SC.8.L.18.1 - Describe and investigate the process of
photosynthesis, such as the roles of light, carbon dioxide, water and
chlorophyll; production of food; release of oxygen.
Teacher Demo: Jeanette's Demo (Joesph Priestley experiments)
What Makes a Seed Breathe Faster?
Teacher’s Guide
by Lynn Vaccaro,
CSIP Graduate Student Fellow, Cornell University
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.
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.
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.
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
carbon-containing (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
high-energy 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.
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 pre-soaked
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.
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.
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
|
|||||||
Description of
each treatment
|
Measurement
|
||||||
Treatment name
(wet seeds or dry
seeds)
|
Seed Type
|
Seed size
|
Seed condition
(soaked or dry)
|
Environmental
conditions (light, temperature)
|
Seed
Weight (g)
|
Final height of
water in tube (cm)
|
Difference
between blank and treatment (cm)
|
1. Blank
|
None
|
None
|
None
|
Sunny, room temperature. Placed near window
|
--
|
1cm
|
--
|
2. Soaked seeds
|
Bean
|
average
|
soaked
|
||||
3. Dry seeds
|
Bean
|
average
|
dry
|
Same as above
|
1.0g
|
1.4cm
|
0.4cm
|
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
|
||||||||||||
Seed Type One
|
Seed Type Two
|
|||||||||||
Light
|
Dark
|
Warm
|
Cold
|
Soaked
|
Dry
|
Light
|
Dark
|
Warm
|
Cold
|
Soaked
|
Dry
|
|
Rise in water (cm)
|
||||||||||||
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.
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