published on the Science NetLinks website by the AAAS.  It also makes use of materials developed by the Society of Toxicology on their website for educators.  We have replicated much of the lesson intact—but have made adaptations and additions to connect it to our focus on mercury.

GETTING READY

Materials

  • Student Worksheets (see below)
  • Teacher's Notes (see below)

ACTIVITY PROCEDURE

Getting Ready:

This activity requires the students to fill out some worksheets. Here are links to online, PDF versions of the worksheets that you can print off. You should make a copy of each worksheet for each student (or group, if you decide to have the students work in groups.)

There are also “Teacher’s Notes” to go along with two of the worksheets.  The Teacher’s Notes will help you make use of the worksheet

Doing the Activity

What follows is a suggested approach to implementing this activity. Teachers in the project will undoubtedly develop other approaches. If you find something that works well, please share it online with others.

Introducing the Key Ideas

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.” (Paraclesus, 1493-1541) 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.

Student Work

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.

Before moving on to the next part of the activity, students need to understand the term xenobiotic. Tell students that the Greek prefix xeno means foreign or different. (Some students might be able to come up with another example of a word using this prefix, such as “xenophobia.”) 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.

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 groups or as homework.

If you are assigning this as homework rather than an “in-class” exercise for which you could provide help, you might find that students will have difficulty with the second question: "Describe, in general terms, the target and/or mechanism of action for one of the poisons you named above." You can tell them to skip this part of the work since we will pick it up later in the “Assessment” section.

In class, review students’ answers to the questions using the Teacher’s Notes.

REFLECTION/ASSESSMENT IDEAS

The students should now be ready to put the knowledge that they gained in Unit 1 to use while extending and solidifying their understanding of mercury’s toxicity. Working individually, in groups, or as a whole class, have them answer the following questions:

  • At what dose is mercury toxic?
  • What organism(s) does (do) mercury affect?
  • Describe, in general terms, the mechanism of action for mercury.
  • Using the diagram Toxicology at Every Level of Biological Organization, state how this toxic chemical affects each level of the biological system.

You might want to ask students to draw a diagram of the mechanism of action of the poison. An additional option is to have students make posters about mercury’s toxicity.

EXTENSION IDEAS

Teachers focused primarily on preparing students to investigate mercury levels in insects or fish might find that this first activity provides all of the introduction to toxicology that students need. Teachers in health occupations, however, might usefully follow this lesson with the next two lessons in the Science NetLinks Toxicology series: Toxicology 2: Finding the Toxic Dose and Toxicology 3: Toxicology and Human Health.

Biology teachers might want to connect this toxicology study to lessons on the structure of the cell.  There are also potential connections to the study of anatomy.

The Society of Toxicology suggests the following topics for more extensive student work on its Topics in Toxicology page:

  • Gulf War Syndrome
  • Thalidomide
  • DDT
  • Aflatoxin
  • Lead
  • 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.

TEACHER'S NOTES

There are also “Teacher’s Notes” to go along with two of the worksheets.  The Teacher’s Notes will help you make use of the worksheet

WORKSHEETS

This activity requires the students to fill out some worksheets. Here are links to online, PDF versions of the worksheets that you can print off. You should make a copy of each worksheet for each student (or group, if you decide to have the students work in groups.)