biological clocks and circadian rhythms
Biological
Clocks &
E
Circadian
Rhythms
very morning when I arrive in my classroom, the leaves of the shamrock plants
unfurl; every night at sunset, these same
leaves close up. What controls this leaf movement? Can the leaf movement be altered? Why does
the leaf close at night? Students can explore these questions and others with this inexpensive houseplant.
Biological clocks are a ubiquitous biological phenomenon
that scaffolds the behavior and regulation of all living systems,
including middle school students. As students explore biological clocks and circadian rhythms, they are provided with opportunities to
connect learning to experiences and observations from their own lives.
The study of biological clocks and circadian rhythms is an excellent way
to address the inquiry strand in the National Science Education Standards
(NSES) (NRC 1996). Students can study these everyday phenomena by designing experiments, gathering and analyzing data, and generating new experiments. These investigations also address the Standard of teaching
unifying concepts and processes though examining form and
function, systems, order, and measurement (NRC 1996).
Overview
Biological clocks control the
circadian rhythms of organisms.
The term circadian means “about a
day” and is used to describe rhythms
that have a 24-hour cycle.
The most obvious circadian rhythm in animals is
the sleep/wake cycle,
but it is only one of
many circadian
rhythms.
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SCIENCE SCOPE
by Laura Robertson and
M. Gail Jones
biological clocks and circadian rhythms
Experimenting with shamrocks
The following section outlines a basic setup for observing the circadian rhythms of shamrock plants. This
basic setup can be used with inquiry projects designed
and completed by small groups of three to four students (see Activity Worksheet). Shamrock plants can
be obtained as either bulbs or ornamental houseplants.
These naturally occurring plants can sometimes become a pest due to their hardy, quick-growing nature,
but that comes in handy for indoor experiments. They
are easiest to find around St. Patrick’s Day, and plants
usually cost $5 or less (they are often on the clearance
rack the day after). These plants can be divided into
four or more smaller plants for student experiments.
Bulbs can be purchased online year round for approximately 30 cents each. If planted as bulbs, small
plants with many leaves can be expected in about two
weeks. Each small group of students needs one or two
shamrock plants, depending on the experiment they
conduct. If the project starts with healthy plants, the
experiments can be completed in four days.
Place the plants in a closet or cabinet with a lamp
(with the shade removed) on a timer (Figure 1). The
light bulb can be a typical indoor light bulb of 60–100
watts. Specialty bulbs for growing plants are available
at home improvement stores and cost approximately
$5, however, these bulbs are not necessary for the
experiment to work. For optimum response, the closet
or cabinet should be as dark as possible and the plants
should be no more than 4 ft. from the light source. Attach the light fixture to a timer. (These are also available at home improvement stores for about $5.) Set the
timer to produce lighting approximately the same as
natural conditions by matching the times of sunrise and
sunset at the time of the experiment. Leave the plants
in the closet with the timer for two days. Use a camera
on a timer to make observations of the plant movements at six-hour intervals. The easiest setup is to use
a web camera built into a laptop computer to take the
pictures. Web cameras usually come with software that
allows the camera to be set on a timer. Free downloads
of timer software for webcams are also available. Other
types of cameras can be used to take the pictures.
Depending on class and teacher schedules, pictures
every six hours may not be possible and the timing of
the pictures may need to be adjusted. The critical times
for pictures are the one- to two-hour periods after the
light comes on and goes off. Alternatively, teachers can
reset the biological clock of shamrock plants so that
they open or close during the school day, in which case
cameras and timers would not be needed (see Resetting
the Biological Clock of Shamrock Plants). On the third
day, remove the timer and leave the light on continuously. Continue to make observations of the plants every six hours using the camera. Sample results from a
similar experiment are shown in Figure 2. As shown in
the photographs, the shamrock leaves continue to open
and close in continuous light at times similar to the
FIGURE 1
Experimental setup
Photo courtesy of the authors
Other examples include daily cycles in blood pressure,
body temperature, heart rate, hormone levels, gene
expression, and cellular division. Biological clocks have
been found in every type of living organism, from bacteria to humans. There is also evidence to suggest that
every cell of an organism contains its own clock that
influences the functioning of the cell.
