Carmen M. Simon Return to Biology Menu
South Shore High School, Brooklyn
Summer 2001
Phototaxis
Introduction:
All
living things strive to survive.
Dictyostelium are soil dwelling amoeba that feed on bacteria.
Most organisms, including Dictyostelium,
thrive at optimal conditions which include temperature, moisture, food
availability, pH, and light intensity.
Dictyostelium prefers relatively dark, warm (20' C), slightly acidic and
moist environments. When food
is plentiful, Dictyostelium cells
live independently of each other as singled celled amoebae.
However, when food is scarce, Dictyostelium
cells aggregate (combine) to form multicellular stationary mounds which
eventually develop into slithering slugs. When
slugs find an appropriate environment for continued development, they form
fruiting bodies. The fruiting
bodies consists of 2 structures, the stalk and the sorus (head) that contain
spores. Fruiting bodies burst to
disperse spores so that they may be carried by the wind or animals to an
environment more suitable for survival. When
spores land in a food-rich, moist environment, they will geminate. Dictyostelium will,
once again, live as unicellular independent amoebae; until the next crisis.
During this investigation, we will observe the behavior of Dictyostelium discoideum when exposed to white light in an
environment that has a limited food supply.
Problem: How
will starving Dictyostelium discoideum
slugs respond to white light?
Hypothesis: Starving Dictyostelium discoideum slugs will be attracted to/repelled by
white
light?
choose 1
Materials:
per class
1
Petri dish containing developing Dictyostelium
discoideum
1
roll of adhesive tape
per group of 3 to 4 students
3
Petri dishes containing non-nutrient agar (NNA)
1
Sharpie marker
1
disposable micropipette
1 microfuge tube containing 100ml bacterial culture
2
strips of aluminum foil 25 cm (10
inches)
1
toothpick
Experimental Design:
1.
Using the Sharpie marker, draw a square (1cm2) at the center of the
outside of the bottom portion of your 3 Petri dishes. Label the poles of the
dishes; north, south, east, and west.
2.
Squeeze the micropipette bulb to expel the air.
Submerge the tip into the microfuge tube containing the bacterial
culture. Release the bulb to take
in 25ml
of bacterial culture.
3.
Open the Petri dish by removing the top portion.
4.
Squeeze the 25ml
of bacterial culture onto the non-nutrient agar at the center of the Petri dish.
Immediately close the Petri dish to reduce contamination of the specimen
(as well as to yourself).
REPEAT
STEPS 2-4 FOR EACH PETRI DISH.
5. Allow bacterial culture to
dry on Petri dishes for approximately 10 minutes.
6.
Using the wide tip of the tooth pick, swipe a small sample of the Dictyostelium
culture.
7.
Gently poke the center of the Petri dish with your sample.
Repeat steps 6 & 7 for each Petri dish.
8. a) Petri dish #1- Tape shut. Incubate at room
temperature in the light.
b)
Petri dish #2- Tape shut. Incubate at room temperature in the dark. That is,
wrap Petri dish completely with foil. Be
certain that no light can enter the dish.
c)
Petri dish #3- Tape shut. Wrap dish
completely with foil. Make a small
hole, about the size of the head of a pin, on the NORTH
side
of the dish. Note: The cells should
perceive the light from the side, not from above. Incubate
at room temperature in the light.
9. Observe in 48 hours for slug development.
10.
Measure the distance of slug movement from the center of the dish and determine
the slug orientation by observing the slug tip direction. Collect data for at
least 5 slugs per direction per plate. Average
the distance traveled for each direction. Graph your results.
Data
Tables
Petri Dish # 1 (5 measurements each)
Direction
Distance Traveled (mm)
Average in mm
North_________________________________________________
South_________________________________________________
East__________________________________________________
West_________________________________________________
Petri
Dish # 2
(5 measurements each)
Direction
Distance Traveled (mm)
Average in mm
North_______________________________________________________
South_______________________________________________________
East________________________________________________________
West_______________________________________________________
Petri
Dish # 3
(5 measurements each)
Direction
Distance Traveled (mm)
Average in mm
North_______________________________________________________
South_______________________________________________________
East________________________________________________________
West_______________________________________________________
Data
Analysis- draw a bar
graph.
a.
Label the x-axis, "direction traveled".
Label the y-axis, "average distance traveled in mm".
b.
Determine and label an appropriate scale for the y-axis.
c.
Your graph should contain 3 bars above each direction. One bar for each Petri
dish.
Conclusion- Based on the data you collected, was your hypothesis correct?
Post-lab questions
1.
Dictyostelium live in the soil where it is relatively dark.
The Dictyostelium cells
in our investigation were placed in an environment that had a limited food
supply.
Based on the results of this investigation, what inference can
be made about the response of starving Dictyostelium
slugs to white light and their need to survive in nature?
2.
What was an external stimulus (change) that occurred within the environment that
caused the Dictyostelium slugs to
respond white light?
3.
What was an internal stimulus (change) that occurred within a Dictyostelium
slug that made it respond to
the white light?
4.
Did Dictyostelium slugs
learn this response to the white light or was it an innate response?
5.How
do you think the starving unicellular amoeba communicated with the other
starving unicellular amoeba?
6.
Think! How did the slugs sense the location of the light?
7.
How does the response of Dictyostelium
to white light make it fit to survive?
8.
Describe 3 similarities Dictyostelium
has with humans.
Correlation to the National
Standards:
Content
Standards
Topic-
The Behavior of Organisms
Description:
Organisms have behavioral responses to internal changes and to external
stimuli. Responses to external
stimuli can result from interactions with the organism's own species and others,
as well as environmental changes; these responses either can be innate or
learned. The broad patterns of
behavior exhibited by animals have evolved to ensure reproductive success.
Animals often live in unpredictable environments, and so their behavior
must be flexible enough to deal with uncertainty and change.
References
Bozzone,
D. M. 1987. A Practical Guide to
the Use of Cellular Slime Molds for
Laboratory
Exercises and Experiments. Department
of Biology, Saint Michael's
College,
Colchester, Vermont
Kessin,
R.H. and Van Lookeren Campagne , M.E.(1992) The Development of a Social
Amoeba.
American Scientist 80: 556-565.
Appendix:
Materials
and kits can be purchased through www.carolina.com.