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What Makes a Seed Breathe Faster?
Teacher’s Guide
by Lynn Vaccaro, CSIP Graduate Student Fellow, Cornell University
Overview
This two-day experiment allows students to detect cellular respiration from a surprising source, a
seed. The activity requires and reinforces a basic understanding of cellular respiration, seed
dormancy and germination, and experimental design. Depending on the course goals, each of
these topics could become the primary focus of class discussions surrounding the experiment.
During the activity, students are presented with an overarching goal: to determine how various
factors influence the respiration rate of a seed. In small groups or pairs they decide on one factor
to test, such as seed size, seed type, or moisture, light, or temperature conditions. Students all
use a standardized protocol for detecting and qualitatively measuring respiration rates. On day
two, students take their final measurements and interpret their results. The data from the class
can be combined within a single data sheet on a computer or on the board to identify broad
patterns. The procedure does not quantify a respiration rate, but it does produce an indicator of
respiration rate that can be compared across different treatments. Although students should see
several patterns in the combined results, the clearest pattern will be that seeds emerging from
dormancy have higher respiration rates. Because all seeds require water for germination, seeds
soaked over night or for a few hours before class typically have higher respiration rates. Pea and
bean seeds may even begin germinating during the experiment.
Main Concepts
 All cells (plants and animals) perform cellular respiration to maintain homeostasis and
grow.
 The unique dormancy and germination cues of a seed are adaptations for a seasonal or
variable environment.
 Experiments can produce valuable insights if a hypothesis is clearly formulated and
variables are carefully controlled across treatments.
Subject
Biology, Environmental Science, Botany or Ecology
Audience: 9-12 grade
Time required: 2-3 periods
Background
In some respects, plants are not that different from people. Although plants can make sugars
using the sun’s energy, when plants need energy they have to metabolize their stored sugars
through cellular respiration, just like we do. Plants need energy to maintain homeostasis, to
perform certain functions like transporting sugars, and to grow. Like us, a plant’s respiration rate
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is not consistent and depends on many factors. Think about when and why we breathe faster.
Do all people respire at the same rate when doing the same activity?
After a seed drops from a plant it usually goes into a resting period called dormancy when it
metabolizes stored energy reserves very slowly. Dormancy is defined as a state during which the
seed is not able to germinate. Very specific cues are needed to break dormancy in some species.
Seeds adapted to fire prone habitats may require high heat or smoke to break dormancy.
Deterioration of the seed coat, elevated soil nutrients, diurnal fluctuations in soil temperature,
prolonged rain, or a change in the quality of light could break seed dormancy. After emerging
from dormancy a seed is able to germinate and will respond to more familiar growth stimulating
factors such as moisture, light and soil nutrients.
The timing of seed germination will strongly influence the success of a seedling. If a seed
emerges too early in the spring it could die of frost. Wildflowers native to deciduous forests
often take advantage of the high light period before trees put out new leaves or after trees lose
their leaves; if these seeds wait too long to germinate they could miss their window of
opportunity. As a result, seeds have evolved complex ways of detecting that window of
opportunity. For example, many seeds can detect the quality or spectral composition of light.
Sunlight that passes through a canopy of leaves is depleted in red light relative to the amount of
far red light (longer wavelength). Thus, the proportion of light of different colors will trigger
germination of wildflower seeds, allowing these seeds to avoid being shaded by trees.
Some seeds have to wait for years before they are able to germinate. During this time, seeds
cannot make their own food because they lack leaves! Therefore, in order for a seed to stay alive
or to grow it needs to use stored energy reserves and undergo cellular respiration. Have you ever
wondered why seeds and nuts have so many calories? The seed will use those calories to survive
during dormancy and to germinate.
To fulfill the high-energy needs of a germinating seedling, cellular respiration increases as a seed
emerges from dormancy and begins germinating. However, seeds respire at a lower rate
throughout dormancy. In fact, seed suppliers measure seed respiration using a highly sensitive
method to determine if dormant seeds are still viable and suitable for cultivation.
When plants use sugars stored in their leaves or seeds they undergo cellular respiration
Sugar + Oxygen  Carbon dioxide + Water + Energy (ATP)
In this experiment you will use a substance called calcium hydroxide that absorbs any carbon
dioxide in the air and converts it to solid calcium carbonate. We will put seeds in test tubes that
contain calcium hydroxide, and then invert the tubes in a beaker of water. The calcium
hydroxide will react with any carbon dioxide that is produced and remove the gas from the test
tube air space. As the seeds respire, they are taking in oxygen and respiring out carbon dioxide,
but the carbon dioxide is absorbed by the calcium hydroxide. As a result, the amount of air in
the sealed test tube actually decreases and water rises in the test tube. This provides a visible
indication that respiration is actually occurring.