Biological clocks are internal, genetic clocks that
respond to the environment. However, because they
are endogenous (internal), circadian rhythms persist in
constant conditions (such as the absence of light). The
adaptive value of an internal rhythm is that an organism is able to predict changes in the environment. For
instance, birds begin foraging for food in the morning
and do not have to wait for the sun to come up. Biological clocks also control circannual rhythms, which have
a yearly cycle. Circannual rhythms allow organisms to
prepare for seasonal cycles through changes in coat
color or migration. The daily movements of shamrock
plants include opening their leaves during the day and
closing their leaves at night. If this is a true circadian
rhythm, the plants will continue to open and close their
leaves in constant lighting conditions. Encourage students to locate additional information about plant circadian rhythms with reference materials and the internet.
An experiment to test this behavior is outlined below.
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biological clocks and circadian rhythms
FIGURE 2
Experimental setup
1:30 p.m.
7:30 p.m.
times that the light would turn on and off (times based
on natural sunrise and sunset). The responses of the
plants do weaken after a day, and the times of the leaf
opening and closing will not correspond as closely to
the natural times of sunrise and sunset. This is typical
of circadian rhythms and should be evident in student
data if the experiment is continued for additional days.
Resetting the biological
clock of shamrock plants
This experiment is designed to be conducted over a
four-day period using a camera on a timer; however,
FIGURE 3
1:30 a.m.
due to the nature of biological clocks, it is possible to
entrain a circadian rhythm to a different light/dark
cycle. For easier use in the classroom, it is possible
to shift the rhythm of shamrock plants by placing the
plants in a dark cabinet with a light on a timer and
phase shifting the light cycle. For example, the previous experiment can be modified by setting the timer so
that light onset begins at 11:00 p.m. and the light turns
off at 11:00 a.m. Placing the plant in these conditions
for about one week can reset the clock. In that way,
when the lights are left on continuously, the leaves will
continue to close at 11:00 a.m. during the school day.
A teacher may wish to reset the clocks of plants prior
Grading rubric
ScaleCriteria
Unacceptable (1 point)
Question and
hypothesis
Question or hypothesis is
missing.
Procedure is incomplete or
vague; little attempt to control
variables; single trial.
Original data are missing
or incomplete; graphs are
missing or incomplete.
Acceptable (3 points)
Question and hypothesis are
present but unspecific or unclear.
Procedure is unclear; some
Experimental
attempt has been made to control
design
variables and use multiple trials.
Original data are adequate; data
representation is adequate;
Data
graphs are present but unclear
or poorly labeled.
Conclusion is only partially related
Conclusion is not related to
to the hypothesis or conclusion is
the hypothesis and is not
Conclusion
not supported by data.
supported by the data.
Written lab/oral report is
Written lab/oral presentation
Communication is incomplete or unorganized; adequately organized;
report contains one or more
multiple grammatical errors
of results
grammatical errors.
Target (5 points)
Question and hypothesis are
specific and clearly written.
Procedure is repeatable; efforts
have been made to control
variables; multiple trials.
Original data provided; data are
represented through appropriate
graph(s); graphs are clearly
labeled and easy to read.
Conclusion is related to the
hypothesis and is supported
by data.
Written lab report/oral
presentation is organized and
free of grammatical errors.
Total score
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Score
/25
biological clocks and circadian rhythms
to student experiments. This would allow students to
observe the closing of the leaves during the school
day and would not require cameras or camera timers.
Once this experimental design is mastered, students
can generate many different testable hypotheses about
shamrocks or other organisms.
With the addition of a few common lab materials,
such as colored plastic wrap, binder clips, and paper
cups, this basic setup can support a variety of student-designed investigations. Allow small groups of students to
design research questions and experiments. Each small
group will need two plants, one for a control group and
one for an experimental group. Example questions for
student investigations include the following questions:
What happens to the circadian rhythm of shamrock
plants when they are covered in black plastic (or colored saran wrap)? Do individual leaves show circadian
rhythms? Do individual leaflets show circadian rhythms?
Do plants that are upside down show circadian rhythms?
Encourage students to design controlled experiments
in which only one variable is changed between the experimental and control groups. Students can summarize
their findings in oral or written reports. Figure 3 is a
sample rubric for assessing student projects.
Researchers now know that the movement of the
shamrock leaf is controlled by a small organ known as
the pulvinus, which is located at the base of the leaf.
