Carolyn observed a freshwater plant, Elodea, in a small aquarium filled with freshwater on her desk. In one five-minute period, Carolyn counted 32 bubbles that formed on the plant’s leaves and were released into the water. She then moved the aquarium from her desk to a sunlit window and observed the Elodea again. In a second five-minute period, Carolyn observed 194 bubbles form and be released by the Elodea into the water. a. Explain how the bubbles released by the Elodea signify that the plant is cycling matter. Provide evidence to support your answer.

Hello there! Photosynthesis

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.

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