Measuring Photosynthetic Rate
Introduction:
Photosynthesis is the ultimate source of energy for most life on the Earth. That is, the
energy that is transferred from one organism to the next can be traced back to the
photoautotrophs (plants, algae, and photosynthetic bacteria) which captured that energy
from sunlight. The process of photosynthesis can broadly be divided into two processes:
the Light-Dependent reactions and the Light-Independent reactions. The LightDependent reactions use energy from sunlight captured by chlorophyll and other
photosynthetic pigment molecules and convert it into chemical (stored) energy in
the form of NADPH and ATP while releasing O2 as a byproduct:
H 2O  ADP  NADP  Light

 O2  ATP  NADPH
The next step, the Light-Independent reactions, takes that chemical energy and stores it in
the form of sugars - primarily glucose - a much more compact and stable energy storage
molecule:
CO2  ATP  NADPH 
 NADP   ADP  C6 H 12O6
Glucose is often converted into starches and fats for longer term energy storage needs.
The two above reactions combine to form the familiar photosynthesis reaction:
6CO2  6 H 2O Light

 C6 H 12O6  6O2
There are several methods for measuring photosynthetic rate. Most of these methods
involve measuring the rate of either product formation (glucose or oxygen) or reactant
usage (carbon dioxide and water). In this activity, the rate of oxygen production by
photosynthesis will be measured by the relative float rates of leaf disks. In this laboratory,
air spaces in leaf tissue will be infiltrated with sodium bicarbonate (NaHCO3) solution.
The mass of the solution increases the mass of the leaf disk such that the leaf discs, which
normally float, now sink. As shown in the equation below, the sodium bicarbonate
(NaHCO3) quickly decomposes into water and carbon dioxide which is consumed by the
leaf disk during photosynthesis.
NaHCO3
+
H2O
H2CO3
+
Sodium bicarbonate
Water
Carbonic acid
H2CO3
Carbonic acid
H2O
Water
+
OH-
+
Hydroxide ion
CO2
Carbon dioxide
While CO2 is consumed O2 is produced, increasing
the buoyancy of the leaf, eventually causing it to rise
to the surface. The rate at which disks rise is therefore
an indirect measure of the net rate of photosynthesis.
Materials (per group):
Spinach leaf disks
hole punch
syringe
1
Na+
Sodium ion
graduated cylinder
2 100 mL beakers
colored cellophane timer
funnel
transparent tape
0.2% sodium bicarbonate/soap solution
light source
soap solution
Activity 1
Lab Protocol
1. Using a hole punch cut leaf disks from a spinach leaf, avoiding the large veins.
Take care not to crush the disks.
2. Remove the piston from a syringe and place the leaf disks inside, near the needle
end. Replace the piston without damaging the leaf disks.
3. Push on the piston until only a small volume of air and the leaf disks remain in the
barrel.
4. Pull approximately 10 ml of 0.2% sodium bicarbonate/soap solution into the
syringe. The soap in the solution will allow the sodium bicarbonate to penetrate
the hydrophobic surface of the leaf disks. Tap the syringe to suspend the leaf
disks in the solution.
5. Hold a finger over the syringe opening and draw back on the piston to create a
vacuum. While holding the vacuum, swirl the leaf disks to suspend them in the
solution. Hold this vacuum for approximately 30 seconds.
6. Remove your finger, releasing the vacuum. Pull the piston almost to the end of the
syringe.
7. Hold a finger over the opening and push the piston in firmly, creating high
pressure for approximately 30 seconds.
8. The bicarbonate solution will infiltrate the air spaces in the leaf causing the disks
to sink. You will probably have to repeat steps 5-7 at least 3 times in order to get
most of the disks to sink.
9. Transfer a minimum of 10 leaf disks to a 100ml beaker to which 60 ml of
bicarbonate solution has been added. Remove any floating disks and discard.
Determine and record the number of disks remaining on the bottom of the beaker.
10. Place the beaker under a light source. Count the number of disks floating at the
end of each one minute period and record in Table 1 below. Continue
observation for 25 minutes, or until all disks float. Discard leaf disks and solution.
11. Determine the percent of leaf disks floating using the following formula:
% 𝑜𝑓 𝑙𝑒𝑎𝑓 𝑑𝑖𝑠𝑘𝑠 𝑓𝑙𝑜𝑎𝑡𝑖𝑛𝑔 =
Table 1: Group Results Activity 1
1 2 3 4 5 6 7 8 9 10
Minutes
# Floating
% Floating
11
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑙𝑒𝑎𝑓 𝑑𝑖𝑠𝑘𝑠 𝑓𝑙𝑜𝑎𝑡𝑖𝑛𝑔
𝑥 100%
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑙𝑒𝑎𝑓 𝑑𝑖𝑠𝑘𝑠
12
13
14
15
16
17
18
19
Activity 2
1. Repeat Activity 1 Protocol, placing the beaker in room light only.
2
20
21
22
23
24
25
2. Record Group data in Table 2.
Table 2: Group Data Activity 2
1 2 3 4 5 6 7 8 9
Minutes
# Floating
% Floating
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
24
25
Activity 3
1. Repeat Activity 1 Protocol. Instead of using 0.2% sodium bicarbonate/soap
solution, substitute with soap solution for both infiltration of leaf disks and beaker
environment (i.e., don’t use sodium bicarbonate solution!). Place the beaker under
a light source.
2. Record Group Data in Table 3.
Table 3: Group Data Activity 3
1 2 3 4 5 6 7 8 9
Minutes
# Floating
% Floating
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Activity 4
1. Repeat Activity 1 Protocol, completely covering the beaker – top and sides - with
red-colored cellophane. Transparent materials, such as cellophane, are materials
which allow one or more of the frequencies of visible light to be transmitted
through them; whatever color(s) is/are not transmitted by such objects, are
typically absorbed by them. Therefore, red cellophane will only transmit red
wavelengths of light energy.
2. Record Group Data in Table 4.
Table 4: Group Data Activity 3
1 2 3 4 5 6 7 8 9
Minutes
# Floating
% Floating
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Activity 5
1. Collect class averages for percent floating per minute for Activities 1-4 and record
in Tables 5 through 8.
Table 5: Class Data Activity 1
1 2 3 4 5 6 7 8
Minutes
% Floating
Table 6: Class Data Activity 2
1 2 3 4 5 6 7 8
Minutes
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3
% Floating
Table 7: Class Data Activity 3
1 2 3 4 5 6 7 8
Minutes
% Floating
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Table 8: Class Data Activity 4
1 2 3 4 5 6 7 8
Minutes
% Floating
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Group Assignment:
1. With a Scatter Plot Graph, a mark, usually a dot or small circle, represents a single data
point. As the data points are plotted, a visual distribution of the data can be
seen. Depending on how tightly the points cluster together, you may be able to discern
a clear trend in the data. Graph the class result averages for each of the four activities
on a single XY Scatter Plot Graph (example below). The completed graph should
possess four sets of plotted points and four lines of best fit. Use a different color for
each set of data and make sure to include a key.
Y
X
2. Turn in a standard lab report. Use the class averages
when reporting results and include your graph for the data representation.
4