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What the Sun Can Do

By Diana Doepken



Type of activity:

  • Hands-on
  • Cooperative group learning

Target Audience:

  • Advanced Biology
  • Biology
  • Life Science
  • Environmental Studies
  • Gifted students

This activity helps students realize that changes in the environment- in this case the amount of ozone - can directly affect living organisms, including humans.


Background

Notes for teacher:

It is helpful to have access to the "Classroom Guide to Yeast Genetics" developed by Tom and Monta Manney, Department of Physics, Kansas State University, Manhattan, Kansas 66506-3601. This guide is extensive in its treatment of yeast experiments and the relationship to UV radiation. This unit follows some of the initial procedures to establish the basic susceptibility of an organism (yeast) to UV radiation.

Common baker's yeast is an organism which is easy to obtain, culture and handle in the high school lab setting. When a UV sensitive strain of this yeast is exposed to ordinary sunlight ( which includes some UV components) some of them die. The amount of radiation which causes this lethal dosage can be experimentally determined and is the basis for the investigation.

UV radiation passes through the earth's atmosphere where it can be absorbed (or deflected) before reaching the surface. Ozone, molecules of oxygen in the form of O3, are particularly good at absorbing the high energy, short wavelength UV radiation, known as UV-A. However, other UV radiation, types known as UV-B and UV-C, can get through the atmosphere and these are the ones that interact with organic molecules in living organisms. Even though the amount of ozone is distributed unevenly over of the earth, the penetration of UV through the atmosphere depends on many factors such as altitude, latitude and extent of atmospheric pollution.

If you are working with advanced or gifted students, they can usually do all the introductory activities, develop a hypothesis and complete the investigation on their own. Other student populations will be more successful if the preliminary activities are demonstrated, allowing them to focus on the hypothesis and investigation only.

Notes for students:

Students need to have sufficient background in cell structure, DNA structure and function. An understanding of UV radiation as a part of the electromagnetic radiation spectrum and its relationship to solar energy would also be helpful. It is also important for students to have information about how ozone prevents some UV radiation from reaching the earth's surface.

Preparation time:

Preparation time varies, depending on how much you assign to the students to do. About 45 minutes is needed to prepare agar plates and to sterilize the equipment for each preliminary lab. As the groups develop their hypotheses, additional plates must be prepared and equipment sterilized.

Class time:

If done in its entirety, this unit should take all or part of seven to nine class sessions of 55 minutes. If procedures are abbreviated, five class sessions are sufficient. Actual project presentation is composed outside of class.


Lesson

Summary:

Students develop and test a unique, personally-relevant hypothesis about the consequences of exposure to UV radiation based units on a living organism, common baker's yeast (Saccharomyces cerevisiae).

Material:

Culture of Saccharomyces cerevisiae (baker's yeast) UV sensitive strain G948-1C (obtainable from Kansas State University, address above)

For Preliminary Investigations (for one lab group):
  • 7 sterile agar plates
  • 7 sterile dropping pipettes
  • Alcohol
  • Bent glass rod to spread yeast
  • 7 sterile culture tubes with caps
  • Test tube rack
  • Sterile tooth picks
  • Masking tape
  • Permanent marking pen
  • Index card
  • Cardboard box bottom
  • Incubator (optional)
  • Active yeast culture
    suspension in sterile water
For Group Investigations (amounts vary, depending on hypothesis):

  • Same as above
  • Aluminum foil
.


Procedure

Preliminary Investigation 1:

Using sterile technique, students inoculate an agar plate with yeast that has been suspended in sterile water. After allowing the inoculate to be absorbed into the agar, the dish is taped shut. An index card is cut and taped over half of the lid of the dish. The plate is exposed to sunlight (with individual groups assigned different exposure times), incubated and then observed for differences in yeast growth.

Preliminary Investigation 2:

Students inoculate 6 sterile agar plates with yeast that has been serially diluted (or they can do this dilution themselves). Five of the plates are exposed to sunlight, varying the amount of exposure time, and the sixth plate is kept as a control. The plates are incubated and growth colonies counted. Class results are averaged and graphed to show percent survivors vs. exposure time. Using this data a "lethal dose" - that is, the time required to kill 90% of the organisms originally present - is determined.

Class Discussion and Formation of Project:

A series of newspaper and/or magazine articles is presented to the class. These relate to studies that have been done on the effects of increased UV radiation on living organisms - skin cancer statistics, effect on midge larvae and the consequences to fish population, the decrease in frog, toad and salamander eggs in the Northwest.

Cooperative groups of students are then formed (assigned or self-selected). The groups are encouraged to speculate on some interesting personal aspects of increased UV exposure - suntans, skiing at high altitudes, wearing sunscreen, the effect of wearing different types of clothing when out in the sun, brands of sunglasses, etc. The group must come up with a hypothesis that can be tested using the yeast organism and the results of learning the lethal dosage for our area. An example of a hypothesis: "The time required for a lethal dose of UV radiation is shorter at an altitude of 9000 feet than at 6600 feet." (We live in an area where nearby communities are at these altitudes).

After a hypothesis/plan is accepted by the teacher the group divides up the responsibilities for the project. A project guide is given to the group explaining the basis for evaluation. They must keep a journal record of their daily work, do a literature search, perform the experiment successfully, analyze the data and present their results to the class. Each component receives a portion of the final points on the project. Each student keeps track of her/his contributions in the journal.

Method of Evaluation:

The group submits a summary of individual responsibilities - who did what - and compares it to guidelines the group was given when their hypothesis was accepted by the teacher. The group also suggests a grade for the project (in points). This is submitted with the project after presentation to the class. The teacher then follows the same procedure - comparison to the guidelines - to arrive at a grade. Discrepancies are resolved by discussion.

Extension/Reinforcement:

Throughout the year, students are asked to bring in any articles from newspapers or magazines that have to do with ozone depletion and its consequences.


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