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The Rate of Photosynthesis at Different Environmental Conditions
Kellie Wong
Section 3
Introduction
Photosynthesis is the process by which plants capture sunlight to produce energy (Mason,
Losos, & Singer, 2014). This energy from photosynthesis is the main source of energy on Earth,
which is why photosynthesis is a very important biological process (Diskin, 2007). In order to
produce energy, plants take in carbon dioxide and water in the presence of light to produce
oxygen and glucose (Diskin, 2007). Though this appears to be a simple chemical reaction, it is in
fact a very complex process (Diskin, 2007). Photosynthesis takes place in an organelle called a
chloroplast (Diskin, 2007). A chloroplast contains a pigment called chlorophyll which allows
light to be captured (Diskin, 2007). The process of photosynthesis can be split into two different
stages: the light-dependent reaction and the light- independent reaction (Mason, Losos, & Singer,
2014).
The light dependent reaction happens in the thylakoid membrane of the chloroplast and it
depends on sunlight (Mason, Losos, & Singer, 2014). The light dependent reaction consists of
two photosystems and an electron transport chain (ETC) (Mason, Losos, & Singer, 2014). A
photosystem is organized into two parts: an antenna complex and a reaction center (Mason,
Losos, & Singer, 2014). The antenna complex is responsible for capturing photons from sunlight
and feeding them to the reaction center (Mason, Losos, & Singer, 2014). In the reaction center
the chlorophyll pigments absorb the photon from the antenna complex and an electron is excited
to a higher energy level (Mason, Losos, & Singer, 2014). The now excited electron will be
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transferred from the reaction center to the next electron acceptor in the ETC (Mason, Losos, &
Singer, 2014). As the electrons travel through the two photosystems and the electron acceptors,
NADP+ is reduced and adenosine triphosphate (ATP) is generated by ATP synthase (Mason,
Losos, & Singer, 2014).
The light- independent reaction takes place in the stroma of the chloroplast and does not
require sunlight (Mason, Losos, & Singer, 2014). The light- independent reaction, also known as
the Calvin cycle, can be split into three phases: carbon fixation, reduction, and regeneration
(Mason, Losos, & Singer, 2014). Carbon fixation combines carbon dioxide and ribulose 1,5biphosphate (RuBP) to produce two molecules of the 3-carbon acid, PGA (Mason, Losos, &
Singer, 2014). PGA is then reduced into glyceraldehyde 3-phosphate (G3P) (Mason, Losos, &
Singer, 2014). RuBP is then regenerated from G3P (Mason, Losos, & Singer, 2014). After six
turns of the Calvin cycle, one molecule of glucose can be produced (Mason, Losos, & Singer,
2014).
In these experiments, the rate of photosynthesis was investigated (Vodopich & Moore,
2011). In experiment one, two variables were tested: light conditions vs. dark conditions and
distilled water vs. sodium bicarbonate (Vodopich & Moore, 2011). In experiment two, the effect
of leaf pigment on the rate of photosynthesis was tested (Vodopich & Moore, 2011). The rate of
photosynthesis can be indirectly measured by the floating leaf method. (Lab 4, 2001). This
method requires gases to be evacuated from the leaf so that the leaf sinks to the bottom of the
solution (either distilled water or sodium bicarbonate) (Lab 4, 2001). Once photosynthesis
occurs, oxygen bubbles will be produced in the layers of the leaves causing the leaves to float.
(Lab 4, 2001). For these experiments the rate of photosynthesis can then be determined by taking
5 divided by the time it took for 5 leaf disks to float (Vodopich & Moore, 2011).
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In reference to experiment one, because photosynthesis requires light, the cups under the
light source will undergo photosynthesis at a faster rate than the cups in the dark. Also recalling
that photosynthesis requires carbon dioxide, the leaf disks in the cup with sodium bicarbonate
will go through photosynthesis faster than the leaf disks in the cups with distilled water. Now
referring to experiment two, the rate of photosynthesis will be faster in the green leaves than the
variegated leaves due to the impression that the chlorophyll pigment is more prominent in the
green leaves.
