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FACTORS AFFECTING PHOTOSYNTHESIS
Part I. Plants need CO2
INTRODUCTION
In order for a plant to produce its own food, it needs raw materials. Plants use energy from the sun to
synthesis water and carbon dioxide into a simple sugar. In Part I of this experiment, you will prove
that carbon dioxide must be present before photosynthesis can take place. Oxygen is a byproduct of
photosynthesis and it is this byproduct that you will use as evidence that photosynthesis has occurred.
MATERIALS
Elodea (2 shoots)
large test tubes (2)
test tube rack
1000 mL beaker
half-meter stick
single edged razor blade
distilled water
0.2% sodium bicarbonate (NaHCO3)
light source
colored filters (red, green, blue)
PROCEDURE
1. Select two healthy shoots of Elodea. Use a sharp razor blade to remove the bottom centimeter of
each shoot using a diagonal cut.
2. Place one shoot into a large test tube containing recently boiled and cooled distilled water. (Keep
plants submerged in solution at all times.)
3. Place the second shoot into a large test tube containing 0.2% sodium bicarbonate solution. (Keep
plants submerged in solution at all times.)
4. Place the test tubes in a test tube rack.
5. Place a 1000-mL beaker full of water in front of the lab light to serve as a heat sink.
(Electrical Shock Potential!! Do NOT spill water on light!)
6. Allow 5 minutes for plants to begin photosynthesis.
7. Record your observations in Student Data Sheet: Part I.
Part II. Plants need light.
INTRODUCTION
In Part I, you learned that plants need carbon dioxide in order to photosynthesize. You also know that
plants need light, but how does the amount of light affect the rate of photosynthesis? In this
experiment you will vary the amount of light the plants receive by varying their distance from the light
source and recording the number of oxygen bubbles released during photosynthesis.
PROCEDURE
1. Using only the test tube containing the 0.2% sodium bicarbonate solution, begin counting the
number of oxygen bubbles that are released from the end of the diagonally-cut shoot.
2. After 1 minute, record the number of bubbles in the Student Data Sheet: Part II under 1-minute at
10-cm. Continuing counting bubbles and recording at 1-minute intervals for a total of 5-minutes.
3. Move the test tube rack 10-cm from the 1000-mL beaker.
(This is 20-cm from the light source.)
4. Repeat the procedure for counting and recording bubbles and record in Student Data Sheet: Part II
at 20-cm.
5. Repeat the procedure for counting and recording bubbles and record in Student Data Sheet: Part II
at 30-cm.
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ASIM Biology: Botany
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Part III: What Part of Light Do Plants Use?
INTRODUCTION
Plants do not use all wavelengths of light. Green plants use all wavelengths of white light except
green, which is reflected. Red plants use all wavelengths of white light except red, etc. In this
experiment, you will determine the various degrees to which Elodea uses the different wavelengths
(colors) of light.
PROCEDURE
1. Using the test tube/Elodea from Part II, place it into the test tube rack and place the rack next to the
heat sink.
(NOTE: Replenish Elodea and NaHCO3 if performing next day)
2. After a continuous stream of bubbles has begun to emit from the shoot, place a piece of red film
between the heat sink and the test tube rack.
3. Count the bubbles emitted and record in Student Data Sheet: Part III. Record number of bubbles
released at 1-2-3-4-5 minute intervals.
4. Repeat Step 2 and Step 3 using a blue film.
5. Repeat Step 2 and Step 3 using a green film.
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ASIM Biology: Botany
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STUDENT DATA SHEET
FACTORS AFFECTING PHOTOSYNTHESIS
Name: _________________________
Date: __________________________
PART I.
Test Tube
(Boiled Water)
Observation:
Test Tube
(NaHCO3)
Observation:
PART II.
Distance
from Light
10-cm
20-cm
30-cm
1-minute
2-minutes 3-minutes 4-minutes 5-minutes Average
1-minute
2-minutes 3-minutes 4-minutes 5-minutes Average
PART III.
Color of
Light
Red
Blue
Green
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ANALYSIS AND CONCLUSIONS
Questions:
Part I.
1. What can be concluded from your observation of Elodea in recently boiled/cooled water.
2. What can be concluded from your observation of Elodea in NaHCO3 solution?
Part II.
1. Construct a graph of average bubbles/minute vs distance. Place distance (cm) on the X-axis and
average bubbles/min on the Y-axis.
Part II Graph
2. What conclusion can you make about the relationship between distance and light intensity?
3. What conclusion can you make about the relationship between light intensity and the rate of
photosynthesis?
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Part III.
1. Construct a graph of average bubbles/minute vs color of light. Place color of light on the X-axis
and average bubbles/min on the Y-axis.
Part III Graph
2. What conclusion can you make about the relationship between color of light and the rate of
photosynthesis?
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ASIM Biology: Botany
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