Activity
The day before prepare the following
solution, being sure to use gloves. To a large test tube
add 5 leaves of spinach. Add 50ml of acetone and 1 tablespoon
of sand. Mix well and place carefully in the freezer
overnight. In the morning you should see a dark liquid
at the top of the test tube; this is chlorophyll that
has been extracted from the chloroplast. In the morning
carefully remove the test tube.
Before the students arrive make sure that the room can
be darkened effectively.
1.
On the overhead or board have the basic photosynthesis
equation we have been using displayed with the new addition
of light driving the reaction. Ask students what ideas
they have about how light helps the photosynthetic process.
Students often connect chlorophyll, the green color of
most leaves with photosynthesis. Guide the students back
to the role of energy in chemical reactions and that
light is energy.
2. Explain that the sun’s energy is captured by
pigments in the leaves and used in a number of ways including
making glucose through photosynthesis, but also explain
that some of the energy is reemitted as red fluorescence.
Explain that you have extracted a pigment, chlorophyll
a, from spinach leaves. As the pigment has been extracted
from the leaves ask the students what they think will
happen to energy that hits the pigments now. Have a student
hold the test tube, turn off the lights, and have another
student shine a flash light at the tube. You should see
a red line of fluorescence around the top of the tube.
Hopefully this is a tangible experience for the student
that connects pigments with the conversion of sunlight
into other forms of energy.
3. Ask students if a plant has to be green to photosynthesize?
Can they think of other colors of leaves? If it is a
different color can it contain pigments that will be
useful for photosynthesis? Have students add light to
their photosynthesis equation if they haven’t already.
4. Share with students a picture of a plant cell that
shows chloroplasts. Remind them that this is the site
of photosynthesis and that it contains special structures
with clusters of pigments.
5. Students are going to explore
the variety of pigments found in plants using paper chromatography.
Give each student team two clear drinking cups, 1 very
green leaf and 1 variegated leaf, a strip of chromatography
paper (a piece of coffee filter that can be cut into
two 2 x 10 cm strips will also work), a pencil, 15 mL
of rubbing (denatured) alcohol, 15 ml water, 1 coffee
stirrer, mortar and pestle to shred or grind the leaf.
Provide
the students with the following instructions:
a.
Grind, tear, and squeeze leaves until a liquid is obtained.
(A little sand can make the grinding more efficient.)
(Another way to do this is to lay the leaf over the
chromatography
paper and rub the leaf with the eraser of a pencil.
The leaf tears, but if you keep going, you transfer enough
chlorophyll to the paper to do the chromatography.).
b. Being careful to touch the paper only on the edges,
draw a faint line using pencil across the narrow part
of the paper, approximately 2 cm from the bottom. Use
a coffee stirrer to place juice onto the chromatography
paper strips 2 cm from bottom in the center. Place
a drop on each strip; let it dry, and then place another
drop until the spot is very dark.
c. Add 15 mL alcohol to one drinking cup, 15 ml water
to the other.
d. Place paper clips through each strip towards the
top at a height that will allow the bottom of the filter
to just touch the alcohol and water without submerging
the pigment spots. The paper clips can be clipped to
a coffee stirrer and hung across the top of the cup.
Do not let the spot wash off in the alcohol. The alcohol
should be about 1 cm below the spot.
e. Observe as alcohol and water rise through the strips,
causing pigments to separate (approximately 15 minutes).
Pull out the strips when the colors have stopped spreading
or just before solvent reaches the top, and let them
dry without touching them. Record the results quickly
as the pigments will quickly fade.
Your
students should see a variety of colors. In the alcohol
solution, they
should find two shades of green, a blue green chlorophyll
a, a yellow green chlorophyll b, a faint yellow band
of xanthophylls, and a thin orange band of carotenes.
In the water solution, blue, red, or brown pigments
may appear. You may want to use this opportunity to discuss
and use Rf values.
7.
Ask students how many colors they saw. Why might the
plant have so many pigments? Guide the students to establishing
that it might be to catch more light by asking “What
is the “color” we see when we look at something?” You
may need to review this. It is the color that bounces
off the leaf and isn’t absorbed. “What color
light would be the least useful to a plant?” That
would be green, because it bounces off. Explain: “Chlorophyll
is a lot like a battery.” The chlorophyll gets
charged in the sun, and then the plant can use its energy.
8.
Referring back to yesterday’s concept ask the students “What
does the plant do with the chlorophyll’s energy?” If
the students can respond, “Make food,” remind
them that food contains energy that the plant can use
to grow.
9. Yesterday students saw the effect of no light on
food production for the leaf when using the geraniums.
How would
they design an experiment to test different frequencies
of light? Set this writing challenge: A new planet
was discovered.
The sun is red. It is very hot and dry on the planet. Most
of the sunlight that reaches the planet is red, orange,
and yellow. Imagine that there is life on the planet.
Draw a
picture in the color of the imaginary plants on this alien
planet. Then write a paragraph about why you imagined the
plants looking as they do. (Students should realize that
the plants could not be red or yellow, because those colors
would bounce back the light they are getting. Plants should
be blue to purple. An additional great extension is for
students to realize that the plants would have to have
small leaves
so they wouldn’t dry out easily, like desert plants. Connections
Social Studies
Students can compare cultures when investigating
plant pigments and photosynthesis. Students can
read about ancient
cultures’ use of natural pigments to create art
and decorate clothing. (Dyes have been derived from plants
for many years. African tie-dyes and Old English lichen
dyes are good resources.)
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Embedded
Assessment |
