Remind students to check on their cotton
1. Ask students “Now that we know that photosynthesis is the energy
source for a plant, how do you think that the energy is used to build the stuff
plants are made of?” “What are the molecules that make up plants?” “Where
do they come from?”
This can be a pre-assessment in students’ notebooks.
2. Using the overhead of the basic plant cell, review with students their knowledge
of basic cell chemistry. Identify the types of molecules that make up the cell
parts. Using the student sheet, review what these molecules are made of. Students
should recognize that photosynthesis produces one huge component of all these
molecules, glucose. Ask students where the additional minerals might come from.
Referring back to van Helmont’s experiment, ask the students “Where
might these minerals have come from?” Students should recognize that
the soil did provide some nutrients. Class discussion reviews students’ ideas
about basic cell chemistry.
3. Using the activity sheet 1, have students identify whether the plant derives
everything from the process of photosynthesis or whether there needs to be
some incorporation of another chemical into the substance on the activity sheet.
4. Remind students that chemical reactions might give off energy or need it.
Ask students how plants might get energy to build the parts of a cell it needs.
While most students will likely respond with photosynthesis at this point,
remind them that the plant needs energy all the time. What happens in the dark?
How does a seed get energy to build cells to become a seedling? The seedling
is in the dark, but it still grows. Hopefully, students will remember the storage
of energy in the insoluble form - starch. Remind them of the geranium exercise
if they don’t remember. Ask them again why the glucose can’t be
stored just as it is.
5. Introduce the idea of respiration as a way of freeing energy from starch.
Students should be familiar with the idea of cellular respiration from the
second quarter. However, reviewing cellular respiration is important as students
often connect the term respiration strictly with the use of lungs to breath.
Aerobic cellular respiration is the oxidation of glucose producing energy,
carbon dioxide and water. So plants breathe too!
6. Challenge students to support the idea that plants respire as well as photosynthesize
with evidence that they really do give off carbon dioxide. Students are to
design an experiment using the materials provided. Before they design the experiment
provide them with some tools to design their experiment.
students that Bromthymol blue (BTB) can be used as
an indicator for carbon dioxide. BTB will turn light
or yellow in the presence of an acid formed by carbon
dioxide and water. Demonstrate how BTB acts as an indicator
through a straw into a solution of water and BTB in a
beaker. The BTB solution will turn from blue to yellow
or light green
with the addition of carbon dioxide. Keep this beaker
out on display; it is important that students see that
time, the yellow color will return to blue once the dissolved
carbon dioxide reaches equilibrium with that in the air.
students to the water plant Elodea, Canadian Pond Weed.
Explain that the cut sprigs of Elodea will
continue to photosynthesize and respire if placed in
test tubes with
water and BTB. Students are to make the assumption
that the BTB in the plant’s environment will not
affect photosynthesis or respiration.
students that they should focus on differing
one variable at a time within their experimental
- Encourage students to incorporate the 24 hour
time period into their experimental design.
Students are provided with an information sheet stating
their challenge, the materials available and safety cautions
and the points that they should include in their experimental
design. They should be given time as a group to design
the experiment. Before starting the experiment students
must first get approval for their experimental design.
8. After students have performed their experiments and analyzed
their data have them present their findings to the class.
Once everyone has presented to the class, establish what
elements might be essential to an experiment that tests the
idea that plants also respire.
homework: Ask students to draw a diagram which demonstrates
the relationship between photosynthesis, respiration and
the nutrition and growth of a plant.
the diagram at the end of this three-day exercise pushes
students to incorporate ideas from the mini-unit as a
whole. The lab is an opportunity to assess students’ ability
to identify variables and create controls in an experiment.
The earlier activity sheet encourages students to articulate
in written form the class discussion.