Name __________________________________________ Date ______________________Period _______
Leaf Prints Lab
Observing the Effects of Light on Starch Production in a Leaf
Question: How does light availability affect starch production in a leaf?
Leaf Overview: In this investigation, you will make a stencil from a piece of plastic tape and place it on
the leaf of a living plant. You will then expose the plant to fluorescent light. On the second day of the
investigation, you will remove the stenciled leaf from the plant and make a “leaf print” to detect which
parts of the leaf contain starch.
Background: During the stage of photosynthesis called the Calvin cycle, plants build molecules of a sugar
called G3P from atoms of carbon, oxygen, and hydrogen. Plant cells use G3P to make glucose and other
organic molecules. In plant cells, energy is stored when glucose molecules are linked together, forming
amylose, a polysaccharide also called plant starch. When needed, plant cells can break down stored
starch into glucose which fuels their life processes.
Indicator used: Iodine solution, which is normally a red-orange color, reacts with plant starch and forms
a substance with an intense blue-black color.
Materials
 Food samples
 Colored plastic tape (duct tape)
 Plastic cutting board
 Scissors/razor
 Potted geranium plant
 Fluorescent light source
 Plastic foam cup
 Boiling water
 Clock or watch
 Forceps or tongs
 Beaker (250 mL)
 70% denatured ethanol
 Plastic wrap
 Hot water bath or microwave oven
 Clear plastic cups
 Iodine solution (IKI)
Procedure
Part A: Making your Leaf Stencil
1. Obtain a strip of plastic tape about 8 cm long. Stick the tape onto a plastic cutting board.
2. With the razor blade or scissors, make cut-outs on the plastic tape to create a stencil of your own
design. CAUTION: Handle the razor blade/scissor carefully to avoid injury. You may want to outline
your design with a pen first. The tape stencil needs to be small enough to fit onto one leaf of a
geranium plant. Use a pen to label a corner of your tape stencil with your initials.
3. Stick the tape stencil onto your upper surface of a leaf on a potted geranium plant as directed by
your teacher. Take care not to crease or detach the leaf. Make sure that the tape stencil lies flat so
that no light can reach the parts of the leaf covered by tape.
4. Place the geranium plant near a fluorescent light source. The light source should be approximately
15-25 cm from the stenciled leaf.
5. Write your predictions in the spaces provided
a. Which parts of the leaf will produce starch?
b. How will the cells in the covered parts of the leaf continue to obtain energy without light?
c. How do you predict your leaf will look if you stain the leaf with iodine solution after the
tape has been on for several days?
Part B: Testing food samples
In Part B of this activity, you will use the iodine test to detect starch in samples taken from common food
plants. Iodine solution, which is normally a red-orange color, reacts with plant starch and forms a
substance with an intense blue-black color.
1. Gather food cubes. Be sure to create a key for the cubes so that you do not forget what each one is.
2. With a dropper, add one drop of iodine solution to each cube. Watch for the color of the iodine
solution to change. Record your observations in Data Table 1 below.
Data Table 1
Food Sample
Observations
Starch Present?
Part B Questions:
Answer the following questions in the spaces provided.
1. How does a plant store the sugars produced by photosynthesis?
2. What is the function of starch in a plant cell?
3. What do the food samples that tested positive for starch have in common?
4. If a plant is kept in darkness and cannot photosynthesize, how might the plant obtain the sugars it
needs for energy?
Part C: Developing Your “Leaf Print”
1. Remove your stenciled leaf from the geranium plant. Carefully remove the tape.
2. To make the leaf print, first whiten your leaf by removing the chlorophyll. Use a hot mitt and
forceps or tongs to place your leaf in a plastic foam cup of boiling hot water provided by your
teacher. CAUTION: Handle hot water carefully to avoid burns.
3. Let the leaf remain in the hot water for 1 minute.
4. Remove the leaf with forceps or tongs. Observe the leaf’s appearance. Record your observations
below.
5. Label a beaker with your group name. Place the leaf in the beaker and pour in enough denatured
ethanol to cover the leaf. Immediately cover the top of the beaker with plastic wrap.
6. Carefully place the beaker containing your leaf and the denatured ethanol inside the hot water
bath or microwave as directed by teacher. Allow the ethanol to boil until the leaf is completely
white. Use a hot mitt to carefully remove the beaker.
7. Use forceps or tongs to remove the leaf from the ethanol. Observe the appearance of the leaf, then
place it in a plastic cup containing iodine solution.
8. Pull the leaf out occasionally with forceps to see how it is developing. Put the leaf into a plastic cup
of water when the leaf print is clearly visible.
9. Draw a sketch below showing what your leaf looked like after staining with iodine solution.
Analysis and Conclusions
1. What was the leaf’s appearance after boiling in water? After boiling in ethanol? Explain.
2. Which parts of the leaf turned black? Explain.
3.
If your leaf print did not appear, suggest a possible reason.