Name: ___________________________________
Period: ____
Biology I Honors – Chapter 7 Photosynthesis Lab
The Floating Leaf Disk Assay:
A Photosynthesis Investigation
PRELAB DISCUSSION
Leaf disks float, normally. When 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 (baking soda.)
Bicarbonate ion serves as the carbon source for
photosynthesis. As photosynthesis proceeds, oxygen is
released into the interior of the leaf, altering the
buoyancy, causing the disks to rise. Because 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
Sodium Bicarbonate
Ivy Plant
Stopwatch or Clock
400 ml Beaker
100 ml Graduated Cylinder
Concentrated Dish Soap
Cork Borer
Clamp Lamp
Stirring Rod
Marker
10cc Plastic Syringe
100 ml Beaker (2)
Balance + Massing Tray
Metric Ruler
Distilled H2O
PROCEDURE
Prepare 150 ml of 0.2 M Sodium Bicarbonate (NaHCO3) Solution
1. Add 2.5 g of baking soda to a clean 400 ml beaker.
Molecular weight of NaHCO3 = 84
150ml (0.15 liter) of a 0.2 M solution...
0.2 x 84 x 0.15 = 2.52 g NaHCO3
2. Use a 100 ml graduated cylinder to attain 150 ml of distilled water. Pour this into the 400
ml beaker and stir.
3. Squirt a drop or two of liquid dish soap into your beaker.
Preparing Specimens and Apparatus
4. Avoiding major veins, use the cork borer to cut eight uniform disks from your ivy leaves.
5. Remove the 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 (< 10% of barrel.)
6. Draw roughly 5 ml of sodium bicarbonate solution into the syringe. Tap the syringe to
suspend the leaf disks in the solution.
7. Holding a finger over the syringe-opening, draw back a bit 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
may need to repeat procedural step #7 again 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 another drop or two
of soap and try again.
8. Pour the disks and solution into a clean 100 ml beaker. Using a clean 100 ml graduated
cylinder, add 80 ml of 0.2 M sodium bicarbonate solution. Label this beaker “CO2.”
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Setting Up a Control
OBSERVATIONS
9. Rinse the 400 ml beaker and add 150 ml
of distilled water. Squirt a drop or two of
liquid dish soap into your beaker.
DATA TABLE ONE
10. Cut and infiltrate eight more leaf disks
using the new solution of soapy water (no
sodium bicarbonate this time.)
11. Pour the disks and solution into a clean
100 ml beaker. Using the 100 ml graduated
cylinder, add 80 ml of the soapy water
solution. Label this beaker “No CO2.”
Observing Photosynthesis
12. Clamp your light source about 6 inches
(15.24 cm) above your beakers, or lay it on
the lab table at that distance and start your
stopwatch.
13. At the end of each 30 second interval,
record the number of floating disks in Data
Table One. After counting, gently swirl the
disks to dislodge any that are stuck against
the sides or bottom of the beaker.
Continue until all of the disks are floating.
Elapsed
Number of
Elapsed
Number of
Time
Floating Disks
Time
Floating Disks
30s
10m 30s
1m
1m 30s
2m
11m
11m 30s
12m
2m 30s
12m 30s
3m
13m
3m 30s
13m 30s
4m
14m
4m 30s
14m 30s
5m
15m
5m 30s
15m 30s
6m
16m
6m 30s
16m 30s
7m
17m
7m 30s
17m 30s
8m
18m
8m 30s
18m 30s
9m
19m
9m 30s
19m 30s
10m
20m
Extend the table if necessary.
AFTER ALL DATA IS COLLECTED, CLEAN ALL MATERIALS AND
LEAVE YOUR LAB STATION LIKE YOU FOUND IT!
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RESULTS
You may use the area below, or create a computer generated graph (eg. Excel) to display your
results from Data Table One. Remember our discussions on variable location and titles when
graphing. If using a computer program, staple your graph to this packet.
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POST LAB QUESTIONS
1. Assume we had more time and wanted our disks to sink again. How could we achieve this?
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2. Discuss the electromagnetic spectrum usage and how photosynthesis may be different if we
used red cabbage in this experiment.
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3. Compare and contrast the energy transfers that occur in plant’s mitochondria during
cellular respiration and in their chloroplasts during photosynthesis.
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4. Describe where, when, and why NADP+ is reduced to form NADPH during photosynthesis.
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5. I selected the presence of sodium bicarbonate (source of CO2) as our independent variable
for this investigation. List four other possible variables that could be tested in a controlled
experiment demonstrating photosynthesis via the floating leaf disk assay. Develop a
hypothesis for each potential experiment.
Alternate Variable: ________________________________________________________________
Hypothesis: ______________________________________________________________________
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Alternate Variable: ________________________________________________________________
Hypothesis: ______________________________________________________________________
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Alternate Variable: ________________________________________________________________
Hypothesis: ______________________________________________________________________
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Alternate Variable: ________________________________________________________________
Hypothesis: ______________________________________________________________________
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This investigation was
grown by Mr. Boylan using
Brad Williamson’s Leaf
Disk Assay Resource Page.
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