Lab Experiment on Light
and Starch Production in Photosynthesis
by
Kathleen Moran (Cornell University)
Cornell
Science Inquiry Partnerships http://csip.cornell.edu
Name_________________________ Period_________ Due Date__________
Grade_________________
____Review Vocabulary New Vocabulary___________ Autotroph Dehydration Synthesis Photosynthesis Producer Monosaccharide Chloroplast Carbohydrates Polysaccharide Chlorophyll
Background
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:
Light
6CO2 + 6H2O à 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
· scissors
· clear tape
Diagram 1.
Step 1.
Step 3.
Step
2.
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Your teacher will shine direct light onto these leaves for 2-4 hours before your class tomorrow.
Day 2.
Materials
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? ______________________
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2. Is there a relationship between the areas
that received light and the areas of starch production?
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3. Think about what you have learned about photosynthesis to fully explain the relationship you observe.
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Discussion
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? ___________________________________________________________________________
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2) Plants don’t 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 à 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__________ Grade_________________
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.
Title:
Hypothesis:
Procedure: (Be sure to specify which plants are your variable and control plants)
Results:
What happened to the control plants?
What happened to your variable plants?
Discussion:
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:
1.
2.