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Photosynthesis: A Controlled Experiment
Photosynthesis:  A Controlled Experiment                      

Coley, Juana
23 W. 143rd Street                Rosa Parks Middle School
Dixmoor, IL  60426                1-708-339-9550


To measure the amount of starch left in a leaf of a geranium plant under the 
following conditions; carbon dioxide increased, decreased and neither increased or 

To prove increased starch increases the process of photosynthesis in the green plant.    
Apparatus Needed

 3  Geranium Plants (same size, shape and color)
 3  2 gallon plastic bags with twist to close
 2  250ml Beakers
 1  500ml Beaker
 1  Hot Plate
 1  Pair of Plastic Tongues
 4  Petri Dishes
 1  1pt. 91% Isopropyl Alcohol
 1  Package of Alka-Seltzer                              
 1  50mL of Soda Lime
 1  Bottle of Potassium Iodide                      
 3  Pieces of Cardboard
 1  Pitcher of Water

Recommended Strategies

1.  Mark plants A, B and C.
2.  Put cardboard pieces at the bottom of each bag.
3.  Put plant A in one bag with one 250mL beaker half filled with water.  Place Alka-
    Seltzer in water, twist close. 
4.  Put plant B in one bag.  Put 50mL of Soda Lime in a Petri dish and place in bag 
    with plant B, twist close. 
5.  Put plant C in one bag. Twist close.  (This is the "control" plant.) 
6.  Find a sunny place in your classroom to place all three plants. (The plants must 
    have same amount of sunlight and water.) The plants are to set for one day. 
7.  After one day, remove plants from bags.  Break off one leaf from each plant put 
    in Petri dishes marked A, B, and C. 
8.  Half fill the 500mL beaker with water.
9.  Fill the 250mL beaker with alcohol.
10. Place beaker with alcohol into beaker with water, on to the hot plate. 
11. Take leaves one at a time and put in beaker with hot alcohol.  Leave in for ten 
12. Remove leaf with plastic tongues.
13. Place leaf on paper towel to dry, then place in Petri dish. 
14. Place several drops of potassium iodide on each leaf.
15. Observe color change of the three leaves.  (the darker the color (purple) the 
    more starch.  The lighter the color, the less starch. 


To determine how much starch is left under three conditions. 
     1.  Carbon Dioxide increased.
     2.  Carbon Dioxide decreased.
     3.  Carbon Dioxide neither increased or decreased.  


1.  What were the results of plant A, with Alka-Seltzer?  Was the carbon dioxide 
    increased, decreased, or remained the same? 
2.  What were the results of plant B, with the soda lime?  Was the carbon dioxide 
    increased, decreased, or remained the same? 
3.  What were the results of plant C, the "control" plant?  Was the carbon dioxide 
    increased, decreased, or remained the same? 

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