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ACTIVIT Y OVERVIEW
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40
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Looking for Signs of Micro-Life
BO RA
T
SUMMARY
Students prepare a wet mount in order to find evidence of microscopic life.
KEY CONCEPTS AND PROCESS SKILLS
1.
The microscope can be used as a tool for investigation.
2.
Microscopes reveal organisms that cannot be seen with the naked eye.
3.
Most infectious diseases are caused by microbes.
4.
Life is composed of a variety of organisms that vary greatly in size.
5.
The process of science utilizes skills in observation and drawing.
KEY VOCABULARY
disease
magnify/magnification
infectious
microbe
MATERIALS AND ADVANCE PREPARATION
For the teacher
*
1
Transparency 35.1, “Bubbles, Dust, or Micro-Life?”
1
Transparency 36.1, “Common Types of Micro-Life”
1
overhead projector
aged tap water (or bottled spring water)
*
hay for preparation of hay infusions (or other source of culture)
rice (optional)
For the class
*
*
1
microscope video camera (optional)
Demoslide® tubes containing culture
paper towels
Teacher’s Guide
C-89
Activity 36 • Looking for Signs of Micro-Life
For each group of four students
1
bottle of methyl cellulose
1
plastic cup containing at least 10 mL of culture (such as a hay infusion or
aquarium water)
plastic cup containing water
droppers
For each pair of students
*
1
microscope slide
1
coverslip
1
microscope
For each student
1
Student Sheet 36.1, “Microbe Drawings”
*Not supplied in kit
Two weeks before conducting the activity, set up hay infusion cultures or order cultures of protists such as Paramecium and Amoeba from a biological supplier. You may
need to allow extra time for hay infusions if the cultures will be in a cool place (such
as in the school over a winter weekend). The debris at the bottom of an aquarium can
also provide a source of protists and other microorganisms. Water from a local pond
or creek may also provide a good source of microbes, and can be used instead of tap
water to culture the hay infusions. Add a few rice grains to each culture to provide a
food source and enhance microbe growth.
To make a hay infusion:
1.
Let one gallon of tap water stand overnight to allow the chlorine to evaporate.
2.
Set up eight cups of hay infusion. Fill each cup 2/3 full with the aged tap
water or bottled spring water. Add a pinch of the hay provided in the kit and a
few grains of rice to each cup.
3.
Cover the cups with a sheet of paper and keep them in a fairly warm place,
preferably in a cupboard or in a prep room away from the classroom. Hay
infusions can smell very unpleasant, but need to be exposed to air.
4.
Periodically check on the cultures to make sure that all of the water has not
evaporated; if significant evaporation is taking place, add more water.
5.
Check the cultures for protists a few days before class to be sure there are
enough protists for the students to observe fairly easily. Water samples taken
from near the decaying matter should have a greater abundance of microbes.
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Science and Life Issues
Looking for Signs of Micro-Life • Activity 36
If you can centrifuge your sample, this will further concentrate the organisms
at the bottom of the tube.
6.
On the morning of the activity, check your cultures. If the microscopic life is
scarce, delay the lab a day or two and try again.
Set up Demoslide tubes from the hay infusions and/or a local pond in addition to having students make their own slides. Fill the tubes to the line, cap, and shake the liquid
down into the transparent, flat part at the base. These specimens will last at least one
day and will frequently show more microscopic life than slides. They are especially
helpful for saving material for absent students. Students should be careful not to
scratch the optical surfaces of the plastic. These tubes are difficult to wash and re-use.
The supplied kit materials include glass coverslips. You may wish to substitute plastic
ones. Plastic coverslips do not break as easily, but they do scratch after one or two uses.
If your school has a microscope video camera, use it for this activity. It will allow you
to project images viewed through the microscope onto a TV monitor.
You may want to keep the hay infusion going because you could use it again in Activity 47, “Reducing Risk.”
TEACHING SUMMARY
Getting Started
1. Prepare students to search for microbes.
Doing the Activity
2. Students prepare and observe slides from an aquatic culture.
Follow–Up
3. The class shares their observations of micro-life.
INTEGRATIONS
Art
Looking through the microscope while drawing observations provides an example of
representational drawing and can be used to discuss careers such as biological and
medical illustration.
Teacher’s Guide
C-91
Activity 36 • Looking for Signs of Micro-Life
BACKGROUND INFORMATION
Microbes
The term microbe is often used in place of the word microorganism. This allows
viruses, which are even smaller than bacteria, to be included in this category.
Viruses are not generally classified as living or as organisms because they lack a
cellular structure. They can be visualized only with an electron microscope.
The category of microbes also includes unicellular life such as bacteria and protists (formerly known as protozoa). Microscopic life also includes some fungi
such as yeasts, small multicellular plants, and small multicellular animals such
as water fleas (Daphnia), larvae of a variety of organisms, and microscopic worms
such as nematodes.
What to expect with classroom hay infusions
The exact species present in your cultures will depend on the source of the hay,
the age of the culture, and the chemical composition of the water you use. The
majority of unicellular organisms visible through the light microscope are likely to be protists (further described below). In addition, your sample may contain
a variety of algae. Multicellular animals such as nematode worms, daphnia, and
rotifers may also be found.
The most common protists in hay infusions appear to be oval creatures that
seem to spin as they move forward. These organisms are probably ciliated protists in the genus Paramecium. Students may be able to see cilia beating (as a faint
shimmer around the edge of the cell) if they reduce the light coming through
the diaphragm. Paramecia usually feed on organic debris and smaller cells,
including other protists, bacteria, and algae. Students may be able to see food
vacuoles as dark specks inside the cell. They may also be able to see the indentation in the center that funnels the food to the “mouth” (the area that forms
the food vacuoles). Other ciliates that students may observe include stalked varieties anchored to bits of detritus and ones that seem to “walk” over the detritus
with specially adapted cilia. Paramecia and other ciliates are important in pond
ecosystems as the food source for many small multicellular organisms such as
water fleas (Daphnia), rotifers, and young fish.
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Science and Life Issues
Looking for Signs of Micro-Life • Activity 36
Some students may also observe blob-like cells moving slowly among the debris.
Amoebas are more difficult to find than the free-swimming ciliates. They move by
cytoplasmic streaming, which gives them the appearance of oozing along. Many
amoebas are scavengers or predators living in fresh water, salt water, or soil. Others are
parasites that cause disease in humans. Entamoeba histolytica is responsible for amoebic dysentery.
Oval or rice-shaped cells that seem to move jerkily may be protists of the genus Euglena: They have a long whip-like structure, called a flagellum, that propels them
through the water. Some euglenoids have an eyespot, which allows them to sense
light. Most euglenoids are free-living (as seen by the presence of chloroplasts).
REFERENCES
Society of Protozoologists, ed. J. Lee, S. Hutner and E. Bovee. An Illustrated Guide to
the Protozoa. USA: Allen Press, Inc., 1985.
Garrett, Laurie. The Coming Plague. New York: Farrar, Straus, and Giroux, 1994.
Teacher’s Guide
C-93
Looking for Signs of Micro-Life • Activity 36
TEACHING SUGGESTIONS
GETTING STARTED
1.
Prepare students to search for microbes.
then doing the entire drawing without referring
back to the image.
n Teacher’s Note: The diagram on page C-28 in the
Student Book shows a labeled chloroplast and cell
wall. Students do not yet know the parts of cells
Have students read the Introduction on page C-27
and can instead label distinctive features such as
in the Student Book. Then tell students that they
“edge of organism,” “clear area,” “dark spot,” etc.
will be searching for living organisms in the cups of
DOING THE ACTIVIT Y
culture at their tables. Ask students, How could you
figure out if something is a microbe? This question
will elicit a variety of responses depending on stu-
2.
Students prepare and observe slides from
an aquatic culture.
dents’ prior experience of microscopy. Some students may say they would look for moving things,
Let students prepare their slides and begin search-
green things, or complex structures. Accept all
ing for microscopic life. Provide students with time
answers, but point out that it may be difficult at first
to simply look for microbes and enjoy microscopy
to distinguish inanimate debris from microbes. If
before having them draw their observations. Some
necessary, use Transparency 35.1, “Bubbles, Dust, or
will require help with rotating the high power
Micro-Life?” to review the different appearances of
objective safely into place. Have students who have
a bubble, dust, and a microbe.
had more success with handling microscopes help
You can also display Transparency 36.1, “Common
others.
Types of Micro-Life,” to help students identify the
If students are having trouble seeing living things,
general types of organisms they may be seeing. Stu-
remind students to:
dents may recognize that many or most of the
•
organisms they see have a structure (the cell membrane) that separates them from the surrounding
environment. Students may also be able to identify
if an object is plant-like (usually by its green color),
Adjust the amount of light with the
diaphragm under the stage.
•
Try making another slide using a small
sample from the bottom of the cup or near
some detritus.
animal-like (typically moving), or not alive. They
do not need to identify organisms or exact species.
After the initial excitement has died down, ask students to find some interesting microscopic organ-
Review “Microscope Drawing Made Easy,” found on
page C-28 in the Student Book, with the class. Com-
isms and to draw two of them on Student Sheet
36.1, “Microbe Drawings.”
mon student errors include preparing a drawing as
tiny as the image, writing the labels on the draw-
Dispose of cultures by pouring them into some dirt
ing, and trying to draw every detail. Another com-
outside of the classroom (and not down the sink).
mon error is taking a quick look at the image and
You may wish to save one culture so that interested
Teacher’s Guide
C-95
Activity 36 • Looking for Signs of Micro-Life
students can track what happens to the populations
n Teacher’s Note: Ecological roles will be explored
of organisms over time.
in more depth in the Ecology unit of Science and Life
As students complete the activity, they can begin to
Issues.
respond to Analysis Question 2, which can be
After students have completed Analysis Question 2,
assessed with either element of the C OMMUNICAT -
you could have students describe an organism to
S CIENTIFIC I NFORMATION (CM) variable. You
the class while others draw what they hear being
may want to first read aloud to students Anton van
described. This will cue students to be sure to write
Leeuwenhoek’s description of the algae Spirogyra,
very detailed and accurate descriptions.
ING
found in Activity 37, “The History of the Germ Theory of Disease,” on page C-33 in the Student Book.
SUGGESTED ANSWERS
TO ANALYSIS QUESTIONS
FOLLOW–UP
1.
3.
The class shares their observations of
micro-life.
Is it possible that microbes smaller than you
observed exist? Explain how you might try
to collect evidence to prove or disprove your idea.
Have students share what characteristics they
Students have no direct evidence of microbes
looked for when trying to decide if something was
smaller than they observed. They may hypoth-
alive. The most obvious will be movement. Ask,
esize that smaller organisms exist, and have
Could something that doesn’t move still be alive?
ideas that can lead to this inference. For exam-
Encourage students to think about plants and dis-
ple, the smallest microbes they observed must
cuss any microscopic plants or algae they may have
themselves have a food source. Some microbes
seen: Were the plants or algae moving? (Be aware
absorb decaying matter or are parasites on larg-
that microscopic plants or algae often appear to
er organisms, while others ingest smaller
move due to water movement on the slide.) Discuss
microorganisms. Many students will have
the types of organisms students observed. Discuss
heard of bacteria and viruses. Viruses are too
Analysis Question 1 with the class.
small to see with the light microscopes, while
Ask students, Where would you expect to find
microbes and what do you think they might be
doing there? Explain that the majority of microbes
are free-living (non-parasitic). They live in almost
stained bacteria can be seen (with some difficulty) on the highest power. (Students will
observe stained bacteria when they do Activity
42, “A Closer Look.”)
every environment on Earth, including deep inside
Some students may suggest that they could use
rocks, in hot springs, in the soil, and even in crude
more powerful light microscopes or electron
oil. They are vital parts of the food web, serving as
microscopes.
decomposers, scavengers, carnivores, herbivores,
parasites, and producers.
C-96
Science and Life Issues
Looking for Signs of Micro-Life • Activity 36
2.
As a scientist, you are asked to
you could only partly see through them, they
CM
describe two of the microbes that you saw to
looked greenish-brown. We also saw a larger
someone who has never looked through a micro-
worm-like creature. It looked like a long empty
scope. Write a short paragraph describing the
tube that was pointed at both ends.
microbes that you observed.
3.
Reflection: Imagine that you are a researcher
A complete and correct level 3 response is
studying microbes. Would you choose to study a dis-
shown below:
ease-causing microbe or one that does not cause dis-
If you looked at the culture with just your eyes,
ease? Explain.
it would look like dirty water with hay in it. But
Many students are likely to decide that study-
a microscope makes everything bigger and
ing disease-causing microbes is the most bene-
shows you things you can’t see with your eyes.
ficial to humans and could help prevent further
We saw tiny round shapes that looked like they
spread of disease. Some students may be con-
were spinning. They moved really fast if you
cerned about the risks involved in studying dis-
didn’t add methyl cellulose. You could kind of
ease-causing microbes and prefer to study one
see their insides, but not too clearly. Because
that did not cause disease.
Teacher’s Guide
C-97
Name
Date
Microbe Drawings
To calculate total magnification, multiply the magnification of the eyepiece
(usually 10x) by the magnification of the objective.
Microbe Drawing 1
Magnification ________
Microbe Drawing 2
©2006 The Regents of the University of California
Magnification ________
Science and Life Issues Student Sheet 36.1
C-99
Common Types of Micro-Life
Single-celled microbes
Microscopic plants
©2006 The Regents of the University of California
Multicellular microbes
Algae
(may appear green)
Science and Life Issues Transparency 36.1
C-101