Materials and Methods
Experiment 1
In experiment one, two variables were tested: light dependency and carbon dioxide
dependency (Vodopich & Moore, 2011). Spinach leaves were cut into circles with a core borer
and then ten to thirteen disks were placed into a syringe (Vodopich & Moore, 2011). This was
done four times (Vodopich & Moore, 2011). Then, two syringes were filled with about four mL
of distilled water and two were filled with about four mL of sodium bicarbonate (Vodopich &
Moore, 2011). In order to create the vacuum, a thumb was placed over the opening of the syringe
and the plunger of the syringe was pulled back (Vodopich & Moore, 2011). This motion, along
with swirling the disks, was continued until at least ten disks sunk to the bottom on the syringe,
indicating that the oxygen had been removed (Vodopich & Moore, 2011). Four cups were
labeled as Light; NaHCO3, Light; H2O, Dark; NaHCO3, and Dark; H2O respectively (Vodopich
& Moore, 2011). Each cup was filled to a 3 cm mark of either NaHCO3 or H2O (Vodopich &
Moore, 2011). Then the contents of each syringe was emptied in its corresponding cup
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(Vodopich & Moore, 2011). Next, the cups labeled as “light” were placed under a lamp and the
cups labeled as “dark” were placed in a compartment in the center of the lab table and the
compartment was covered with a piece of cardboard to ensure darkness (Vodopich & Moore,
2011). The initial amount of floating leaf disks was recorded and a timer was started (Vodopich
& Moore, 2011). After each minute, the number of floating leaf disks would be checked for a
total of twenty minutes (Vodopich & Moore, 2011). After twenty minutes, the cups were
disposed of down the sink and cleanup took place (Vodopich & Moore, 2011). With the data, the
rate of photosynthesis could be determined.
Experiment 2
In experiment two, only one variable was tested: the effect of pigment on the production
on oxygen (Vodopich & Moore, 2011). For this experiment, Coleus leaves were used (Vodopich
& Moore, 2011). Using the core borer, about twenty disks were cut from the green part of the
leaf and about twenty disks were cut from the lighter, variegated part of the leaf (Vodopich &
Moore, 2011). Once again, ten disks were placed into four syringes, making sure that green and
variegated leaves were seperated (Vodopich & Moore, 2011). Each syringe was filled with
NaHCO3 and the same vacuuming process from experiment one was used again (Vodopich &
Moore, 2011). Once the oxygen had been removed, the contents of each syringe was placed into
one of four cups that all contained NaHCO3 (Vodopich & Moore, 2011). Then, one cup with
green leaves and one cup with variegated leaves were placed under lamps (Vodopich & Moore,
2011). The other two cups were placed in the dark compartment (Vodopich & Moore, 2011). The
initial amounts of floating leaf disks were taken and then the timing process began again
(Vodopich & Moore, 2011). After each minute, the number of floating disks were counted for a
total of twenty minutes (Vodopich & Moore, 2011). After twenty minutes, the cups were
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disposed of down the sink and clean up took place (Vodopich & Moore, 2011). With the
collected data, the rate of photosynthesis could be determined.
Results
Table 1: Experiment 1
Time
Number of Floating Leaf Disks (out of 10)
Experimental Groups
Control Groups
H20
NaHCO3
H2O
NaHCO3
4
0
5
5
5
1
8
7
5
1
9
7
5
2
8
7
0
1
2
3
4
5
5
10
8
6
7
8
7
7
8
8
7
8
9
7
8
10
7
8
11
7
8
12
7
7
13
7
8
14
7
8
15
7
7
16
7
8
17
7
8
18
7
8
19
7
8
20
7
8
*No data was recorded at the four minute interval due to a missed reading
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
6
Graph 1: Experiment 1- Experimental Group (Light)
Experimental Group (Light)
Number of Floating Leaf Disks
50
40
30
Legend
20
H20
10
NaHCO3
0
0
5
10
-10
15
20
25
Time (min)
Graph 2: Experiment 1- Control Group (Dark)
Number of Floating Leaf Disks
Control Group (Dark)
10
9
8
7
6
5
4
3
2
1
0
Legend
H20
NaHCO3
0
5
10
15
Time (min)
Table 2: Experiment 2
20
25
7
Time
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Number of Floating Leaf Disks (out of 10)
Experimental Groups
Control Groups
Green
Variegated
Green
Variegated
1
2
5
3
2
2
6
3
2
2
6
3
2
2
6
3
2
2
6
4
2
3
6
4
2
3
6
4
2
3
6
4
2
3
6
4
3
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
4
3
6
4
5
3
6
4
7
3
6
5
Graph 3: Experiment 2- Experimental Group (Light)
Experimental Group (Light)
Number of Floating Leaf Disks
8
7
6
5
Legend
4
Green
3
2
Variegated
1
0
0
5
10
15
Time (min)
20
25
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Graph 4: Experiment 2- Control Group (Dark)
Control Group (Dark)
Number of Floating Leaf Disks
7
6
5
4
Legend
3
Green
2
Variegated
1
0
0
5
10
15
20
25
Time (min)
In order to determine which conditions cause the fastest rate of photosynthesis, the
equation, {Rate of Photosynthesis = 5/ time for 5 disks to float}, can be used (Vodopich &
Moore, 2011). However, this equation assumes that all disks were at the bottom of the cup at
time zero and that at least five disks floated by the end of the twenty minutes. Unfortunately, the
results of the experiment do not meet these conditions. But, there is another way to analyze the
results. By graphing each trail and producing a best fit line, the rate of photosynthesis can be
determined by the slope of the line. In experiment one, the first variable tested was the
dependency on light. By comparing the slopes of graph one and two, it is obvious that graph one,
which used a light source, had a larger slope and therefore a higher rate of photosynthesis. Also
in experiment one, the dependency of carbon dioxide was also investigated. Referring to graph
one, it is clear that the cup that contained the sodium bicarbonate had a higher rate of
photosynthesis. So, the results that can be drawn from experiment one is that the presence of
light and carbon dioxide speed up the rate of photosynthesis, which supports the stated
hypothesis.