When the pulvinus bends, the leaf folds up. The bending of the pulvinus is a result of changes in the potassium channels in the cells. In the morning, the potassium
channels are open and water moves into the pulvinus
cells, causing them to swell, and the organ straightens
out and the accompanying leaf opens up. When the
potassium channels close, the water moves out of the
pulvinus cells and the cells shrink, causing the pulvinus
to bend, and the leaf folds up. Currently, researchers
are studying the plant’s genes to determine the mechanisms that are used to control the opening and closing
of the potassium channels. Challenge students to find
other plants, such as bean plants and the mimosa tree,
that exhibit circadian movements. Many flowers, such
as morning glories, flower at set times of day.
Because circadian rhythms are present in all living things, there are many other rhythms that can be
easily observed or tested. Figure 4 provides a list of
additional questions related to biological clocks around
which students can design experiments. Students can
determine if rhythmic behaviors of classroom pets or
plants are true circadian rhythms by exposing the organisms to constant lighting conditions. Other human
circadian rhythms can be discovered through surveys
of classmates, friends, and parents. Teachers can al-
FIGURE 4
Extended biological-clock
investigations
•Chart daily rhythms of classroom or family pets.
• Measure the time to complete a word search or
multiplication facts (morning vs. afternoon).
•Test reflex speed (morning vs. afternoon).
•Collect classroom data about time of birth.
•Survey parents about the age that children sleep
through the night.
•Survey students and/or parents about seasonal
affective disorder.
•Research strange rhythms in nature.
•Chart migrating species in your area or online at
www.JourneyNorth.org.
• Put a bird feeder on the window; record feeding times
of different birds.
• Grow caterpillars and record time of eclosion
(emergence from cocoon or chrysalis).
•Record daily changes in body temperature.
• Plant seeds and record the time seedlings emerge.
•Research and/or survey possible effects of daylight
saving time.
•Test your ability to wake up without an alarm clock
on the weekends.
low students to select projects based on their interests
without worrying about expensive or difficult materials.
The needed materials can be as simple as a light source
and a dark cabinet. Topics might even lead to science
fair projects or more in-depth classroom studies.
Applications and criticalthinking questions
While experimenting with circadian rhythms, teachers should challenge students to think about the applications, impacts, and adaptive values of circadian
rhythms. In the case of the shamrock plant, the opening of the leaves maximizes the surface area of each
leaf to capture sunlight for photosynthesis, while the
folded leaves at night reduce the surface area of the
leaf and conserve water.
Figure 5 highlights some of the more important
aspects of biological clocks, including medical applications. Encourage students to locate information related
to these challenge questions on the internet. Current
medical practices do not always take circadian rhythms
into consideration. Research has shown that the posi-
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biological clocks and circadian rhythms
FIGURE 5
Applications and critical-thinking questions
What are applications of circadian rhythms?
Medications can be more effective and have fewer side effects if administered at certain times of the day.
How are circadian rhythms helpful adaptations?
Circadian rhythms allow animals and plants to prepare for cyclic changes in their environment. For example, many birds
wake up before sunrise and are ready to forage in the early morning.
What are environmental situations that an organism might need to prepare for or avoid?
Circannual rhythms help organisms time behaviors to optimize beneficial environmental conditions (e.g., abundant food
sources) and avoid difficult conditions (e.g., exceptionally cold or dry seasons). Examples include trees shedding their
leaves, hibernation, reproduction, and migration.
What is necessary to prove something is a circadian rhythm?
To prove something is a circadian rhythm, it must have a period of approximately 24 hours and persist in constant conditions.
How do circadian rhythms complicate the idea of homeostasis?
Instead of maintaining a constant internal environment, the levels of many chemicals and behaviors vary during a 24-hour
cycle. For example, levels of melatonin, an important chemical involved in maintaining circadian rhythms, peak at night.
When do you think circadian rhythms develop in organisms?
In humans, circadian rhythms develop around 15 weeks of age.
How might you change a circadian rhythm? Describe an experiment to test your idea.
One way to change a circadian rhythm would be to expose an organism to lighting conditions that were phase shifted
from natural conditions.
tive and negative effects of many drugs and medical
procedures are impacted by the time of day that they are
administered. Circadian rhythms also add an interesting
and complicating dimension to the concept of homeostasis. While homeostasis is generally defined as the maintenance of a stable internal environment, the levels of
many chemicals and physiological processes have a daily
cycle. Hormones, enzymes, ions, blood volume, temperature, and mitosis all have daily rhythms. The hormone
melatonin, which peaks at night and drops to very low
levels during the day, is very important in regulating
biological clocks. (See Resources for helpful websites
containing in-depth information and teaching resources.)
