Biology/Chemistry

Life in a Drop of Water

Objective(s):

Grade Level: Middle/ Upper Level -Elementary School

Periods: 4-40 minutes

To observe the wealth of living organisms in a drop of water from our freshwater ecosystems

To classify the types of organisms observed as single-celled or multi-cellular

Use a key or chart to identify and name the organisms found

Background:

            Ecologists divide freshwater ecosystems into bodies of running water and bodies of standing water.  In the latter the current slows and most suspended particles settle to the bottom. Lakes and ponds are bodies of standing water that will serve as the sources for water samples to investigate freshwater ecosystems.

            Protists, which comprise many of the organisms found in the water samples, are mostly single-celled microscopic organisms.  They have a nucleus and other cell parts. There are three groups of protists: protozoans (animal-like protists), algae (plant-like protists), and fungi.

Materials:

Pond water, lake water, microscopes, slides, cover slips, droppers, paper towels, small dishes, key of freshwater microorganisms (obtained from a book such as one of the references listed below)

Strategy:

1. Teacher will collect enough pond/lake water in order for each student to have his/her own sample.
2. Review usage and handling of a microscope during period 1.
3. Explain the steps for preparing a slide (period 1).  Each student will be required to prepare a slide using the water sample(s).  Place a drop of water on the slide and cover with coverslip, make sure no air bubbles are present.
4. Observe the slide under the low power objective lens and search for protists. Watch also for movement.
5. When you find some protists that are not moving or moving slowly switch to the high power objective lens.
6. Draw your protists within circles (about 8 cm in diameter) drawn on sheets of paper.
7. Note the eyepiece and objective lens magnifications and calculate total magnification. This is the eyepiece magnification times the magnification of the objective lens (both should be marked right on the lenses).

NOTE: Pond water can be simulated by preparing a culture of hay infusion.   Method:  Fill a large jar about two-thirds full of tap water. Let it stand uncovered for three or four days so that air can dissolve in it. Cut some hay or dried grass into short pieces and drop two handfuls into the water. Cover the jar and allow the culture to incubate about two weeks.  When the culture is ready, examine several drops taken from different parts of the jar.

Performance Assessment:

Draw and label living organism observed under the microscope.

Use the key to identify and group organism(s) found in the water samples.

Conclusions:

With all life, irrespective of scale, there is a continual struggle and competition for survival.  A pond is a miniature cosmos.  A diverse range of living forms, both animal and plant, compete for energy supplies, food, space, and other resources.

References:

Daniels, Lucy, Hummer, Paul J. Jr., Kaskel, Albert. 1988. Biology Laboratory Experiences, Merrill Publishing Company.

Alexander, Peter, Fiegel, Marilyn, Harris, Anne F., May, Kenneth W. 1990. Life Science, Silver Burdett and Ginn, Morristown, N.J.

Popular Science Encyclopedia, Volume 6. 1994.