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Karen F. Adams Burnside Scholastic Academy
650 East 91st Place
Chicago IL 60619
(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
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.
(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
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
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
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?
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
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.