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Treatment 1
Treatment 2
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Blank
Calcium hydroxide powder
Cotton plug
Seeds
Cotton plug
Ruler
Rubber band
Air space within tube
Beaker with water
Figure 1. Diagram of the protocol for measuring respiration
The reaction
As the seeds respire they take in oxygen and release carbon dioxide at roughly the same rate. If
left alone in a sealed test tube, the carbon dioxide would replace any oxygen utilized by the seeds
and the air pressure would remain relatively constant. In this experiment, any carbon dioxide
released in the test tube reacts with the calcium hydroxide to form solid calcium carbonate, also
known as calcite or limestone. This process essentially removes all gaseous carbon dioxide from
the air space in the test tube and converts it to a solid. As more carbon dioxide is produced, more
carbon dioxide is removed from the air and the air pressure in the test tube declines, essentially
sucking water up into the test tube. If atmospheric pressure is higher outside the test tube than
inside the test tube, water will rise in the test tube. Theoretically, the difference in air pressure
should equal the weight of the water that rose in the test tube (P1- P2 = weight of water). Thus,
the height of the water in the test tube is an indicator of the amount of respiration that occurred.
It is theoretically possible to calculate a respiration rate from the change in the volume of air in
the test tube, but changes in humidity and barometric pressure could complicate the calculations.
P2 Air pressure within test tube
P1 Atmospheric pressure
(14.7 lbs/in2 at sea level)
Figure 2. Explaining the forces that cause water to rise in the test tube.
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Learning and Behavioral Objectives
 Students will develop their own hypothesis related to seed respiration.
 Students will design a controlled experiment that tests a single variable.
 Students will observe a tangible indication of cellular respiration.
 Students will use their results to construct an explanation about how seed characteristics
and environmental factors influence seed respiration.
National Science Education Standards
Depending on the focus of the course, respiration, plant adaptations, or experimental design
could be emphasized in order to teach some of the following standards.
Matter, Energy, and Organization in Living Systems
 Living systems require a continuous input of energy to maintain their chemical and
physical organizations. With death, and the cessation of energy input, living systems
rapidly disintegrate.
 The energy for life primarily derives from the sun. Plants capture energy by absorbing
light and using it to form strong (covalent) chemical bonds between the atoms of carboncontaining (organic) molecules. These molecules can be used to assemble larger
molecules with biological activity (including proteins, DNA, sugars, and fats). In
addition, the energy stored in bonds between the atoms (chemical energy) can be used as
sources of energy for life processes.
 The chemical bonds of food molecules contain energy. Energy is released when the
bonds of food molecules are broken and new compounds with lower energy bonds are
formed. Cells usually store this energy temporarily in phosphate bonds of a small highenergy compound called ATP.
Biological Evolution
 Species evolve over time. Evolution is the consequence of the interactions of (1) the
potential for a species to increase its numbers, (2) the genetic variability of offspring due
to mutation and recombination of genes, (3) a finite supply of the resources required for
life, and (4) the ensuing selection by the environment of those offspring better able to
survive and leave offspring.
 The great diversity of organisms is the result of more than 3.5 billion years of evolution
that has filled every available niche with life forms.
Abilities Necessary to do Scientific Inquiry
 Identify questions and concepts that guide investigations.
 Formulate a testable hypothesis and demonstrate the logical connections between the
scientific concepts guiding a hypothesis and the design of an experiment.
 Develop and revise scientific explanations using logic and evidence.
Assessment strategy
Students can be assessed using the provided project worksheet. Their ability to form relevant
hypotheses, to design a controlled experiment, and to generate and interpret their results can all
be assessed via the worksheet. In addition, students could engage in a peer review process
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whereby groups present their results to the class while other students evaluate their work based
on pre-determined criteria.
Teaching tips
Materials
Test tubes: Medium sized test tubes, 25- 50mL work well, but other sizes could be used
depending on the size of the seeds. If your test tubes are larger you may want to use 5-6 seeds in
each test tube. Run a pilot test with your materials in advance.
Seeds: Any type of seed could be used for this experiment, but only viable (living) seeds will
respire, and small seeds that are still dormant may not respire enough to detect the process.
Students could collect a variety of seeds; however some may not be ready to break dormancy and
respiration will be low; test a variety of seeds with the students or test a few in advance. Seeds
purchased at a garden store are more likely to be viable, and enough water and light should
initiate germination, yielding good respiration measurements. If a seed requires additional cues
in order to break dormancy, such as a period of cold temperatures, or physical damage to a seed
coat, seed suppliers will simulate these conditions before packaging the seeds. This ensures
better germination success for gardeners and for you!
Ensure that wet seeds are available for students by soaking half (or more) of the seeds for a few
hours or overnight before Day One. The moist seeds should begin breaking dormancy and will
have higher respiration rates than their dry counterparts. Encourage students to use the presoaked seeds, even if they are testing other factors (e.g., seed type or environmental conditions)
because respiration rates should be higher, allowing them to better detect differences between
their treatments.
