Lab Experiment on Light and Starch Production in Photosynthesis

by Kathleen Moran (Cornell University)

Cornell Science Inquiry Partnerships



Name_________________________ Period_________ Due Date__________



____Review Vocabulary New Vocabulary___________

Autotroph Dehydration Synthesis Photosynthesis

Producer Monosaccharide Chloroplast

Carbohydrates Polysaccharide Chlorophyll









In an earlier unit we learned that autotrophs are organisms capable of making their own food. Autotrophs are also called producers because they can use the energy in sunlight or chemicals to produce food. Now we are going to explore how autotrophs use the energy in sunlight to produce food. In the process of photosynthesis plants use the energy in sunlight to convert water and carbon dioxide into oxygen and high-energy carbohydrates. Photosynthesis occurs inside a cell organelle called the chloroplast. The chloroplast contains green pigments called chlorophyll, which capture the energy in sunlight. The equation for photosynthesis is as follows:


6CO2 + 6H2O C6H12O6 + 6O2


One of the products of photosynthesis is C6H12O6, which represents a molecule of glucose. As the plant receives sunlight it is constantly producing glucose and then transporting it outside of the chloroplast. However, the plant cannot transport glucose out of the chloroplast as fast as it is produced. To solve this problem the plant combines the glucose molecules into a larger molecule called starch. The glucose molecule is a monosaccharide that through the process of dehydration synthesis, can form a larger molecule called a polysaccharide. This polysaccharide, starch, is stored inside the chloroplast until it can be transported. Iodine is a specific stain for starch, producing a black color (actually a very dark blue) where starch is present.


Part 1



Day 1

Each lab group will get:

        one plant (that has been kept in the dark for 24 hours)

        four clear plastic squares slightly larger than the leaf

        paper clips

        two paper cut outs and/or film negatives smaller than the leaf


        clear tape


  1. Take one paper cut out and tape it to one side of the clear plastic square. Then take the other paper cut out and tape it to one side of another clear plastic square.
  2. Cut out a large leaf shaped whole in the two other plastic squares
  3. Take one plastic square with a cut out on it and one plastic square with a hole cut out of it and use the paper clips to sandwich the leaf between both squares. The square with the hole cut out goes on the underside of the leaf and the square with the cut out goes on the upper side of the leaf. Repeat this step on another leaf using the other two squares.


Diagram 1.


Step 1.

Step 3.





Step 2.







  1. Read the procedure for day two and make a prediction for how you think the leaf will look at the end of the experiment.______________________________________________





Your teacher will shine direct light onto these leaves for 2-4 hours before your class tomorrow.


Day 2.



  1. Carefully remove the plastic squares from the plant
  2. Cut the leaf from the plant.
  3. Hold the leaf with forceps, tweezers or tongs, and
    1. Drop it in a beaker of boiling water to kill the cells.
    2. Place it in a beaker of hot ethanol for two minutes to remove most of the chlorophyll. (This breaks down the cellulose in the cell walls so that the ethanol can get into the cells and destroy chlorophyll and other pigments).
    3. Place it in a beaker of room temperature ethanol for one minute. The leaf should be almost white.
  4. Blot the leaf on paper towels.
  5. Place the leaf in iodine solution on a square of aluminum foil for one minute or less. Remove it when you can see the picture. Remove both the leaf and the foil together and place in water to rinse. You may blot the leaf and press it in a book if desired.




Draw a picture of your leaf indicating the different shades you observe.


Picture of Leaf

1.      Why are certain parts of the leaf darker than others? ______________________








2.      Is there a relationship between the areas

that received light and the areas of starch production?











3.      Think about what you have learned about photosynthesis to fully explain the relationship you observe.











Discuss the following questions with your lab partners and write down your conclusions.


1) Photosynthesis requires the plant to take in gases such as carbon dioxide and release gases such as oxygen. Why did we have to cut a whole out of the plastic square placed on the underside of the leaf? ___________________________________________________________________________









2) Plants dont absorb all sunlight equally. Chlorophyll is a green pigment that absorbs light in the blue and red regions of the visible spectrum. Plants are green because they have a pigment (chlorophyll) that absorbs blue and red light and reflects green light. We can now modify the photosynthesis equation to the following:

red and blue light

6CO2 + 6H2O C6H12O6 + 6O2



What colors of light does a red object absorb?______________________________


What colors of light does a blue object absorb?_____________________________



What do you think you would have observed about starch production if the leaf received only:


Green light_________________________________________________________________


Red light___________________________________________________________________


Blue light__________________________________________________________________



Name_________________________ Period_________ Due Date__________







Part 2. Design Your Own Experiment


In the lab in part 1, we demonstrated how light influences starch production in plants.

What are some other factors that might influence starch production?






Pick one factor. Hypothesize about how this factor might influence starch production.

How might you test this hypothesis? Together with your lab group design an experiment to test your hypothesis. Turn in this sheet to your teacher when you have completed your experiment.









Procedure: (Be sure to specify which plants are your variable and control plants)













What happened to the control plants?






What happened to your variable plants?



Do your results support your hypothesis? Why or why not?










In doing an experiment scientists often begin to think about other questions. After doing your experiment, write down two questions you thought about while carrying out your experiment: