Name _____________________________ Period ___________________ Date __________________
Rate of Photosynthesis Lab
BACKGROUND: Photosynthesis converts some of the energy absorbed from sunlight into
the chemical energy of sugars. The process is also the major source of oxygen in Earth's
atmosphere. By measuring oxygen production indirectly, you can measure the rate of
photosynthesis. Remember that a rate describes how one quantity changes compared with
another. Leaf disks float, normally. When the air spaces are infiltrated with the solution, the
overall density of the leaf disk increases, causing the disk to sink. The infiltration solution
includes a small amount of sodium bicarbonate, which we will be using baking soda. The
Bicarbonate ions serve as a carbon source for photosynthesis (dissolved source of carbon
dioxide). As photosynthesis continues, oxygen is released into the interior of the leaf causing it
to float (buoyancy) or rise. Since cellular respiration is taking place at the same time,
consuming oxygen, the rate that the disks rise is an indirect measurement of the net rate of
photosynthesis.
Pre-lab questions:
1. Define photosynthesis.
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2. Write the chemical equation of photosynthesis.
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3. What causes the leaf to float?
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4. What does the bicarbonate ion serve as?
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5. What other process is occurring at the same time as photosynthesis?
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Holt McDougal Biology
Chapter Investigation
1
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
PURPOSE: Design an experiment to test the effect of different light sources on the rate of
photosynthesis
PROBLEM: How does a light source affect the rate of photosynthesis?
HYPOTHESIS:
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MATERIALS

Young ivy leaves or fresh spinach

Hole punch

100 mL beakers

Plastic cups

Sodium bicarbonate/detergent solution or baking soda

Water

Liquid soap

Plastic 10 cc syringe

Forceps

Strong light sources

Stopwatch
OPTIONAL:

Buffer Solutions

Colored Cellophane or filters

Leaf Material of different ages
PROCEDURE
1. Prepare 300 ml. of bicarbonate solution for each trial (1/8 teaspoon of baking soda in 300
ml. of water).
2. Add 1 drop of diluted liquid soap to the bicarbonate solution. The soap wets the
hydrophobic surface of the leaf allowing the solution to be drawn into the leaf. It’s difficult
to quantify this because different soaps have different concentrations. Just try to avoid
suds. If there are too many suds then dilute it with more bicarbonate solution.
3. Use the hole punch to make 10 disks from an ivy leaf. Make sure that you punch holes
that are smooth and not too thick.
4. Fill one beaker halfway with the sodium bicarbonate/detergent solution. Fill a second
beaker with water.
5. Remove the plunger from the syringe and place the ivy leaf disks into the syringe. Insert
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Holt McDougal Biology
Chapter Investigation
2
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
the plunger and draw 5 cc (5 mL) of the sodium bicarbonate/detergent solution into the
syringe as shown below. Be careful when you do this so that you don’t crush the disks.
5. Hold the syringe so that the tip is pointing upwards. Push on the plunger to squirt out any
air in the syringe.
6. Place your finger on the tip of the syringe, as shown below. Withdraw the plunger to form a
vacuum, but be careful to not pull the plunger all the way out of the syringe. When the
vacuum is formed, the gases in the air spaces in the leaf disks move into the syringe and
the solution diffuses into the air spaces.
7. Take your finger off of the tip of the syringe. This causes the leaf disks to sink to the
bottom of the syringe because they become more dense from the diffusion of solution into
the air spaces. You may have to do this several times. If after three times and your disks
do not sink, it may be because there is not enough soap in the solution. Add a few more
drops.
8. Open the syringe by pulling the plunger almost all the way out. Place your finger over the
tip of the syringe and turn it so the tip is pointing down. Carefully remove the plunger and
pour the contents of the syringe into the beaker of water. Use the forceps to remove the
leaf disks if they stick to the walls of the syringe. For a control infiltrate the leaf disks with a
solution of only water with a drop of soap, no bicarbonate.
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Holt McDougal Biology
Chapter Investigation
3
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
9. Place the beaker with the leaf disks under the light source and immediately start the
stopwatch. At the end of each minute record the number of floating disks. As the leaf disks
begin to photosynthesize, the production of oxygen replaces the solution in the air spaces
and the disks become less dense and float to the top of the water.
Data Collection:
Minutes
Disks Floating
1
2
Independent Variable=
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Dependent Variable=
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3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
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Holt McDougal Biology
Chapter Investigation
4
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
ANALYZE AND CONCLUDE
1. Identify What are the independent and dependent variables in your experiment? Also,
what were your constants and control?
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2. Calculate Determine the mean rate of photosynthesis in your experiment. To calculate the
rate of photosynthesis, use the formula below:
5 (# disks floating)
 ____________ disks/sec
total time (sec)
3. Analyze Make a line graph to represent your data. Explain your choice and construct the
graph. Be sure to carefully label the axes of the graph.
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Holt McDougal Biology
Chapter Investigation
5
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
4.Conclude Based on your data, what can you conclude about how your independent
variable affects the rate of photosynthesis?
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5. Infer Why do you think sodium bicarbonate was used in this investigation? (Hint: Think
about the equation for the overall process of photosynthesis.)
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6. Experimental Design Identify possible reasons for inconsistent results.
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6. Why did you record the time it took for the ivy discs to float? What is the significance of
them floating?
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7. Why do you think light source affects the rate of photosynthesis?
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Holt McDougal Biology
Chapter Investigation
6
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
Extension:
Remove the disks from the light and place the cups of floating disks in the dark (perhaps use
a shoebox). Every minute remove the box, count how many are still floating, and then record
the time on the data table. Make sure you start off on whatever minute you left off on prior to
the darkness. For example if they all were floating on minute 15, then minute 16 would start
off this portion of the experiment.
Minute
Disks Still Floating
Make any notes or observations you see while this happens.
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Holt McDougal Biology
Chapter Investigation
7
Cells and Energy
Section 2: Overview of Photosynthesis
Name _____________________________ Period ___________________ Date __________________
Graph the results from both the light and dark.
Question:
1. At what time did the disks begin to sink? Why did this happen?
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Holt McDougal Biology
Chapter Investigation
8
Cells and Energy
Section 2: Overview of Photosynthesis