Cells or unicellular organisms that move
in response to light exhibit
phototaxis. Phototaxis is
easily demonstrated using
Euglena,
Chlamydomonas, or
Volvox. The
following 2 demonstrations can be done in your classroom with
inexpensive materials you probably already have on hand. If
you’re pressed for time, check out our
Euglena Phototaxis Kit (shown right). It’s got all the materials you
need to demonstrate algal phototaxis—plus the
Carolina™ Protozoa
and Invertebrates Manual.
Background
Because light is essential for photosynthesis, many algae have
developed phototactic responses. An algal cell can have either a
positive phototactic response (it moves toward light) or
a negative phototactic response (it moves away from
light). Many algae respond negatively under bright light and
positively under dim light. Interestingly, Euglena
displays a positive phototactic response in daylight, but at
night, even when light is introduced, it is unresponsive.
Demonstration 1
Materials
Concentrated motile algae (e.g., Euglena)
Large test tube
Black construction paper
Scissors
Clear tape
Procedure
- Cut narrow slits in the black construction paper.
- Tape the paper to the outside of the test tube.
- Pour a concentrated culture of algae into the test
tube.
- Allow the tube to remain in normal classroom light
undisturbed for 10–15 minutes.
- Remove the paper without jarring or moving the tube,
and observe that the algae are clustered where light
filtered through the slits in the paper.
Optional: To establish at what light intensity algae
react negatively and to what wavelength they respond,
perform the demonstration using several different light
intensities and wavelengths (colors).
Demonstration 2
Materials
Square petri dish
Black paint
Volvox culture
Stereomicroscope
Two microscope point lights or penlights
Procedure
- Paint the outside of a square petri dish black except
for a horizontal slit on each of 2 opposing sides.
- Place the painted dish on the stereomicroscope's stage,
and set up 2 microscope point lights (or penlights) for
illumination.
- Dim the room lights and add Volvox and medium to the
dish until it is about a quarter full.
- Move the point lights so that one is focused on one slit
and the other on the opposing slit. Keep one light close
to the dish and the other several centimeters away to
set up a light gradient, i.e., one side of the dish at
high light intensity and the other at low.
- Using the microscope, study the movement of Volvox and
its response to the lights. Intersect the light paths,
vary the distance between the lights, and note any
changes in the algae's response.
Additional resources
Further your students’ knowledge of
Euglena and other
protozoans with the following kits, cultures, and equipment.
Protozoa
Survey Set
Protozoa
Demoslide Set
Carolina™ Protozoa Slide Sets (Euglena)
Euglena BioKit®
Euglena Phototaxis Kit
Carolina™ Minipond Ecosystem Kit
Protist Observation System
Carolina™ Deep-Well Slides
Demoslide Stage Adapter
