Photosynthesis

Karen F. Adams                 Burnside Scholastic Academy
                               650 East 91st Place
                               Chicago IL 60619
                               312-535-3300

Objectives:

(Adaptable to grade levels 6-9)

The student will:
1.  Observe evidence of photosynthesis in a water plant.
2.  Assemble the equipment needed to measure the rate of photosynthesis in
    elodea (water plant).
3.  Count bubbles of oxygen gas given off by elodea to determine the rate of
    photosynthesis.
4.  Change the conditions of photosynthesis by altering light intensity and 
    carbon dioxide amount, and determine the effects on the photosynthesis rate.
5.  Prepare a graph of the collected data and analyze it.

Materials Needed:

(For each group of four students)

elodea (water plant)                       lamp (40 watt)
test tube                                  razor blade (single-edge)          
dechlorinated water (room temperature)     tape
sodium bicarbonate powder (baking soda)    clock or timer
metal stand with rod or test tube rack     metric ruler
                    
Strategy:

PART A. Setting Up the Experiment
1.  Obtain a sprig of elodea.  Remove several leaves from around the cut end of 
    the stem.  Slice off a portion of the stem at an angle and lightly crush the
    cut end of the stem. 
2.  Place the plant into the test tube, stem end up, filled with water.
3.  Secure the test tube to a metal stand with tape or place the test tube in a
    test tube rack.
PART B. Running the Experiment
1.  Place a 40 watt lamp 5 cm from the plant.  After one minute, count and 
    record the number of oxygen bubbles rising from the cut end of the stem.
    Count bubbles for five minutes.  If bubbles fail to appear, cut off more of 
    the stem and recrush.
2.  Run a second five-minute trial.  Record and average your results.
3.  Move the lamp so it is 20 cm from the plant.  After one minute count and
    record bubbles for two five-minutes trials.  Again, average and record your 
    results.          
4.  Add a pinch of sodium bicarbonate powder to the test tube.  Place the lamp 
    5 cm from the test tube.  After one minute, record bubbles for two five- 
    minute trials.  Average and record your results.
5.  Prepare a graph of your results.  Use the average number of bubbles for the 
    vertical axis.  Use the type of environmental condition for the horizontal 
    axis.

Performance Assessment:                                            

The students will answer these questions using specific values from the 
investigation.  Diagrams may be included.

1.  How does this investigation demonstrate that plants give off oxygen during 
    photosynthesis?  Explain your answer based on your observations.
2.  How does the rate of photosynthesis change when the light source is moved 
    from a distance of 5 cm to 20 cm?
3.  How does the rate of photosynthesis change when sodium bicarbonate is added 
    to the water?  

Conclusions:

Plants use green pigments called chlorophylls to trap light energy.  The 
chlorophylls give a plant its green color.  Inside the cells that have 
chloroplasts, the light energy is used to make a simple sugar called glucose.  
The process by which plants use light energy to make glucose is called 
photosynthesis. 
 
During this process of sugar production, carbon dioxide combines with water to 
form glucose and oxygen is released.  Oxygen that is produced in photosynthesis 
is given off as a gas.  If a lot of oxygen is being given off, photosynthesis is
occurring rapidly.  If little oxygen is being given off, photosynthesis is  
occurring slowly.  The amount of trapped light energy and the amount of carbon 
dioxide available affects the rate of photosynthesis.  

The purpose of adding sodium bicarbonate powder to the water increases the 
amount of carbon dioxide in the water.

This investigation can be performed with water plants grown in many parts of 
the world, except regions that have permanent ice.