Activity
Remind students to check on their cotton
seeds.
1. Yesterday students explored common misconceptions that challenge their ability
to understand photosynthesis. Today, continue to challenge their understanding
by presenting them with a bottle of sparkling water. Yesterday they were told
that carbon dioxide and water produced sugars and here they have water with
bubbles, which is actually carbon dioxide. Based on what they have learned
over the past few days, mixing these two substances should produce sugar, right?
Share with the students that you have drunk a little and it wasn’t sweet
at all. You’re thinking that maybe you should mix it a little, perhaps
shake the bottle up. What do they think? What do they predict will happen?
Will it be sweet then? If not, why not? If students do not recognize the need
for another form of energy to progress the reaction this is the time to lead
them through the use of light as a source of energy. They are going to be exploring
the connection between light and the production of sugar in plants today.
2. Present students with the geranium plants whose leaves you partially covered
up on Day 1 of ‘Jolly Green Thumb’. Explain to the students that
you placed the foil or paper on the leaves two days ago. Given the connection
between the photosynthesis reaction and light, what do students think has happened
to the part of the leaf that has not been receiving light? Have students make
predictions in their notebooks? What might they expect to happen if an area
of a leaf is covered up? Why?
3. Explain that while sugar is a product of photosynthesis, it is stored in
the leaf as starch. Share with the students that starch can be detected using
Lugol’s Iodine Reagent. Demonstrate the detection of starch using a slice
of potato, something that the students identify as being “starchy.”
4. Give each group a leaf that has been partially covered for the past two
days and one from the same plant that was not covered. Students should sketch
each leaf in their notebooks, illustrating where on the leaf the foil or paper
is positioned.
5. Provide students with the following information (also attached as the student
protocol sheet)
Safety:
1. Ethyl alcohol is very flammable and should not be in direct contact with
the hot plate.
2. Do not leave the ethyl alcohol unattended.
3. Everyone should be wearing safety goggles.
1. Set up a boiling water bath: Put a 1000ml beaker on the hot plate, which
should be off, and fill with 300-400ml of water. Carefully place a 400ml beaker
containing 200ml of 80% ethyl alcohol inside the 1000ml beaker. (This will
function like a double boiler.) Turn on the hot plate and bring the alcohol
to a gentle boil. Once the alcohol begins to boil, turn down the hotplate so
the alcohol maintains a gentle boil.
2. Remove the paper clip and paper or foil from the leaves. Do you notice any
difference between the area that was covered and the area which was not covered
? Using forceps drop both leaves into the boiling alcohol. What do you notice?
3. As the leaves whiten remove them from the boiling alcohol using the forceps
and place in separate dishes. Turn off the hot plate. Using distilled water,
rinse the leaves off and add just enough water to cover the leaves.
4. Drop by drop add Lugol’s Iodine reagent to the water until the water
is a very pale amber.
5. After 5 minutes note the coloration pattern on each leaf. Sketch the leaves
and the patterns of coloration into your notebooks.
6.Based upon their prior knowledge and this experience, students write what
they think the relationship between sunlight and starch is.
7.Have students take out their photosynthesis jigsaw and assemble the sugar.
Present the molecular structure of starch, what do they notice? Are there any
similarities between starch and glucose? Students should recognize that there
is some similarity. Explain that starch is actually hundreds of glucose molecules
joined up.
8. Why might the plant store starch rather than sugar? Ask students for possible
ideas. Then, demonstrate the solubility of sugar and the lack of solubility
of starch. A couple of teaspoons of each in separate beakers should be sufficient
to demonstrate their solubility. Ask students why it might be better to store
energy in an insoluble form. Keeping its store of energy in an insoluble form
means a plant can keep this store in a discrete place within the cell. If the
energy were stored as sugar, a soluble format, it would impact the concentration
of dissolved solutes in the cytoplasm, which would impact water movement.
9. Using the overhead with the basic photosynthesis equation written on it,
add light to the equation. Walk students through the equation. If there is
no light, then there is no photosynthesis, no sugar is produced and converted
to starch.
10.
Students should write a summary describing the relationship
between photosynthesis, light, sugar and starch. Within that summary they
should also describe why the sugar is converted to starch,
possibly using an analogy.
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Embedded
Assessment
Summary describing relationship between photosynthesis, light,
sugar and starch |