Conclusion
Biological clocks are an excellent way to engage students
in the processes of inquiry. Not only are circadian rhythms
present in all types of organisms, they can be easily tested
and manipulated in the classroom with relatively simple
materials. This ease of access and availability provides an
environment in which teachers can allow students the freedom to design their own studies to answer personally relevant and engaging questions. Once students start looking,
they may be surprised at how many aspects of daily life are
impacted by the internal rhythms of biological clocks. n
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Acknowledgment
Appreciation is extended to Dr. Herb Underwood for his assistance developing the plant investigation.
References
National Research Council (NRC). 1996. National science
education standards. Washington, DC: National Academies Press.
Resources
Cockroaches Are Not Morning People—www.livescience.
com/animals/070930_cockroaches_learning.html
A Live Science article on training cockroaches, includes
links to other articles.
HHMI’s Biological Clocks—www.hhmi.org/biointeractive/
clocks
Includes lectures with discussion questions and student activities, and a downloadable teacher’s manual.
National Geographic’s Pacific Salmon—www.national
geographic.com/xpeditions/lessons/09/gk2/
migrationsalmon.html
Allows students to chart the migration of salmon.
Neuroscience for Kids—http://faculty.washington.edu/
chudler/clock.html
Includes a list of five activities for students related to
biological clocks and circadian rhythms
Activity Worksheet: Experimenting with shamrocks
Materials
• Shamrock plants, 2 per small group of students
• Light fixtures (with the shade removed) with indoor light bulbs, 60–100 watts, 1 per closet
• Light timer, 1 per light fixture
• Camera with timer, 1 per closet
• Dark closet or cabinet
• Extension cord for light and/or camera (depending on classroom organization)
• Black plastic or colored plastic wrap to cover plants
• Binder clips or rubber bands to attach plastic cover
• Paper cups for individual leaf stems in water
Setup
1.Select a dark cabinet(s) that can contain plants for each small group of students.
2.Set up light fixtures, timers, and cameras in experiment closets.
3.Set the light timer so that lights turn on and off at the times of natural sunrise and sunset. (Students can check the local
weather report to determine current sunrise and sunset times.)
4.Set the camera timer to take pictures every six hours. Pictures should be taken after changes in lighting. If the timer is set
to go on at 8:00 a.m. and off at 8:00 p.m., pictures should be taken at approximately 9:00 a.m., 3:00 a.m., 9:00 p.m., and
3:00 a.m.
Procedure
Day 1
1. Describe for students the position of shamrock leaves that
you have noted at different times of the day. Challenge
students to design an experiment to determine what
factors might influence the rhythm of leaf movement.
2.Allow students time to observe shamrock plants in small
groups. Students should sketch the plants and one of the
leaves making note of the position of the leaves.
3.Students should generate a research question and design
a controlled experiment to test one factor that might
influence leaf movement.
4. Students should prepare their plants and place the
shamrock plants in the cabinet with the light, timer, and
camera. Leave the plants overnight with the camera
taking pictures every six hours.
biological rhythms.
NIH’s Sleep, Sleep Disorders, and Biological Rhythms—
http://science.education.nih.gov/supplements/nih3/
sleep/default.htm
Offers a downloadable curriculum and a database students can access to answer hypothesized questions.
Science NetLink’s Biological Clock—www.sciencenetlinks.
com/lessons.cfm?DocID=286
Provides two lessons and materials for introducing
Day 2
1.The teacher may want to access the stored pictures and
share them with the class as a daily up-date, but the plants
should be disturbed as little as possible. Leave the plants
in the cabinet on with the light timer for another 24 hours.
Day 3
1.Again, the teacher may want to share pictures from
the previous 24-hour period. For the next 24 hours, the
light should be left on continuously. The camera should
continue to take pictures every six hours.
Day 4
1.Student groups can remove their plants from the cabinet
and make general observations about the plants and leaf
position. The teacher should share the pictures with the
class. Students can summarize their findings in written or
oral reports.
students to biological clocks in humans, plants, and
migration.
Laura Robertson ([email protected]) is a research
assistant and M. Gail Jones ([email protected])
is a professor in the science education department
at North Carolina State University in Raleigh, North
Carolina.
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