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In experiment two, the effect of leaf pigment was investigated. Using the same idea that
the slope of the best fit line corresponds to the rate of photosynthesis, it can be determined that
green pigmented leaves undergo photosynthesis at a faster rate than variegated pigmented leaves.
This conclusion is supported by graph 3. It is obvious that the slope that represents the green
leaves is higher than the slope that represents the variegated leaves, therefore, indicating a faster
rate of photosynthesis. Though graph four shows that the slope of the line of the variegated
leaves is higher, this is irrelevant due to the fact that photosynthesis performs more efficiently in
the light than the dark. So, from experiment 2, the conclusion can be made that photosynthesis is
more efficient in green pigmented leaves than variegated pigmented leaves, which supports the
stated hypothesis.
Discussion
From the results of the two experiments, it was found that photosynthesis is more
efficient in green leaves in the presence of sunlight and carbon dioxide. The explanation for these
results is simple. Photosynthesis takes place in the chloroplasts of a plant (Diskin, 2007). In the
chloroplasts is the pigment chlorophyll which is responsible for capturing light to begin the
photosynthesis process (Diskin, 2007). Chlorophyll is responsible for capturing the red-orange
and blue-violet light and reflecting green light (Diskin, 2007). Because it reflects green light,
leaves appear to be green. (Diskin, 2007). This gives the evidence that green plants contain more
chlorophyll than variegated plants and therefore is able to undergo photosynthesis at a faster rate.
The reasoning behind the dependence on sunlight and carbon dioxide are also
straightforward. Photosynthesis begins as capturing a photon from sunlight and having an
electron travel through the two photosystems and the ETC to produce ATP and NADPH (Mason,
Losos, & Singer, 2014). Without sunlight, this whole process could not be possible and therefore
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supports why photosynthesis works better in sunlight. Photosynthesis also requires carbon
dioxide to go through the Calvin cycle (Mason, Losos, & Singer, 2014). Without carbon dioxide,
the Calvin cycle would not be able to function properly (Mason, Losos, & Singer, 2014). By
using sodium bicarbonate, it introduced the carbon dioxide component into the experiment
(Vodopich & Moore, 2011). When the oxygen was evacuated out of the leaf, sodium bicarbonate
filled the empty spaces in the leaf (Vodopich & Moore, 2011). However, in the distilled water
solution, no carbon dioxide was present and therefore no Calvin cycle could occur (Mason,
Losos, & Singer, 2014).
To conclude, green pigmented leaves in sunlight and in the presence of carbon oxygen
produce the fastest rate of photosynthesis, which supports the hypotheses. Though these
hypotheses are supported, there are more that can be tested to see how the rate of photosynthesis
changes in other conditions. One investigation that could be tested is to see if leaves perform
photosynthesis better in hotter temperatures or cooler temperatures. Another factor to be tested,
is which species of leaves undergo photosynthesis at a faster rate? There are many factors that
can be tested, but these are just a few to name.
Literature Cited
Diskin, B. (2007). How Does Photosynthesis Work. Science 101, April/May, pp. 60-63.
Mason, K. A., Losos, J. B., & Singer, S. R. (2014). Biology. New York, NY: McGraw- Hill.
Vodopich, D.S. & Moore, R. (2011). Biology laboratory manual. New York, NY: McGraw- Hill.
(2001). Lab 4: Plant Pigments and Photosynthesis. AP Biology Lab Manual, The College Board.