The Millennium Seed Bank Project run by the Royal Botanical Gardens provides additional
information about seed collection and storage and includes links to relevant educational
resources. http://www.rbgkew.org.uk/msbp/index.html The backyard gardener website includes
information about seed germination conditions for a variety of plants used in horticulture.
http://www.backyardgardener.com/tm.html
CO2 absorbent: Calcium hydroxide (also called hydrated lime) or soda lime (a mixture of
calcium hydroxide and sodium hydroxide) can be used to absorb CO2 in this experiment. The
calcium in either substance will react with and absorb any carbon dioxide produced by the seeds
during respiration. Both are available from most chemical suppliers. Be sure to carefully read
the proper handling and disposal procedures included with the agent. Calcium hydroxide is a
strong base and will cause irritation and burns upon contact with skin. Gloves and eye protection
should be worn and contact with skin and clothing should be avoided. Because inhalation of
airborne particles will irritate lungs, using a fume hood when transferring the calcium hydroxide
is advised. There are no carcinogenic or toxic byproducts of calcium hydroxide. It should be
stored in an airtight container, as exposure to air will degrade its ability to absorb CO2. Excess
calcium hydroxide should be mixed with water, neutralized with vinegar or other acid, and
poured down the drain.
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Supporting student inquiry
This guide suggests several factors that might influence seed respiration; however, students will
generate some unanticipated research questions. Inevitably some students will want to test the
effect of putting seeds in Kool-Aid, or in the microwave before measuring their respiration rate.
Encourage students to take their experiment seriously and frame such questions within a realistic
context. For example, students could ask, how does microwave radiation affect the activity of a
seed, and consider the similarities between solar and microwave radiation. Students might
wonder how different contaminants, such as spilled soda, affect seed activity. They could
investigate how dissolved carbohydrates, like the sugar in Kool-Aid, affect seed respiration. If
improved inquiry skills are an important outcome of the lesson, then unanticipated research
questions should be encouraged but framed within the context of science.
Talk with students to make sure they can articulate which variable they are testing. Ensure that
students are testing only one variable (e.g., seed moisture content) and all other factors are held
constant (e.g., seed type, temperature) in their experiment. If students are putting seeds in
different conditions (e.g., in front of a window and in a cabinet) they will need two different
blanks to ensure that water isn’t rising in the test tube due to abiotic processes such as heating
and cooling. Below is an example of a completed data table for an experiment testing the effect
seed moisture content.
Seed Respiration Experimental Treatments and Results
Treatment
name
(wet seeds
or dry
seeds)
1. Blank
Description of each treatment
Seed
Seed size
Seed
Environmental
(large,
Type
condition
conditions
(bean,
small)
(soaked
(light,
pea)
or dry)
temperature)
--
soaked
Sunny, room
temperature.
Placed near
window
Same as above
Measurement
Final
Difference
height
between
of water
blank and
in tube
treatment
(cm)
(cm)
1cm
--
2.5g
2.6cm
1.6cm
dry
Same as above
1.0g
1.4cm
0.4cm
None
None
None
2. Soaked
seeds
Bean
average
3. Dry
seeds
Bean
average
Seed
Weight
(g)
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This protocol does not refer to an explicit “control”. All groups will isolate the effect of one
variable (e.g., seed size) by controlling for other the effect of other variables (e.g., seed type). A
true control group becomes important in experiments in which a treatment is being applied; the
control is usually the baseline condition without the treatment. Observational studies often do
not have a control. Because students will design a variety of comparisons to test their own
question, the “control group” will differ among groups and some projects will not include a
control. For example, a group testing seed moisture could consider the dry seed a type of
control, but a group comparing different seed types or different seed sizes is not applying an
experimental treatment, and thus a control is unnecessary. Some seed respiration studies use
boiled seeds as a control. Boiled seeds would allow students to see that any observed respiration
is due to the metabolic process occurring in a living seed. Boiled seeds could be easily included
in this protocol if the concept of a control group is an important learning objective.
Encourage students to really think about their data. What does this experiment tell us about seed
dormancy and seed respiration? If students were unable to detect any respiration, encourage
them to use part of the class’s data to answer the final questions in the project worksheets.
Discussion of results could be completed within a single period, or if time is available students
could present their results and interpretations to the class.
Compiling and comparing results
Results from one or more classes could be compiled on a few datasheets similar to the one
below. Students should only record the difference in water height between the treatment and the
blank test tubes. This will account for changes in humidity or temperature that could also cause
water to rise in the test tube. Encourage students to think about interesting comparisons: which
seed type exhibited the highest respiration under similar conditions? What conditions caused the
beans to respire the most; did this factor have the same effect on the pea seeds?
Comparing Seed Respiration Rates
Light
Rise
in
water
(cm)
Dark
Seed Type One
Warm Cold
Soaked Dry
Light
Dark
Seed Type Two
Warm Cold
Soaked
Dry
Potential problems
This procedure will not detect really low rates of respiration. The respiration rate of some small,
dormant seeds or of seeds under certain conditions (like a cold treatment) may be below
detection limits. If you want to ensure positive results, encourage students to use pre-soaked
seeds for comparisons of light, temperature or seed type. Peas and beans work particularly well.
This material was developed through the Cornell Science Inquiry Partnership program (http://csip.cornell.edu), with support
from the National Science Foundation’s Graduate Teaching Fellows in K-12 Education (GK-12) program (DGE # 0231913 and #
9979516) and Cornell University. Any opinions, findings, and conclusions or recommendations expressed in this material are
those of the author(s) and do not necessarily reflect the views of the NSF.
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