Download By Rachel Hughes and Kirstin Bittel

Without pigments we’re nothing!
Modified from:
Demonstrating that Light is dissipated as Chlorophyll a Fluoresces by H. Alexandria
Bodha - Michigan State Sample Core Curriculum, Lesson 8, True Colors
By Rachel Hughes and Kirstin Bittel
Time :
Preparation Time :
Materials :
1 class period
30 minutes plus time for solution to chill
4 pieces of chromatography paper or coffee filters
Coffee stirrers (1 per group)
Cups (6 ounce plastic, two per group)
Denatured alcohol (15 ml per group, 100 ml for teacher
demonstration)
Leaves (fresh spinach or geranium, and a variegated leaf
like Coleus or red maple)
Mortar and pestle with sand, or other device for mashing
leaves
Ruler (cm, per group)
Scissors (per group)
Water (about 15 ml per group)
(Safety Precautions: Alcohol is a toxic and flammable substance. Its use should be carefully controlled. Students may
associate the denatured ethyl alcohol with consumable alcohol, or fail to understand the process of “denaturing” ethyl
alcohol. Boil alcohol in a protected, ventilated area (preferably a fume hood) using a double boiler. Have MSDS sheet
available.)
Abstract
Students have, in previous experiments, established that light is necessary for photosynthesis to
happen, and that light drives the reaction, but how? In this exercise students find out about the
pigments present using paper chromatography and explore their role in providing energy for the
combination of carbon dioxide and water to produce sugar.
Objectives
Students will be able to:
i.
Identify chloroplasts as the primary site of photosynthesis
ii.
Identify chlorophyll pigments as the primary pigments that absorb light as
part of photosynthesis
iii.
Explain the role chlorophyll plays in ‘capturing’ the light energy of the sun
in a written form
National Science Education Standards
Plant cells contain chloroplasts, the site of photosynthesis. Plants and many microorganisms use
solar energy to combine molecules of carbon dioxide and water into complex, energy rich organic
compounds and release oxygen to the environment. This process of photosynthesis provides a
vital connection between the sun and the energy needs of living systems.
Teacher Background
Photosynthesis depends upon the energy supplied by the sun. Electrons within pigment
molecules, such as chlorophyll a and b, are excited from a ground state to a higher energy state
when photons of light hit the molecule. In the leaf these high energy electrons are transferred
from the chlorophyll to another molecule via a carrier molecule ex. NADP+ with a hydrogen ion to
areas where the energy is used to create different types of molecules such as glucose. At the
beginning of this lesson, acetone extracted chlorophyll a receives light that is not used in the
photochemical processes of photosynthesis. Instead, the electrons return to the ground state and
energy is re-emitted as fluorescent light. Students use a flashlight to observe this fluorescence.
http://www.ab.ipw.agrl.ethz.ch/~yfracheb/flex.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chlorophyll.html
Related URLs
http://washpost.com/nielessonplans.nsf/0/818BCB4B7CE60E5285256AF000647E62?OpenDocu
ment&sol=0
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.
Embedded Assessment
The writing challenge provides an opportunity for students to demonstrate an understanding of
form and function. Knowing the relationship between different frequencies of light and the
capture of energy is important in understanding food production for the plant.
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.)