The Floating Leaf Disk Lab
The biology behind the procedure:
Leaf disks float, normally. When the air spaces are infiltrated with solution the overall density of the leaf disk
increases and the disk sinks. The infiltration solution includes a small amount of Sodium bicarbonate.
Bicarbonate ion serves as the carbon source for photosynthesis. As photosynthesis proceeds oxygen is
released into the interior of the leaf which changes the buoyancy--causing the disks to 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.
Materials:
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Sodium bicarbonate (Baking soda)
Liquid Soap
Plastic syringe (10 cc or larger)—needle removed.
Leaf material (spinach)
straw
beaker
Timer
Light source
test tube
test tube rack
Ruler
Procedure:
1. Prepare the bicarbonate solution. This has already been done for you.
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The bicarbonate serves as an alternate dissolved source of carbon dioxide for photosynthesis. It’s
about a 0.2% solution. (This is not very much: only about 1/8 of a teaspoon of baking soda in 300
ml of water.)
2. One drop of dilute liquid soap was added to the 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 since liquid
soaps vary in concentration. Avoid suds. If your solution generates suds then dilute it with more
bicarbonate solution.
3. Using a straw, cut 10 uniform leaf disks. Avoid major veins
4. Infiltrate the leaf disks with sodium bicarbonate solution.
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Remove the piston or plunger and place the leaf disks into the syringe barrel. Replace the plunger
being careful not to crush the leaf disks. Push on the plunger until only a small volume of air and
leaf disk remain in the barrel
Pull a small volume of sodium bicarbonate solution into the syringe. Tap the syringe to suspend
the leaf disks in the solution.
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Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold this
vacuum for about 10 seconds. While holding the vacuum, swirl the leaf disks to suspend them in
the solution. Let off the vacuum. The bicarbonate solution will infiltrate the air spaces in the leaf
causing the disks to sink. You will probably have to repeat this procedure 2-3 times in order to get
the disks to sink. If you have difficulty getting your disks to sink after about 3 evacuations, it is
usually because there is not enough soap in the solution. Add a few more drops of soap.
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Pour the disks and solution into a test tube. Add bicarbonate solution to a depth of about 9
centimeters. Use the same depth for each trial
5. Place the test tube into the test tube rack. Place the light source 12 inches away from the test tube
and turn on the light. Record how many leaf discs reached the surface of the solution during every
minute of your experiment. This should be recorded in a table like the one on the next page. Your
numbers should be the cumulative amount per minute. Periodically swirl the disks to dislodge any
that are stuck against the sides of the test tube. Continue until all of the disks are floating or 20
minutes has elapsed.
6. Repeat steps 3 and 4 to collect another set of 10 discs. For this trial you will be using a light bulb
with a different wattage. Continue as before. You will be asked to do this activity using 3 different
light bulbs.
7. A control group is needed to make this a true experiment. For this, follow the procedure above but
wrap your test tube with several layers of paper towel. Remove a small amount of paper towel from
the top of the test tube so that you can see if any leaf discs have risen. You may do this at the
beginning of the experiment.
Data Collection and Analysis
The time required for a leaf disk to float is an index of the net rate of photosynthesis in that leaf disk. However,
since some leaf disks will be "early floaters" and others will be "late floaters", this variable can be reduced in
significance by plotting the percentage of leaf disks floating as a function of time. The time required for 50
percent of the leaf disks to float is called the photosynthetic effective time, shortened to PS ET-50, sort of an
average rate. The larger the PS ET-50, the slower the rate of PS; the smaller the PS ET-50, the faster the rate of
PS.
RATE OF PS = 1 / PS ET-50
Sample Data (testing light intensity)
Time (min)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
0 watt (dark)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
25 watt
0
0
0
0
1
1
1
1
1
2
2
2
3
4
5
5
5
5
6
6
7
75 watt
0
0
1
1
1
2
2
3
3
4
5
5
5
5
5
5
6
7
8
9
10
150 watt
0
0
1
4
5
6
6
6
7
7
8
8
8
9
10
10
10
10
10
10
10
Figure the rate of PS (1/ET-50) for each of the above light intensities (note: since the dark never had half of the
disks floating, the value will be zero).